Welding of Stainless Steels

Stainless steels and heat-resistant steels can be welded using the fusion and pressure welding methods employed for unalloyed and low-alloy steels, subject to certain limitations. The welding process for these steels may vary depending on the expected properties of the base metal to be welded, such as corrosion and heat resistance. The welding material used should have the same composition as the base metal, or a higher-alloyed material may be preferred for certain applications.

Important Notes:

The surfaces of stabilized stainless steels and the weld metal must not be excessively polished.

Stabilized stainless steels can be welded using both welding materials with the same composition and low-carbon welding materials.

Low-carbon stainless steels should be welded only with low-carbon welding consumables whenever possible.

Nitrogen (N)-alloyed stainless steels should be welded with standard (non-N-alloyed) welding consumables that possess sufficient mechanical properties. The mixing ratio with the base metal should be kept as low as possible.

Because of their high thermal expansion, tack welding should be performed in short passes to minimize deformation.

Due to its low thermal conductivity, it causes excessive heat buildup in the weld zone; therefore, heat input must be limited.

Post-weld heat treatment should be applied to obtain a clean metallic surface, thereby ensuring a defect-free passivated surface.

Welding of Standard Austenitic Stainless Steels

The weld metal is resistant to hot cracking because it contains between 4% and 12% (5–15 FN) delta ferrite. If properties such as non-magnetic behavior, high corrosion resistance, or toughness at low temperatures are required in welded joints, fully austenitic weld metals should be preferred.

The mixing ratio with the base metal should be kept below 40%, and to avoid reducing the delta ferrite content too much, nitrogen absorption should be kept to a minimum during welding, if possible. No preheating should be performed, and the interpass temperature should not exceed 150°C. Arc strikes should be made within the weld groove.

Delta ferrite is a magnetic phase. Cr-Ni austenitic stainless steels can be joined with Cr-Ni-Mo austenitic stainless steels; however, it is important to select a filler metal with the same composition to ensure corrosion resistance.

Welding of Fully Austenitic Stainless Steels

It should be noted that fully austenitic weld metals have a high tendency for hot cracking. The following points must be observed during welding:

• The weld area must be absolutely clean; substances such as sulfur, which can cause hot cracking, must not be allowed to enter the weld area.
• Craters must be filled and, if necessary, ground out.
• To prevent the formation of longitudinal cracks, the root pass must be of sufficient thickness.
• During the design phase, the creation of localized stresses and the use of thick-sectioned material should be avoided.
• To keep grain size small and minimize post-weld stresses, large weld pools and high heat input should be avoided. This requires the following:
  • Ensuring a limited heat input (maximum 10–15 kJ/cm),
  • Performing a straight weld bead or using very limited oscillation,
  • Avoiding preheating and ensuring the interpass temperature does not exceed 130 (150)°C.

Welding of Ferritic-Austenitic Stainless Steels

Steels of this type, which contain delta-ferrite and austenite phases, are called duplex stainless steels. These steels can be joined using fusion welding, and the welded joints can operate at temperatures up to 250°C. Due to the formation of brittle phases at 475°C, toughness decreases at temperatures between 250°C and 900°C.

To limit the delta ferrite content in the weld metal, the nickel content in the welding material—which has the same nitrogen (N) content as the base metal—should be slightly higher than that of the base metal. In joints made with low-alloy stainless steels, the mixture should be kept below 40%. Welding without filler metal is possible; however, only a solution annealing followed by a quenching process should be performed.

Welding should be performed without a preheat, and the interpass temperature should not exceed 250°C for steels containing 23% Cr and 150°C for steels containing 25% Cr. A slightly higher heat input may be selected compared to austenitic stainless steels. Depending on the welding method used and the material thickness, welding can be performed with a heat input of 5–25 kJ/cm for steels containing 23% Cr and 2–15 kJ/cm for steels containing 25% Cr.

Steels containing high amounts of delta-ferrite are prone to hydrogen cracking; therefore, hydrogen absorption must be kept as low as possible during welding. Electrodes must be dried before use, and gases containing hydrogen must not be used.

Welding of Semi-Ferritic Chromium-Containing Stainless Steels

Weld metals and heat-affected zones with the same chemical composition may have a martensitic or tempered microstructure. The pre-weld annealing and interpass temperature should be 200–300°C. Post-weld annealing at 700–800°C ensures the tempering of martensite, the rounding of chromium carbides to increase toughness, and improved resistance to intergranular corrosion.

Due to the risk of cold cracking, hydrogen absorption must be kept low during welding; electrodes must be dried before use, and gases containing hydrogen must not be used. If the same color and thermal expansion properties as the base metal are desired, filler metal of the same composition and free of nickel should be used.

If high toughness is expected from the weld metal and post-weld heat treatment is not possible, a welding material different from the base metal (e.g., austenitic stainless steel or a Ni-Cr alloy) may be used.

Welding of Fully Ferritic Chromium-Containing Stainless Steels

Fully ferritic stainless steels tend to exhibit grain growth above 950°C. Their coarse-grained structure leads to a decrease in toughness, and this toughness cannot be restored to the same level by any heat treatment. Therefore, welding must be performed with low heat input (low welding current, use of small-diameter electrodes, and straight or low-oscillation welding).

The transition temperature from ductility to brittleness, as determined by the notch impact test, is around room temperature in ferritic stainless steels. To minimize cracking in the heat-affected zone and keep post-weld stresses low, preheating and interpass temperatures of 200–300°C should be applied.

Due to the risk of cold cracking, hydrogen absorption must be kept low during welding; electrodes must be dried before use, and gases containing hydrogen must not be used. When using high-toughness welding materials with different chemical compositions (austenitic or Ni-Cr alloys), multi-pass welding is preferred. If color matching with the base metal or a lower nickel content in the weld metal is desired, the cover pass should be made with a welding material having the same composition as the base metal.

Annealing at 700–800°C increases the toughness of the heat-affected zone and the weld metal, reduces residual stresses after welding, and restores intergranular corrosion resistance to its previous level.

Welding of Martensitic Chromium-Containing Stainless Steels

These steels exhibit air-hardening properties and have limited weldability. To keep the hardness low in the heat-affected zone, pre-weld annealing and interpass temperatures of 200–300°C should be selected. Steels containing more than 0.2% carbon are not suitable for welded structures.

Tempering at 700°C immediately after welding increases the toughness of the welded joint and reduces residual stresses. Due to the risk of cold cracking, hydrogen absorption must be kept low during welding; electrodes must be dried before use, and gases containing hydrogen must not be used.

If a weld metal that is the same color as the base metal and nickel-free is desired, the cover pass can be made using welding material of the same composition. For high-carbon steels, austenitic welding materials with different compositions according to DIN 8556 are generally used; Ni-Cr alloy welding materials according to DIN 1736 may also be preferred.

Welding of Cr-Ni Soft Martensitic Stainless Steels

A carbon content limited to 0.05% ensures the formation of a ductile martensite phase in the heat-affected zone and in the base metal of the same composition. For thick-section materials, a preheat at 100°C is required, and interpass temperatures should be maintained between 100 and 150°C.

Due to the risk of cold cracking, hydrogen absorption must be kept low during welding; electrodes must be dried before use, and gases containing hydrogen must not be used. Welding materials with the same composition as the base metal produce a weld metal containing 0.04% carbon and 5% delta ferrite. Post-weld tempering at 580–620°C increases ductility.