Ferritic stainless steel is divided into two categories: ordinary ferritic stainless steel and ultra-pure ferritic stainless steel. Ordinary ferritic stainless steel has Cr12~Cr14 type, such as 00Cr12, 0Cr13Al; Cr16~Cr18 type, such as 1Cr17Mo; Cr25~30 type.
Due to the high carbon and nitrogen content in ordinary ferritic stainless steel, it is difficult to process, form and weld, and the corrosion resistance is difficult to guarantee, so its use is restricted. In ultra-pure ferritic stainless steel, the carbon and nitrogen content in the steel are strictly controlled. The total nitrogen content is generally controlled at three levels: 0.035% to 0.045%, 0.030%, and 0.010% to 0.015%. Necessary alloy elements are also added to further improve the corrosion resistance and comprehensive performance of the steel. Compared with ordinary ferritic stainless steel, ultra-pure high-chromium ferritic stainless steel has good resistance to uniform corrosion, pitting corrosion and stress corrosion, and is mostly used in petrochemical equipment. Ferritic stainless steel has the following welding characteristics:
① Under the action of high welding temperature, the grains in the heat-affected zone, especially near the seam area, will grow rapidly when the heating temperature reaches 1000°C or above. Even if it is cooled quickly after welding, the sharp decrease in toughness and the toughness caused by the coarsening of the grains cannot be avoided. Higher tendency of intergranular corrosion.
② Ferritic steel itself contains higher chromium, more harmful elements such as carbon, nitrogen, oxygen, etc., has a higher brittle transition temperature, and has stronger notch sensitivity. Therefore, post-weld embrittlement is more serious.
③ When heated and cooled slowly at 400°C to 600°C for a long time, embrittlement will occur at 475°C, seriously reducing the toughness at room temperature. After long-term heating at 550°C to 820°C, the σ phase is easily precipitated from the ferrite, which also significantly reduces its plasticity and toughness.