After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?
What is Austenitic stainless steel?
Austenitic stainless steel is the most widely used stainless steel, and the high Cr-Ni type is the most common. At present, austenitic stainless steel can be roughly divided into Cr18-Ni8 type, Cr25-Ni20 type, and Cr25-Ni35 type.
Austenitic stainless steel has the following welding characteristics:
Weld hot crack
Austenitic stainless steel has low thermal conductivity and a high coefficient of linear expansion, so during the welding process, the high-temperature residence time of the welded joint is long, and the weld is easy to form a coarse columnar crystal structure. If the content of phosphorus, tin, antimony, niobium and other impurity elements is high, a low melting point of eutectic will be formed in the intergranular. Liquefaction cracks are formed, which belong to welding hot cracks. The most effective way to prevent hot cracks is to reduce impurity elements that are prone to low melting point eutectic in steel and welding consumables and to make chromium-nickel austenitic stainless steel contain 4% to 12% of ferrite.
According to the theory of depletion of chromium, chromium carbide is precipitated on the intergranular, resulting in the depletion of chromium at the grain boundary, which is the main cause of intergranular corrosion. For this reason, the selection of ultra-low carbon welding consumables or welding consumables containing niobium, titanium and other stabilizing elements is the main measure to prevent intergranular corrosion.
Stress corrosion cracking
Stress corrosion cracking usually manifests as a brittle failure, and the processing time of failure is short, so the damage is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. The microstructure change of welded joints or the existence of stress concentration and local concentration of corrosive medium are also the reasons that affect stress corrosion cracking.
Sigma-phase embrittlement of welded joints
The σ phase is a kind of brittle and hard intermetallic compound, which mainly gathers at the grain boundaries of columnar grains. The sigma phase transition can occur in both the gamma phase and the delta phase. For example, when the Cr25Ni20 type weld is heated at 800 ℃ ~ 900 ℃, a strong γ→δ transformation will occur. For chromium-nickel austenitic stainless steel, especially chromium-nickel-molybdenum type stainless steel, δ→σ phase transformation is easy to occur. At 12%, the transformation of δ→σ is very obvious, resulting in obvious embrittlement of the weld metal, which is why the surfacing layer on the inner wall of the hot-wall hydrogenation reactor controls the content of δ-ferrite at 3%~10%. reason.