Austenitic stainless steel is the most widely used stainless steel, with high Cr-Ni type being 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:
① Welding hot cracks Austenitic stainless steel has low thermal conductivity and large linear expansion coefficient. Therefore, during the welding process, the high temperature residence time of the welded joint part is long, and the weld seam is easy to form a coarse columnar crystal structure. During the solidification and crystallization process, , if the content of impurity elements such as sulfur, phosphorus, tin, antimony, niobium, etc. is high, a low melting point eutectic will be formed between the crystals. When the welded joint is subjected to high tensile stress, solidification cracks will easily form in the weld. Liquefaction cracks are formed in the heat affected zone, which are all welding hot cracks. The most effective way to prevent hot cracks is to reduce impurity elements in steel and welding materials that are prone to produce low melting point eutectics and to make chromium-nickel austenitic stainless steel contain 4% to 12% ferrite structure.
② Intergranular corrosion According to the chromium depletion theory, chromium carbide precipitates on the intergranules, resulting in chromium depletion at the grain boundaries, which is the main cause of intergranular corrosion. For this reason, choosing ultra-low carbon welding materials or welding materials containing stabilizing elements such as niobium and titanium is the main measure to prevent intergranular corrosion.
③ Stress corrosion cracking Stress corrosion cracking usually manifests as brittle damage, and the damage process occurs in a short time, so the harm is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. The structural changes of welded joints or the existence of stress concentration and local corrosion medium concentration are also factors that affect stress corrosion cracking.
④ σ phase embrittlement of welded joints σ phase is a brittle and hard intermetallic compound that mainly accumulates at the grain boundaries of columnar crystals. Both γ and δ phases can undergo σ phase transformation. For example, when the Cr25Ni20 type weld is heated at 800°C ~ 900°C, a strong γ→δ transformation will occur. For chromium-nickel austenitic stainless steel, especially chromium-nickel-molybdenum stainless steel, the δ → σ phase transformation is prone to occur. This is mainly due to the obvious σ transformation of chromium and molybdenum elements. When the δ ferrite content in the weld exceeds At 12%, the transition from δ to σ is very obvious, causing obvious embrittlement of the weld metal. This is why the cladding layer on the inner wall of the hot wall hydrogenation reactor controls the δ ferrite content to 3% to 10%. reason.