Stainless Steel

Stainless steel is an iron alloy that is resistant to rust and corrosion, and is the abbreviation for stainless acid-resistant steel. According to the definition of GB/T20878-2007 standard, the main characteristics are stainless performance and corrosion resistance, and the chromium content is at least 10.5%, the carbon content is not more than 1.2%, and there may be other non-metallic and metal elements. to obtain other desired properties. The corrosion resistance of stainless steel comes from chromium, which forms a passive film that protects the material and heals itself in the presence of oxygen.

Steel grades that are resistant to weak corrosive media such as air, steam, and water or have stainless properties are called stainless steels; and those that are resistant to chemical corrosion media (acid, alkali, salt, etc.) are called acid-resistant steels. Due to the difference in chemical composition of the two, their corrosion resistance is different. Ordinary stainless steel is generally not resistant to chemical corrosion, while acid-resistant steel generally has stainless properties. There are five main families of stainless steels, classified primarily by their crystal structure: austenitic, ferritic, martensite, duplex, and precipitation hardening.

Austenitic stainless steel

It contains more than 18% chromium, about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and is resistant to corrosion by various media. The commonly used grades of austenitic stainless steel are 1Cr18Ni9, 0Cr19Ni9, etc. The Wc of 0Cr19Ni9 steel is less than 0.08%, which is marked as “0” in the steel number. This type of steel contains a large amount of Ni and Cr, which makes the steel in an austenitic state at room temperature. This type of steel has good plasticity, toughness, weldability, corrosion resistance and non-magnetic or weak magnetic properties. It has good corrosion resistance in oxidizing and reducing media. It is used to make acid-resistant equipment, such as corrosion-resistant containers and equipment. Linings, conveying pipes, nitric acid-resistant equipment parts, etc., can also be used as the main material of stainless steel watch accessories. Austenitic stainless steel is generally solution treated, that is, heating the steel to 1050-1150 °C, and then cooling it with water or air to obtain a single-phase austenite structure.

Ferritic stainless steel

Contains 15% 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. Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Because of the high chromium content, ferritic stainless steel has good corrosion resistance and oxidation resistance, but poor mechanical properties and process performance. This kind of steel can resist the corrosion of the atmosphere, nitric acid and brine solution, and has the characteristics of good high temperature oxidation resistance and small thermal expansion coefficient.

400 series ferritic stainless steel

– No manganese, can replace 304 stainless steel to a certain extent

408 stainless steel: good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
409 Stainless Steel: The cheapest model (British and American), commonly used as a car exhaust pipe, is a ferritic stainless steel (chromium steel).
430 Stainless Steel: Ferritic stainless steel, used for decoration, such as for car accessories. Good formability, but poor temperature and corrosion resistance.

Martensitic stainless steel

High strength, but poor plasticity and weldability. The commonly used grades of martensitic stainless steel are 1Cr13, 3Cr13, etc. Due to the high carbon content, it has high strength, hardness and wear resistance, but the corrosion resistance is slightly worse, and it is used for high mechanical properties and corrosion resistance. Some general parts are required, such as springs, turbine blades, hydraulic valves, etc. This type of steel is used after quenching and tempering. Annealing is required after forging and stamping.

400 series martensitic stainless steel

– No manganese, can replace 304 stainless steel to a certain extent

410 stainless steel: martensitic (high-strength chromium steel), with good wear resistance and poor corrosion resistance.

416 Stainless Steel: The addition of sulfur improves the machinability of the material.

420 stainless steel: “cutting grade” martensitic steel, the earliest stainless steel similar to Brinell high chromium steel. Also used in surgical knives, can do very bright.

440 stainless steel: high-strength cutting tool steel with slightly higher carbon content, high yield strength can be obtained after proper heat treatment, and hardness can reach 58HRC, which is one of the hardest stainless steels. The most common application example is “razor blades”. There are three commonly used models: 440A, 440B, 440C, and 440F (easy to process).

Austenitic-ferritic duplex stainless steel

It has the advantages of both austenitic and ferritic stainless steels, and has superplasticity. Austenitic and ferritic structures each account for about half of stainless steel. In the case of low carbon content, the chromium (Cr) content is 18% to 28%, and the nickel (Ni) content is 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel has the characteristics of both austenitic and ferritic stainless steels. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance. The 475 ℃ brittleness and high thermal conductivity of element stainless steel have the characteristics of superplasticity. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

200 Series Stainless Steel:

– Chromium-manganese-nickel, with manganese instead of nickel, has poor corrosion resistance, and is widely used as a cheap substitute for 300 series in China

201 stainless steel
202 stainless steel

300 Series Stainless Steel:

– Chromium-Nickel Austenitic Stainless Steel

301 stainless steel: good ductility, used for shaped products. It can also be hardened by mechanical processing. Good weldability. Wear resistance and fatigue strength are better than 304 stainless steel.
302 stainless steel: The corrosion resistance is the same as 304, and the strength is better due to the relatively high carbon content.
303 stainless steel: It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.
304 Stainless Steel: General purpose model; ie 18/8 stainless steel. Products such as: corrosion-resistant containers, tableware, furniture, railings, medical equipment. The standard composition is 18 % chromium plus 8 % nickel. It is a stainless steel that is non-magnetic and cannot be changed by heat treatment. The GB grade is 06Cr19Ni10.
304L stainless steel: The same characteristics as 304, but with low carbon, it is more corrosion-resistant and easy to heat treatment, but it has poor mechanical properties. It is suitable for welding and products that are not easy to heat treatment.
304N stainless steel: The same characteristics as 304, it is a nitrogen-containing stainless steel, nitrogen is added to improve the strength of the steel.
309 stainless steel: Compared with 304, it has better temperature resistance, and the temperature resistance is as high as 980 ℃.
309S stainless steel: It has a lot of chromium and nickel, so it has good heat resistance and oxidation resistance. Products such as heat exchangers, boiler components, and jet engines.
310 stainless steel: excellent high temperature oxidation resistance, the maximum operating temperature is 1200 ℃.
316 stainless steel: after 304, the second most widely used steel grade, mainly used in the food industry, watch accessories, pharmaceutical industry and surgical equipment, adding molybdenum element to obtain a special corrosion-resistant structure. Because of its better resistance to chloride corrosion than 304, it is also used as “marine steel”. SS316 is usually used in nuclear fuel recovery units. Grade 18/10 stainless steels generally also meet this application level.
316L stainless steel: low carbon, so it is more resistant to corrosion and easy to heat treatment. Products such as chemical processing equipment, nuclear power generators, and refrigerant storage tanks.
321 stainless steel: Similar to 304 except that the risk of corrosion of the material weld is reduced due to the addition of titanium elements.
347 stainless steel: adding stabilizing element niobium, suitable for welding aircraft parts and chemical equipment.

Precipitation hardened stainless steel

The matrix is austenite or martensite, and the commonly used grades of precipitation hardening stainless steel are 04Cr13Ni8Mo2Al and so on. Stainless steel which can be hardened (strengthened) by precipitation hardening (also known as age hardening)

Precipitation hardening stainless steel has good formability and good weldability, and can be used as an ultra-high-strength material in the nuclear industry, aviation and aerospace industries. According to the composition, it can be divided into Cr series (400 series), Cr-Ni series (300 series), Cr-Mn-Ni (200 series), heat-resistant chromium alloy steel (500 series) and precipitation hardening series (600 series).

Stainless steel plates and strips for pressure equipment

Stainless steel for pressure vessels has clear requirements for its classification and code, size, shape and allowable deviation, technical requirements, test methods, inspection rules, packaging, marking and product quality certificates. The commonly used grades are 06Cr19Ni10, 022Cr17Ni12Mo2, and the digital codes are: S30408, S31603, etc. Mainly used in food machinery, pharmaceutical machinery and other sanitary equipment.

The properties of this alloy, such as gloss and corrosion resistance, are useful in many applications. Stainless steel can be rolled into sheet, plate, bar, wire and tube. These can be used in cookware, tableware, surgical instruments, major appliances, vehicles, construction materials in large buildings, industrial equipment (eg, paper mills, chemical plants, water treatment), and storage tanks and tankers for chemicals and food.

Stainless steel biocleans better than aluminum and copper and has biocleanability comparable to glass. The cleanability, strength and corrosion resistance of stainless steel have prompted the use of stainless steel in pharmaceutical and food processing plants.

Different types of stainless steel are marked with AISI three-digit numbers, and the ISO 15510 standard lists the chemical composition of stainless steels sized in existing ISO, ASTM, EN, JIS and GB standards in a useful interchangeable table.

The term “stainless steel” does not simply refer to one type of stainless steel, but to more than one hundred industrial stainless steels, each developed to perform well in its specific field of application. The key to successful material selection is to first understand the purpose and then to determine the correct steel grade. There are usually only six steel grades related to building construction applications, and they all contain 17 to 22% chromium, and the better grades also contain nickel. The addition of molybdenum can further improve atmospheric corrosion resistance, especially corrosion resistance to chloride-containing atmospheres.

Generally speaking, the hardness of stainless steel is higher than that of aluminum alloy, and the cost of stainless steel is higher than that of aluminum alloy.