The selection of stainless steel welding materials generally follows the "homogeneity" principle.
1. Ferritic stainless steel: If it is heated at 430-480 ℃ for a long time and cooled slowly during welding, it will easily produce 475 ℃ brittleness, and impurities will play a catalytic role. Welding process is recommended for low current, fast welding, no swing, and low interlayer temperature for multilayer welding.
2. Martensitic stainless steel: The embrittlement of HAZ and cold cracks during welding are to be avoided as much as possible during welding. Generally, corresponding pre-weld (150-300 ° C) and post-weld heat treatment (700-750 ° C) measures are taken. Use larger wire energy, if necessary, use austenitic welding materials.
3. Austenitic stainless steel: The influencing factors of hot cracking and intergranular corrosion are the first consideration when welding this type of stainless steel. In view of its physical characteristics, small linear energy should be used for welding, rapid cooling, and control during multilayer welding. Low interlayer temperature. Strive to contain the appropriate amount of ferrite in the weld to prevent the occurrence of hot cracks. In the petrochemical industry, such as hydrogenation reactors and other equipment, the number of ferrites (FN) in the weld is clearly specified, and generally 3-10 is required.
4. Duplex stainless steel: Compared with austenitic steel, it has a lower tendency to crack; compared with ferritic steel, it has lower embrittlement after welding, so it has excellent weldability and is less prone to welding cracks. However, there is a tendency for grain growth during high temperature heating. During welding, appropriate line energy and interlayer temperature should be adopted to control the dual-phase structure after welding. In short, duplex stainless steel has good weldability. Although its solidified crystals are single-phase ferrite, under general restraint conditions, the sensitivity of the weld metal to thermal cracking is very small. Its cold crack sensitivity is also very low. However, it should be noted that after all, duplex stainless steel has high ferrite. When the degree of restraint is high and the hydrogen content of the weld metal is high, there is also the danger of welding-induced cracks. Therefore, the source of hydrogen should be controlled in the selection of welding materials and welding process.
5. Precipitation hardening stainless steel: Due to its high hardness, poor toughness and plasticity, and large welding residual stress, it is easy to produce cracks. A welding process similar to that of martensitic stainless steel is recommended, but the preheating temperature must be increased.