Ferritic stainless steel (marketing department of Shanghai HY Industry Co., Ltd)
Chromium content of 12% to 30%, at high temperature and room temperature are body-centered cubic lattice of ferrite as the matrix organization of stainless steel. This type of steel generally does not contain nickel, some contain a small amount of molybdenum, titanium or niobium and other elements, with good oxidation resistance, corrosion resistance and resistance to chloride corrosion rupture. Ferritic stainless steel according to the chromium content can be divided into low chromium, chromium and high chromium three categories, according to the purity of the steel, especially carbon, nitrogen impurity content can be divided into ordinary ferritic stainless steel and ultra-pure ferriticstainless steel. Ordinary ferritic stainlesssteel has low temperature and room temperature brittleness, gap sensitivity and higher intergranular corrosion tendency, poor weldability and other shortcomings, although the development of this type of steel earlier, in industrial applications have been greatly restricted. These shortcomings of ordinary ferriticstainless steel, and the purity of the steel, especially with the high content of carbon, nitrogen and other interstitial elements in the steel. As long as the steel carbon, nitrogen is low enough, for example, not more than 150 × 10-6 ~ 250 × 10-6, basically can overcome the above defects. 1970s due to smelting technology, especially vacuum metallurgy and the development of secondary refining process, has been able to produce carbon + nitrogen ≤ 150 ~ 250 × 10-6 high purity ferritic stainlesssteel, so that this type of steel in industry has been widely used.
Generally can be divided into ordinary ferritic stainless steel and high purity ferritic stainless steel two categories.
ordinary ferritic stainless steel. Such steels include low, medium and high chromium content. Low chromium ferritic stainless steel, containing about 11% to 14% chromium, such as China’s 00Cr12, 0Cr13Al. the United States of AIS1400, AIS405, AIS406MF-2. such steel has good toughness, plasticity, cold deformability and weldability. AIS405 can be used as petroleum refining tower, tank lining, steam turbine blades, high temperature sulfur corrosion resistant devices, etc. AIS400 is used for home and office appliances, etc. AIS409 is used for automotive exhaust muffler system devices and cold, warm water pipes, etc. AIS429, AIS430, AIS433, AIS434, AIS435, AIS436, AIS439. These steels have good rust and corrosion resistance. AIS430 is used for architectural decoration, automotive decoration, kitchen equipment, gas burners and parts of nitric acid industrial installations, etc. AIS434 is used for exterior decoration of automobiles and buildings. AIS439 is used for gas water heaters, hoses for coal and gas pipelines, etc. High chromium ferritic stainlesssteel containing 19% to 30% chromium, such as China’s Cr18Si2, Cr25, the United States of AIS1442, AIS443, AIS446. such steel has good oxidation resistance. ais442 in the atmosphere intermittent use, the upper temperature of 1035 ℃, continuous use of the maximum temperature of 980 ℃. ais446 has better oxidation resistance.
High purity ferritic stainlesssteel. Such steel contains very low carbon, nitrogen; high chromium, molybdenum, titanium, niobium and other elements. Such as China’s 00Cr17Mo, 00Cr18Mo2, 00Cr26M01, 00Cr30Mo2, international 18-2, Cr26Mo1, 25Cr-5Ni-4Mo-Nb, MoNiT, A129-4, A129-4-2. such steel has good mechanical properties (especially toughness), welding performance, resistance to intergranular corrosion. resistance to pitting and crevice corrosion, excellent resistance to stress corrosion rupture. For example, 18-2 has good corrosion resistance in nitric acid, acetic acid and NaOH, and pitting resistance in 3% NaCl and FeCl3. 26Cr-1Mo steel has good corrosion resistance in many media, especially organic acids, oxidizing acids and strong bases. It has good pitting resistance in strong chloride media. No stress corrosion cracking in chlorides, hydrogen sulphide, excess sulphuric acid and strong alkalis. 30Cr-2Mo maintains stress corrosion resistance while offering higher resistance to pitting and crevice corrosion. Steel with a small amount of nickel to improve performance in reducing acids.
Uniform corrosion. Chromium is the element that is most easily passivated. In the atmospheric environment, chromium content of 12% or more of the iron-chromium alloy can be passivated since. In oxidizing media, chromium content of 17% or more can be passivated. In some aggressive media, high chromium and add molybdenum, nickel, copper and other elements can obtain good corrosion resistance.
intergranular corrosion. Ferritic stainless steel and austenitic stainless steel will occur as intergranular corrosion, but the sensitization process and avoid this corrosion heat treatment is the opposite. Ferritic stainless steel from 925 ℃ above the rapid cooling susceptible to intergranular corrosion, and susceptible to intergranular corrosion of the state (sensitized state) after 650 ~ 815 ℃ for a short time tempering, it can be eliminated. Ferritic steel crystalline corrosion is also due to carbide precipitation caused by the results of poor chromium. So reduce the carbon and nitrogen content of steel and add titanium, niobium and other elements, can reduce intergranular corrosion sensitivity.
pitting and crevice corrosion. Chromium, molybdenum is to improve the resistance of stainless steel pitting and crevice corrosion of the most effective elements. Chromium content increases the chromium content of the oxide film also increases, the chemical stability of the film increases. Molybdenum in the form of MoO4 adsorbed on the active metal surface, inhibit the dissolution of the metal, to promote re-passivation and prevent the destruction of the film, so high chromium, molybdenum ferritic stainless steel has excellent resistance to pitting and crevice corrosion.
Stress corrosion rupture resistance. Due to the characteristics of the organizational structure, ferritic stainless steel in austenitic stainless steel produces stress corrosion rupture in the medium is corrosion-resistant.
Ferritic stainless steel can not be strengthened by heat treatment because there is no phase change. Generally used after annealing at 700 to 800°C. As the iron-chromium atomic size is similar to the solution strengthening effect is small, ferritic stainless steel yield strength, tensile strength is slightly higher than low carbon steel, ductility is lower than low carbon steel.
Ordinary ferritic stainless steel susceptible to brittleness:.
room temperature brittleness. Ordinary ferritic stainless steel is notch sensitive, brittle transition temperature in addition to low chromium (such as AIS 405) are above room temperature, the higher the amount of chromium cold brittleness is greater. This cold brittleness and steel carbon, nitrogen and other interstitial elements, and ultra-pure ferritic steel due to carbon, nitrogen and other interstitial elements containing carbon is very low, can get good toughness, brittle transformation temperature can be reduced to room temperature below.
high temperature embrittlement. Ordinary ferritic stainless steel heated to more than 927 ℃ after acute cooling to room temperature, plasticity and toughness significantly reduced. This high temperature embrittlement and carbon (nitride) in 427 ~ 927 ℃ temperature rapidly in the grain boundaries or dislocation precipitation related. Reducing the carbon and nitrogen content of the steel (using ultra-pure technology) can greatly improve this embrittlement. In addition, ferritic steel heated to 927 ℃ above the grain capacity coarsening, coarse grains will make the steel plasticity, toughness deterioration.
The formation of σ-phase. According to the iron-chromium phase diagram (see figure), in 500 ~ 800oC insulation, containing 40% to 50% chromium alloy will form a single phase σ, containing less than 20% chromium or more than 70% of the alloy, will form a “σ + σ” two-phase organization. σ-phase formation will significantly reduce the plasticity and toughness of steel. So such steel should not be used for a long time in 500 ~ 800 ℃.
475 ℃ brittleness. High chromium (> 15%) ferritic steel, in 400 ~ 500 ℃ insulation will be strongly embrittlement. This embrittlement takes less time than σ-phase precipitation, for example, 0.080C-0.4Si-16.9Cr steel in 450 ℃ insulation 4h, room temperature impact toughness almost down to zero. Embrittlement increases with increasing Cr content, but treatment above 600°C restores toughness. 475°C embrittlement is due to the precipitation of Cr-rich α’ phases. Such steels should be avoided by heating near 475°C.
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