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Since the invention of stainless steel at the beginning of this century, stainless steel has combined the image of modern materials with its outstanding reputation in architectural applications, making its competitors envy.
As long as the steel is selected correctly and maintained properly, stainless steel will not cause corrosion, pitting, rust or wear. Stainless steel is also one of the strongest materials among metal materials for construction. Because stainless steel has good corrosion resistance, it can make structural components permanently maintain the integrity of engineering design. Chromium-containing stainless steel also integrates mechanical strength and high extensibility, easy to process and manufacture parts, and can meet the needs of architects and structural designers.
Stainless steel grade group
200 series—chromium-nickel-manganese austenitic stainless steel
300 series—chromium-nickel austenitic stainless steel
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, and the strength is better due to the relatively high carbon content.
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-has better temperature resistance than 304.
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 a 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—"cutting 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.
Why is stainless steel corrosion resistant?
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 forming holes. You can use paint or oxidation-resistant metals (such as zinc, nickel and chromium) for electroplating to ensure the surface of carbon steel, but, as people know, this protection is only a thin film. If the protective layer is damaged, the steel below will begin to rust.
First of all, let's first understand what stainless steel is. Generally speaking, the steel that will not rust is called stainless steel, but in an academic sense, it is resistant to weak corrosive media such as air, steam, and water, and chemical corrosion such as acid, alkali, and salt. Medium corroded steel. 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.
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 addition amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel is significantly increased, but when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that when chromium is used to alloy steel, 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, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself and re-form this "passivation film" to continue to protect it.
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