Steel resistant to weak corrosive media such as air, steam, and water, and chemical corrosive media such as acids, alkalis, and salts. Also known as stainless acid-resistant steel. In practical applications, steel that is resistant to corrosion by weak corrosive media is often called stainless steel, and steel that is resistant to chemical media is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter is generally non-corrosive. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element for the corrosion resistance of stainless steel.
When the chromium content in the steel reaches about 12%, the chromium interacts with the oxygen in the corrosive medium to form a very thin oxide film (self-passivation film) on the surface of the steel. , Can prevent further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements include nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of various uses for the structure and performance of stainless steel. Stainless steel is usually divided into: ① Ferritic stainless steel. Containing 12% to 30% chromium.
Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is better than other types of stainless steel. ②Austenitic stainless steel. The chromium content is more than 18%, and it also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, resistant to corrosion by various media. ③Austenitic-ferritic duplex stainless steel. It has the advantages of austenitic and ferritic stainless steel, and has superplasticity. ④Martensitic stainless steel. High strength, but poor plasticity and weldability.
Model 301—Good ductility, used for molded products. It can also be hardened by mechanical processing. Good weldability. Abrasion resistance and fatigue strength are better than 304 stainless steel.
Model 302—The corrosion resistance is the same as that of 304, because the carbon content is relatively high, the strength is better.
Model 303—It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.
Model 304—general model; that is, 18/8 stainless steel. The GB grade is 0Cr18Ni9.
Model 309—Compared with 304, it has better temperature resistance.
Model 316-after 304, the second most widely used steel grade, mainly used in food industry and surgical equipment, adding molybdenum to obtain a special corrosion-resistant structure. Because it has better resistance to chloride corrosion than 304, it is also used as “ship steel”. SS316 is usually used in nuclear fuel recovery devices. 18/10 grade stainless steel usually also meets this application level. 
Model 321—Except for the addition of titanium to reduce the risk of corrosion of the material welds, other properties are similar to 304.
400 series-ferritic and martensitic stainless steel
Model 408—Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
Model 409—the cheapest model (British and American), usually used as car exhaust pipe, is a ferritic stainless steel (chrome steel).
Model 410—Martensite (high-strength chromium steel), with good wear resistance and poor corrosion resistance.
Model 416—Add sulfur to improve the processing performance of the material.
Model 420—”tool grade” martensitic steel, similar to the earliest stainless steel such as Brinell high chromium steel. It is also used for surgical knives, which can be very bright.
Model 430—Ferritic stainless steel, for decoration, such as car accessories. Good formability, but poor temperature resistance and corrosion resistance.
Model 440—High-strength cutting tool steel with slightly higher carbon content. After proper heat treatment, higher yield strength can be obtained. The hardness can reach 58HRC, which is among the hardest stainless steels. The most common application example is “razor blades”. There are three commonly used models: 440A, 440B, 440C, and 440F (easy processing type).
500 series—heat-resistant chromium alloy steel.
600 series—Martensitic precipitation hardening stainless steel.
Model 630-the most commonly used precipitation hardening stainless steel model, usually also called 17-4; 17% Cr, 4% Ni.