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Avoidance of hydrogen hydrogen cracking cracking in ferritic steels Method to determine preheat temperature Weld temperature cycle Heat input Carbon equivalent Combined Thickness Diffusible H Content Transition thickness Calculating preheating temperature according AWS D1.1 Weld shape factor Grouping system system for steels (groups (groups 1-4) 1 -4) Disclaimer All informati information on obtained obtained from this this program program shall shall be considered considered as a guideli guideline. ne. Under Under no circumstances, the author can be hold liable for any situation resulting from using this program. All rights rights reserved, reserved, including including the right of reproduction reproduction in whole or in part in any form. form. §
§
Avo idance of crackin g in ferritic steels The scope of this program is to give guidance for avoiding hydrogen cracking (cold cracking) in unalloyed and low-alloyed ferritic steels. Cold cracking in ferritic steels can occur when there are three combined factors: Hydrogen generated by the welding process A microstructure, susceptible to cracking Residual stresses in the welded joint § § §
In unalloyed en low alloyed steels, most of the hydrogen cracks are in the HAZ, however cracks can also occur in the weld metal, especially in low alloyed steel. To avoid this cracking you can minimize the combined contribution of the factors. Weld metal hydrogen content The principal source of hydrogen is the moisture in the consumables. Basic stick electrodes normally generated less hydrogen than rutile or cellulosic types. For cored wires, basic0, rutile- and metal cored wires, all can deposit weld metal with low hydrogen. In sub arc welding, basic fluxes typically give a low hydrogen weld metal. Parent metal composition The hardenability of a material is usually expressed in terms of its carbon content or, when other elements are taken into account its carbon equivalent (like CE). The higher the carbon equivalent the greater the risk of hydrogen cracking. Generally, steels with a CE < 0,4 are not susceptible to hydrogen cracking, as long as low hydrogen welding consumables are used.
Parent material thickness The parent material thickness influences the cooling rate, and therefore the hardness of the HAZ, but also the amount of hydrogen retained in the weld and the residual stresses.
Stresses acting on the weld The stresses generated across the welded joint will be greatly influenced by external restraint, material thickness, joint geometry and fit up. Areas of stress concentration are more likely to initiate a crack at the toe and root of the weld. Heat input The heat input, together with the material thickness and the preheat temperature will determine the thermal cycle and the resulting microstructure, the hardness and the remaining hydrogen content. A high heat input will reduce the hardness and hydrogen content, but increases the width of the heat affected zone and decreases the Charpy toughness. Preheating When it is not possible to avoid cold cracks by lowering the hydrogen content, preheat is a necessity. In EN 1011-2 (2001) recommendations for the preheating temperature of ferritic steels are given.
typical cold crack, due to high stresses in the root (misalignment), as well as high hardness in the HAZ (from: Bailey, Welding of ferritic steels)
Method to determine preheat temperature of ferritic steels The methods, described in EN 1011-2:2001, are recommendations to avoid hydrogen cracking (also known as cold cracking) in ferritic steels. Many methods have been proposed for predicting preheat temperature to avoid hydrogen cracking in non-alloyed, fine grained and low alloy steel weldments. Examples are given in IIW documents. Two of those methods are described in this standard: Method A is based on extensive experience and data which is mainly, but not exclusively, for carbon-manganese type steels. Method B is based on experience and data which is mainly, but not exclusively for low alloy high strength steels. Beside these two methods there are tables which shall be used for creep resisting and low temperature steels. (method C). The recommendations apply only to normal fabrication restraint condition. Higher restraint situations may need higher preheat temperature or other precautions to prevent hydrogen cracking. The methods A and B refer to welding of parent metal at temperatures above 0 °C. When welding is carried out below this temperature it is possible that special requirements will be needed. Otherwise lower preheat temperatures are possible, if this is supported by procedures. To calculate the preheat temperature for method A or B you have to know The hydrogen content of the consumable (HD) The composition of the parent metal (CE of CET); The plate thickness and joint geometry The heat input § § § §
In this program, a method D is added, based on the standard AWS D1.1 To calculate the preheat temperature for this method you have to know The hydrogen content of the consumable (HD) The composition of the parent metal (Pcm); The plate thickness The restraint level § § § §
Weld temp erature cycle The calculation of the welding temperature cycle is based upon the simplified formulas of Rykalin. The formulas used here for respectively 3- and 2-dimensional cooling of a bead on plate are the following.
3-Dimensional: The temperature as a function of time and place is given by
R 2 × exp − + T0 . T(t , R ) = 2π λ t 4 at Q
The cooling time from 800°C to 500°C then is
∆t 8 / 5 =
1 1 . × − 2π λ 500 − T0 800 − T0 Q
2-Dimensional: The temperature as a function of time and place is given by
T(t , R ) =
R 2 × exp − + T0 . 4 at 4 πλρc t Q
d
The cooling time from 800°C to 500°C then is
∆t 8 / 5 =
Q2 4 πλρ cd
2
1 1 . × − 2 2 ( 500 − T0 ) ( 800 − T0 )
Here R represents the distance to the center of a point (3D) or line (2D) shaped heat source, λ , c en ρ are the physical “constants”, d is the plate thickness and T 0 the preheat and interpass temperature.
The relations for the cooling time t 8/ 5 have been empirically adapted to steel by Uwer et. al. (IIW doc. IX 1631-91), obtaining the formulas below. Here there is no need for the values of λ, ρ en c, which are often difficult to obtain. Furthermore the weld shape factor for three- or two-dimensional heat flow (F3 respectively F 2) has been introduced. This enables one to calculate more situations than a bead on plate. These new formulas for ∆t8/ 5 are described in EN 1011-2.
3-Dimensional: t8/ 5
=
(6700 − 5 T0 ) × Q ×
1 1 − × F3 . − − 500 T 800 T 0 0
2-Dimensional: t8/ 5
= (4300 − 4, 3 T0 ) × 10 × 5
Q2 d
2
1 1 × F2 . − 2 2 ( 500 − T0 ) ( 800 − T0 )
The transition thickness dt is the plate thickness at which the transition from threedimensional to two-dimensional heat flow takes place. In that case F2 = F3 and both values of t8/5 are equal, also:
dt
=
( 4300 − 4,3 × T0 ) × 10 5 6700 − 5 × T0
1 × Q × 1 + 500 − T0 800 − T0
Some values of the transition thickness are below:
Preheating temperature
Q. 20°C.
100°C.
200°C.
0,5
10.4
11.1
12.3
1
14.7
15.7
17.4
1,5
18
19.2
21.3
2
20.7
22.2
24.6
2,5
23.2
24.8
27.5
3
25.4
27.1
30.1
Heat Inpu t The heat input is defined as
Q
= k ×
U×I
(kJ/mm)
v where k = relative thermal efficiency for the applicable process (see table); U = arc voltage in V I = welding current in mm/s v = welding speed in mm/s; Often the welding speed is given in cm/min. In case of shielded metal arc welding; it may be difficult to use the above formula, so you can use the data of the tables listed in EN 1011-2, in which the run out length is expressed in terms of electrode diameter and heat input, by different efficiencies and a consumed electrode length of 410 mm (when the electrode length is 450 mm). Otherwise you can use the following formula:
Q
=
D2
×L× F rol
(kJ/mm)
where D = electrode diameter L = the consumed length of the electrode (mm). Normally this is the originally length less 40 mm for the stub end rol = run out length 3 F = factor in kJ/mm depending on the electrode efficiency Efficiency approx. 95% F = 0,0368 95% < efficiency ≤ 110% F = 0,0408 110%< efficiency ≤ 130% F = 0.0472 efficiency > 130% F = 0,0608 This formula is normally used when the electrode length differs from 450 mm, but is also used in this program
Carbon equivalent Hardness and hardness penetration of steel (the hardenability) depends on the carbon content, the alloying elements, the cooling rate and the grain size. The effect of the alloying elements on the hardenability, and thus on the weldability of steel is usually expressed in a carbon equivalent. In a carbon equivalent formula the hardening effect of each alloying element is compared to that of carbon. Because it is an empirical formula, there are a number of carbon equivalents. In this program there are three formulas used (CE, CET and Pcm).
1.
CE = C +
Mn 6
+
Cr + Mo + V 5
+
Ni + Cu 15
in %
This carbon equivalent is applicable in the range of 0,30 to 0,70 and may be used for unalloyed, fine grained and low alloy steels within the following range of composition (weight %) Carbon 0,05 to 0,25 % Silicium 0,8% max. Manganese 1,7% max. Chromium 0,9% max. Copper 1,0% max. Nickel 2,5% max. Molybdenum 0,75% max. Vanadium 0.20% max. The formula is not suitable for boron-containing steels When, of the elements in this formula, only carbon and manganese are stated on the mill sheet, then 0,03 should be added to the calculated value . (This is corrected in the program)
2.
CET
= C + Mn
+ Mo
10
+ Cr
+ Cu 20
+ Ni 40
in %
This carbon equivalent is applicable in the range of 0,30 to 0,70 and may be used for unalloyed, fine grained and low alloy steels within the following range of composition (weight %). Carbon 0,05 to 0,32% Silicium 0,8% max. Manganese 0,5 to 1,9% Chromium 1,5% max. Copper 0.7% max. Nickel 2,5% max. Molybdenum 0,75% max. Vanadium 0.18% max. Niobium 0,06% max. Titanium 0,12% max Boron 0,005% max
The relationship is valid for structural steels with Rp 02 < 1000 N/mm2 , and CET = 0,2 to 0,5% The CET of the parent material exceeds that of the weld metal by at least 0,03% Otherwise the calculation of the preheat temperature has to be based on a CET of the weld metal, increased by 0,03% (This can not be corrected by the program)
3.
Pcm
= C + Si + Mn 30
+ Cr + Cu 20
+ Ni + Mo + V + 5B 60
15
10
This carbon equivalent, according Ito and Bessyo, for low alloyed steels is valid within the following composition(weight %): C 0,07 to 0,22% Mn 0,4 to 1,40 % Si 0,6% max. Ni 1,2% max Cr 1,2% max Mo 0,7% max V 0,12% max Cu 0,5% max. B 0,005% max. This formula is used, together with the hydrogen content, plate thickness and restraint condition to calculate a preheating temperature from a table. (method D in this program)
Combined thickness
The combined thickness (tg) is the sum of the parent metal thickness averaged over a distance of 75 mm from the weld line. Combined thickness is used to assess the heat sink of a joint for the purpose of determining the cooling rate In a fillet weld, the heat sink is greater than in a butt weld with the same thickness. The preheating temperature is higher because of the greater combined thickness.
Tg = d1 + d2 + d 3
Tg = D1 + D2
Diffusible H Content In fusion welding the hydrogen content, immediately after solidification, is very high, but most of it diffuses out of the weld This diffusible hydrogen moves not only into the air, but also into the HAZ. The remaining diffusible hydrogen can be high resulting embrittlement. It is necessary to know the amount of diffusible hydrogen. Sources are not only the consumables, but also the plate surface and the atmosphere. The hydrogen content is usually expressed in ml/100 g deposited weld metal, known as HD . In setting up welding procedures, the hydrogen content in the weld metal as a result of supported by the consumable used, is divided in classes: hydrogen scales for A to E Diffusible hydrogen content ml/100g of deposited metal > 15 10 ≤ 15 5 ≤ 10 3 ≤ 5 ≤ 3
Hydrogen scale A B C D E
Transition thickn ess The transition thickness dt is the plate thickness at which the transition from threedimensional to two-dimensional heat flow takes place. In that case F2 = F3 and both values of t8/5 are equal, also:
dt
=
( 4300 − 4,3 × T0 ) × 10 5 6700 − 5 × T0
1 × Q × 1 + − − 500 T 800 T 0 0
Some values of the transition thickness (in mm) are below: Preheating temperature
Q.
20 °C.
100 °C.
200 °C.
0,5
10.4
11.1
12.3
1
14.7
15.7
17.4
1,5
18
19.2
21.3
2
20.7
22.2
24.6
2,5
23.2
24.8
27.5
3
25.4
27.1
30.1
Weld shape factor The influence of the weld shape on the cooling time has been investigated by Uwer et al. and is used in the calculations given in EN 1011-2. The shape factor for two dimensional is F2, for three dimensional F3
F2 Form of weld
F3
Two dimensional heat flow Three dimensional heat flow 1
1
0.9
0.9
0.9-0.67
0.67
0.45-0.67
0.67
Calculating preheating temperature acco rding to AWS D1.1 Here the calculation of the preheating temperature from Pcm (method D) according to awsD1.1 is explained First calculate the Pcm value and determine the hydrogen content. A “Susceptibility Index Grouping ” A-G is then derived from a table, or by calculation. Then a restraint condition and plate thickness is chosen. In the second table the advised minimum preheating temperature derived at the crossing of the Susceptibility Index and the plate thickness by the given restraint factor To calculate the susceptibility index grouping there are two methods 1. AWS formula method The formula susceptibility index = 12*Pcm +10log HD Pcm is the calculated value and the following value of HD, given in ml/100 g of weld metal: H1 = 5, H2 = 10, H3 =30 This gives values for the SI, which are converted to a susceptibility index grouping: For greater convenience , the Susceptibility Index Groupings have been expressed in the table by means of letters A through G, to cover the following range: susceptibility index <3 3,1 - 3,5 3,6 -4,0 4,1 - 4,5 4,6 -5,0 5,1 - 5,5 5,6 -7,0
susceptibility index grouping A B C D E F G
Now table 2 gives the minimum preheat and interpass temperatures. 2. AWS table method In this case a Susceptibility Index Grouping is found in the table 1. It is possible that this method gives higher preheat temperatures because of the big steps of the Pcm value in the table Hydrogen content HD (ml/100 g)
P CM <0,18
<0,23
<0,28
<0,33
<0.38
H1 ≤ 5
A
B
C
D
E
H2 ≤ 10
B
C
D
E
F
H3 =30
C
D
E
F
G
Table 1 Susceptibility Index Grouping A-G as a function of Pcm and HD.
Hydrogen level H1 Extra-Low Hydrogen The consumables give a diffusible hydrogen content of less than 5 ml/100 g deposited weld metal
H2
H3
Low Hydrogen The consumables give a diffusible hydrogen content of less than 10 ml/100 g deposited weld metal Hydrogen not controlled
Restraint Low restraint This level describes common fillet and groove welded joints in which a reasonable freedom of movement of members exists. Medium restraint This level describes common fillet and groove welded joints in which, because of members being already attached to structural work, a reduced freedom of movement exists. High restraint This level describes welds in which there is almost no freedom of movement for members joined (such as repair welds, especially in thick material). Example, using the table. Suppose PCM = 0,24 and HD = 7 ml, Then the susceptibility index is D (Pcm <0,28, H2) Suppose a high restraint in 18 mm plate, then the advised preheating temperature is 105 °C. When using the formula the preheat temperature is 70 (susceptibility index is C).
Degree of restraint
mm Low restraint
Medium restraint
High restraint
Advised minimum preheating temperature
Plate thickness
Temperature in °C A
B
C
D
E
F
G
< 10
< 20
< 20
< 20
< 20
60
140
150
10-20
< 20
< 20
20
60
100
140
150
20-38
< 20
< 20
20
80
110
140
150
38-75
20
20
40
95
120
140
150
>75
20
20
40
95
120
140
150
< 10
< 20
< 20
< 20
< 20
70
140
160
10-20
< 20
< 20
20
80
115
145
160
20-38
< 20
20
75
110
140
150
160
38-75
20
80
110
130
150
150
160
>75
95
120
140
150
160
160
160
< 10
< 20
< 20
< 20
40
110
160
160
10-20
< 20
20
70
105
140
160
160
20-38
20
85
115
140
150
160
160
38-75
115
130
150
150
160
160
160
>75
115
130
150
150
160
160
160
Table 2 Advised minimum preheating temperature as a function of restraint, plate thickness and Susceptibility Index (Pcm, HD).
Group ing sys tem for steels (grou ps 1 -4) Method B is valid for steel of groups 1-4 according to CR ISO/ TR 15608 (Welding – Guidelines for a metallic material grouping system , 1999) Groups 1-4 are listed below Group
Subgroup
Type of steel 2
Steels with a specified minimum yield strength ReH ≤ 460 N/mm and with analysis in %:
1
C
≤ 0.25
Si
≤ 0.60
Mn
≤ 1.70
Mo*
≤ 0.70
S
≤ 0.045
P
≤ 0.045
Cu*
≤ 0.40
Ni*
≤ 0.5
Cr*
≤
Nb
≤ 0.05
V*
≤ 0.12
Ti
≤ 0.05
0.3**
* A higher value is accepted provided that Cr + Mo + Ni+ Cu + V ≤ 0,75 %
** for castings ≤ 0,4
2
1.1
Steels with a specified minimum yield strength ReH ≤ 275 N/mm
1.2
Steels with a specified minimum yield strength 275 N/mm < R eH ≤360 N/mm
1.3
Normalized fine grain steels with a specified minimum yield strength ReH ≤ 360 2 N/mm
1.4
Steels with improved atmospheric corrosion resistance whose analysis may exceed the requirements for the single elements as indicated under 1
2<
2
Thermomechanically treated fine grain steels an cast steels with a specified 2 minimum yield strength ReH ≤ 360 N/mm
2 2.1
Thermomechanically treated fine grain steels an cast steels with a specified 2 2 minimum yield strength 360 N/mm < ReH ≤ 460 N/mm
2.2
Thermomechanically treated fine grain steels an cast steels with a specified 2 minimum yield strength ReH > 460 N/mm Quenched and tempered steels and precipitation hardened steels except 2 stainless steels with a specified minimum yield strength ReH >360 N/mm
3
3.1
Quenched and tempered steels and precipitation hardened steels except 2 stainless steels with a specified minimum yield strength 360 N/mm < ReH ≤ 690 2 N/mm
3.2
Quenched and tempered steels and precipitation hardened steels except 2 stainless steels with a specified minimum yield strength ReH >690 N/mm
3.3
Precipitation hardened steels except stainless steels Low vanadium alloyed Cr-Mo-(Ni) steels with Mo≤ 0,7 % and V ≤ 0,1%
4 4.1
Steels with Cr ≤ 0,3 % and Ni ≤ 0,7 %
4.2
Steels with Cr ≤ 0,7 % and Ni ≤ 1,5 %
