① Welded hot-cracked austenitic stainless steel has low thermal conductivity and large linear expansion coefficient, so in the welding process, the high temperature residence time of welded joints is long, and coarse columnar crystal structure is easy to form in the weld. In the process of solidification crystallization, if the content of impurity elements such as sulfur, phosphorus, tin, antimony and niobium is high, crystals with low melting point will be formed between grains, and solidification cracks will easily form in the weld when the welded joint bears high tensile stress. The most effective way to prevent hot cracking is to reduce the impurity elements that are easy to produce low melting point crystals in steel and welding materials, so that Cr-Ni austenitic stainless steel contains 4% ~ 12% ferrite structure.
② Intergranular corrosion According to the theory of chromium deficiency, chromium carbide precipitates between grains, which leads to chromium deficiency at grain boundaries, which is the main reason for intergranular corrosion. Therefore, choosing ultra-low carbon solder or solder containing stable elements such as niobium and titanium is the main measure to prevent intergranular corrosion.
③ Stress corrosion cracking Stress corrosion cracking usually shows brittle failure, and the process of failure is short, so it is very harmful. 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 corrosion medium concentration are also the reasons that affect stress corrosion cracking.
④ σ phase embrittlement of welded joints σ phase is a brittle and hard intermetallic compound, which mainly precipitates on the grain boundaries of columnar crystals. Both γ phase and δ phase can undergo σ phase transition. For example, when the weld of Cr25Ni20 is heated at 800℃ ~ 900℃, a strong γ→δ transformation will occur. For Cr-Ni austenitic stainless steel, especially Cr-Ni-Mo stainless steel, δ→σ phase transition is easy to occur, which is mainly due to the obvious σ effect of Cr and Mo elements. When the δ ferrite content in the weld exceeds 12%, the δ→σ phase transformation is very obvious, which leads to the obvious embrittlement of the weld metal, which is also the reason why the δ ferrite content is controlled between 3% and 65438% by the surfacing layer on the inner wall of the hot wall hydrogenation reactor.
Austenitic stainless steel welding:
① Austenitic stainless steel has small thermal conductivity and large thermal expansion coefficient, which is easy to produce large deformation and welding stress, so the welding method with concentrated welding energy should be chosen as far as possible.
(2) Due to the low thermal conductivity of austenitic stainless steel, at the same current, it can obtain greater penetration than low alloy steel. At the same time, because of its high resistivity, in covered electrode arc welding, compared with carbon steel or low alloy steel covered electrode with the same diameter, the welding current is smaller in order to avoid covered electrode from turning red.
③ Welding specifications. Welding generally does not use large line energy. In covered electrode arc welding, small diameter covered electrode and fast multi-pass welding should be adopted. For demanding welds, even cold water is poured to accelerate cooling. For pure austenitic stainless steel and super austenitic stainless steel, the welding line energy should be strictly controlled to prevent the serious growth of weld grains and the occurrence of welding hot cracks.
(4) In order to improve the hot cracking resistance and corrosion resistance of the weld, special attention should be paid to the cleanliness of the welding zone during welding to avoid harmful elements from infiltrating into the weld.
⑤ Austenitic stainless steel generally does not need preheating when welding. In order to prevent grain growth and carbide precipitation in weld and heat affected zone and ensure the plasticity, toughness and corrosion resistance of welded joints, the interlayer temperature should be controlled at a low level, generally not exceeding 150℃.
0Cr 18Ni9Ti covered electrode/welding wire/welding material stainless steel welding wire model.
ER307 stainless steel welding wire TGS-307 MIG -307
Usage: Suitable for welding non-magnetic steel, high manganese steel and carbon steel.
ER307Si stainless steel welding wire TGS-307Si MIG -307Si
Uses: High manganese content leads to low crack sensitivity, and it is suitable for steels that are difficult to weld, such as nonmagnetic steel, high manganese steel and hardened wear-resistant steel.