A1 – Welding of Standard Austenitic Stainless Steel
The welding metal contains 4% to 12% (5 -15 FN) delta ferrite, so it is resistant to hot crack
Fully austenitic welding metals should be preferred if the welded connection is desired to be non-magnetic, with high corrosion resistance or have a toughness at low temperatures.
The mixture ratio with the base metal should be lower than 40% and the nitrogen absorption during welding should be low, if possible, not to over-reduce the delta-ferrite ratio.
Pre-heating should not be performed and the temperature between the passes should be maximum 150°C.
Arc inceptions should be made within the welding bend.
Delta-ferrite is a magnetic phase.
While the austenitic stainless steels with Cr-Ni may be combined with austenitic stainless steels with Cr-Ni-Mo, the welding materials having the same composition should be preferred regarding the corrosion resistance.
A2 – Welding of Fully Austenitic Stainless Steels
It should be noted that the hot crack tendency is high while performing welding works on fully austenitic welding metals. Also the following should be considered.
The welding area should be perfectly clean and the ingression of materials causing hot crack (i.e. sulphur) into the welding area should be avoided.
The craters should be filled up, or removed by grinding if necessary,
The root pass should be sufficiently thick to prevent possible formation of longitudinal cracks.
It should be avoided to create local tensions and use of thick sectioned materials during the designing phase.
Large welding baths and high heat inputs should be avoided in order to keep post-welding tensions in welded connections low and the grain size small. This means,
- A limited heat input (max. 10 -15 kJ/cm).
- Making flat weld beads or very limited oscillation,
- Not performing a pre-heating means that the temperature between passes is maximum 130°C (150).
F - A - Welding of Ferritic-Austenitic Stainless Steels
These types of steels with two phases as "delta-ferrite" and "austenite" are called "duplex stainless steels". These steels may be jointed with fusion welding. Welded joints may only work for up to 250°C. Their toughness reduces between 250°C and 900°C due to the formation of delicate phases at 475°C.
The nickel content in welding materials with a same nitrogen-alloy ratio with the base metal is slightly higher than that of the base metal in order to limit the delta-ferrite ratio in the welding metal. For the joints to be made with stainless steels having a low nickel ratio the mixture should be kept lower than 40%. Welding without using additional metal is possible only if solution treatment and respectively quenching is performed.
Welding should be completed without pre-annealing and the temperature between passes should not exceed 250°C (for the steels containing nearly 23% Cr) or 150°C (for the steels containing nearly 25% Cr).
A slightly higher heat input when compared to that of austenitic stainless steels may be selected. Welding with 5 - 25 kJ/cm heat input for the steels containing 23% Cr and 2 - 15 kJ/cm heat input for the steels containing 25%Cr is possible according to the selected welding method and the material thickness.
The steels containing a high amount of delta-ferrite are prone to hydrogen crack. Therefore, the hydrogen absorption during welding should be kept as low as possible (the electrodes should be dried before use and the gases containing hydrogen should not be used).
F1 - Welding of Stainless Steels with Semi-Ferritic Cr
The welding metals having the same chemical composition and the areas effected from heat may have a martensite or tempered structure.
The temperature between passes and pre-welding annealing should be between 200°C and 300°C.
The post-welding annealing process at 700 - 800°C ensures that the martensites are tempered, chromium carbides are rounded and toughened and the inter-grain corrosion resistance is increased.
Due to the risk of formation of cold cracks, the hydrogen absorption during welding should be kept low (the electrodes should be dried before use and the gases containing hydrogen should not be used).
Additional metal with same composition and not containing nickel should be used if the color and thermal expansion are desired to be the same with those of the base metal.
If high toughness is desired from the welding metal and it is not possible to perform post-welding heat treatment, a welding material different than the base metal (such as austenitic stainless steel or Ni-Cr alloy) may be used.
F2 - Welding of Stainless Steels with Full-Ferritic Cr
The full-ferritic stainless steels are prone to grain enlargement at temperatures over 950
°C. This large grained structure causes a decrease in toughness and the toughness can not be brought to same levels by any heat treatment process.
Therefore the welding should be performed with a low heat input (low welding current, use of small diameter electrode, and flat or low-oscillating welding).
The ductile-brittle transition temperature measured with a notch pulse test is around room temperature in ferritic stainless steels. Pre-annealing and interpass temperatures should be applied at 200-300°C in order to keep the post-welding tension and breakage at the heat effected area.
Due to the risk of formation of cold cracks, the hydrogen absorption during welding should be kept low (the electrodes should be dried before use and the gases containing hydrogen should not be used).
Multi-pass welding is preferred when using highly tough welding materials with different chemical compositions (austenitic or Ni-Cr alloys). If color match with the base metal is desired or it is desired that the welding metal contains lower nickel content the welding pass is created with a welding material having the same composition with the base metal.
Annealing at 700-800°C increases the toughness of the heat-impacted area and welding metal, reduces post-welding residual voltages and brings the inter-grain corrosion resistance to the previous level.
M - Welding of Stainless Steels with Martensitic Cr
These steels have air-hardening properties and limited welding capabilities. Pre-annealing and interpass temperatures at 200-300°C should be selected in order to keep the toughness at the heat-effected area low.
The steels containing carbon more than 0.2% are not suitable for welded structures.
The tempering process performed at 700°C immediately after the welding increases the toughness of welded joint and reduces the post-welding residual voltages.
Due to the risk of formation of cold cracks, the hydrogen absorption during welding should be kept low (the electrodes should be dried before use and the gases containing hydrogen should not be used).
If a welding metal that has the same color with the base metal and that does not contain nickel is desired, the cap pass may be created with a welding material having the same composition.
Austenitic welding materials with a different composition acc. to DIN 8556 are used for high-carbon steels, and Ni-Cr alloy welding materials may also be used acc. to DIN 1736.
Welding of Soft Martensitic Stainless Steels with Cr-Ni
The carbon amount limited to 0.05% provides a ductile martensite phase in the heat-effected area and on the base metal having the same composition.
Pre-annealing should be performed at 100°C in case of thick-sectioned materials and the interpass temperatures should be at 100 - 150°C.
Due to the risk of formation of cold cracks, the hydrogen absorption during welding should be kept low (the electrodes should be dried before use and the gases containing hydrogen should not be used).
The welding materials having the same composition with the base metal give a welding metal containing 0.04% carbon and 5%delta-ferrite.
Post-welding tempering at 580-620°C increases the ductility.