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The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element to make stainless steel corrosion resistance, when the chromium content in the steel reaches about 12%, chromium and the oxygen in the corrosive medium, the formation of a thin oxide film on the surface of the steel (self-passivation film), can prevent further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of various uses on the structure and properties of stainless steel.
Stainless steel is usually divided into:
1. Stainless steel, containing 12%-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 kinds of stainless steel.
2. Austenitic stainless steel, containing more than 18% chromium, also contains about 8% nickel and a small amount of molybdenum, titanium, and other elements. Good overall performance, can resist a variety of media corrosion.
3. Austenite-ferrite 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.
300 series stainless steel:
301- Good ductility for molding products. It can also be quickly hardened by machining. welding
Good connection. The wear resistance and fatigue strength are better than 304 stainless steel.
302- Corrosion resistance is the same as 304, and the strength is better because the carbon content is relatively higher
303- By adding a small amount of sulfur, phosphorus to make it easier to cut than 304.
304- General purpose model; 18/8 stainless steel. GB brand 0Cr18Ni9,
309 has better temperature resistance than 304.
316 is the second most widely used steel grade after 304, mainly used in the food industry and surgical equipment, adding molybdenum element to make it obtain a special structure resistant to 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 units.
321- Other properties similar to 304 except that the addition of titanium reduces the risk of corrosion in the weld of the material.