AISI has classified Stainless steels into several groups which includes, austenitic, ferritic, martensitic, austenitic-ferritic (Duplex) and precipitation hardening steels.
At the end of this article, the reader should have basic understanding on the following stainless steels listed below:
- Welding austenitic stainless steels
- Welding ferritic stainless steels
- Welding martensitic stainless steels
- Welding austenitic-ferritic (Duplex) stainless steels
- Welding precipitation hardening stainless steels
Welding austenitic-ferritic (Duplex) stainless steels
Austenitic-ferritic has good weld ability of Duplex stainless steels. Shielding gas containing nitrogen should be used in order to prevent nitrogen loss. austenitic-ferritic (Duplex) stainless steels (examples: 2205 ) have a mixed austenitic-ferritic structure and commonly contain 0.1-0.3% of Nitrogen.
In order to obtain balance between the austenitic and ferritic phases proper filler materials and controlled heat is required
Welding precipitation hardening stainless steels
This stainless steel is categorized into three groups which includes, martensitic (17-4PH , 15-5PH), semiaustenitic (17-7PH, PH 15-7Mo) and austenitic (17-10P, A286). Composition of filler materials used must be close to the composition of the welded parts. austenitic precipitation hardening stainless steels has poor wettability because of their susceptibility to hot cracks which required limited heat input and Solution treatment to the welded part for diminishing risk of cracks. The austenitic precipitation hardening stainless steels uses Nickel alloys as its filler material during welding.
Welding ferritic stainless steels
Ferritic stainless steels include part of the steels from the series 400 for examples 405, 409, 430 , 442, 446. One of welding disadvantage of ferritic stainless steel is growth of ferrite grains caused by heating and coarse grain structure results in low toughness of the weld material. Prevention grain coarsening, the preheat is limited from 150-450°F (65-230°C). Low carbon ferritic steel of (405, 409) are welded without being preheated. Application of heat should be limited most especially when thick parts are welded. Austenitic stainless steels filler material should be used to improves the weld toughness.
Welding martensitic stainless steels
Martensitic stainless steels has low wettability because of their sensitivity to cold crack formation. It steel structure is determined by high carbon content which reduces the steel ductility and increases the susceptibility to cracks. Martensitic stainless steels include series 500 and part of the steels from the series 400, for examples: 403, 410 , 414, 416, 420, 422, 431, 440, 501, 502, 503, 504.
Transformation of martensitic causes changes of grains volume because of the changes of crystal lattice which produces internal stresses and increase risk of cracks. Hydrogen inflexibility is caused when martensitic stainless steels pick-up and dissolve hydrogen from the atmosphere and other sources. Preventing cracks formation from martensitic stainless steels, the welded parts should be preheated from 400-570°F (204-300°C). This should be noted in order to avoid hydrogen pick-up during the welding precess. Martensitic steel required post-weld heat treatment at 1000-1200°f (540-650°c) to improve toughness of steel. The weld toughness may be improved if an Austenitic stainless steels filler material (308, 309) is used. Filler materials such as chromium and carbon matches composition of the welded parts which are used in welding martensitic stainless steels
Welding austenitic stainless steels
Austenitic stainless steels has low thermal conductivity and required lower heat input for welding due to their austenitic structure. Stainless steels include series of 200 and 300 for examples: 201, 202, 216, 302, 304, 310, 316, 321, 347). The coefficient of thermal expansion of austenitic stainless steels is relatively high arises at internal stresses and larger thermal distortion of the welded parts. This could increase suceptibility of the weld to hot cracks, but can be reduced when about 5% of ferrite is added to the steel. Because ferrite dissoves in low melting impurities. Austenitic stainless steels can be characterized by higher viscosity and lower wettability in a liquid metal, which occasionally causes welding defeats.
Addition compositions of filler materials for welding austenitic stainless steels matches the steel compositions. For example, 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 suppression of martensite formation and controllable amount of ferrite limiting suceptibility of the weld to hot cracks.
Sensitization is one of the welding defects of austenitic stainless steels. It has the temperatures of 900-1400°F (482-760°C) chromium carbides. Sensitization causes depletion of chromium from grains decreasing the corrosion protective passive film. It also depressed in low carbon steels (0.03%) designated with suffix L (304L, 316L).
And that is it for welding stainless steel. We’ve discussed on all stainless steels classified by AISI with the aid of diagrams. Kindly share this post on your social media handles and learn other welding processes listed below.