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STAINLESS STEEL WELDING

Except for some restrictions, austenitic stainless steels can be welded employing the same fusion and pressure welding procedures as for welding unalloyed or low-alloy structural steels. Weld processing of these steels will be effected with a view to obtain welded joints that will meet the requirements, as for instance, corrosion and heat resistance, of the base metals to be welded. Welding consumables to be used, shall be of the same composition or higher alloyed for special applications.

Notice

• Stabilized steels and weld metal cannot be high-luster polished,
• Stabilized steels can be welded using either consumables having the same composition or LC-consumables,
• If possible, LC-steels should be welded using LC-consumables,
• Nitrogen alloyed standard austenites are welded using normal consumables, who's tensile properties being sufficiently high. Admixture with base metal should be kept low,
• A higher thermal expansion coefficient will lead to greater warpage, therefore tack-welding at short spaces,
• Low heat conductivity results in heat accumulation or overheating within the weld area, therefore heat input should be limited,
• Post-weld heat treatment of the weld joint is absolutely indispensible in order to obtain a clean metallic surface so that the formation of a faultless passive layer will be possible.

Schaeffler Diagram

A1 - Welding of Standard Austenites

• Weld metal of the same composition contains 4 to 12% (5 to 15 FN) delta ferrite, thus being resistant to hot-cracking,
• In the case of special requirements, such as welded joints required to be non-magnetic, highly corrosion resistant or tough at subzero temperatures, a fully austenitic weld metal should be chosen,
• Admixture from the base metal should be below 40% and if possible nitrogen pick-up during welding should be kept low, in order to not lower the delta ferrite too much,
• No preheating, interpass temperature max. 150°C
• Arc striking only within weld groove
• Delta ferrite is a magnetic phase
• Cr-Ni-austenites may also be joined by using Cr-Ni-Mo-consumables, but with regard to corrosion resistance, weld metal of the same composition should be preferred.

A2 - Welding of Full-Austenites

The strong tendency of fully austenitic weld metal to hot-cracking should be considered when welding such steels.

Above all, the following items should be observed:

• Absolutely clean weld area, in order to avoid those agents producing hot-cracking, and particular sulphur, do not enter the weld pool,
• Avoid local stress concentrations and great wall thickness by design considerations,
• Avoid a large and overheated weld pool, in order to keep grain size small and weld residual stresses of the weld joint low.

This means;

• Limited heat input (max. 10 to 15 kJ/cm)
• Using stringer beads or slight weaving
• No preheating, interpass temperature max. 130 (150)°C
• Filling-up end crater, if necessary grinding out
• Welding root with sufficient section, in order to avoid longitudinal stress cracking.

F - A - Welding of Ferritic-Austenitic Steels

These steels with a two-phase structure of delta-ferrite and austenite are defined as Duplex-steels. They are well-suited for fusion welding.

• Highest admissible operating temperature for welded structures is 250°C. In the temperature range between 250 to 900°C there will occur a decline in toughness due to the 475°C embrittlement and the formation of brittle intermetallic phases.
• Weld consumable of the same nitrogen-alloyed composition, the nickel content of which being slightly increased for limiting the delta-ferrite content in the weld metal. Admixture from the less nickel containing steel should not exceed 40%. Welding without adding weld metal only possible with subsequent solution annealing and quenching.
• Welding without preheating, interpass temperature max. 250°C (steels having about 23% Cr) or max. 150°C (steels having about 25% Cr).
• Heat input is chosen a bit higher as in welding austenitic steels. Depending upon welding procedure, thickness of workpiece etc., welding is carried out at 5 to 25 kJ/cm (steels with about 23% Cr) or at 2 to 15 kJ/cm (steels with about 25% Cr).
• Possessing high contents of delta-ferrite, steels are susceptible to hydrogen induced cracking. Therefore, hydrogen pick-up during welding shall be kept low (e.g. by redrying covered electrodes, no hydrogen bearing shielding gas).

F1 - Welding of Semi Ferritic Chromium Steels

• Weld metal of the same composition and the HA-zone exhibit a structure consisting of martensite or structure as tempered, resp., delta-ferrite and finely distributed carbides.
• Preheating and interpass temperature is 200 to 300°C.
• Annealing at 700 to 800°C after welding will result in tempering of martensite and enhancing toughness by coagulation of the chromium carbides and restoring resistance to intergranular corrosion (stabilizing).
• Due to the tendency of forming cold-cracks, pick-up of hydrogen during welding should be kept low (redrying covered electrodes, no hydrogen bearing shielding gases).
• Weld consumable of the same composition, if matching the color of the base metal, identical thermal expansion coefficient and nickel-free weld metal are required.
• If a tough weld metal is required and heat treatment after welding is not possible, dissimilar welding consumables (austenite or nickel-chromium alloy) can be used.

F2 - Welding of Fully Ferritic Chromium Steels

• At temperatures of over 950°C the pure ferritic structure has a tendency to grain coarsening. A coarse grain will result in a loss of toughness and cannot be restored by any heat treatment.
• Therefore, welding should be done with low heat input (low amperage, small electrode diameter, stringer beads or only slight weaving).
• In ferritic steels, the transition temperature from the ductile to the cleavage fractures, determined by the impact test, is situated in the room temperature range. In order to avoid cracking in the heat affected zone (HAZ) and keeping the weld residual stresses low, preheat and interpass temperature of 200 to 300°C must be chosen.
• Because of the tendency to form cold cracks, the hydrogen pick-up during welding should be kept low, if possible (redrying covered electrodes, no hydrogen bearing shielding gas).
• Multipass welds are preferably made using dissimilar tough weld metal consumables (austenite or nickel-chromium alloys). If matching the color of the base metal or weld metal, poor in nickel is required, cover layer is welded using weld metal of the same composition as base metal.
• Annealing at 700 to 800°C after welding improves toughness of heat affected zone (HAZ) and the identical weld metal, reduces weld metal residual stresses and restores resistance to intergranular corrosion (stabilizing).

M - Welding of Martensitic Chromium Steels

• These steels are air-hardenable and posses only restricted weldability. In order to keep hardening of the heat affected zone (HAZ) low, a preheat and interpass temperature of 200 to 300°C has to be chosen.
• Steels with C > 0.2% are not suitable for welded structures
• Tempering at 700 to 800°C immediatelly after welding will raise the toughness of the welded joint and reduces the weld residual stresses.
• Because of the tendency to produce cold cracking, hydrogen pick-up during welding should be kept low (redrying covered electrodes, no hydrogen bearing gases).
• Weld consumables of the same composition as base metal are preferably used for cover layers, if matching the color of the base metal or nickel-poor weld metal, is required.
• Dissimilar austenitic weld consumables according to DIN 8556 are mainly used, and for steels having a higher carbon content, also Ni-Cr-alloy welding consumables according to DIN 1736 can be employed.

Welding of Soft Martensitic Stainless Chromium-Nickel Steels

• A carbon content limited to 0.05 % leads to the formation of ductile martensite in the heat affected zone (HAZ) and the weld metal of the same composition.
• Preheating to 100°C of thick-walled workpieces, interpass temperature should be 100 to 150°C.
• Because of the tendency to produce cold-cracking, hydrogen pick-up during welding should be kept low (redrying covered electrodes, no hydrogen bearing shielding gas).
• Weld consumables of the same composition will produce a weld metal with max. 0.04% carbon and 5% delta-ferrite.
• Tempering after welding at 580 to 620°C for enhanced ductility.