Basic knowledge of stainless steel material classification

Since the invention of stainless steel at the beginning of this century, stainless steel has combined the image of modern materials and the excellent reputation in architectural applications, making its competitors envious.

Stainless steel grade grouping

200 series—chromium-nickel-manganese austenitic stainless steel

300 series—chromium-nickel austenitic stainless steel

Model 301—Good ductility, used for molding products. It can also be hardened by mechanical processing. Good weldability. Wear resistance and fatigue strength are better than 304 stainless steel.

Model 302-Corrosion resistance is the same as 304, because the carbon content is relatively high and the strength is better.

Model 303-By adding a small amount of sulfur and phosphorus, it is easier to cut than 304.

Model 304—Universal model; namely 18/8 stainless steel. GB grade is 0Cr18Ni9.

Model 309-better temperature resistance than 304.

Model 316—after 304, the second most widely used steel grade, mainly used in the food industry and surgical equipment. The addition of molybdenum makes it obtain a special structure that resists corrosion. Because it has better resistance to chloride corrosion than 304, it is also used as \"marine steel\". SS316 is commonly used in nuclear fuel recovery devices. 18/10 grade stainless steel usually also meets this application level.

Model 321-Other properties are similar to 304 except that the addition of titanium reduces the risk of corrosion of the material weld. Stainless steel decorative tube, 201 stainless steel tube, 304 stainless steel tube.

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 an automobile exhaust pipe, is ferritic stainless steel (chrome steel).

Model 410—Martensite (high-strength chromium steel) has good wear resistance and poor corrosion resistance.

Model 416-Adding sulfur improves the processing performance of the material.

Model 420—\"Cutting grade \" martensitic steel, similar to the earliest stainless steel such as Brinell high-chromium steel. It is also used for surgical knives and can be made very bright. Model 430—Ferritic stainless steel, for decoration, for example for car accessories. Good formability, but poor temperature resistance and corrosion resistance.

Model 440—High-strength cutting tool steel with a slightly higher carbon content. After proper heat treatment, a higher yield strength can be obtained, and the hardness can reach 58HRC, which is among the hardest stainless steels. The most common application example is \"razor blade \". There are three commonly used models: 440A, 440B, 440C, and 440F (easy processing type).

500 series-heat-resistant chrome alloy steel.

600 series-martensitic precipitation hardening stainless steel.

Model 630—The most commonly used precipitation hardening stainless steel model, also commonly called 17-4; 17% Cr, 4% Ni.

Why is stainless steel resistant to corrosion?

All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing the corrosion to continue to expand and eventually form holes. Paint or oxidation-resistant metals (for example, zinc, nickel and chromium) can be used for electroplating to ensure the carbon steel surface, but, as is known, this protection is only a thin film. If the protective layer is damaged, the underlying steel begins to rust.

First of all, let's first understand what is stainless steel. Popularly speaking, steel that does not rust is called stainless steel, but in an academic sense, it is resistant to weak corrosive media such as air, steam, water, and chemical erosion such as acids, alkalis, and salts. Medium corroded steel. Also known as stainless acid-resistant steel. In practical applications, steel that is resistant to corrosion by weak corrosion media is often called stainless steel, and steel that is resistant to corrosion by chemical media is called acid-resistant steel. Due to the difference in chemical composition of the two, the former is not necessarily resistant to chemical media corrosion, while the latter are generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element for stainless steel to obtain corrosion resistance. When the chromium content in the steel reaches about 12%, chromium reacts with the oxygen in the corrosive medium to form a thin oxide film (self-passivation film) on the steel surface. , Can prevent the steel matrix from further corrosion. In addition to chromium, commonly used alloy elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of stainless steel structure and performance for various uses.

Stainless steel is usually divided according to the matrix structure: 201 stainless steel tube, 304 stainless steel tube

①Ferritic stainless steel. Contain chromium 12% ～ 30%. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is superior to other types of stainless steel.

② Austenitic stainless steel. It contains more than 18% chromium and about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and is resistant to various media.

③ Austenitic-ferritic duplex stainless steel. It has the advantages of austenitic and ferritic stainless steel, and has superplasticity. ④ Martensitic stainless steel. High strength, but poor plasticity and weldability.

The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different.

When the amount of chromium added reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly, but when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that when alloying steel with chromium, the type of surface oxide is changed to a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface and prevents further oxidation. This oxide layer is extremely thin, through which you can see the natural luster of the steel surface, giving stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, re-forming this \"passivation film \", and continue to play a protective role.