What is ferritic stainless steel
What is ferritic stainless steel?
Ferritic stainless steel (400 Series) contains 15% ~ 30% chromium and has body centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel has the characteristics of high thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc., and is mostly used to manufacture parts resistant to atmospheric, steam, water and oxidizing acid corrosion. The price of ferritic stainless steel is not only relatively low and stable, but also has many unique characteristics and advantages. It has been proved that ferritic stainless steel is an excellent substitute material in many application fields that originally thought that only austenitic stainless steel (300 Series) can be used. Ferritic stainless steel does not contain nickel, and the main elements are chromium (> 10%) and iron, Chromium is a corrosion resistant element of stainless steel, and its price is relatively stable.
The chromium content is 12% ~ 30%. The ferrite of the cubic lattice is used as the base structure of stainless steel at high temperature and normal temperature. Generally, this kind of steel does not contain nickel, some contain a small amount of molybdenum, titanium or niobium, and have good oxidation resistance, corrosion resistance and chloride corrosion cracking resistance. 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 steel, especially the content of carbon and nitrogen impurities, it can be divided into ordinary ferritic stainless steel and ultra pure ferritic stainless steel. The common ferritic stainless steel has the disadvantages of low temperature and room temperature brittleness, notch sensitivity, high intergranular corrosion tendency, poor weldability, etc. Although the development of this kind of steel is earlier, it has been limited in industrial application. These deficiencies of ordinary ferritic stainless steel are related to the purity of steel, especially the high content of carbon and nitrogen in steel. As long as carbon and nitrogen in steel are low enough, for example, not more than 150 × 10~250 × 10-6, basically, the above shortcomings can be overcome. Since 1970s, due to the development of smelting technology, especially vacuum metallurgy and secondary refining process, carbon + nitrogen ≤ 150-250 can be produced × The high purity ferritic stainless steel of 10-6 has made this kind of steel widely used in industry.
Classification of ferrite
Generally, it can be divided into ordinary ferritic stainless steel and high-purity ferritic stainless steel.
This kind of steel includes low, medium and high chromium content. The low chromium ferritic stainless steel contains about 11% ～ 14% chromium, such as 00Cr12 and 0Cr13Al in China. Aisi400, 405 and 406mf-2 in the United States (see table). This kind of steel has good toughness, plasticity, cold deformation and weldability. Because the steel contains a certain amount of chromium and aluminum, it has good oxidation resistance and rust resistance. 405 can be used as oil refining tower, tank lining, steam turbine blade, high temperature sulfur corrosion resistance device, etc. 400 for home and office appliances, etc. 409 is used in automobile exhaust silencing system and cold and warm water pipes. Medium chromium ferritic stainless steel with chromium content of 14% ～ 19%, such as 1Cr17 and 1cr17mo in China. Aisi 429, 430, 433, 434, 435, 436, 439 in the United States. This kind of steel has good rust resistance and corrosion resistance. Its work hardening coefficient is small (n ≈ 2), it has good deep drawing performance, but its ductility is poor. 430 is used for building decoration, automobile decoration, kitchen equipment, gas burner and nitric acid industrial device. 434 is used for exterior decoration of automobiles and buildings. 439 is used as the hose of gas water heater, coal and gas pipeline. The chromium content of high chromium ferritic stainless steel is 19% ～ 30%, such as cr18si2 and Cr25 in China and aisi442, 443 and 446 in America. This kind of steel has good oxidation resistance. 442 is used in the atmosphere, the upper limit temperature is 1035 ℃, and the maximum temperature for continuous use is 980 ℃. 446 has better antioxidant activity.
High purity ferrite
This kind of steel contains very low carbon and nitrogen; High chromium, molybdenum, titanium, niobium and other elements. For example, China’s 00cr17mo, 00Cr18Mo2, 00Cr26Mol, 00cr30mo2, foreign 18-2, cr26mol, 25cr-5ni-4mo-nb, monit, al29-4, al29-4-2. This kind 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, pitting corrosion resistance in 3% NaCl and FeCl3 is equal to or more than 18-8 austenitic steel, and SCC resistance is much more than 18-8 steel. 26crmo steel is resistant to corrosion in many mediums, especially in organic acid, oxidizing acid and strong alkali. It has good pitting resistance in strong chloride medium. There is no stress corrosion cracking in chloride, hydrogen sulfide, excessive sulfuric acid and strong alkali. 30cr-2mo has higher pitting and crevice corrosion resistance while maintaining stress corrosion resistance. The steel with a small amount of nickel can improve its performance in reducing acid.
Corrosion resistance of ferrite
- (1) Uniform corrosion. Chromium is the most easily passivated element. In atmospheric environment, the Fe Cr alloy with more than 12% Cr can be self passivated. The passivation can be achieved when the chromium content is more than 17% in the oxidizing medium. In some corrosive medium, the high chromium and the addition of molybdenum, nickel, copper and other elements can obtain good corrosion resistance.
- (2) Intergranular corrosion. Both ferritic stainless steel and austenitic stainless steel have intergranular corrosion, but the sensitization treatment and heat treatment to avoid this corrosion are just the opposite. Ferritic stainless steel is easy to suffer intergranular corrosion when quenched above 925 ℃, and the state (sensitized state) which is easy to suffer intergranular corrosion can be eliminated by tempering at 650 ~ 815 ℃ for a short time. The intergranular corrosion of ferritic steel is also the result of chromium deficiency caused by carbide precipitation. Therefore, the susceptibility to intergranular corrosion can be reduced by reducing the content of carbon and nitrogen and adding titanium and niobium.
- (3) Pitting and crevice corrosion. Chromium and molybdenum are the most effective elements to improve pitting and crevice corrosion resistance of stainless steel. With the increase of chromium content, the chromium content in the oxide film also increases and the chemical stability of the film increases. Molybdenum is adsorbed on the active metal surface in the form of MoO4, which inhibits the dissolution of metal, promotes re passivation and prevents the damage of film. Therefore, high chromium and molybdenum ferritic stainless steel has excellent pitting and crevice corrosion resistance.
- (4) Stress corrosion cracking resistance. Due to the characteristics of microstructure, ferritic stainless steel is corrosion resistant in the medium of stress corrosion cracking of austenitic stainless steel.
Mechanical properties of ferrite
Ferritic stainless steel can not be strengthened by heat treatment because it has no phase transformation. It is generally used after annealing at 700-800 ℃. Due to the similar size of Fe and Cr atoms, the effect of solid solution strengthening is small. The yield strength and tensile strength of ferritic stainless steel are slightly higher than those of low carbon steel, while the ductility is lower than that of low carbon steel.
Ordinary ferritic stainless steel is prone to brittleness:
- (1) Brittleness at room temperature. Ordinary ferritic stainless steel is sensitive to notch. The brittle transition temperature is above room temperature except for low chromium (such as 405). The higher the chromium content is, the greater the cold brittleness is. The cold brittleness is related to the interstitial elements such as carbon and nitrogen in the steel, while the ultra pure ferritic steel can obtain good toughness due to the very low carbon content of interstitial elements such as carbon and nitrogen, and the brittle transition temperature can be reduced below room temperature.
- (2) High temperature embrittlement. After heating to above 927 ℃ and quenching to room temperature, the ductility and toughness of ordinary ferritic stainless steel decrease significantly. This high temperature embrittlement is related to the rapid precipitation of carbonitrides on grain boundaries or dislocations at 427-927 ℃. The brittleness can be greatly improved by reducing the content of carbon and nitrogen in the steel. In addition, when the ferrite steel is heated to above 927 ℃, the grain volume is coarsened, and the coarsening grain will deteriorate the plasticity and toughness of the steel.
- （3） σ- Phase formation. According to the Fe Cr phase diagram (see Fig. 1), the alloy containing 40% ～ 50% Cr will form a single phase at 500 ～ 800 ℃ σ， When the chromium content is less than 20% or more than 70%, the alloy will be formed α+σ Biphasic structure. σ- Phase formation can significantly reduce the plasticity and toughness of steel. Therefore, this kind of steel should not be used at 500 ~ 800 ℃ for a long time.
- (4) It is brittle at 475 ℃. High chromium (> 15%) ferritic steel will embrittle strongly at 400 ~ 500 ℃. This embrittlement takes less time σ For example, when 0.080c-0.4si-16.9cr steel is kept at 450 ℃ for 4h, the room temperature impact toughness is almost zero. The degree of embrittlement increases with the increase of chromium content, but the toughness can be restored after treatment above 600 ℃. The embrittlement at 475 ℃ is due to chromium enrichment α′ Phase precipitation. This kind of steel should avoid heating near 475 ℃.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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