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According to the AISI classification Stainless steels are divided onto groups: austenitic, ferritic, martensitic, austenitic-ferritic (Duplex) and precipitation hardening steels.
Most stainless steels may be welded by different welding processes:
Austenitic stainless steels include series 200 and 300 (examples: 201, 202, 216, 302 ,304, 310, 316, 321 ,347).
Due to their austenitic structure the steels have low thermal conductivity (half of that of ferritic steels) and therefore lower heat input is required for welding.
coefficient of thermal expansion of austenitic stainless steels is relatively high resulting in larger thermal distortions and internal stresses of the welded parts, which increase suceptibility of the weld to hot cracks. The presence of small amount of ferrite (about 5%) decreases the risk of hot cracks due to the ability of ferrite to dissolve low melting impurities.
Austenitic stainless steels are also characterized by lower wettability and higher viscosity of the liquid metal in weld pool, which may cause welding defects.
Commonly the compositions of filler materials for welding austenitic stainless steels match the steels compositions. The un-stabilized steels 201, 202,301, 302, 304, 305 are welded by the filler material of type 308 (21%Cr, 10%Ni).
308 alloy contains more chromium and nickel than 304, which results in:
SENSITIZATION
One of the possible welding defects of austenitic stainless steels is sensitization.
At the temperatures 900-1400ºF (482-760ºC) chromium carbides form along the austenite grains. This causes depletion of chromium from the grains resulting in decreasing the corrosion protective passive film.
This effect is called sensitization.
Sensitization is depressed in low carbon steels (0.03%) designated with suffix L (304L, 316L).
Formation of chromium carbides is also avoided in stabilized austenitic stainless steels (321, 347) containing carbide forming elements like titanium, niobium, tantalum, zirconium. Stabilization heat treatment of such steels results in preferred formation of carbides of the stabilizing elements instead of chromium carbides.
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Ferritic stainless steels include part of the steels from the series 400 (examples: 405, 409, 430, 442, 446).
The main welding problem of ferritic stainless steel is growth of ferrite grains caused by heating. Coarse grain structure results in low toughness of the weld material.
In order to prevent the grain coarsening the preheat is limited to 150-450ºF (65-230ºC). Low carbon ferritic steels (405, 409) are welded without preheating. Heat input should also be limited particularly when thick parts are welded (thicker, than 0.25”/6mm). Weld toughness may be improved if an Austenitic stainless steels filler material is used.
Commonly filler materials containing equal or excessive amount of chromium are used for welding ferritic stainless steels.
Some parts may be welded without filler material.
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Ferritic stainless steels include series 500 and part of the steels from the series 400 (examples: 403,410, 414, 416, 420, 422, 431, 440, 501, 502, 503, 504).
Weldability of martensitic stainless steels is low because of their sensitivity to cold cracks formation:
In order to prevent cracks formation the welded parts should be preheated to 400-570ºF (204-300ºC). Measures should be taken in order to diminish hydrogen pick-up during the welding process (dry flux, shielding gas). Post-weld heat treatment at 1000-1200ºF (540-650ºC) of high carbon (> 0.2%) martensitic steels is required for improvement of the steel toughness.
Commonly filler materials, in which contents of chromium and carbon match the composition of the welded parts, are used for welding martensitic stainless steels.
Weld toughness may be improved if an Austenitic stainless steels filler material (308, 309) is used.
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austenitic-ferritic (Duplex) stainless steels (examples: 2205) have a mixed austenitic-ferritic structure and commonly contain 0.1-0.3% of Nitrogen.
Weldability of Duplex stainless steels is good.
In order to prevent nitrogen loss during welding shielding gas containing nitrogen is used.
Proper filler materials and controlled heat input help to obtain the required balance between the austenitic and ferritic phases.
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Precipitation hardening stainless steels are categorized into three groups: martensitic (17-4PH, 15-5PH), semiaustenitic (17-7PH, PH 15-7Mo) and austenitic (17-10P, A286).
Commonly filler materials, composition of which close to the composition of the welded parts, are used for welding precipitation hardening stainless steels.
Weldability of austenitic precipitation hardening stainless steels is poor because of their susceptibility to hot cracks. Limited heat input and welding of parts in solution treated condition are required for diminishing risk of cracks. Nickel alloys (nickel-chromium-iron) are used as filler materials for welding austenitic precipitation hardening stainless steels.
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