1.2.3 Classification by organization
According to the microstructure of stainless steel, it is divided into five categories: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, duplex stainless steel, and precipitation hardening stainless steel. It is a widely used classification method. (The following content will be introduced in detail according to the classification of stainless steel microstructure.) For the composition and properties of commonly used stainless steel, see GB / T4237-2007 “Stainless Steel Hot Rolled Steel Plate and Steel Strip”.
(1) Martensitic stainless steel Martensitic stainless steel contains 11.5% to 18% chromium, 0.08% to 1.2% carbon, and other alloy elements less than 2% to 3%. Common steel grades are 08Cr13, 15Cr13, 21Cr13, 14Cr17Ni2 Wait. They exist as austenite at high temperature and transform to martensite structure after proper cooling to room temperature, but steel often contains a certain amount of residual austenite, ferrite or pearlite. Martensitic stainless steel is characterized by high hardness, strength, wear resistance, good fatigue resistance and certain corrosion resistance. In the 1950s, in order to improve the welding performance of martensitic stainless steel, the carbon content of the steel was reduced to less than 0.07%, and a certain amount of nickel was added to obtain the possibility of martensitic transformation, thereby forming a new series. With the development of refining technology and the introduction of stainless steel production, the carbon content in steel can be reduced to below 0.03%, and the composition of the steel can be optimized according to needs, forming a series of super martensitic stainless steels, such as 00Cr13Ni2Mo, 00Cr13Ni5Mo, etc. Its welding performance has been greatly improved.
(2) Ferritic stainless steels generally do not contain nickel, and the chromium content is 12% to 30%, and some also contain a small amount of elements such as molybdenum, titanium, or niobium. They have good oxidation resistance, corrosion resistance, and Resistance to chloride corrosion and fracture. There are currently two classification methods for ferritic stainless steels. One type is divided into ordinary type and high purity type according to the purity of steel, especially the content of carbon and nitrogen impurities. Ordinary ferritic stainless steels have the disadvantages of low temperature and room temperature brittleness, high notch sensitivity, intergranular corrosion tendency, and poor weldability, while high purity ferritic stainless steels have extremely low carbon and nitrogen content (0.015% to 0.025%). ), Not only good performance, but also welding performance, such as 03Cr17Mo, 01Cr27Mo and so on. The other is divided into three types of low chromium, medium chromium and high chromium according to the chromium content. Among them, the chromium content of the low chromium ferritic stainless steel is 11% to 14%, such as 03Cr12, 08Cr13Al, etc., which has good toughness, plasticity, cold deformability and welding performance. Medium chromium ferritic stainless steel has a chromium content of 14% to 18%, such as 12Cr17, 12Cr17Mo, etc., and has good corrosion resistance and rust resistance. High chromium ferritic stainless steel has a chromium content of 18% to 30%, such as Cr18Si, Cr25, etc., has good oxidation resistance and can be continuously used at 980 ° C. During the smelting process, the total amount of C, N and other elements is strictly controlled to obtain high-purity or ultra-pure ferritic stainless steels, which contain extremely low carbon and nitrogen (“high-purity” number 000, C + N ≤ 0.030%, “super “Pure” number 0000, C + N≤0.015%, C can reach ≤0.005%), high chromium content, and also contains molybdenum, titanium, niobium and other elements, this type of steel has good mechanical properties (especially toughness), welding Performance, resistance to intergranular corrosion, resistance to pitting and crevice corrosion, excellent resistance to stress corrosion fracture. It is possible to replace austenitic stainless steel in a wide range in the future, but its 475 ° C brittleness still needs attention, such as 000Cr18Mo (Ti), 0000Cr18Mo2 (Ti), 0000Cr26Mo1, 0000Cr30Mo2 and so on.
(3) Austenitic stainless steel can be divided into two categories: chromium-nickel system (3XX series) and chromium-manganese system (2XX series) according to the different austenitizing elements. Chromium nickel system uses nickel as the main austenitizing element, and its content must be at least 8% and up to 30%. In order to ensure the rust and corrosion resistance of steel, the chromium content is generally not less than 17%. Chromium-manganese austenite uses manganese as the main austenitizing element, but the austenitizing capacity of manganese is much lower than that of nickel (only about half of nickel), and in steels with chromium content exceeding 15%, Relying on manganese alone, steel cannot be completely austenitized even at higher heights. Therefore, this system usually contains a sufficient amount of nitrogen, and some must retain appropriate nickel, so the system actually becomes chromium-manganese-nitrogen or Cr-Mn-Ni-N austenitic stainless steel. ①Cr-Ni austenitic stainless steel It is the main body of austenitic stainless steel. Its basic grade is type 188 stainless steel. The chromium-nickel content in the steel is 19% and 10% respectively, and it has excellent corrosion resistance in oxidizing media. In order to improve the corrosion resistance under various conditions of use and in strong corrosive environments, the alloy composition of steel has been developed and improved in two aspects: First, increase the content of chromium and nickel, and chromium can be increased to more than 25%. Nickel can reach about 30%; the second is to add alloy elements such as Mo, Cu, Si, N, Ti and Nb to the steel. The carbon content is low, the commonly used grades are mostly below 0.08%, and more and more grades have reached ultra-low carbon (≤0.03%) or even lower (≤0.02%). The most commonly used steel types and their representative grades are as follows. a Basic steel types: 03Cr18Ni9, 03Cr19Ni10. b Steels stabilized with titanium and niobium: 07Cr18Ni9Ti, 08Cr18Ni11Nb. c. Steels with increased chromium and nickel content: 08Cr23Ni13, 08Cr25Ni20, 08Cr18Ni35Si. d Steel alloyed with molybdenum and copper: 08Cr17Ni12Mo2, 0Cr18Ni10Cu3, 03Cr18Ni14Mo2Cu2, 00Cr20Ni29Mo2Cu3Nb. e. High silicon or nitrogen-containing steel: 08Cr18Ni13Si4, 08Cr19Ni9N, 03Cr17Ni13Mo2N, etc. ② Chromium-manganese austenitic stainless steel Manganese is an alloying element that maintains the austenite matrix, and its content is 5% to 18%; the content of chromium is more than 17%, up to 22% to ensure its rust resistance and resistance Corrosive. The austenitizing capacity of nitrogen is 30 times that of nickel, and its content is generally above 0.2%, and sometimes it can reach 0.5% to 0.6%. Simple chromium-manganese-nitrogen steel is only resistant to corrosion by oxidizing media. Adding molybdenum, copper and other elements to the steel can improve the corrosion resistance of the steel in a variety of non-oxidizing corrosive environments; sometimes a small amount of niobium or vanadium (<1 %) To improve the resistance to intergranular corrosion. The following grades have been used. a CrMnMn steel: 0Cr18Mn15N. bCrMnNiNi steel: 0Cr18Ni3Mn13N, 0Cr21Ni6Mn9N. c. Cr-Mn steels containing molybdenum or copper: 0Cr18Mn13Mo2N, 0Cr18Ni5Mn10Mo3N, 0Cr17Ni5Mn6Cu, 0Cr17Ni6Mn6MoCu2, etc. Recently, a class of high-molybdenum (about 6%), nitrogen-containing (about 0.20% to 0.40%) chromium-nickel austenitic stainless steel has been developed, and this type is commonly called super austenitic stainless steel. In addition to excellent corrosion resistance in reducing media, and good resistance to stress corrosion, pitting and crevice corrosion, its representative steel grades are: 00Cr20Ni18Mo6N, 00Cr24Ni22Mo6N and Avesta254SMO with carbon content <0.020% (6% Mo), 654SMO (7% Mo) and other steels.
(4) The ferritic and austenitic phases in duplex stainless steel and stainless steel each account for about half of each stainless steel, and the content of less phase should be above 30%. Duplex stainless steel has the advantages of both ferritic and austenitic stainless steels. It has both high strength and resistance to chloride stress corrosion, and excellent toughness and welding performance. It has been used in the chemical, petroleum, paper and energy industries, especially in chlorine-containing media. According to the classification of main elements in steel, duplex stainless steels can be divided into two types: CrNi-based and CrMnN-based, but CrNi-based duplex stainless steels are widely used. In order to obtain proper two comparison examples, the chromium content in CrNi-based steel is higher, while the nickel content is lower or even lower. In order to obtain more ideal corrosion resistance, Mo, N, Cu, W, Nb, Ti and other elements are also added to the steel. According to the level of chromium content, it is generally divided into three types: 18Cr type, 22Cr type, and 25Cr type. The nickel content in each type is about 5% to 7%. Duplex stainless steel has excellent pitting resistance. The larger the pitting resistance equivalent value PRE (PRE = Cr% + 3.3Mo% + 16N%), the better the corrosion resistance. 18Cr type (18Cr5Ni3Mo) and 22Cr type (22Cr5Ni3Mo) PRE = 29 ～36; 25Cr type (25Cr5Ni3Mo) PRE = 32 ～40; PRE> 40 is called super duplex stainless steel, such as 25Cr type (25Cr7Ni4Mo0.3N) . Duplex stainless steel has been developed for three generations. The first generation is represented by 329 steel in the United States. Due to the high carbon content (C≤0.1%), the phase balance is lost during welding and carbides are precipitated along the grain boundaries, resulting in corrosion resistance. And toughness decrease, heat treatment must be performed after welding, and the application is limited. With the emergence and popularization of secondary refining technologies such as AOD and VOD, it is easy to produce ultra-low carbon (C≤0.03%) steel. At the same time, it has been found that nitrogen as an austenite forming element has an important role in duplex stainless steel. During rapid cooling in the heat-affected zone, nitrogen promoted the reverse transformation of ferrite formed at high temperature into a sufficient amount of secondary austenite to maintain the necessary phase balance and improve the corrosion resistance of the welded joint, thereby developing a second Generation of new nitrogen-containing duplex stainless steel. The super duplex stainless steel developed in the late 1980s belongs to the third generation. It is characterized by low carbon content (0.01% to 0.02%), high molybdenum content (about 4%), and high nitrogen content (about 0.3%). ), The ferrite content in steel is 40% to 45%. Commonly used duplex stainless steel grades are: 03Cr19Ni5Mo3Si2N, 03Cr26Ni5Mo2N, 03Cr22Ni5Mo3N, 04Cr25Ni6Mo3Cu2N, etc. (5) Precipitation hardening stainless steel: It is a type of iron-chromium-nickel alloy containing precipitation-hardening elements (Cu, Al, Ti, Nb), and is strengthened by heat treatment. This type of steel has high strength, sufficient toughness and suitable corrosion resistance, and is mainly used in the aerospace industry and some high-tech industries, referred to as PH steel. According to the structure of steel, it can be divided into the following three categories: martensitic precipitation hardening stainless steel, semi-austenitic precipitation hardening stainless steel, and austenitic precipitation hardening stainless steel. The different types of stainless steel were introduced above. In general, the weldability of most stainless steels is good, but corresponding technological measures need to be adopted to avoid the occurrence of cold and hot cracks, embrittlement, and deterioration of resistance to intergranular corrosion and stress corrosion. For stainless steels such as high S, high P, and Se containing 303, 416, 416Se, 430F, 430FSe, etc., and stainless steels such as 440 with high C content, welding is very difficult.
