Classification of stainless steel According to different microstructure, stainless steel can be divided into martensitic, austenitic, ferritic, and duplex stainless steel (duplex of austenite and ferrite). Different types of stainless steel with chromium and nickel content Stainless steel family Austenitic stainless steel and duplex stainless steel Austenitic stainless steel (1) Austenitic stainless steel Austenitic stainless steel contains at least 16% chromium and 6% nickel. For example, the well-known 304 stainless steel is austenitic stainless steel, which contains 18% chromium and 8% nickel, also known as 18/8 stainless steel. The addition of nickel can make austenitic stainless steel can be used at low temperature without brittle fracture (hard and brittle at low temperature). Austenitic stainless steel is divided into 3 series and 2 series stainless steel, 300 series stainless steel is stainless steel containing chromium and nickel, while 200 series stainless steel is stainless steel containing chromium, nickel and manganese. These stainless steels can be hardened by cold working but not by heat treatment. In the annealed state, austenitic stainless steel is basically non-magnetic, but some may become slightly magnetic due to cold working. Austenitic stainless steel has excellent corrosion resistance, excellent formability. Commonly used austenitic stainless steels are 303, 304, 316, 317, etc. 304 Stainless Steel: It can withstand common rust in construction, it can resist food processing environments (except high acid and chloride high temperature conditions), it can also resist organic chemicals, dyes and many inorganic chemicals. 304 L stainless steel (L=Low, low carbon): It can resist nitric acid and sulfuric acid well at moderate temperature and concentration, and is widely used to store liquefied gas, household appliances and other consumer products, like kitchen equipment, hospital equipment. 316 stainless steel: This type of stainless steel contains more nickel and 2-3% molybdenum than 304 stainless steel, so it has better corrosion resistance than 304 stainless steel, especially in chloride environments that are prone to pitting corrosion. 317 stainless steel: Contains 3-4% molybdenum and more chromium than 316, which can better resist pitting and crevice corrosion. (2) Ferritic stainless steel Ferritic stainless steel is straight chromium 400 series stainless steel, which contains 10.5%-25% chromium, has medium strength corrosion resistance and poor processability, such as 430, 409 is ferrite Body stainless steel. Such stainless steels cannot be hardened by heat treatment but only moderately hardened by cold working, they are magnetically conductive, have good ductility, and are resistant to corrosion and oxidation. 430 is the most basic ferritic stainless steel. Ferritic and martensitic stainless steel Ferritic stainless steel (3) Martensitic stainless steel Martensitic stainless steel is 400 series stainless steel, its main alloying element is also chromium, compared with ferritic stainless steel, carbon of martensitic stainless steel The content is higher, and the chromium content is lower, for example, 410 and 416 only contain 12% chromium. These alloys can be hardened by heat treatment, are magnetic, and have moderate corrosion resistance. And they have quite good ductility, and the tensile strength can reach 1379 MPa after heat treatment. Martensitic stainless steel 410 stainless steel is the basic alloy of the martensitic stainless steel family. It has the lowest alloy content among the three general-purpose stainless steels and is generally used for high-stress parts that require a combination of strength and corrosion resistance, such as fasteners. 410 stainless steel is resistant to corrosion in mild atmospheres, steam and many mild chemical environments. (4) Duplex stainless steel (austenite + ferrite) The microstructure of duplex stainless steel is a mixture of austenite and ferrite, so it has both austenitic stainless steel and ferritic stainless steel. For example, although not as resistant to stress corrosion as ferritic stainless steel, it also has high resistance to chloride stress corrosion cracking, has excellent resistance to pitting and crevice corrosion, and its yield strength is about twice that of conventional stainless steel . Type 329 and Type 2205 are typical duplex alloys. In addition, the corrosion resistance of general duplex stainless steel is equal to or better than 304 and 316, and its pitting corrosion resistance is better than 316 in general, because these alloys contain very high chromium content, about 22% to 25% Cr. (5) Precipitation-hardening stainless steels These are also chromium-nickel steels, and the biggest advantage is that they can provide high tensile strength under “solution treatment” conditions. The most common of this group is “17-4 PH,” also known as 630 stainless steel, which has a composition of 17% chromium, 4% nickel, 4% copper, and 0.3% niobium.