Acid-resistant stainless steel 
 Acid two. Arrived in the air can be called out of stainless steel corrosion; strong in a variety of aggressive media can arrive out of steel called acid corrosion of steel. Stainless steel is not necessarily acid, and acid-resistant stainless steel is generally good performance there. Such steel is mainly chromium, nickel and other alloying elements, and also contains a small amount of molybdenum, vanadium, copper, manganese, nitrogen or other elements. Chromium content, some as high as 25% (13% chromium content in the following steel, corrosion not only in the case is strong corrosion), the nickel content of up to 20%. Type of steel is mainly used to manufacture chemical equipment, medical equipment, food industry equipment and other requirements such as stainless steel device. 
 The role of alloying elements 
 Stainless steel is the most important technical requirements for corrosion resistance, suitable mechanical properties, good cold, thermal processing and welding process performance. Corrosion resistance of chromium stainless steel to obtain the basic elements. When the chromium content in steel up to 12%, the steel in oxidizing medium corrosion resistance mutations of the rise. At this point the steel surface to form a thin, dense chromium oxide film, prevent the continued erosion of the metal matrix is ??(see metal corrosion). In addition to chromium, the steel also contains other elements, some are added as the main ingredient, and some are residual impurities; its impact below. 
 Category 
 Acid-resistant stainless steel wide range of different characteristics, classification and characteristics according to the organization as follows: 
 Ferritic stainless steel 
 With chromium as the main alloying elements, with Cr12 ~ 30%, C ≤ 0.25%; some steel also contains Mo, Ti and other elements, such as 1Cr17, 1Cr25, 0Cr18Mo2Ti and so on. Generally showed a single-phase ferrite or ferrite and a half. Since these are single-phase structure steel, no phase change, which can not be enhanced by heat treatment to make it. Such a large thermal conductivity of steel and a smaller coefficient of thermal expansion, oxidation resistance, strength, and corrosion resistance of steel with chromium content increased with the increase, it is used for manufacture of anti-air, steam, water and oxidizing acids and organic acids corrosion resistant parts and components. Their resistance to chloride stress corrosion better than ordinary Cr-Ni austenitic steel, but more sensitive to intergranular corrosion. When the increase in sulfur content in steel (S0.35%), also has a good and easy cutting. If the steel contains chromium ≥ 15%, the toughness of ferritic stainless steel - brittle transition temperature will rise above room temperature, and heating at ≥ 900 ℃ grain growth will occur and result in brittleness. At 550 ~ 750 ℃ ??and 475 ℃ σ-phase long-term stay will also appear brittle brittle and 475 ℃. However, carbon, nitrogen, total less than 150ppm of pure high-chromium ferritic stainless steel such as high purity Cr18Mo2, purity Cr26Mo1, etc. Basically, does not produce brittle at room temperature. If used as a welding parts, welding process to take to prevent the increase of carbon, nitrogen measures in order to obtain satisfactory performance. 
 Austenitic stainless steel 
 Cr steel containing about 18%, Ni8 ~ 10%, C 0.1%, the stable austenite. Austenitic chromium-nickel stainless steel, including the famous "18-8" on the basis of steel and an increase in Cr, Ni content and adding Mo, Cu, Si, Nb, Ti and other elements developed system of high-Cr-Ni steel. Austenitic steel is nonmagnetic and has high toughness and ductility, but lower strength, and can not be enhanced by making the phase transition can only be strengthened by cold working. If S, Ca, Se, Te and other elements added steel, it will be good and easy cutting. In addition to these steel corrosion resistant to oxidizing acid medium, but also resistant to sulfuric acid, phosphoric acid and formic acid, acetic acid, urea and other corrosion. Stainless steel in carbon, such as less than 0.03% or with Ti, Nb, can significantly improve their resistance to intergranular corrosion. High silicon austenitic stainless steel of concentrated nitric acid has good corrosion resistance. To save and to steel price remain high Ni austenite to Mn, N on behalf of Ni, the development of a Cr-Mn-Ni-N and Cr-Mn-N stainless steel. In addition they are resistant to oxidizing acids, urea, acetic acid and other media, but also has a yield strength much higher than the Cr-Ni austenitic steels advantages under certain conditions, can replace the system Cr-Ni stainless steel. 
 Austenitic ferritic duplex stainless steel 
 Current mass production and application of duplex stainless steel structure is characterized by both austenitic ferritic there. Chemical composition is characterized in containing C lower case, with Cr content of 18 to 28%, Ni content in the 3 to 10%. Some brands also contain Mo, Cu, Si, Ti, Nb, N and other alloying elements. The performance characteristics of duplex stainless steel is both austenitic and ferritic stainless steel features. And compared to ferritic stainless steel, duplex stainless steel plastic, higher toughness, non-brittle at room temperature, its resistance to intergranular corrosion and welding performance are significantly improved; while maintaining a ferritic stainless steel of 475 ℃ and brittle σ phase brittleness and high thermal conductivity, linear expansion coefficient is small, with superplastic characteristics. Compared with the austenitic stainless steel, duplex stainless steel of high strength and resistance to intergranular corrosion and chloride stress corrosion has significantly improved. With Mo, N and other elements of the duplex stainless steel alloy also has excellent resistance to pitting corrosion performance. As low Ni content of duplex stainless steel, so it is also a section of nickel stainless steel; mainly used in chemical, petroleum, atomic energy industry as a structural material to use, typical use is to create a variety of heat transfer equipment. 
 Heat treatment 
 Ferritic stainless steel heat treatment temperature should be selected to avoid the brittle zone, generally carried out in 780 ~ 870 ℃. In order to prevent carbide precipitation during cooling, rapid cooling after heating requirements (such as water cooling). Austenitic stainless steel heat treatment, mainly to ensure that complete solid solution carbide in austenite and prevent the formation of chromium carbides in order to obtain good corrosion resistance organization. Usually heated to high temperatures (such as 1000 ~ 1150 ℃), and then quickly cooled to prevent carbide precipitation and intermediate phase, this method is called solution heat treatment. Containing Ti, Nb stainless steel can be heated to 800 ~ 900 ℃ and holding a certain time, so a lot of carbon steel in the formation of Ti and Nb carbides, followed by welding in order to prevent (or 450 ~ 850 ℃ heating) during the chromium carbide material along the grain boundary precipitation caused by intergranular corrosion, this method is known as the stabilization process. Austenitic - ferritic duplex stainless steel and austenitic stainless steel generally use the same solution treatment of the heat treatment method. However, for the control of the appropriate proportion of each phase and prevent precipitation of σ phase, heat treatment temperature and cooling rate must be strictly controlled. Martensitic stainless steel hardened and tempered with the general method. Austenitizing temperature is usually about 1000 ℃. Hardenability of the steel with carbon content in steel increased with the increase, more oil cooling or air cooling. Sub-cryogenic tempering temperature (150 ~ 370 ℃) and high temperature (450 ~ 560 ℃) two; low temperature mainly to eliminate stress, high temperature is to ensure good corrosion resistance while obtaining good mechanical properties. For the martensitic precipitation hardening stainless steel, usually the first to solution treatment, to obtain supersaturated solid solution, after cooling with martensite; then the precipitation hardening treatment, the use of small and aging effects have dispersed precipitates to improve the steel strength. Semi-austenitic (or semi-martensitic) precipitation hardening stainless steel, heat treatment including solution treatment, intermediate treatment and precipitation hardening phases of aging (see metal enhanced).