Metallographic Printing Methods Sulphur Printing
Aim: Determination of sulphur in iron and steel Introduction: Impurities may exist in steel products. It is desirable that their amounts should be minimal and homogeneously distributed within the product. These impurities degrade the mechanical properties of the steel; especially sulphur content in steel makes it brittle. brittle. Sulphur may exist chemically in steel in one of two forms, either as manganese sulphide or as Iron sulphide. Experimental Procedure: Sulphur printing detects and permanently records the distribution of sulphur in Steel. The surface of interest to be tested for distribution of sulphur should be reasonably smooth and free from foreign matter such as dirt and grease. Grinding the surface on papers, numbers 400 to 1200 and subsequent thorough washing will generally produce a surface satisfactory for the purpose.
Photographic bromide paper is soaked in a 2% aqueous solution of sulphuric acid for approximately 3 or 4 min. The paper is removed from the acid solution and allowed to drain free from excess solution. The emulsion side of the paper is then placed in direct contact with the prepared specimen surface and allowed to remain in contact under moderately applied pressure for 1 or 2 min. Care must be taken that all entrapped air bubbles between the paper and the specimen surface are eliminated. The reaction of the sulphuric acid with the sulphide regions of the steel produces hydrogen sulphide gas, which reacts with the silver bromide in the paper emulsion, forming a characteristic brown to gray-black deposit of silver sulphide. These reactions may be expressed as follows: + 2 4 → 4 + 2 Or + 2 4 → 4 + 2 2 + 2 → 2 + 2 When the reaction has proceeded for approximately the recommended length of time, the photographic paper is removed from the surface of the specimen, rinsed in clear running water, and then fixed permanently by placing it in a photographic fixing solution for about 15 min. When fixation is completed, the print is again washed in running water for approximately approx imately 30 min. and subsequently dried in the usual manner. Fixing Solution composition: 600 mlWater 240 g Sodium Thiosulfate (hypo) 15 g Sodium Sulfate 45 ml Acetic Acid (28%) * 7.5gBoricAcid
15 g Potassium Alum Enough water to make one liter *To make 28% acetic acid from glacial acetic, dilute 3parts glacial acetic to 8 parts water. Store unused fixing bath in a dark place. Result: The examination of properly prepared sulphur print will disclose quite clearly, because of the presence of darkly colored areas of silver sulphide, the precise location of sulphur inclusions on the prepared surface of the metal. A grouping or gathering of such silver sulphide areas indic ates the presence of sulphur segregation, whereas a random dispersion of the spots denotes a more uniform, distribution of the sulphur inclusions. Oxide printing: First method: A technique for showing the distribution of oxide inclusion containing iron was developed by Niessner. Gelatin paper is moistened in an aqueous solution containing5 % HCL acid for about 5 min. The gelatinized side of the paper is blotted dry and placed on to the polished surface of steel sample for about 5 seconds. The paper is then removed and place in the aqueous solution containing potassium ferrocyanide (20 gm 4 ()6 to 1000 ml of water). This solution develops the image. The print ex hibits a light blue colour over the contact area. Dark blue spots are present at place where iron containing inclusions are present. Second method: Soak the photographic paper in the solution of 15gm of sodium chloride to the dilute hydrochloric acid solution. The rest process is same as first method. Third method: Cellophane 0.025 mm thick is soaked for few minutes in water. After the c ellophane dries, a few drops of 20 % aqueous solution of quinoline-Etl are distributed over the surface of the film. After about a minute, the solution is wiped off. The film is placed in the polished sample, and an etching solution consisting of equal part of a 1.2 % ferrocyanide solution and a 0.25% HCL acid solution is applied. After 2 to 3 min., the etching solution is washed off and the film is removed and developed in a 2.5% solution of potassium ferrocyanide in water and then rinsed. The film image is then oxidized in a solution containing 1ml of 3% H2O2 in 100 mL water. Phosphorus printing:
A sample with a ground surface is immensed in a solution consisting of 5 gm Ni(NO3)2 and 1 .5 gm CuCl2 dissolved in 12 mL hot water, 6 gm FeCl3 and 150 mL methanol(a few ml HNO3 1/2 may be added.). After 1 to 3 min, a coloured surface began to form. This layer can exhibit a wide range of colours from pale brown to purplish red. Segregation shows as white strikes and spots. This pattern can reportedly be transferred to p hotographic paper. A sheet of photographic paper is soaked for several minutes in a 5 % solution of potassium ferricyanide inwater The paper is placed face up on the blotting paper and the coated steel surface is placed against the paper for about a minute. The paper is rinsed and fixed. The segregated areas should show up as a blue colour.
The filter paper is soaked in an ammonium molybdenum and nitric acid solution (5 gm ammonium molybdenum dissolved in 100 mL of cold water, which is then added to 35 mL nitric acid). The excess solution is drained off, and the paper is placed on the surface of polished sample. After a contact time of 3 to 5 minutes, the paper is removed and placed in the developing solution. The paper is immersed in this solution for 3 to 4 minutes. Initially the print should appear yellow but after 45 seconds, a blue colour is observed where phosphorus is present. Because the filter paper is attacked by the solution of SnCl2 and HCL, a pinch of alum is added to the developer to harden the paper. After development, access acid is washed off in flowing water and paper is dried. Developing solution composition: By Niessner and Feigl 50 mL SnCl2 50 mL HCL 100mL H2O
Enos and khel 5 mL of saturated SnCl2 solution to a mixture of 50 mL HCL and 100 mL H2O
