ASNT Level II Study Guide
Liquid Penetrant Testing Method
Published by The American Society for Nondestructive Testing, Inc. 1711 Arlingate Lane PO Box 28518 Columbus, OH 43228-0518 Copyright © 2002 by The American Society for Nondestructive Testing, Inc. ASNT is not responsible for the authenticity or accuracy of information herein. Published opinions and statements do not necessarily reflect the opinion of ASNT. Products or services that are advertised or mentioned do not carry the endorsement or recommendation of ASNT. 1RRSP, The NDT Technician and www.asnt.org are trademarks of The American Society for Nondestructive Testing, Inc. ACCP, ASNT, Level III Study Guide, Materials Evaluation, NDT Handbook, Nondestructive Testing Handbook, Research in Nondestructive Evaluation and RNDE and are registered trademarks of The American Society for Nondestructive Testing, Inc. ASNT Mission Statement ASNT exists to create a safer world by promoting the profession and technologies of nondestructive testing. IBN-13: 978-1-57117-092-7 ISBN-10: 1-57117-092-8 Published by the American Society for Nondestructive Testing, Inc. PRINTED IN THE UNITED STATES OF AMERICA. first printing 1998 second printing with revisions 10/02 third printing 08/07
Contents Contents.....................................................................................................................................................iii Introduction.............................................................................................................................................vii Chapter 1, Overview of Liquid Penetrant Testing....................................................................................1 History of Liquid Penetrant Testing .....................................................................................................1 Principles of Liquid Penetrant Testing ...................................................................................................1 Advantages of the Penetrant Process ...............................................................................................I Disadvantages of the Penetrant Process...............................................................................................1 Basics of the Penetrant Process ..............................................................................................................2 Preparation of Parts for Penetrant Testing ........................................................................................2 Surface Condition .......................................................................................................................2 Penetrant System.................................................................................................................................3 System Sensitivity ........................................................................................................................3 Type of Penetrant............................................................................................................................3 Advantages of Fluorescent Liquid Penetrants ...........................................................................4 Advantages of Visible Dye Penetrants .....................................................................................4 Selection of Liquid Penetrant Material and Process .....................................................................5 Fluorescent Liquid Penetrant ...................................................................................................5 Visible Dye Liquid Penetrant ...................................................................................................5 Penetrant Application...........................................................................................................................5 Penetrant Dwell ............................................................................................................................5 Temperature ..................................................................................................................................6 Penetrant Removal Method .............................................................................................................6 Water Washable (Method A) .......................................................................................................6 Lipophilic Emulsifier (Method B) ...............................................................................................7 Solvent Removable (Method C) .................................................................................................8 Hydrophilic Emulsifier (Method D) ...........................................................................................8 Drying ................................................................................................................................................ .8 Develop the Indication ......................................................................................................................8 Inspection............................................................................................................................................8 Importance of the Inspector's Evaluation .....................................................................................9 Penetrant Inspection Documents .................................................................................................9 Visual Requirements.......................................................................................................................9 Classification of Discontinuities by Origin ..................................................................................9 Identifying Indications ...............................................................................................................10 Postcleaning ....................................................................................................................................11 Chapter 2, Penetrants, Developer and Equipment ..............................................................................13 Penetrants ............................................................................................................................................13 Characteristics of Penetrants ...........................................................................................................13 Surface Tension ..........................................................................................................................13 Wetting Ability..............................................................................................................................13 Capillary Action ..........................................................................................................................13
Physical Properties of Penetrants ...................................................................................................14 Viscosity ....................................................................................................................................14 Volatility ....................................................................................................................................14 Specific Gravity .........................................................................................................................14 Flash Point ..................................................................................................................................14 Thermal Stability ........................................................................................................................14 Chemical Properties of Penetrants................................................................................................... .14 Inertness ....................................................................................................................................14 Removability ..............................................................................................................................15 Water Tolerance............................................................................................................................15 Developer ............................................................................................................................................15 Dry Powder ....................................................................................................................................16 Water Soluble ..................................................................................................................................16 Water Suspended ............................................................................................................................16 Nonaqueous ....................................................................................................................................17 Specific Application .......................................................................................................................17 Material and Process Control Tests .....................................................................................................17 Ultraviolet Light Intensity .............................................................................................................19 Emulsifier Concentration...................................................................................................................19 Ambient White Light Intensity..........................................................................................................19 Penetrant Inspection Area Cleanliness ...........................................................................................19 Penetrant Materials Testing...............................................................................................................19 Water Content...............................................................................................................................19 Separation of Constituents and Loss of Fluorescence ................................................................19 Washability ...............................................................................................................................19 Sensitivity Comparison .............................................................................................................19 Penetrant System Monitor......................................................................................................................19 Equipment ..........................................................................................................................................20 Ultraviolet Lighting .......................................................................................................................20 Stationary Processing Equipment ...................................................................................................21 Portable Processing Equipment.........................................................................................................21 Automated Equipment ....................................................................................................................22 Chapter 3, Testing in Primary Metals Production.................................................................................23 Producing Metals and Alloys .............................................................................................................23 Processing Discontinuities ..................................................................................................................23 Discontinuities Related to Metal Solidification.................................................................................23 Discontinuities in Metal Ingots ........................................................................................................23 Discontinuities in Metal Castings ...................................................................................................23 Discontinuities in Blooms and Billets .............................................................................................23 Discontinuities in Hot Rolled Bars, Shapes, Plate or Strip .............................................................24 Discontinuities in Forgings .............................................................................................................24 Discontinuities in Rolled and Pierced Products ..............................................................................24 Discontinuities in Extrusions .........................................................................................................24 Discontinuities in Cold Worked Metal Products .............................................................................24 Discontinuities in Fusion Welds .....................................................................................................25 Discontinuities in Heat Treating Processes .....................................................................................25 Discontinuities in Machined and Ground Surfaces .........................................................................25 Cracks During Plating or Chemical Treatment ..............................................................................25
Service Discontinuities..........................................................................................................................25 Fatigue Cracks ................................................................................................................................25 Significance of Surface Discontinuities under Repeated Loading or Vibration ........................25 Prevention of Fatigue Failure on Service ...................................................................................25 Stress Corrosion Cracking ................ ..............................................................................26 Other Causes of Service Damage....................... ..................................................................26 Continuous Line Indication.......................... ..................................................................27 Intermittent Line Indications ................... .............................................................................27 Rounded Areas of Indications ........... .................................................................................27 Small Dot Indications........................................................................................................................27 Diffuse Indications ........................ . . . .......................................................................................27 Edges of Indications .......................... ........................................................................................27 Chapter 4, Appearance of Liquid Pentrant Indications . . . . . ............................................................27 Brilliance and Extent of Indications .....................................................................................28 Persistence of Indications......................................................................................................................28 Other Variables Affecting Indication Appearance................................................................……...:... .28 Surface Conditions............................................................................................................................28 Temperature ...................................................................................................................................28 Dwell Time and Washing ............... .....................................................................................28 Developer..........................................................................................................................................29 Previous Testing.............................................................................. .......................................29 Common Causes of Nonrelevant and False Indications ......................................................................29 Poor Process Control.........................................................................................................................29 Part Geometry and Surface Condition...............................................................................................30 Evaluation of Nonrelevant Indications............................................... .....................................30 Review Questions.....................................................................................................................................31 Answers.....................................................................................................................................................37 Appendix, Standard Terminology for the Liquid Penetrant Examination..........................................39
VI
Introduction Overview of the Study Guide
Resource Materials
This study guide contains basic information intended to prepare a candidate for Level II penetrant inspection examinations. This study guide does not present all of the knowledge necessary for certification; the candidate is expected to supplement this guide with the recommended references that follow. "Recommended reading" boxes, which list additional areas of study and which Recommended References contain the information, appear throughout the text.
ASM Handbook, 9th edition, Volume 17, Nondestructive Evaluation and Quality Control, Materials Park, OH, ASM International, 1989.
Acknowledgments The authors acknowledge the support of the AlliedSignal Aerospace NDT Network and the ASNT Technical Services Department staff for assisting with the first edition of this book. This revision was possible thanks to the contributions of Bill Chedister, William Cook, Vilma Holmgren, Bill Mooz and Thorn Schafer.
Recommended References A. Tracy, Noel A., technical editor, Patrick O. Moore, editor, Nondestructive Testing Handbook, third edition: Volume 2, Liquid Penetrant Testing, Columbus, OH, American Society for Nondestructive Testing, Inc., 1999.
Recommended Practice No. SNT-TC-1A, 2001, Columbus, OH, American Society for Nondestructive Testing, Inc., 2001. ANSI/ASNT CP-189-2001: ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel, Columbus, OH, American Society for Nondestructive Testing, Inc., 2001. ASTM International, ASTM E 165, Standard Test Method for Liquid Penetrant Examination, West Conshohocken, PA, ASTM, 2001. ASTM International, ASTM E 1208, Standard Test Method for Fluorescent Liquid Penetrant Examination Using the Lipophilic PostEmulsification Process, West Conshohocken, PA, ASTM, 2001. ASTM International, ASTM E 1209, Standard Test Method for Fluorescent Liquid Penetrant Examination Using the Water-Washable Process, West Conshohocken, PA, ASTM, 2001.
B. Boisvert, Bernie, Principles and Applications of Liquid Penetrant Testing: A Classroom Training Text, Columbus, OH, American Society for Nondestructive Testing, Inc., 1993.
ASTM International, ASTM E 1210, Standard Test Method for Fluorescent Liquid Penetrant Examination Using the Hydrophilic PostEmulsification Process, West Conshohocken, PA, ASTM, 2001.
C. General Dynamics, Liquid Penetrant Testing Programmed Instruction Handbook, PI-4-2, Columbus, OH, American Society for Nondestructive Testing, Inc., 1977.
ASTM International, ASTM E 1219, Standard Test Method for Fluorescent Liquid Penetrant Examination Using the Solvent-Removable Process, West Conshohocken, PA, ASTM, 2001.
viii ASTM International, ASTM E 1219, Standard Test Method for Fluorescent Liquid Penetrant Examination Using the Solvent-Removable Process, West Conshohocken, PA, ASTM, 2001. ASTM Internationa], ASTM E 1220, Standard Test Method for Visible Penetrant Examination Using Solvent-Removable Process, West Conshohocken, PA, ASTM, 2001. ASTM Internationa], ASTM E 1417, Standard Practice for Liquid Penetrant Examination, West Conshohocken, PA, ASTM, 2001. ASTM International, ASTM E 1418, Standard Test Method for Visible Penetrant Examination Using the Water-Washable Process, West Conshohocken, PA, ASTM, 2001. SAE International, SAE AMS 2644, Inspection Materials, Penetrant, Warrendale, PA, SAE (Society of Automotive Engineers) International, 2002.
Chapter 1 Overview of Liquid Penetrant Testing History of Liquid Penetrant Testing Liquid penetrant testing is one of the oldest methods of nondestructive testing. Its origin is generally attributed to the inspection of wheel axles in the railroad industry in the 1890s. It was noticed that after oil was wiped off of a part surface, the oil would reappear where a crack or other type of discontinuity was located. This discovery led to the oil-and-whiting test. Parts were immersed in oil and then wiped with rags dampened with kerosene. Powdered chalk was used to dust the surface to increase the visibility of any oil leaking back onto the part surface. Today's liquid penetrant testing evolved from the oil-andwhiting test through continual improvements to the type of oil used, the addition of dyes and the creation of special removers and developers.
Principles of Liquid Penetrant Testing Liquid penetrant testing is based on the ability of a liquid to seep into cavities that are open to the surface and remain in the cavity when the excess liquid is removed. The liquid remaining in the cavity is then drawn out to form an indication that is much more visible than the cavity itself. Liquid penetrant testing is used to detect surface discontinuities in solid, nonporous materials. It is used on wrought, cast metal and metal alloys; weldments, powdered metals, plastics, ceramics and glass. A variety of materials and application methods allow for some adjustment of the process sensitivity to certain types of discontinuities, surface conditions and material types. Liquid penetrant testing is one of the most widely used nondestructive testing methods in industry. It is able to detect small discontinuities open to the surface regardless of their orientation
or the shape of the part. With proper sequencing of the process in manufacturing and the use of appropriate cleaning methods with trained and certified operators, liquid penetrant testing is a very reliable, accurate and effective surface inspection process.
Advantages of the Penetrant Process The following are advantages of the liquid penetrant process: 1. very small surface discontinuities can be detected, 2. a wide variety(sự đa dạng) of materials can be tested, 3. complex geometrical shapes can be inspected in a single operation, 4. various sensitivities and processing methods are available, 5. it is relatively (tương đối)easy to use, 6. it can be portable(xách tay, di chuyển được), and 7. when compared to other testing methods, it has a low cost.
Disadvantages of the Penetrant Process The following are disadvantages of the liquid penetrant process: 1. porous(xốp) materials cannot be tested; 2. surfaces must be clean and free of paint, coatings, soil, moisture(hơi ẩm) and contaminants; 3. it cannot be used following metal smearing operations such as shot peening, sand blasting, certain machining operations (unless material is acid etched to remove smeared metal); 4. some materials (rubber and plastics) may be adversely affected by the penetrant's petroleum base; 5. it is very dependent on operator skill; and 6. some penetrant products use ingredients that may cause skin irritation under prolonged contact if proper protective clothing is not worn.
ASNT Level II Study Guide: Liquid Penetrant Testing
Basics of the Penetrant Process The basic penetrant process can be divided into three steps: application of the penetrant, removal of the excess surface penetrant and development of the indication. However, the process must be preceded by appropriate surface preparation and cleaning methods, parts must be dried after penetrant removal, indications must be evaluated after processing and parts must be cleaned after testing.
Preparation of Parts for Penetrant Testing Final examination by the penetrant process should be performed following all operations that may cause or expose discontinuities on the part surface. Examples of these operations are heat treating, machining, welding, bending and straightening. The penetrant process should be performed before any treatments that smear the surface of the part such as: honing, lapping, sanding, deburring, sand blasting or vapor honing. If these types of operations have been performed, the parts must be chemically etched before the penetrant process in order to obtain any meaningful results. Penetrant inspection also should be performed before operations such as peening, plating, painting, anodizing, or any other surface coating operations.
Surface Condition The surface condition of a part must be evaluated before the penetrant process begins. The condition of the surface plays a critical role in the success or failure of the liquid penetrant test to form an indication from a discontinuity.
Coatings such as paint and plating interfere with the penetrant's ability to enter a discontinuity by covering or filling the cavity. Indications produced on coated surfaces are generally from discontinuities in the paint or plating rather than those in the base material. Anodized coatings are very porous. Absorption of the penetrant can mask discontinuities by making relevant indications indistinguishable from the anodized surface of the part. Operations that deform the surface, such as grinding, cutting, shot peening, sand blasting and vapor honing, can cover discontinuities by smearing a thin layer of metal over the opening, reducing its size or sealing it completely. Because liquid penetrant testing cannot detect anything not open to the surface, this thin coating of metal prevents the penetrant liquid from entering the cavity and forming an indication. These operations also can cause discontinuities in the material and performing the liquid penetrant test before the metal smearing operation is not always an option. In such cases, the material should be chemically etched following the mechanical operation and before the liquid penetrant test. The most common surface condition that adversely affects the liquid penetrant testing process is the presence of contaminants such as oils, grease, dirt, rust, scale, carbon deposits or others. These materials may prevent the penetrant from entering the discontinuity, absorb the penetrant and mask indications, or chemically react with the penetrant materials reducing the penetrant's effectiveness. Precleaning of components using the appropriate method before liquid penetrant testing is critical. Table 1.1 lists common contaminants that affect the penetrant process.
Recommended reading Subject
Reference*
history of liquid penetrant testing
A,B,C
basic principles of liquid penetrant testing
A,B,C
advantages and disadvantages of liquid penetrant testing
A,B,C
*See introduction for explanation of references.
Penetrant System Liquid penetrant testing is defined by the materials used to perform the process. The variations in the testing process produce several system sensitivities and types of penetrants. System Sensitivity The sensitivity of a system is defined as a measure of the smallest discontinuity that will produce a discernible indication. The sensitivity of a system must meet the needs of the circumstances. If a system is too sensitive, the results may be misleading and time will be wasted in the evaluation process. If the system is not sensitive enough, rejectable defects could remain undetected. The cost of materials can be directly related to system sensitivity and the use of a sensitivity higher than required would increase the cost unnecessarily. The appropriate liquid penetrant testing system is determined by considering the following: 1. governing standard or specification - certain industries require specific penetrant methods, or specify restrictions on the materials that may be used; 2. surface condition and the type of discontinuities associated with the part - rough surfaces, threads, keyways, internal cavities, wide, shallow or tight crack discontinuities are conditions that can affect the optimum choice for a penetrant system; and
3 3.
location, size of the part, quantity and coverage required - field inspections are most often performed with solvent removable visible dye and nonaqueous developer because of portability and no special lighting is needed.
Type of Penetrant Penetrant can be fluorescent, visible or dual mode dye that has both visible and fluorescent properties. Fluorescent penetrant is further defined by a sensitivity level category. The sensitivity of a penetrant is related to the brightness of an indication when measured with a photometer and compared to a standard. There are five levels of sensitivity for fluorescent dyes: 1. Level 1/2: ultralow sensitivity, 2. Level 1: low sensitivity, 3. Level 2: medium sensitivity, 4. Level 3: high sensitivity, and 5. Level 4: ultrahigh sensitivity. Fluorescent penetrant contains a dye that is visible under UV-A light. To view the indications, inspection is performed in a darkened area with an ultraviolet light source. Visible penetrant uses a red, orange or purple dye that is highly visible when contrasted against a white developer coating the part surface. Inspection must be performed under white light.
Table 1.1: Precleaning Contaminant Carbon, varnish Heavy oils Light oils Oil with solid particles Paint Penetrant material residue Scale, oxide, corrosion Smeared metal
Effects on the Penetrant Process Bridge/fill voids, produce residual background, cause false indications Fill voids, fluoresce Fill voids, fluoresce Fill voids, fluoresce, cause false indications Bridge voids, produce residual background, cause false indications Fill voids, interfere with indication formation Bridge/fill voids, produce residual background, cause false indications Bridge/fill voids
ASNT Level II Study Guide: Liquid Penetrant Testing Dual mode penetrants contain dyes that are both colored under white light and fluorescent under ultraviolet light. However, the intensities are less than the single mode penetrants. Tables 1.2 and 1.3 compare the types of penetrant and sensitivities. Advantages of Fluorescent Liquid Penetrants Fluorescent indications viewed in a dark test booth are more readily seen than indications produced by visible dye liquid penetrants under normal light conditions. This is especially true in the case of liquid penetrant indications of minute cracks or pinholes. Because fluorescent liquid penetrant indications are more visible, it is generally accepted that fluorescent liquid penetrant testing is more sensitive to small discontinuities
than visible dye. Also, because luminous discontinuity marks can be located easily, the fluorescent dye lends itself to faster readout. Aerospace industry specifications often call for fluorescent testing of critical parts because very small discontinuity indications must be detected reliably. Advantages of Visible Dye Penetrants Though its overall performance is considered lower and production testing of small parts is slower, the visible dye liquid penetrant process has some advantages of its own. It does not require a darkened area or a source of electric power for illumination. For this reason, it is more often used as a portable testing tool in the shop or field. Large test parts, difficult to move to a dark booth, can be
Table 1.2: Comparison of fluorescent penetrant sensitivity levels Level
Primary Uses and Advantages
1/2 and 1 In process inspection - minimal background Complex geometry Porous surfaces 2 3
4
In process inspection Final inspection In process and final inspection Sensitive to small, tight discontinuities In process and final inspection Very sensitive to small, tight discontinuities
Disadvantages Least sensitive
May produce moderate background on rough surfaces High background on rough surfaces
High background on rough surfaces
Table 1.3: Comparison of penetrant types Type Primary Uses and Advantages 1 In process and final inspections Wide, shallow and tight crack discontinuities Failure analysis 2 In process inspection of surfaces machined in further processing. Field inspections with portable kit Localized area of large parts
Disadvantages Requires special lighting Requires electricity Less sensitive than Type 1 Does not perform well for wide, shallow discontinuities
5
inspected where they are located with visible dye liquid penetrants. The visible dye liquid penetrant is widely used for inspection of weldments both in the shop and in the field. Selection of Liquid Penetrant Material and Process The techniques capable of the greatest sensitivity are often the most costly and many tests require the ultimate sensitivity. However, high sensitivity is frequently not required and may even produce misleading results. The following factors should be considered before deciding on the type of liquid penetrant testing system to install and/or use: 1. composition of parts to be inspected; 1. number of parts or test areas to be inspected per unit of time; 2. size and weight of parts to be handled; 2. location at which testing is required (that is, receiving department, in process along a production line, during assembly, as final inspection, in the field during erection, at maintenance facilities or in service); 3. types of discontinuities to be expected, for example, porosity, cracks, seams or laps; 4. size of discontinuities to be detected (for example, small, deep, large, shallow, clustered or scattered); 5. surface condition of parts to be inspected (such as as-cast, as-forged, machined, ground, lapped, polished, plated, painted, corroded, oily, covered with combustion, scratched or scaled parts); and 6. condition to which parts will be subjected after testing (for example, medical, nuclear or oxygen systems; welding, plating or finishing operations; high temperature, aerospace, industrial or transportation uses; inaccessible locations; or consumer products). The selection may be influenced by specification requirements, education and experience of personnel and the quality level of test required. The quality of the test parts to be inspected should never be allowed to influence the quality level of testing. The test quality is determined by the nature of test parts and the service conditions for which they are intended.
Fluorescent Liquid Penetrant The regular performance water washable, fluorescent liquid penetrant is generally more practical for rough surface parts such as sand castings. The higher performance liquid penetrants are favored for inspecting forgings, machined surfaces and more critical load carrying parts. Visible Dye Liquid Penetrant The selection of a visible dye liquid penetrant is relatively uncomplicated when compared to selecting fluorescent liquid penetrants. If the test is to take place in the field, a portable system using a solvent remover wipe-off technique is usually the first choice. Selection of a visible dye liquid penetrant is largely dependent on economic factors and the facilities available. The visible dye liquid penetrant class is not divided into performance grades as is fluorescent liquid penetrants. The visibility of the indications with respect to the white developer background is a key factor in successfully revealing tight discontinuities. The visibility of colored contrast liquid penetrants depends on the color dye concentration. Because visible dye liquid penetrants are not categorized with respect to visibility or performance, these dyes are selected based on need.
Penetrant Application Penetrant may be applied by various methods as listed in Table 1.4. Care should be taken to ensure complete coverage of the areas of interest, regardless of how the penetrant is applied. Brushing and spraying can fail to cover areas completely and immersion can trap air and prevent penetrant (or emulsifier when rinsing) from contacting the part surface. Penetrant Dwell The total time the penetrant remains on the part surface is the penetrant dwell. The time required for a penetrant to enter a discontinuity is dependent on temperature, the type of discontinuity and the possibility of contaminants entrapped within discontinuities. Large, shallow discontinuities require less dwell time than small, tight cracks. Discontinuities free from contaminants require less dwell time than discontinuities with entrapped soil because it is easier for the penetrant to enter them. The standard minimum dwell is 5 min unless testing in service parts. If service fatigue cracks are
ASNT Level II Study Guide: Liquid Penetrant Testing suspected, the minimum dwell time required is often 2 h, but for stress corrosion up to 4 h may be required. When the dwell exceeds 1 h, applying fresh penetrant every hour improves the rate of penetration. It is important to allow sufficient time for the appearance of minute indications from fine discontinuities, such as tight fatigue cracks. To use the time required for an indication to develop as a measure of the extent of the discontinuity, other variables such as type of liquid penetrant, sensitivity of technique, temperature of part, dwell time and condition of examination must be controlled. Fine liquid penetrant indications usually denote fine discontinuities, however, fine indications may also be caused by insufficient dwell time. Temperature The temperature of the penetrant materials and the part must be within an acceptable range for processing. ASTM E 1417 specifies a temperature range of 4 to 52 °C (40 to 125 °F). The viscosity of most liquids, including penetrants, increases at low temperatures. If a part is quite cold, under 16 °C (60 °F) for example, the liquid penetrant may become chilled and thickened when applied, making the dwell time for very fine discontinuities longer than for parts at higher temperatures. If a part or liquid penetrant is too hot, the volatile components of the liquid penetrant may evaporate, changing the ability for revealing small discontinuities. Material temperatures from 10 to 52 °C (50 to 125 °F) produce optimum results with conventional liquid penetrants, however, special penetrants developed for
aerospace and nuclear power programs can be used in extremely cold or relatively high temperatures.
Penetrant Removal Method Excess penetrant is removed by water wash, hydrophilic emulsification, lipophilic emulsification, or solvent wipe. The penetrant must be removed from the part surface without removing it from within a discontinuity. An optimum amount of removal must be obtained. Too little removal will cause an excessive penetrant background and indications will be indistinguishable from the part surface. If too much is removed from the discontinuity, a weak indication or no indication will be formed. The methods for removing surface penetrant and the SAE AMS 2644 designation are: 1. Method A - Water Washable 2. Method B - Lipophilic Emulsifier 3. Method C - Solvent Removable 4. Method D - Hydrophilic Emulsifier Fluorescent penetrant should be removed in a darkened area equipped with ultraviolet lighting regardless of the removal method. Table 1.5 lists the advantages and disadvantages of the various removal methods. Water Washable (Method A) Water washable penetrants contain emulsifying agents that make the oil miscible (mixable) with water. The penetrant may be removed with a manual or automatic coarse water spray, by immersion in an air agitated water wash or by manually wiping.
Table 1.4: Penetrant application Type of Penetrant
AMS-2644 Designation
Fluorescent
1
Visible
2
Dual mode
Method of Application* Spray, dip, brush, Spray, dip, brush Spray, dip, brush
Dwell Time Min./Max.
Lighting Required
10min/2h
Ultraviolet
10min/2h
Visible
10 min/2 h
Visible/ Ultraviolet
* Spray may be manual, pressurized or electrostatic.
7 The size of the droplets, pressure, temperature and angle of a water spray rinse can affect removal of the penetrant. Most industry and military specifications limit acceptable water temperature to 10 to 38 °C (50 to 100 °F) at a pressure not to exceed 276 kPa (40 psi). The spray nozzle should be a minimum of 30 cm (12 in.) from the part at an angle between 45 and 70 degrees. An air agitated immersion rinse is a removal method that must maintain adequate water circulation and air agitation. Experiments to determine the amount of rinsing required for specific processes should be performed before the actual inspection. The water temperature requirements are the same as for the water spray method.
The rinsing operation must be monitored closely to avoid overwashing. Washing should be stopped as soon as an acceptable background level is observed. Lipophilic Emulsifier (Method B) Removal is performed by introducing an oilbased emulsifier as a separate process step. Lipophilic emulsifier diffuses in to the surface penetrant. It is usually applied by immersion; brushing or wiping may cause streaking due to uneven emulsification and are not recommended. This system offers more control and has fewer problems with over and under washing compared to the water washable method. The emulsifier dwell may be anywhere between 5 s up to 5 min.
Table 1.5: Method of removal Remover Material Water
AMS-2644 Designation A
Lipophilic Emulsifier
B
Solvent
C
Hydrophilic Emulsifier
D
Advantages and Primary Uses Fewer processing steps Reduced processing time Lower cost Elimination of the emulsifier station Wide, shallow discontinuities Supplied ready for use
Portable Localized areas, field applications Wash station not required Small, tight discontinuities Wide, shallow discontinuities Post rinse time less critical Reduced background on rough surfaces Emulsifier touchup during final rinse
Disadvantages No control of emulsifier Critical rinse time Effects of water contamination Extra processing steps Critical rinse time Poor results on rough surfaces Water and penetrant contamination Over-removal of penetrant Time consuming for large parts or high volume May be flammable Extra processing steps Must maintain concentration levels Contamination from penetrant
8 ASTM E 1417 limits lipophilic emulsifiers to a 3 min maximum dwell time. Finding the optimum time for specific conditions is generally accomplished through experimentation. Experience under similar circumstances related to the type and size of suspected defects, surface roughness of the part, temperature of materials, water contamination levels of the penetrant and emulsifier can be used for a guideline. If the emulsification time is either too short or too long, the part must be entirely cleaned and reprocessed with an adjusted emulsifier dwell. Following the appropriate dwell, the part should be quickly sprayed with water wash to stop the diffusion process. Once the diffusion process is halted, the emulsified penetrant is rinsed using the same methods as the water washable method. Solvent Removable (Method C) Solvent remover is usually supplied in aerosol spray cans. The solvent works by diluting the surface penetrant. The part is first wiped with clean, dry, lintfree cloths or towels. After the major portion of penetrant has been wiped from the surface, the surface should be wiped with a clean cloth, lightly sprayed with solvent, until just a trace of penetrant is observed on the cloth. Extreme caution must be used to avoid excessive removal. Hydrophilic Emulsifier (Method D) This process is similar to method B except that the emulsifier is waterbased and uses detergent action rather than diffusion to remove the surface penetrant. Emulsification of the penetrant within a discontinuity is less likely to occur with hydrophilic emulsifier than with lipophilic emulsifier. This allows complete removal of surface penetrant and improves contrast for inspection. Prerinsing the part following the penetrant dwell is recommended for hydrophilic emulsification processing. The prerinse is performed using the water spray techniques described for water washable penetrant. Prerinsing should be held to the minimum time required to remove the bulk of surface penetrant. Immediately following the prerinse, the hydrophilic emulsifier should be applied by spraying or immersing. Agitation of the emulsifier is required to produce the detergent (scrubbing) action that removes the penetrant. The optimum emulsification time must be determined through experimentation for the
particular application. The emulsifier concentration for spray methods is limited to 5%. Immersion method concentration levels can vary from 5% to 35% depending on the manufacturer. Do not exceed the concentration levels suggested by the manufacturer. The emulsifier dwell time should not exceed 2 min.
Drying The need to dry parts after penetrant removal depends on the removal method used and the type of developer that will be applied. When dry or nonaqueous developer is used, parts are dried before the developer is applied. When aqueous developer is used (water soluble and water suspended), parts are dried after the developer is applied. For water washable and emulsified applications, parts are dried at room temperature or in a recirculating forced air oven. Parts should be rotated during drying and pools of water should be removed. Oven drying time should not be any longer than the minimum time necessary to dry the part. Oven temperature should never exceed that specified in the written procedure or in ASTM E 1417(71 °C or 160 °F).
Develop the Indication An indication may be selfdeveloped, where the penetrant seeps to the surface after the excess is removed. Development is normally enhanced with a developing agent. Developer is available in dry, water soluble, water suspended or solvent suspended forms. Developers are described in more detail starting on page 15.
Inspection A vital part of the liquid penetrant testing process is the evaluation of the results. Indications must be reviewed for their relationship to discontinuities in the material. Consideration must be given to the type of material, size, shape and processing history of the part that is being inspected. Processing history includes information specific to the part and may include forming, casting, welding, machining, heat treating or other manufacturing operations.
9 When properly performed, the penetrant process will provide visible indications at all relevant surface discontinuities. Importance of the Inspector's Evaluation Inspector training and experience are necessary for determining the relevancy of an indication. Evaluating indications correctly is a skill learned by working with experienced inspectors and observing the interpretation process until sufficient knowledge is gained. Effective evaluation cannot be mastered from classroom training alone. In some mass inspection operations, the person who operates the test equipment merely segregates the parts that show indications. Others decide their disposition. In most cases, however, the inspector who first sees the indication is also expected to interpret it. Skill and proficiency in developing and then interpreting indications must be acquired by experience. A skilled and experienced inspector can add greatly to the effectiveness of liquid penetrant testing. The inspector must also know the materials under test: what they are, how they are made and how they have been processed. In addition, the inspector must understand what types of discontinuities may be introduced during manufacturing and processing. Inspectors who know what discontinuities to look for and where they are likely to occur are able to select the best techniques available for their detection. Penetrant Inspection Documents Penetrant testing is conducted with a specific technique in accordance with a written procedure. The technique defines the process variables (dwell, emulsification, etc.) specifically for the part being inspected. The technique should reference the acceptance criteria as per contract, engineering drawing, specification or standard. The procedure is a general inspection instruction that defines the materials, equipment, cleaning and process methods that may be used. It defines the minimum and maximum times, temperatures, pressures, etc., for each step in the penetrant process. Requirements for personnel qualification and certification are also defined. Procedures are based on the requirements of industry or military specifications, standards or codes. These specifications define the requirements
for personnel, materials, processing methods and/or acceptance criteria. Examples include ASTM E 165, ASTM E 1417, MIL-STD-6866, MIL-STD271, MIL-STD-410, NAV-SEA 250-1500 and ASME Boiler and Pressure Vessel Code, Section V. Acceptance criteria generally indicate the maximum size of allowable indications. They may define a total number or a total area for acceptable indications over a specified surface area of a part. A minimum distance for separation between rounded indications may also be defined based on the size of the largest indication. Weldments and castings are often photographed for use as accept and reject examples of various types of indications. These reference standards also assist in determining the type of discontinuity that caused the indication. Visual Requirements Penetrant inspection standards and specifications have minimum requirements for an inspector's visual acuity and color perception. Visual acuity (the ability to resolve details) decreases as light levels decrease. The ability to distinguish between colors is necessary to locate indications. An individual with red-green color blindness may not be able to distinguish indications at various levels of brightness because of the penetrant's colors. Fluorescent penetrant inspection requires an inspector to allow for visual adjustment to the darkened area. Eyes become adapted to the dark after approximately 5 min. Periodic eye examinations by qualified medical personnel are recommended for those involved in the interpretation of penetrant indications. Classification of Discontinuities by Origin There are three times during the life of a part where discontinuities can be introduced. They may exist in the raw material, they may result from forming and machining, or they may be caused by service conditions. Table 1.6 lists common discontinuities that can be detected with penetrant inspection. This classification is as follows. 1. Inherent discontinuities are introduced into the basic materials as the result of its initial production from ore to raw components up to the point where it is ready for initial processing. Inherent discontinuities can form during the solidification process of molten metals in
10
ASNT Level II Study Guide: Liquid Penetrant Testing castings and ingots. Welding produces many of the same discontinuities when the metal becomes molten and flows during the process. Discontinuities occur when the material, or a portion thereof, cools too slowly or too fast, is subjected to localized moisture or other material that creates gas pressure buildup, or develops internal stresses after solidification. Processing or forming discontinuities fall into two groups: (a) discontinuities introduced during primary processes, such as casting, rolling, forging, drawing, welding, etc. and (b) discontinuities introduced during finishing processing, such as machining, heat treating, grinding, plating, etc. Forming operations that involve hot and cold working can produce new discontinuities or alter the appearance of
existing discontinuities. Machining can expose subsurface discontinuities or contribute to the formation of new discontinuities during grinding, lapping, milling, cutting and lathing operations. 3. Service discontinuities develop as the result of the material or part being in service. This group includes fatigue from repetitive cyclic loads, service loads, corrosion, residual stress and stress corrosion. Identifying Indications Indications as small as 0.4 mm (0.015 in.) may be considered relevant and may affect the serviceability of a part. Inspection may result in multiple indications being observed that must be
Table 1.6: Types of discontinuities Discontinuity Burst Center bead crack Cold shut Crater crack Fatigue crack Fold Grinding crack Heat affected zone (HAZ) crack Hot tear Inclusion Lack of fusion or penetration Lamination Lap Microshrinkage Pipe Porosity Quench crack Shrinkage crack Stress corrosion crack
Product Forge Weld Cast Weld Service related Cast Any form machined Weld (in the base metal) Cast All Weld Forge, roll Forge Cast Forge Cast, weld Any form heat treated Cast Service related
Appearance Broad starburst Continuous/intermittent linear, tight, or open Half moon to linear, smooth Starburst Continuous linear, tight, or open L/I, L/C, tight Linear, tight, random multiple Linear, tight, emanating from weld Linear, tight or open Near-rounded Linear parallel to weld Linear, tight Linear, tight Round, porous Linear, tight Round, randomly dispersed Linear, tight or open Linear emanating from void Linear, tight or wide
11 determined to be relevant or nonrelevant and interpreted as acceptable or rejectable. According to ASTM 1316, a relevant indication is "an NDT indication that is caused by a condition or type of discontinuity that requires evaluation." The relevancy of an indication is determined by the inspector's judgment alone. The acceptance or rejection of relevant indications is determined by criteria specified on the part drawing, in referenced inspection specifications or standards, or in a contractual agreement. Relevant indications are the result of penetrant bleedout from an unintentional change or discontinuity on the part surface. Relevant indications must be evaluated for size, location and type according to the specified acceptance criteria to determine if the discontinuity is cause for rejection of the part. No all discontinuities are necessarily cause for rejection. According to ASTM 1316, a nonrelevant indication is "an NDT indication that is caused by a condition or type of discontinuity that is not rejectable. False indications are nonrelevant." Nonrelevant indications can be caused by intentional changes of the part surface. Sheet metal fittings, rolled flanges, threads and splines are examples of features inherent in a part's design that may form nonrelevant indications. According to ASTM 1316, a false indication is "an NDT indication that is interpreted to be caused by a discontinuity at a location where no
discontinuity exists." False indications are often caused when penetrant comes into contact with the part after processing. Penetrant on the inspection table surface or on the operator's hands can create false indications. False and nonrelevant indications may potentially mask relevant indications. Cleaning and reprocessing a part for inspection is required when false indications are noted.
Postcleaning Residue from the penetrant process can be harmful to the part during further processing or the service of a part. It can also interfere with future penetrant inspections. Penetrants remaining on a part may adversely affect plating, anodizing and welding operations. Residual oils on liquid oxygen system components can result in an explosion. Penetrant remaining in discontinuities can dry or disrupt the mechanics of penetrant in later inspections. Developers may act as an abrasive when left on contact areas of moving parts, they can absorb moisture and cause corrosion, and they may absorb penetrant in subsequent processing masking indications or creating a high residual background. Developers become harder to remove with time. Most developers can be removed with water and wiping or by scrubbing.
Recommended reading Subject
Reference*
surface condition, cleaning
A, B, C
penetrant application removal
A, B, C
drying__________________________
A, B,C
inspection ________________________________ visual requirements ________________________________ identifying indications ________________________________ formation of discontinuities ________________________________ post cleaning __________________
A, B,C
*See Introduction for explanation of references.
A, B, C B,C A, B,C A, B, C A, B, C
Chapter 2 Penetrants, Developer and Equipment Penetrants Characteristics of Penetrants To detect discontinuities, the penetrant must coat the area of interest in a continuous and uniform manner and must migrate into small cavities that are open to the surface. These cavities are generally invisible to the naked eye. Surface tension, wetting ability and capillary action are the mechanisms that enable a liquid to perform well as a penetrant material. Light oils are generally used for penetrants because of these characteristics. Surface Tension Molecules of a liquid are subjected to the attraction of surrounding molecules. The attraction of like molecules is called cohesion. This attraction has no effect at the interior of the liquid because the force is equalized by the attraction from all directions. However, molecules at the surface are pulled back toward the interior of the liquid by the molecules below. This contraction of the surface creates what is called surface tension. As the cohesive force increases, the surface tension also increases. Wetting Ability The attraction of dissimilar molecules is called adhesion. If the adhesive force between a liquid and a surrounding solid is weaker than the
14
cohesive force of the liquid, the liquid will contract and pull away from the surface, forming a contact angle greater than 90 degrees. If the adhesive force is stronger than the cohesive force, the liquid will flow and adhere to the solid to a point where the adhesive and cohesive forces are equalized in the liquid. If the liquid forms an angle of less than 90 degrees to the surface, the individual droplets will merge into a uniform coating wetting the surface (Figure 2.1). Capillary Action Capillary action is easily illustrated by inserting a capillary tube into a clear container of liquid. Inside the tube, a liquid with high surface tension will rise above the liquid level outside of the tube. The low contact angle causes the liquid to curve up the tube wall while the surface tension pulls the liquid downward at the center of the liquid. These forces working together create capillary rise (Figure 2.2a). A liquid that does not have sufficient wetting ability will keep the liquid inside the tube below the level outside (Figure 2.2b), or at the same level (Figure 2.2c). Capillary rise enables the penetrant to enter discontinuities. A discontinuity acts like a capillary tube and the ability of a penetrant to climb the walls of a discontinuity is the mechanism used to enter the discontinuity. Capillary rise will occur even though the discontinuity is not open on both ends like the tube. The penetrant will not completely fill the cavity due to trapped air, which
ASNT Level II Study Guide: Liquid Penetrant Testing will exert an outward force on the penetrant (Figure 2.3).
Specific Gravity
Physical Properties of Penetrants Physical properties of penetrants include viscosity, volatility, specific gravity, flash point and thermal stability. Viscosity Viscosity measures the flow rate of a liquid. Higher viscosity liquids flow more slowly than less viscous liquids. Increasing the temperature of a liquid lowers its viscosity. Viscosity does not affect the ability of the penetrant to enter a discontinuity; however, the viscosity of a penetrant does affect the rate of penetration and the penetrant dwell time. Penetrants with a higher viscosity will require longer dwell times than less viscous penetrants. Volatility Volatility indicates the boiling point of a liquid. Low volatility means a high boiling point. Highly volatile liquids are not desirable for penetrants because of their rapid evaporation, which results in the drying of the penetrant on a part during the dwell and the loss of penetrant materials in open tanks.
The ratio of a liquid's density to that of distilled water is the liquid's specific gravity. A penetrant's specific gravity may be less than 1 if it has an oil base. Flash Point The temperature at which a liquid's vapors become flammable when a source of ignition is present is called its flash point. The flash point does not affect the performance of a penetrant but it is considered for safety reasons.
Thermal Stability Thermal stability is related to the tendency of some dyes used in penetrant materials to lose their brightness or color when exposed to elevated temperatures for an extended time. This loss of color is referred to as heat fade. The ability of a penetrant to resist this loss is its thermal stability.
Chemical Properties of Penetrants Inertness Penetrant processing materials must be inert to the material composition of the part being processed. Nickel based alloys, titanium and austenitic stainless steels are susceptible to hydrogen embrittlement and intergranular
Figure 2.2: Contact angle in a capillary tube Capillary Tubes
Chapter 2, Penetrant Characteristics corrosion from small amounts of sulfur and halogen residues. Nuclear and pressure vessel codes limit the sulfur or chlorine residual content to 1 % by weight maximum because of their extensive use of these metals and the risks involved with any system failure. Removability Removability is the ease with which excess surface penetrant is sufficiently removed without removing it from discontinuities. Good removability is critical for reducing or eliminating residual background.
15
4. form d, nonaqueous developer Type I, 5. form e, nonaqueous developer Type II, 6. specific application developer. Table 2.1 compares the various developers.
Figure 2.3: Penetrant over discontinuity Penetrant Discontinuity
Water Tolerance Water washable penetrants must be capable of tolerating a certain amount of water contamination.
Developer Penetrant remaining in a discontinuity is drawn out by the same capillary forces that draw it into the cavity. The adhesive force between the penetrant liquid and the solid surface will cause the penetrant to be drawn back out of the discontinuity. The cohesive force of the liquid will stop its rise out of the discontinuity at the point where it is balanced by the adhesive force. If the discontinuity is large, the penetrant drawn to the surface is visible without further processing. Developer is applied to aid the detection of smaller discontinuities. Developers provide an absorbent surface containing fine capillary paths that assist in drawing penetrant out of a discontinuity. This provides a much larger penetrant indication surface area (Figure 2.4). Observation of indications is further enhanced by the contrasting background that the developer provides. The following are types of developers: 1. form a, dry powder developer, 2. form b, water soluble developer, 3. form c, water suspendible developer,
Figure 2.4: Developer enhancement of discontinuity Penetrant
Developer
Discontinuity
Recommended reading Subject penetrant properties systems "See Introduction for explanation of references.
Reference* B,C C
16
Dry Powder
Water Soluble
Dry developer is a light, fluffy powder that clings to the part in a fine film. Dry developer is easy to use and has good development properties for fluorescent penetrant. Under ultraviolet light, metal surfaces appear deep violet in color, which contrasts well with the bright yellow-green fluorescence of the penetrant. Dry developer does not interfere with this contrast because it dusts the surface lightly but does not coat it. It does not work well for visible penetrant because it does not provide a contrasting background. If dry developer becomes wet, clumping may occur. Clumping limits or eliminates the developer's capillary action. Dry developer in tanks may become fluorescent if it is recycled excessively. Fluorescent specks or particles may cause false indications. Dry developer in a powder bulb is often kept in the inspection area and applied to a part surface that has been wiped during evaluation.
This type of aqueous developer is dissolved in water and forms a translucent film. Water soluble developer can be used for fluorescent dyes, but it is not recommended for use with visible dye or the water washable removal method. Water in the developer may remove water washable penetrant from discontinuities. Water soluble developer can save time during processing because drying is included in the developer dwell time. Water soluble developer is supplied as a dry concentrate that must be mixed and maintained at the proper ratio of developer to water. The developer should be checked at regular intervals for penetrant contamination and water evaporation.
Water Suspendible Water suspendible developer is supplied as a dry concentrate that must be mixed and maintained at the proper ratio of developer to water. Suspended aqueous developer does not dissolve in water so it must be thoroughly agitated just before application
Table 2.1: Developer types Material Form/ AMS-2644 Designation Dry powder
Method of Application Dust/fog, dip,
Application Sequence After drying
spray
Water soluble
Spray, flow, dip
Water suspendible
Spray, flow, dip
Nonaqueous Type I and Type 11
Spray
Before drying
Before drying
After drying
Dwell Time
Advantages and Primary Uses
1/2
Easy to use
Penetrant dwell to 4h 1/2 Penetrant dwell to 2h
Easy to remove Good for Type I
1/2. Penetrant dwell to 2h 10 min to 1h
Disadvantages
Not for Type II
Parts must dry first Penetrant contaminated Needs no Acts as detergent agitation Not for Method A Easy to remove Not for Type II Applies thin coat Bacteria/fungus Evaporation Visible'coating , Critical immersion Surface contact Frequent agitation
Most sensitive High contrast Portable
Bacteria/fungus Difficult to remove Expensive Flammable Not for large areas Frequent agitation Parts must dry first
17 to suspend the particles in water. The development of indications does not begin until the moisture is completely evaporated from the developer. The thickness of the coating and its white color work well with visible dyes. Parts should be checked to ensure a uniform coating of developer has been applied. The developer should be checked at regular intervals for penetrant contamination and water evaporation.
Nonaqueous Nonaqueous solvent suspended developer is supplied in bulk containers and in aerosol spray cans. It may be used for all penetrant types and methods. This type of developer is applied after drying and is considered to be the most sensitive developer method available. When nonaqueous developer is supplied in aerosol spray cans, process controls are not required, but to ensure the proper coat of developer, it is necessary to shake the can to be sure the developer particles are resuspended. When the developer is supplied in bulk, it is important that the suspension be thoroughly mixed before use. The developer must be thoroughly agitated before spraying and should
be applied in a thin, uniform coat. The spray does not cover larger surfaces quickly and each part must be separately sprayed while the developer is constantly agitated. A consistent, even coating of developer should be maintained.
Special Application Special application developers are designed for use with only specified penetrant materials and must be qualified with a particular penetrant system. An example would be qualification of a specific manufacturer's visible dye with a dry developer.
Material and Process Control Tests Successful penetrant processing relies on properly functioning material and equipment. Process system variables of time, temperature and pressure along with material concentration, contamination, consistency, etc., must be verified as acceptable at regular intervals. Equipment used for verification may also require scheduled
Table 2.2: Typical equipment calibration Equipment Temperature gage Pressure gage Timers White light meter Ultraviolet light meter Refractometer Temperature gage Measuring equipment
Calibration Tolerance Spray water rinse/wash station ±1 °C at 21 °C (±2 °F at 70 °F) Spray water rinse/wash station ±2% of scale Penetrant, emulsifier, rinse, ±1 s at 10 min dry stations Type 2 lighting/ambient light ±10% of scale for Type 1 Ultraviolet light intensity ±10% of scale Where Used
Hydrophilic emulsifier concentration Drying oven Inspection booth — measuring indications
Item 3 months 3 months 3 months 6 months 6 months
±2% of scale
1 year
±5.6 °C at 66 °C (±10°F at 150 °F) ±0.1 % of scale
6 months 1 year
18 calibration. Tables 2.2 and 2.3 contain standard process control requirements and equipment calibration. Ultraviolet light intensity, emulsifier concentration, ambient white light intensity and
penetrant area cleanliness are examples of typical process controls that may be specified in the penetrant inspection procedure.
Table 2.3: Typical process control requirements Process Element Water wash pressure Water wash temperature Oven temperature Systems performance
System Requirement Method A, B, D 276 kPa (40 psi) maximum Method A, B, D 10 to 38 °C (50 to 100°F)
Method A, B, D 71 °C (160°F) maximum meet minimum code All requirements Ultraviolet intensity Type 1 1,000 (J.W/cm2 minimum at part surface 1,076 lx (100 ftc) minimum White light intensity Type 2 at part surface Ambient light intensity Type 1 20 lx (2 ftc) maximum at part surface no fluorescent Area cleanliness Type 1 contamination Method B <5% Lipophilic emulsifier water content immersion: < than specified Method D Hydrophilic emulsifier on the AMS-2644 QPL, concentration spray < 5% check concentration with Aqueous developer hydrometer Dry developer no fluorescent condition contamination (fluffy not caked) Fluorescent brightness Type 1 >90% of unused material sample Penetrant water content Method A <5% comparison to unused Penetrant removability Method A material sample comparison to unused Penetrant sensitivity All material sample
Item Each shift Each shift Each shiftDaily Daily Daily Weekly Daily Monthly Weekly
Weekly Daily
Quarterly Monthly Monthly Weekly
19
Ultraviolet Light Intensity Intensity of the ultraviolet light in the inspection booth is checked with a calibrated ultraviolet light intensity meter. Ultraviolet light intensity should be 1000 (iW/cm2 minimum at the part surface.
Emulsifier Concentration To check emulsifier concentration (refractive method) of hydrophilic postemulsifiable penetrant systems the following procedure should be used: 1. Lift the refractometer cover plate to expose the prism. 2. Use the plastic dip stick supplied with the instrument to spread a small amount of emulsifier over the prism surface. 3. Close the cover plate and point the instrument toward a light source. Look through the eyepiece and take the reading at the point where the dividing line between light and dark crosses the scale. (Tilting the instrument with respect to the light source may be required to obtain optimum contrast between light and dark boundary.) 4. Clean the instrument after each use. Wipe the prism and cover plate with a soft cloth or tissue paper moistened with water, then wipe dry. If the prism surface and cover plate are not well cleaned before the next use erroneous or fuzzy readings may result.
Ambient White Light Intensity The ambient white light intensity in the darkened inspection booth is verified by the use of a white light meter. The ambient white light intensity should not exceed 22 lx (2 ftc) at the inspection surface.
Penetrant Inspection Area Cleanliness The cleanliness of the inspection booth is checked for any contamination of fluorescent penetrant, developer powder or other residue that could affect the interpretation.
Penetrant Materials Testing Water Content Water washable penetrants are analyzed for water content each month or any time stratification or cloudiness is noted. Water content should not exceed 5% by volume. Separation of Constituents and Loss of Fluorescence Penetrants shall be compared with unused samples for separation of constituents and loss of fluorescence each month or whenever stratification or cloudiness is noted. Separation and loss of fluorescent brightness or cloudiness shall not be permitted. Washability The washability of penetrants shall be determined each month. Materials not passing the washability test shall be discarded and replaced with fresh material. Sensitivity Comparison Penetrants shall have a sensitivity comparison performed each month. The sensitivity of penetrant samples shall be comparable to the unused reference sample.
Penetrant System Monitor The testing and monitoring panel (Figure 2.5) is a commonly used reference standard for monitoring the overall performance of the penetrant system. It is processed through all stations of the penetrant production line to verify that all aspects of the process (dwell time, temperatures, removal, lighting, drying, developing, etc.) are functioning together. The following is typical of procedure requirements for processing the panel: 1. Ensure the panel is dry. 2. Examine under ultraviolet light for residual penetrant. Residual fluorescence shall be cause for cleaning or replacement. 3. Process the panel per standard procedure and production technique using minimum process times. 4. Examine the processed panel under ultraviolet light. Compare results to the correspondingly
20
ASNT Level II Study Guide: Liquid Penetrant Testing
numbered reference photograph or to previous test results. Results shall be as follows: a. removability: At least 75% of the grit blasted panel surface shall show no evidence of background fluorescence. b. sensitivity: The minimum number of crack indications visible shall be the same as the reference photograph or the same as previous test results. 5. If the system performance is unacceptable, replace materials (for example, penetrant, emulsifier, developer) as applicable and recheck. 6. After completion of the test, place the panel in an ultrasonic cleaner for at least 5 min. Store panel until next usage in isopropanol or in a clean dry container. Keep lid on container to minimize evaporation.
Equipment Penetrant processing equipment can vary from a few spray cans and cloths to computer controlled, fully automated systems. Table 2.4 lists materials Figure 2.5: Testing and monitoring panel
common to portable and stationary penetrant process lines.
Ultraviolet Lighting Ultraviolet light is required when Type 1 (fluorescent) penetrant is used. The fluorescence of the penetrant assists in locating small, difficult-tosee discontinuities. The most common UV-A light consists of a mercury bulb housed with a filter that is powered by a constant voltage transformer producing wavelengths in the range of 320 to 400 nm. The minimum intensity of the ultraviolet light should be 1000 uAV/cm2 at the inspection surface. A warmup time is required for the mercury bulb to reach full intensity and the light should remain on to avoid cycling, which reduces the life of the bulb. Regular cleaning of the bulb and filter is required to remove dust, oil, dirt and fluorescent contamination that can reduce the ultraviolet light intensity. For remote processing when electricity is available, portable enclosures or heavy tarps may
The testing and monitoring panel features five crack centers of different sizes for evaluating sensitivity and includes a grit blasted section for judging wash characteristics of liquid penetrant system: (a) drawing; (b) under white light; (c) under ultraviolet light. (Reprinted from Nondestructive Testing Handbook, third edition: Volume 2, Liquid Penetrant Testing. Courtesy of Boeing Company, St. Louis, MO.
Chapter 2, Penetrant Characteristics
21 be used to obtain sufficient darkness to perform evaluations.
Stationary Processing Equipment Requirements vary depending on the type of penetrant and removal method used. The system may consist of a deep sink next to a small tank of penetrant or can be more elaborate with electrostatic sprayers for penetrant application, semiautomatic emulsification and/or rinse and developer stations run by the push of a button. Parts may be moved by hoists or conveyors,
automatically lowered and processed in emulsifier immersion stations, or automatically processed through a water rinse operation.
Portable Processing Equipment Penetrant is available in kits packaged with spray cans of penetrant, remover and developer. They are generally a visible, solvent removable system, but they are also available with water washable penetrant, lipophilic emulsifier and fluorescent penetrant. The developer is almost always nonaqueous.
Table 2.4: Manual processing equipment Stationary
Portable
Penetrant Application Spray booth —• standard or electrostatic Penetrant spray can Immersion tanks Brushes, cotton swabs, etc. Dwell/drain area Timer for penetrant dwell Penetrant Removal Coarse water spray Solvent spray can Ultraviolet light (Type 1) Cloths/rags White light (Type 2) Portable ultraviolet light (Type 1) Emulsifier tanks and/or spray apparatus Tarp Water rinse spray and/or tank Water pressure and temperature gages Timer for emulsifier dwell Drying/Development Hot-air recirculating oven Developer spray can or powder bulb Oven temperature gage Developer dip tank, fog chamber, or spray area Developer dwell area Timer for developer dwell Inspection Ultraviolet light and darkened booth (Type 1) Portable ultraviolet light (Type 1) White light (Type 1 and 2) Tarp Magnifier, measuring devices Magnifier, measuring devices Solvent Solvent Developer powder bulb or spray can Developer spray can or powder bulb
22
ASNT Level II Study Guide: Liquid Penetrant Testing
Automated Equipment Fully automated liquid penetrant testing systems process materials through cleaning, penetrant application, dwell and removal, drying and developer application and dwell time without an operator. Automatic loading and unloading capabilities also can be incorporated in some systems. Automated systems are custom designed for specific applications of high volume processing. The elimination of inconsistencies in dwell and rinse time associated with human processing is the major advantage. The
disadvantages are the cost and inflexible nature of the system. Semiautomated systems may have conveyors to move parts to the various stations where processes are manually performed, or may include automated emulsifier, rinse, and/or oven and developer stations. Automated processing ends at the inspection area. Human inspectors are almost always required to evaluate indications resulting from the process.
Recommended reading Subject developer equipment materials and process controls *See Introduction for explanation of references,
Reference*
A,B,C A,B,C A,B
Chapter 3 Testing in Primary Metals Production By far the largest field of application of nondestructive tests is in the metal making industry, including ferrous alloys and the growing list of nonferrous metals and alloys being produced and used. For the usual metals and alloys, there is a considerable similarity of processes and discontinuities that are encountered.
Producing Metals and Alloys Iron and the nonferrous metals are refined from their ores by a variety of processes but most of them are ultimately produced in molten form, the molten metal is poured into molds and allowed to solidify. These molds may be ingot molds from which the solidified ingot is taken through a variety of forming processes or they may be molds for casting the metal into final shape.
Processing Discontinuities Discontinuities Related to Metal Solidification In spite of precautions to produce and solidify metal free of foreign materials and gasses, most metals contain nonmetallic inclusions of some type. These inclusions may be (1) dissolved or entrapped gasses; (2) various nonmetallic materials such as oxides and sulfides of the metal; and (3) finely dispersed particles of slag. Their presence is not in itself an indication of poor quality; this is true of steel, copper, brass, aluminum and other metals. However, if not well distributed or if the segregates are large, they may be very objectionable. They will appear as stringers or discontinuities in the final rolled or forged product, which may seriously affect the suitability of the metal for many purposes.
Discontinuities in Metal Ingots Metal shrinks during solidifying and cooling. This shrinkage may cause shrink cavities in the center of the ingot, usually confined to the top or last-to-freeze portion. This section of the ingot is normally cropped and discarded. If not completely eliminated, these types of internal discontinuities may show up in the finished product as pipe in rolled bars or as laminations in plate or strip. Cracking also may occur on the surface of ingots. These cracks are removed as completely as possible by chipping or scarfing to prevent the forming of seams on the finished product.
Discontinuities in Metal Castings In the case of castings where the metal solidifies in final form, gates and risers allow surplus metal to form at favorable locations so that impurities can float out of the casting proper and gas can have a chance to escape. Porosity and gas pockets along with slag inclusions are a major source of discontinuities in castings and much testing focuses on locating them. This is true of nonferrous castings as well as steel. Shrinkage on cooling results in thermal cracks in castings, most often on the surface and related to the shape of the casting. Other casting discontinuities are hot tears and cold shuts, both being surface discontinuities. Vibration and external stress would cause these cracks to become more extensive, resulting in failure of the part. Indications of cracks are considered a cause for rejection in almost all cases. Cracks not only reduce the strength of a part but they also may propagate, especially under alternating or fatigue loads.
Discontinuities in Blooms and Billets After being cast into ingots, a metal is then further worked into usable form by forging, rolling
24 or pressing. A steel ingot, for example, is reheated to the proper temperature and passed between heavy rolls to form blooms or billets. The size of ingots varies widely. Ingots of alloy and tool steel may be quite small, weighing perhaps 100 kg (220 lbm), whereas ingots from which large forgings or large thick plate are to be made may weigh many tons. Blooms and billets are formed in an intermediate step between the ingot and the finished size or product. The surface of blooms and billets may contain seams due to rolled out ingot cracks resulting from cooling stresses, bursts, tears, rolled-in scale or metal particles. All of these surface imperfections should be removed before the final finishing operation. The surfaces of blooms and billets may be freed of such objectionable conditions by chipping, grinding or flame scarfing. Billets, especially when intended for seamless tube piercing, are often cleaned up by scaling, by taking a cut off the surface, or by planing the surface.
Discontinuities in Hot Rolled Bars, Shapes, Plate or Strip Blooms and billets are often reheated to proper working temperature and rolled into bars, shapes, plate or strip. Seams, stringers and laminations may appear in the finished product as a result of rolling, forging or extrusion. The rolling operation itself may introduce certain discontinuities, the most common of which are laps resulting from too much or too little metal to fill the rolls. Too rapid reduction of cross section, especially if the metal is at too low a temperature, may produce tears and internal rupture called cupping. Internal shrinkage cavities or gas pockets may result in laminations in plate, strip, pipe, bars or other shapes. Uneven cooling of some types of steel may cause cooling cracks to appear in bars and shapes.
Discontinuities in Forgings If an ingot or billet is used for forging, a variety of discontinuities can be produced from improper handling of the metal under the hammer or from working at too low a temperature. Common discontinuities are laps and folds. Forging bursts, both external and internal, can also occur.
In the case of die or drop forging, laps, surface tears and bursts may be produced. In die forgings, excess metal is forced out between the two halves of the die forming the flash and this excess metal is subsequently trimmed off. If not properly done, flashline cracks or tears may be formed. Another type of forging discontinuity is the internal rupture, usually called flake, which occurs in the process of cooling relatively large forgings. These may be very severe and extremely objectionable.
Discontinuities in Rolled and Pierced Products In the production of seamless tubing, the billet is pierced. Discontinuities on the inside surface of the pierced billet may take the form of tears. Seams may occur on the outside surface, especially if the surface of the billet or blank had not been properly cleaned.
Discontinuities in Extrusions Nonferrous seamless tubing is often made by extrusion, starting with hollow or pierced billets, or from solid ingots. Discontinuities that occur may be checks and tears, or imperfections may result from uncovering internal discontinuities of the billets or ingots. Many shapes and products are made by various extrusion processes. Because of flow of the metal of the original ingot during extrusion, oxide and surface skin sometimes become folded into the interior. A form of pipe also results from this action, which is most objectionable when it appears in a finished shape.
Discontinuities in Cold Worked Metal Products Cold rolling and cold drawing of strip, rod and wire is a process for improving the surface finish of steel products. Close dimensional tolerances can thus be maintained and the metal becomes somewhat work hardened. Die marks, scratches and surface imperfections sometimes make their appearance in this process. The process does not remove seams or other discontinuities present in the material before the cold finishing is applied.
Chapter 3, Testing in Primary Metals Production
Discontinuities in Fusion Welds In fusion welding processes, where liquid metal solidifies in the weld, the same sort of discontinuities may occur as in castings — namely, slag inclusions, gas porosity and thermal cracks. In addition, cracking in the parent metal may occur because of thermal stresses or as a consequence of hydrogen pressure. There are also discontinuities from a lack of penetration of the weld and failure to get proper fusion of the weld metal to the parent metal.
Discontinuities in Heat Treating Processes Most metal products undergo some form of heat treatment to produce desirable physical properties. Heat treatment discontinuities may be produced as the result of warping or cracking. Quenching cracks occur in steel when the process is not properly carried out or when the design of the part or the steel used is not adapted to the operation. Quenching cracks are apt to appear in connection with changes in cross section of the material that cause excessive cooling stresses or at locations where the contour of the surface permits stress concentrations to occur. Sometimes hardened parts are cracked when an attempt is made to straighten those that have warped slightly out of shape. Special nondestructive test techniques have been devised for checking hardness against specification tolerances and for unwanted conditions such as abnormal grain size, segregations, etc.
Discontinuities in Machined and Ground Surfaces Machining operations may cause surface damage, leaving discontinuities in the form of surface tears. Grinding of surfaces for accurate dimensions or for finish is a common source of surface cracking, especially on hardened steel surfaces. Parts that have been hardened may have residual internal stresses, produced in the quenching process and not subsequently removed by drawing or stress relieving. Such parts are particularly sensitive to the formation of grinding checks.
25
Cracks During Plating or Chemical Treatment Parts containing residual stresses from heat treatment or cold working may crack during plating processes or during the pickling that precedes plating. Pickling etch removes some of the metal surface containing compressive stresses, permitting the internal tensile stress to be relieved through cracking. Embrittlement by absorbed hydrogen during pickling may sometimes be a factor in such cracking.
Service Discontinuities Fatigue Cracks Fatigue cracking and failure resulting from fatigue is by far the most important group of service discontinuities. Fatigue occurs during repeated or cyclic loading or stressing of materials. A fatigue failure starts with a crack initiated by a variety of causes. Under cyclic loads, this crack progresses through the cross section of the metal until only a small portion of the original section remains sound. Finally failures occur even when calculated or average stresses are far below the elastic limit and do not depend directly on whether the tensile strength of the metal, as determined by static tests, is either high or low. Significance of Surface Discontinuities under Repeated Loading or Vibration Fatigue cracks almost invariably start at the surface and are initiated by conditions that bring about locally high stresses. These stress concentrations involve stresses above average for the part. The stresses may, in their local area, be above the fatigue strength of the metal. Surface scratches, cracks or other discontinuities may cause these local areas of high stress. Localized high stresses may also be caused by design features of the part — for example, high stresses occur around holes, fillets and stiffening members. Prevention of Fatigue Failures in Service One of the main purposes of liquid penetrant testing is to locate and eliminate discontinuities likely to lead to fatigue failures. Fatigue cracking is a serious indication sought by nondestructive
26 techniques when the machine or structure is inspected at intervals during its service life. Fortunately, fatigue cracks do not often propagate so rapidly that they cannot be located early in their existence and the part replaced before failure.
Stress Corrosion Cracking If a part that is subject to frequent stress reversals or fluctuations is at the same time subject to corrosion, fatigue cracks may appear very rapidly and may progress to failure in a very short period of time. Corrosion leads to service failure through cracking of metal subject to corrosion at the same time it is under tensile stress. Stress corrosion
cracking of steel beams around bolt holes, leading to bridge collapse, is an example. The corrosion allows the tension stress at the surface to be relieved by cracking.
Other Causes of Service Damage Other sources of service damage causing cracking are abnormally high stresses caused by vibration, as in steam turbine blades and jet engine buckets; impact due to wrecks and other sudden stoppages; and overloads due to abnormal service conditions or simple abuse of the engine or machine.
Recommended reading Subject
Reference
producing metal processing discontinuities service discontinuities
A A A
* See Introduction for explanation of reference
Chapter 4 Appearance of Liquid Penetrant Indications Several factors influence the exact appearance of individual liquid penetrant indications. However, there are certain general trends that hold true for all sorts and forms of materials.
Continuous Line Indications
irregular in outline. Deep crater cracks in welds frequently show up as rounded indications because a large amount of liquid penetrant is entrapped and the individual indications merge. Porosity resulting from shrinkage cavities may produce rounded liquid penetrant indications or may give the appearance of cracks. Usually, shrinkage cavities have a dendritic pattern that helps the inspector identify them. For high stress applications, shrinkage cavities are rejectable discontinuities.
A crack usually shows up as a continuous linear liquid penetrant indication. The width and brightness of fluorescence or color depend on the volume of the crack. The line may be fairly straight or jagged. A cold shut on the surface of a casting also appears as a continuous line, generally a relatively narrow one. Because cold shuts are caused by imperfect fusion where two streams of metal meet but do not merge, the indication is likely to be smooth in outline rather than jagged. A forging lap may also cause a continuous line indication.
Liquid penetrant indications in the form of small dots result from a porous condition. Such indications may denote small pin holes, excessively coarse grains in castings, or may be caused by a shrinkage cavity.
Intermittent Line Indications
Diffuse Indications
Many forging laps are partially welded during subsequent blows of the forging hammer. The liquid penetrant indication caused by such forging laps is therefore an intermittent linear indication. A subsurface crack that does not reach the surface for its entire length, or a seam that is partially filled, also produces an intermittent line indication.
Sometimes a large area presents a diffused appearance. If fluorescent liquid penetrants are used, the whole surface may glow feebly; if visible dyes are used, the background may be pink instead of white. This diffused condition may result from very fine, widespread porosity, such as microshrinkage in magnesium. It may also be caused by insufficient cleaning before testing, by incomplete removal of excess liquid penetrant, or by too thick a coat of developer. Weak indications extending over a wide area should be regarded with suspicion. It is wise to repeat the test and to eliminate any false indications, rather than to attempt immediate evaluation of a diffused indication.
Rounded Areas of Indications Liquid penetrant indications of gas holes are round blobs of rich color or fluorescence. Large gas holes are a basis for rejection because they reduce strength and produce a rough surface. Small gas holes may not be rejectable discontinuities depending on the effect on the strength of the part. If pressure tightness or a smooth surface is required, then even tiny pinholes are considered a basis for rejection. The indications appear rounded because of the volume of liquid penetrant entrapped; the actual discontinuities may be
Small Dot Indications
Edges of Indications The sharpness of liquid penetrant indications is affected by the volume of liquid retained in the discontinuity, the test conditions, such as
28 temperature and time allowed for indications to develop, and the type of liquid penetrant used. Generally, clear cut indications come from narrow linear discontinuities.
Brilliance and Extent of Indications The color or fluorescent brightness of liquid penetrant indications can be useful in estimating the seriousness of the discontinuity. Brightness is directly related to the amount of penetrant present and therefore to the size of the discontinuity. It is difficult for the human eye to detect slight differences in color of dye or brilliance of fluorescence. Fortunately, larger discontinuities usually produce larger indications in addition to increased brightness.
Persistence of Indications One good way to estimate the size of a discontinuity is by the persistence of the indication. If it reappears after the developer has been removed and reapplied, there must be a reservoir of liquid penetrant present. Where faint or weak indications leave some doubt as to the type or even the existence of a discontinuity, it is good practice to repeat the entire liquid penetrant test. If the indication reappears, it is probably due to a small discontinuity rather than to incomplete cleaning.
Other Variables Affecting Indication Appearance The following variables have a marked effect on the size, brilliance and appearance of liquid penetrant indications: condition of surface, temperature of the part or the liquid penetrant, dwell times, amount of washing, developer, conditions of examination and previous testing.
Surface Conditions There are four general ways in which the surface conditions can interfere with the proper results from liquid penetrant testing. 1. Surface openings may be closed. This can occur when lubricants, polishing compounds, dirt, scale or other contaminants are forced into
cracks or holes. Or metal can be peened over surface discontinuities by forging, polishing or shot blasting. 2. Rough or porous areas may retain liquid penetrant, producing irrelevant indications. Or naturally fluorescent materials (such as many oils and greases) may cause confusing indications. 3. Deposits on the surface or in openings may dilute the liquid penetrant reducing its effectiveness, or such materials may react with the liquid penetrant, destroying fluorescence or dye color. 4. Water or moisture within crack discontinuities can prevent the liquid penetrant from entering the crack. Proper cleaning before liquid penetrant testing will remove any contaminants, leaving surface openings clear. If it is impossible to adequately clean the surface, this fact must be taken into account when weighing the importance of indications.
Temperature The viscosity of most liquids increases at low temperatures and liquid penetrants are no exception. If a part is quite cold, under 16 °C (60 °F) for example, the liquid penetrant may become chilled and thickened when applied, making the dwell time for very fine discontinuities longer than for parts at higher temperatures. If a part or liquid penetrant is too hot, the volatile components of the liquid penetrant may evaporate, changing the capability for revealing small discontinuities. Material temperatures from 10 to 52 °C (50 to 125 °F) produce optimum results with conventional liquid penetrants.
Dwell Time and Washing Fine liquid penetrant indications usually denote fine discontinuities. However, fine indications may also be caused by insufficient dwell time. A diffused indication may mean a generally porous condition but may also imply incomplete washing. If excess liquid penetrant is not removed, false indications are evident. On the other hand, washing too long and too hard can remove liquid penetrant from large or shallow discontinuities, resulting in less intense indications.
29
Developer The developer makes indications more visible by: 1. providing a contrasting background, 2. pulling liquid penetrant out of openings by a blotting action, 3. making the indication easier to read by enlarging it as the penetrant spreads into the developer and 4. reducing glare from reflected light during observations of fluorescent indications. A thick film of developer may absorb liquid penetrant, resulting in faint indications or even masking very fine discontinuities. Most faulty liquid penetrant processing techniques are because of loss of indications.
Previous Testing In judging the presence or absence of liquid penetrant indications, the inspector must know whether the parts have been subjected to other test processes. If so, extreme care must be taken to clean parts thoroughly before the additional liquid penetrant testing. Test processes, such as fabrication, can change the surface condition. It is not possible to determine the relative efficiency of various test processes by subsequent checks of the same part because many of the materials used in magnetic particle, fluorescent liquid penetrant and visible dye liquid penetrant techniques are incompatible. Discontinuities may remain undetected by liquid penetrant testing if magnetic particle testing has been previously used. The residual iron oxide may fill or bridge the discontinuity. Similarly, fluorescent liquid penetrant will often fail to show discontinuities previously found by visible dye liquid penetrant because the visible dye may interfere with fluorescence. Subsequent visible dye liquid penetrant testing may miss discontinuities indicated by fluorescent liquid penetrant because the later affects the visible dye color.
Common Causes of Nonrelevant and False Indications Nonrelevant indications may be due to misapplied or improper testing procedures or to
actual discontinuities that do not affect the usability of a part. False indications, a subcategory of nonrelevant indications, are best defined as indications that distract from the quality of the inspection but do not indicate a discontinuity in a part. False indications have several causes but are usually caused by a deficiency in process control.
Poor Process Control Regardless of the liquid penetrant technique used, the level of sensitivity can be adversely affected by poor process control, such as improper application of liquid penetrant, improper removal of liquid penetrant, lack of cleanliness in the inspection area and poor handling of parts being processed. Any of these can cause false indications that will interfere with or cause a distraction during the visual detection and interpretation of indications. Improper application can cause pooling of the liquid penetrant. This produces an uneven coating of liquid penetrant on the surface of a part and, on certain types of materials, results in areas of high background bleedout. Improper removal can cause overremoval or underremoval of excess surface liquid penetrant. Overremoval is indicated by a complete lack of background or indications; a slight background is normal. Overremoval will not result in a false indication but increases the chances of removing a valid discontinuity indication. Underremoval of excess liquid penetrant can cause a high background intensity and decrease the contrast needed to reveal small indications. In addition, the remaining liquid penetrant background may mask a relevant indication. In either case, a part that has been subject to improper removal of excess surface liquid penetrant must be cleaned and reprocessed. Lack of cleanliness in the inspection area is often a cause of false indications. An inspection area should be free of all liquid penetrant. Liquid penetrant on roller assemblies, dryer shelves or inspection tables should be cleaned up before parts are processed. Otherwise the penetrant can be absorbed by the developer and cause nonrelevant indications. Proper handling of parts during all phases of the liquid penetrant process is important. Rubber gloves should be cleaned between processing steps. Small parts processed together should not
30
ASNT Level II Study Guide: Liquid Penetrant Testing
rub against each other. Lint from towels or rags will fluoresce under near ultraviolet light and give the appearance of a crack. False indications due to improper handling of parts can be easily prevented.
Part Geometry and Surface Condition The geometry of a part or assembly can cause nonrelevant indications. The indication of an interface between a turbine blade and a disk can not only produce a nonrelevant indication, but can also mask possible small indications if the bleedout from the interface is excessive. In many cases inspection procedures will require that an assembly be completely disassembled before inspection. Press fittings are a source of nonrelevant indications caused by part geometry. It is sometimes impossible to completely disassemble a part that must be inspected. The bleedout of the liquid penetrant around press fittings can be considerable and can mask relevant indications. In these situations the part must be monitored very closely during the developer dwell time to properly evaluate the indications as they form. Another cause of false and nonrelevant indications is a rough part surface. Rough weld bead surfaces and weld spatter make welds very
difficult to inspect, especially with solvent removable method. The major concern on a weld bead is masking of relevant indications. As-cast and as-forged surfaces may also lead to nonrelevant indications because of surface roughness or other irregularities. Machined parts may have machining grooves or marks, or scratches from rough handling.
Evaluation of Nonrelevant Indications One technique that can be used to help identify a nonrelevant indication is wiping off the indication with a solvent dampened, lint free, swab or cloth and reapplying the developer. The original developer dwell time must be repeated; however, the bleedout should be monitored to help evaluate the indication. If there is no bleedout, the original indication can be considered nonrelevant or false. This technique may also reveal multiple relevant indications that had previously merged into one or had been hidden by excessive bleedout from an adjacent nonrelevant indication during the developer dwell.
Recommended reading Subject
Reference*
appearance of liquid penetrant indications
A
variables affecting indication appearance
A
causes of nonrelevant indications
A
*See Introduction for explanation of references.
Review Questions
1. One disadvantage of liquid penetrant inspection is that:
5. A penetrant inspection cannot find:
a. penetrant materials may cause drying and cracking of the skin. b. large subsurface discontinuities with openings to the surface will not form indications. c. it can only be used with nonferrous material. d. the materials are only available in aerosol spray cans. Ref B 2. Which of the following is not a physical property associated with penetrant materials? a. b. c. d.
Solvent resistance. Flash point. Viscosity. Chemical inertness.
grit or sand blasting. machining. etching. quenching.
Ref B
water soluble. water washable. nonfluorescent. solvent removable.
Ref B
6. Which of the methods listed below is most applicable for detecting fine, tight surface cracks? a. b. c. d.
Visible dye, water washable. Visible dye, postemulsifiable. Fluorescent, water washable. Fluorescent, postemulsifiable.
Ref B
a. b. c. d.
immersion. drain. dwell. presoak.
Ref B;Ref C
8. Developer assists in the detection of discontinuities by: Ref B
4. A standard classification for a type of developer is: a. b. c. d.
surface porosity. surface cracks. an internal cavity. a surface forging lap.
7. The term used to describe the total time that penetrant remains on the workpiece is:
3. The removal of smeared metal by chemical processes is called: a. b. c. d.
a. b. c. d.
Ref B
a. b. c. d.
providing a contrasting background. emulsifying the penetrant bleedout. providing a clean surface. providing a dry surface. Ref B
32
9. Difficulties with removal of a postemulsifiable penetrant during the rinse operation can be overcome by: a. reapplying a coating of emulsifier. b. using conditioned (soft) water during the washing operation. c. cleaning, then completely reprocessing the part using a longer emulsification time. d. increasing the water temperature during the washing operation. Ref B;Ref C 10. Excessive removal of penetrant may result in: a. undetected discontinuities during evaluation. b. an excessive background that inhibits the evaluation process. c. the need to apply the developer twice to obtain significant bleedout. d. small, tight cracks that are easily detected on parts with a smooth surface. Ref B 11. Which of the following is an advantage of a fluorescent water washable penetrant system compared to a fluorescent postemulsifiable penetrant system? a. Water washable is superior for testing critical in service parts. b. Water washable has fewer processing steps. c. Only wet developer may be used with postemulsifiable penetrants. d. Water washable penetrants will more readily detect fine, tight cracks in weldments. Ref B
12. Which of the following is not an advantage of postemulsifiable fluorescent penetrants? a. High sensitivity level to detect very fine defects. b. They are easily removed with water after proper emulsification dwell time. c. They are easily removed from sand castings after proper emulsification dwell time. d. They have higher tolerance to variations in the removal process when compared to water washable penetrants. Ref B 13. When using the visible, solvent removable process, after the penetrant dwell time, excess penetrant is removed by: a. spraying solvent on the part and then wiping with a clean, dry rag or paper towel. b. wiping off excess penetrant with a clean, dry rag or paper towel, moistening the part with solvent and rewiping the part. c. wiping off excess penetrant with a clean, dry rag or paper towel, moistening a clean cloth with solvent and rewiping the part. d. wiping off excess penetrant with a clean, dry rag or paper towel, then rewiping with a cloth soaked with solvent. Ref B 14. When using a fluorescent, postemulsifiable penetrant, the best emulsification time is: a. 10 s. b. 5 s. c.
2 to 3 s.
d. determined by experimentation Ref B
33
15. Which type of developer is generally considered the most sensitive for locating fine discontinuities? a. b. c. d.
Solvent suspended. Water soluble Water suspended. Dry powder.
Ref C
16. Which of the following penetrant systems is the least sensitive? a. b. c. d.
fluorescent, postemulsifiable visible, solvent removable. fluorescent, solvent removable. fluorescent, water washable.
Ref B
17. One method for applying dry developer to a localized area of a large part is with: a. b. c. d.
a brush. a powder bulb. an aerosol can. an immersion tank.
Ref B
18. Prewash and postwash stations are an advantage when using which of the following penetrant methods? a. Visible, lipophilic postemulsifiable. b. Fluorescent, hydrophilic postemulsifiable. c. Visible, water washable. d. Fluorescent, lipophilic postemulsifiable. Ref B 19. The specific gravity of wet developer is measured with a: a. b. c. d.
developer gage. hydrogage. balance scale. hydrometer.
Ref C
20. Which of the following describes the comparator that is used to monitor the process performance of the penetrant system? a. A panel with one half chrome plated containing five crack centers and the other half grit blasted. b. A cracked aluminum block with a groove in the center. c. A chrome plated panel with fine cracks across the face to a depth equal to the plating thickness. d. Ceramic coated block with fine cracks extending around the entire panel. Ref B 21. Ultraviolet light intensity is measured with: a. b. c. d.
a digital or analog UV-A meter. a photographic light meter. an infrared meter. a standard white light meter.
Ref B
22. What instrument is used to check hydrophilic emulsifier concentration? a. b. c. d.
A hydrometer. A capillary fill barometer. A centrifuge tube. A refractometer.
Ref B
23. The penetrant performance characteristic known as sensitivity is defined as the: a. the contrast between an indication and the background. b. ability to produce a visible indication from a small, tight linear discontinuity. c. volume percentage of penetrant that remains in a discontinuity after processing. d. ability to enter a discontinuity to form an indication. Ref B
34
ASNT Level II Study Guide: Liquid Penetrant Testing
24. A group of nearly rounded indications located in an area that transitions between thick and thin sections of aluminum castings is known as: a. b. c. d.
shrinkage cracks. gas holes. laminations. cracks due to heat treating.
Ref B
25. Processing a part with developer residue remaining from a previous penetrant inspection may result in: a. an enhanced background contrast. b. a more rapid formation of indications. c. a greatly reduced formation of indications. d. the elimination of a second application of developer. Ref B 26. For intermediate dwell times, applying fresh penetrant 15 min before removal assists the process by increasing the: a. rate of evaporation. b. time required for the removal operation. c. rate of penetration. d. sensitivity of the penetrant.
28. Repeated or cyclic loads below the yield strength of a material can cause: a. b. c. d.
service cracking. fatigue cracking. intergranular cracking. thermal cracking. Ref B
29. Grinding checks or cracks are caused by: a. b. c. d.
localized overheating. a slow feed rate. unequal heating or cooling. rapid cooling from high temperatures. Ref B
30. Elongated inclusions found in bar stock are called: a. b. c. d.
seams. laps. stringers. contaminants.
Ref B
31. An advantage of the postemulsification fluorescent penetrant process is that it: Ref B
27. How does an increase in temperature affect the viscosity of a penetrant? a. It becomes more viscous and decreases in flow rate. b. It becomes less viscous and decreases in flow rate. c. It becomes more viscous and increases in flow rate. d. It becomes less viscous and increases in flow rate. Ref B
c. d.
is very good on rough surfaces such as sand castings. has fewer operations than other penetrant processes. can be used on anodized surfaces. can be used to detect wide, shallow discontinuities. Ref B;Ref C
32. What comparison standard has control over the depth and quantity of cracks? a. b. c. d.
A stainless steel test panel. A cracked aluminum test block. A ceramic test block. A grit blast panel.
Ref B
Review Questions 33. Compared to a certified Level I inspector, a Level II inspector: a. can process all forms of a product. b. is authorized to approve all penetrant procedures. c. requires more formal training hours and work experience. d. can determine the acceptance criteria for a part. Ref B 34. A limitation on materials used to inspect nickel alloy welds in a nuclear or pressure vessel system is that the penetrant materials should have: a.
sulfur and halogen residual of 10% or less by weight. b. sulfur, chlorine and fluorine residual of 1 % or less by weight. c. chlorine and halogen residual of 5% or less by weight. d. halogen and fluoride residual of 10% or less by weight. Ref B 35. A weld discontinuity that appears starshaped with small indications emanating from a central point is a: a. b c. d
35
37. Which of the following would not be the cause of a nonrelevant indication? a. b. c. d.
Penetrant on the inspection table. An improperly cleaned surface. A corrosion crack. Penetrant trapped in the threads of a part. Ref C
38. The periodic checks performed to assure that the penetrant testing system is functioning are called: a. b. c. d.
process control tests. maintenance tests. certification tests. performance evaluations.
Ref C
39. A relevant indication in a casting may be caused by: a. b. c. d.
a burst. a crater crack. laminations. porosity.
Ref C
40. The relative size of a surface discontinuity is estimated based on the indication burst. intensity and the: grinding crack. crater crack. a. amount of bleedout. heat affected zone (HAZ) crack. b. size and shape. Ref B c. direction of bleedout Ref C 36. Multiple round or nearly round indications scattered on a weld are the result of: 41. Which of the following is a nonrelevant indication found during penetrant a. crater pits. inspection? b. undercut. c. porosity. a. Marks on the part due to penetrant on d. weld laps. Ref B the inspector's hands. b. A press fit insert ring in the bore of a shaft. c. Linear indications on a heat treated weldment. d. Dry developer residue. Ref B
36
ASNT Level II Study Guide: Liquid Penetrant Testing
42. A visual method used as acceptance criteria for a part uses: a. b. c. d.
sized wires. a calibrated scale. a testing and monitoring panel. referenced photographs.
46. Ultraviolet lights over a fluorescent penetrant wash station are used to:
Ref C
43. The purpose of a penetrant system monitor is to provide: a. a method of estimating the cost of inspection. b. verification of the overall processing system performance. c. standardization for performing process controls. d. the means to reject test objects. Ref B 44. Using a weld standard for interpretation of indications provides: a. reliable acceptance criteria. b. an accurate method to determine repair costs. c. the format for a test report. d. the means to reject linear indications. Ref B; Ref C 45. Dry developer is checked for fluorescent contamination by: a. comparing the in-use developer to new material. b. processing a testing and monitoring panel. c. a laboratory examination for chemical composition. d. viewing under an ultraviolet light for fluorescent specks. Ref B
a. verify a uniform coating of penetrant has been applied to the part. b. verify the emulsifier covers the entire surface. c. monitor the removal of surface penetrant. d. check the cleanliness of parts before processing. Ref C 47. An advantage of inspecting forgings with the penetrant process is that: a. it can detect any type of discontinuity associated with forgings. b. there are no limitations to the part size or shape. c. forging scale does not interfere with the test. d. penetrant materials do not have any adverse affects on any alloy that is forged. Ref B 48. Discontinuities that may be found in aluminum extrusions are: a. b. c. d.
porosity and inclusions. laminations. cracks. laps and seams.
Ref B
49. Which of the following discontinuities are associated with forgings? a. b. c. d.
Tears. Cracks. Bursts. Blow holes.
Ref B
37
Review Question Answers 1. 2. 3. 4. 5. 6. 7. .8. 9. 10. 11. 12. 13.
a a c a c d c a c a b c c
14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
d a b b b d a a d b a c c
27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.
d b a c d a c b c c c a d
40. 41. 42. 43. 44. 45. 46. 47. 48. 49.
a b d b a d c b d c
38
ASNT Level II Study Guide: Liquid Penetrant Testing
Appendix Standard Terminology for the Liquid Penetrant Examination This standard terminology is adapted from ASTM E 1316-94. Angstrom unit - A unit of length that may be used to express the wavelength of electromagnetic radiation, that is light. One angstrom unit is equal to 0.1 nanometers. Background -~ The surface of the test part against which the indication is viewed. It may be the natural surface of the test part or the developer coating on the surface. Ultraviolet light - Electromagnetic radiation used for nondestructive testing in the range of wavelength 330 to 390 nm (3300 to 3900 A), also known as UV-A. Ultraviolet light filter - A filter that transmits near-ultraviolet radiation while absorbing other wavelengths. Bieedout - The action of an entrapped liquid penetrant in surfacing from discontinuities to form indications. Blotting - The action of the developer in soaking up the penetrant from the discontinuity to accelerate bieedout. Carrier - A liquid, either aqueous or nonaqueous, in which liquid penetrant examination materials are dissolved or suspended. Clean - Free of contaminants. Contaminant ~ Any foreign substance present on the test surface or in the inspection materials that will adversely affect the performance of liquid penetrant materials. Contrast - The difference in visibility (brightness or coloration) between an indication and the background. Defect - One or more flaws whose aggregate size, shape, orientation, location, or properties do not meet specified acceptance criteria and are rejectable. Developer - A material that is applied to the test surface to accelerate bieedout and to enhance the contrast of indications. Developer, aqueous - A suspension of developer particles in water. Developer, dry powder - A fine, freeflowing powder used as supplied. Developer, liquid film - A suspension of developer particles in a vehicle that leaves a resin/polymer film on the test surface after drying. Developer, nonaqueous - Developer particles suspended in a nonaqueous vehicle before application.
40
ASNT Level II Study Guide: Liquid Penetrant Testing
Developer, soluble - A developer completely soluble in its carrier, not a suspension of powder in a liquid, which dries to an absorptive coating. Developing time - The elapsed time between the application of the developer and the examination of the part. Discontinuity - A lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component. Dragout - The carryout or loss of penetrant materials as a result of their adherence to the test piece. Drain time - That portion of the dwell time during which the excess penetrant or emulsifier drains from the part. Drying oven - An oven used for increasing the evaporation rate of rinse water or an aqueous developer vehicle from test parts. Drying time - The time required for a cleaned, rinsed, or wet developed part to dry. Dwell time - The total time that the penetrant of emulsifier is in contact with the test surface, including the time required for application and the drain time. Electrostatic spraying - A technique for attaining a uniform coating in which the material sprayed is given an electrical charge. Emulsification time - The time that an emulsifier is permitted to remain on the part to combine with the surface penetrant before removal. Also called emulsification dwell time. Emulsifier - A liquid that interacts with an oily substance to make it water washable. Emulsifier, hydrophilic - A waterbased liquid used in penetrant examination that interacts with the penetrant oil rendering it water washable. The hydrophilic is supplied in a concentrate and is mixed with water to a concentration not exceeding that listed in the QPL. Emulsifier, lipophilic - An oil based liquid used in penetrant examination that interacts with the penetrant oil rendering it water washable. Etching - The removal of surface material by chemical or electrochemical methods. Evaluation - A review, following interpretation of the indications noted, to determine whether they meet specified acceptance criteria. False indication - A nondestructive testing indication that is interpreted to be caused by a discontinuity at a location where no discontinuity exists. Family - A complete series of penetrant materials required for the performance of a liquid penetrant examination. The term Family only applies to Method B and Method D and only includes the penetrant and the emulsifier. Developers are not part of the Family. Flaw - An imperfection or discontinuity that may be detectable by nondestructive testing and is not necessarily rejectable. Flaw characterization - The process of quantifying the size, shape, orientation, location, growth, or other properties of a flaw based on nondestructive testing response.
Appendix 1, Standard Terminology for the Liquid Penetrant Examination
41
Fluorescence - The emission of visible radiation by a substance as a result of and only during the absorption of ultraviolet light radiation. Footcandle - The illumination on a surface, 1 ft2 in area, on which is uniformly distributed a flux of 1 lm (lumen). It equals 10.8 lm/m2. Immersion rinse - A means of removing surface penetrant in which the test part is immersed in a tank of either water or remover. Imperfection -A departure of a quality characteristic from its intended condition. Indication - Evidence of a discontinuity that requires interpretation to determine its significance. Inspection - Visual examination of the test part after completion of the liquid penetrant processing steps. Interpretation - The determination of whether indications are relevant or nonrelevant. Liquid penetrant examination - A nondestructive test that uses suitable liquids that penetrate discontinuities open to the surface of solid materials and, after appropriate treatment, indicate the presence of discontinuities. Nondestructive testing (NDT) - The development and application of technical methods to examine materials or components in ways that do not impair future usefulness and serviceability in order to detect, locate, measure and evaluate discontinuities, defects and other imperfections; to access integrity, properties and composition, and to measure geometrical characteristics. Nonrelevant indication -A nondestructive testing indication that is caused by a condition or type of discontinuity that is not rejectable. False indications are nonrelevant. Over emulsification - Excessive emulsifier dwell time that results in the removal of penetrants from some discontinuities. Over washing - Too long or too vigorous washing, or both, which results in removal of penetrants from some discontinuities. Penetrant - A liquid which has been formulated to enter surface discontinuites and which contains a tracer dye. Penetrant comparator - An intentionally flawed specimen having separate but adjacent areas for the application of different liquid penetrant materials so that a direct comparison of their relative effectiveness can be obtained. It can also be used to evaluate liquid penetrant techniques, liquid penetrant systems, or test conditions. Penetrant, fluorescent - A penetrant that emits visible radiation when excited by an ultraviolet light. Penetrant, postemulsifiable - A liquid penetrant that requires the application of a separate emulsifier to render the excess surface penetrant water washable. Penetrant, solvent removable - A liquid penetrant so formulated that most of the excess surface penetrant can be removed by wiping with lintfree material, with the remaining surface penetrant traces removable by further wiping with a lintfree material lightly moistened with solvent remover.
42
ASNT Level II Study Guide: Liquid Penetrant Testing
Penetrant, visible - A liquid penetrant that is characterized by an intense color, usually red. Penetrant, water washable - A liquid penetrant with a built-in emulsifier. Pooling - The existence of excessive amounts of penetrant, emulsifier, or developer in an incompletely drained area. Post cleaning - The removal of residual liquid penetrant examination materials from the test part after the penetrant examination has been completed. Post emulsification - A penetrant removal technique employing a separate emulsifier. Precleaning - The removal of surface contaminants from the test part so that they will not interfere with the examination process. Relevant indication - A nondestructive testing indication that is caused be a condition or type of discontinuity that requires evaluation. Rinse - The process of removing liquid penetrant examination materials from the surface of a test part by means of washing or flooding with another liquid, usually water. The process is also termed wash. Solvent remover - A volatile liquid penetrant used to remove excess from the surface being examined. Temperature envelope - The temperature range over which a particular penetrant inspection test will operate. Viscosity - The property of a fluid that presents a resistance to shearing flow. Visible light - Electromagnetic radiation in the 400 to 700 nm wavelength range. Visual adaptation - The adjustment of the eyes when one passes from a bright to a darkened place. Water tolerance - The amount of water that a penetrant or emulsifier can absorb before its effectiveness is impaired. Wetting action - The ability of a liquid to spread over and adhere to solid surfaces.
