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Stainless Steel Welding

TECH. INFO.

STAINLESS STEEL WELDING

Excluding some limitations, the stainless steels and heat-resistant steels may be welded with melting and pressure welding methods used on non-alloy and light alloy steels. The welding process for stainless and heat resistant steels may vary depending on the anticipated properties of base metal such as corrosion and heat resistance. The welding material to be used should have the same composition with the base metal or should have a higher alloy content for some specific applications.

Important Notes

  • The surfaces of stabilized stainless steels as well as welding metal may not be highly polished,
  • The stabilized stainless steels may be welded with both welding materials having the same composition and the low-carbon welding materials,
  • Low-carbon stainless steels should only be welded with low-carbon welding materials, if possible,
  • The nitrogen (N) alloy stainless steels should be welded with regular (non nitrogen alloy) welding materials having sufficiently high mechanical properties. Mixing with the base metal should be as low as possible.
  • Since its thermal expansion is high, centering should be made with short passes in order to reduce deformation,
  • It causes excess heat accumulation in welding area since it has a low thermal conductivity, therefore its heat input should be limited.
  • Post-welding heat treatment should be carried out to obtain a clean metallic surface, so a non-active smooth surface is obtained.

SCHAEFFLER DIAGRAM

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 150C.
  • 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 130C (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 250C. Their toughness reduces between 250C and 900C due to the formation of delicate phases at 475C.
  • 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 250C (for the steels containing nearly 23% Cr) or 150C (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 200C and 300C.
  • The post-welding annealing process at 700 - 800C 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 200-300C 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 700-800C 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 200-300C 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 700C 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 100C in case of thick-sectioned materials and the interpass temperatures should be at 100 - 150C.
  • 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 580-620C increases the ductility.

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