The chromium content is 12% to 30%, and the body-centered cubic lattice of ferrite is used as the matrix structure of stainless steel at high temperature and normal temperature. This type of steel generally does not contain nickel, and some contain a small amount of molybdenum, titanium or niobium and other elements, and has good oxidation resistance, corrosion resistance and resistance to chloride corrosion cracking. According to the chromium content, ferritic stainless steel can be divided into three categories: low chromium, medium chromium and high chromium. According to the purity of the steel, especially the content of carbon and nitrogen impurities, it can be divided into ordinary ferritic stainless steel and ultra-pure ferritic stainless steel. . Ordinary ferritic stainless steel has shortcomings such as low temperature and room temperature brittleness, notch sensitivity, high intergranular corrosion tendency, and poor weldability. Although this type of steel developed earlier, its industrial application has been greatly limited. These deficiencies of ordinary ferritic stainless steel are related to the purity of the steel, especially the high content of interstitial elements such as carbon and nitrogen in the steel. As long as the carbon and nitrogen in the steel are sufficiently low, for example, not more than 150×10 to 250×10-6, the above disadvantages can basically be overcome. After the 1970s, due to the development of smelting technology, especially vacuum metallurgy and secondary refining process, high-purity ferritic stainless steel with carbon + nitrogen ≤ 150 ~ 250 × 10-6 has been produced, making this type of steel industrially obtainable. widely used.
Generally, it can be divided into two categories: ordinary ferritic stainless steel and high-purity ferritic stainless steel.
Ordinary ferrite
Such steels include low, medium and high chromium content. Low chromium ferritic stainless steel contains about 11% to 14% chromium, such as 00Cr12 and 0Cr13Al in China. AISI400, 405, 406MF-2 in the United States (see table). This type of steel has good toughness, plasticity, cold deformability and weldability. Since steel contains a certain amount of chromium and aluminum, it has good oxidation resistance and rust resistance. 405 can be used as petroleum refining towers, tank linings, steam turbine blades, high temperature sulfur corrosion resistance devices, etc. 400 is used for home and office appliances, etc. 409 is used in automobile exhaust muffler system devices and cold and warm water pipes, etc. Medium chromium ferritic stainless steel, the chromium content is 14% to 19%, such as 1Cr17 and 1Cr17Mo in China. AISI429, 430, 433, 434, 435, 436, 439 in the United States. This type of steel has good rust resistance and corrosion resistance. Its work hardening coefficient is small (n≈2), and it has good deep drawing performance, but poor ductility. 430 is used as building decoration, automobile decoration, kitchen equipment, gas burner and parts of nitric acid industrial equipment, etc. 434 is used for exterior decoration of automobiles, buildings. 439 is used as a hose for gas water heaters, coal and gas pipelines, etc. High-chromium ferritic stainless steels contain 19% to 30% chromium, such as Cr18Si2 and Cr25 in China, and AISI442, 443, and 446 in the United States. This type of steel has good oxidation resistance. 442 is used continuously in the atmosphere, the upper limit temperature is 1035℃, and the maximum temperature for continuous use is 980℃. 446 has better antioxidant properties.
High Purity Ferrite
This type of steel contains extremely low carbon, nitrogen; high chromium, molybdenum, titanium, niobium and other elements. Such as China's 00Cr17Mo, 00Cr18Mo2, 00Cr26Mol, 00Cr30Mo2 foreign 18-2, Cr26Mol, 25Cr-5Ni-4Mo-Nb, MoNiT, Al29-4, Al29-4-2. This type of steel has good mechanical properties (especially toughness), weldability, intergranular corrosion resistance, pitting corrosion resistance, crevice corrosion resistance, and excellent stress corrosion cracking resistance. For example, 18-2 has good corrosion resistance in nitric acid, acetic acid and NaOH. In 3%NaCl and FeCl3, its pitting corrosion resistance is equivalent to or exceeds that of 18-8 austenitic steel, and its SCC resistance is much higher than that of 18-8 steel. 26CrMo steel is resistant to corrosion in many media, especially in organic acids, oxidizing acids, and strong alkalis. Good pitting resistance in strong chloride media. Stress corrosion cracking does not occur in chloride, hydrogen sulfide, excessive sulfuric acid and strong alkali. 30Cr-2Mo has higher resistance to pitting and crevice corrosion while maintaining stress corrosion resistance. Steel with a small amount of nickel added to improve performance in reducing acids.