Stainless steel is a type of high alloy steel with FeCr, FeCrC and FeCrNi as alloy systems. As a type of stainless steel, chromium must be contained in a mass fraction of not less than 10.5%. The steel containing this minimum chromium content can form an inert oxide layer on its surface. This inert oxide layer can protect the inner metal from being oxidized and corroded in the air without corrosive medium. If a certain amount of alloy elements such as Ni, Mo, Cu, Nb, Ti, W are added on this basis, the steel can have properties such as special corrosion resistance, high temperature oxidation resistance, or certain high temperature strength. Characteristics and developed into various types of stainless steel.
Stainless steel not only has obvious corrosion resistance, but also has excellent strength and plastic toughness. It has good performance at high and low temperatures. Therefore, people not only use its corrosion resistance, rust resistance, oxidation resistance, heat resistance, but also use its good processing and mechanical properties, in the petrochemical, fiber industry, food industry, power station, aviation industry , Atomic energy industry, pharmaceutical, dairy and food industry, as acid-resistant, alkali-resistant, high-temperature and low-temperature resistant materials, it has been widely used. In short, stainless steel is a collective name for a series of iron-based alloys with stainless and acid resistance. There are many varieties and different performances. There are many classification methods. The following classification methods are mainly introduced.
1.2.1 Classified according to the main chemical composition (main plus elements in stainless steel)
(1) Chrome type stainless steel Chrome type stainless steel mainly contains chromium (≥12%). It has certain corrosion resistance to oxidizing acids such as nitric acid, but has no corrosion resistance to non-oxidizing acids such as sulfuric acid and hydrochloric acid. Its basic type is Cr13.
(2) If a certain amount of nickel is added to the chromium-nickel stainless steel, it can also show excellent corrosion resistance in non-oxidizing acids, so it has been developed into a chromium-nickel stainless steel. Its basic type is Cr18Ni9 steel.
(3) Cr-Mn-N stainless steel: In order to save alloy element nickel and obtain some special properties, CrMnN and CrMnNiN stainless steels have also been developed. Chromium manganese nitrogen stainless steel belongs to nickel-saving austenitic stainless steel. Some of the nickel in the chemical composition is replaced by manganese and nitrogen, which can reduce the content of nickel. As a solid solution strengthening element, nitrogen can increase the strength of austenitic stainless steel without significantly damaging the plasticity and toughness of the steel, and at the same time improve the corrosion resistance of the steel, especially resistance to local corrosion, such as intergranular corrosion, pitting corrosion and crevice corrosion. Wait. This type of steel is 1Cr18Mn8Ni5N, 1Cr18Mn6Ni5N, etc.
1.2.2 Classification by purpose
(1) Stainless steel (referring to the customary meaning, also known as acid-resistant stainless steel) Including steel used in atmospheric environments and in corrosive chemical media, the working temperature generally does not exceed 500 ℃, corrosion resistance is required, and the strength is not high. The most widely used are high-Cr steel (such as 1Cr13, 2Cr13) and low-carbon CrNi steel (such as 0Cr19Ni3, 1Cr18Ni9Ti) or ultra-low-carbon CrNi steel (such as 00Cr25Ni22Mo2, 00Cr22Ni5Mo3N, etc.). Stainless steel for urea equipment with high corrosion resistance requirements is often limited to w (C) ≤0.02%, w (Cr) ≥17%, w (Ni) ≥13%, and w (Mo) ≥2.2%. For stainless steels with higher corrosion resistance requirements, the purity must also be improved, such as w (C) ≤0.02%, w (P) ≤0.01%, w (S) ≤0.01%, w (Si) ≤0.1%, which is called high Pure stainless steel, such as 000Cr19Ni15, 000Cr25Ni20 and so on.
(2) Anti-oxidation steel: Steel that has anti-oxidation performance at high temperature. It does not require high-temperature strength, and the working temperature can be as high as 900 ~ 1100 ℃. Commonly used steels are high Cr steels (such as 1Cr17, 1Cr25Si2) and CrNi steels (such as 2Cr25Ni20, 2Cr25Ni20Si2). (3) Hot-strength steel reeds must have both oxidation resistance and high-temperature strength at high temperatures, and the working temperature can be as high as 600-800 ° C. CrNi steel is widely used, such as 1Cr18Ni9Ti, 1Cr16Ni25Mo6, 4Cr25Ni20, 4Cr25Ni34 and so on. Cr12-based multi-alloyed high-Cr steels (such as 1Cr12MoWV) are also important hot-strength steels. Because the Cr content of heat-resistant stainless steel is more than 12%, this kind of steel has both heat resistance and certain corrosion resistance. Therefore, many types of heat-resistant stainless steel are also acid-resistant stainless steel. In terms of welding materials, the welding materials of acid-resistant stainless steel and heat-resistant stainless steel are commonly referred to as stainless steel welding materials.
