Stainless steel refers to the main element Cr is higher than 12%, can make the steel in the passivation state, and has the characteristics of stainless steel steel. According to its microstructure, stainless steel is divided into ferrite type, Martensite type, austenite type, austenite + ferrite type and precipitation hardening type stainless steel. Austenitic stainless steel is usually pure austenite at room temperature, but some are austenite + a small amount of ferrite, which helps prevent hot cracking.
A, austenitic stainless steel welding characteristics:
1, easy to appear hot crack.
Preventive measures :(1) try to make the weld metal dual-phase structure, ferrite content control in 3-5% below. Ferrite can dissolve harmful S and P impurities in large quantities. (2) as far as possible to choose the quality of alkaline coating electrode, in order to limit the content of S, P, C and so on in the weld metal.
2, intergranular corrosion: according to the theory of poor chromium, weld and heat affected zone in heating to 450-850℃ sensitization temperature zone precipitates chromium carbide on the grain boundary, resulting in poor chromium grain boundary, not enough to resist the degree of corrosion.
Prevention measures :(1) the use of low carbon or ultra-low carbon welding materials, such as A002, etc.; The electrode containing titanium, niobium and other stabilizing elements, such as A137, A132, etc. (2) by welding wire or electrode to weld into a certain amount of ferrite forming elements, so that the weld metal becomes austenite + ferrite two-phase structure,(ferrite is generally controlled in 4-12%). (3) Reduce the overheating of the welding pool, choose smaller welding current and faster welding speed, and speed up the cooling speed. (4) The welding parts with high intergranular corrosion resistance should be stabilized and annealed after welding.
3. Stress corrosion cracking: Stress corrosion cracking is the delayed cracking phenomenon of welded joints under tensile stress in a specific corrosion environment. Stress corrosion cracking (SCC) of austenitic stainless steel welded joints is a serious failure mode, which is brittle failure without plastic deformation.
