A wide range of stainless steel
Stainless steel is usually divided into ferrite, martensite, austenite, biphase and precipitation hardening. More than 100 varieties of stainless steel are classified into these five categories, each unique in terms of composition, structure and organization.
Stainless steel is an alloy composed mainly of iron, at least 10.5% chromium, ≤1.2% carbon and other alloying elements. The argon-oxygen decarburization (AOD) process is generally considered to be the most efficient process for producing different types of stainless steel. In contrast, the vacuum deoxidation (VOD) process can produce ultra-low carbon (=0.02%) stainless steel at almost no additional cost, and alloying elements such as nickel, molybdenum, nitrogen, titanium, niobium, manganese enhance the corrosion resistance and mechanical properties of stainless steel. Chromium forms a stable oxide film (Cr2O3) on the surface of stainless steel, and the continuous, dense passivation film prevents further reactions between the steel and the surrounding atmosphere, protecting the steel from further oxidation. Other alloying elements affect formability, weldability, strength, oxidation resistance and cold rolling deformation rate.
Ferrite stainless steel
Ferritic stainless steel is composed of a microstructure called a ferritic phase, with a body-centered cubic lattice. The ferritic stainless steel contains more than 12% chromium and less than 0.20% carbon. They cannot be hardened by heat treatment and can only be slightly hardened by cold rolling. The body-centered cubic lattice is the reason why ferritic steel is magnetic, in this regard, unlike all other types of stainless steel.
Although ferritic stainless steel is not as strong as martensitic stainless steel, it has high corrosion resistance. Commonly used in kitchenware, industrial machinery and automotive industries. Ferritic stainless steel is an alternative to austenitic stainless steel and can be used to manufacture sheet products such as automobile exhaust pipes, washing machine rollers, containers, buses, train cars, liquid crystal displays, microwave ovens and solar water heaters.
Compared with austenitic stainless steel, ferritic stainless steel does not add expensive nickel, the price is low, because of its good cold formability, ferritic stainless steel is easy to design and process. Chrome-rich ferritic stainless steels have good oxidation resistance at high temperatures and can be used to manufacture industrial furnace components.
Martensitic stainless steel
Martensitic stainless steel contains 12% to 17% chromium and 0.10% to 1.20% carbon. This steel is quenched in oil or air at 1050 ° C (when fully austenitizing) and then tempered. Low temperature tempering can obtain low toughness, high tensile strength and high yield strength, and high temperature tempering can obtain higher toughness. Under normal circumstances, the yield strength of martensitic stainless steel can be obtained by tempering treatment of 550 ~ 1860MPa, and the hardenability increases with the increase of chromium content. These steels are generally air hardened steels with large sections. According to the carbon content, martensitic stainless steel can be divided into two categories:
1) Low carbon high strength martensitic stainless steel: low carbon content (about 0.10%), chromium content ranging from 11.50% to 18.00%. These are mainly high-strength structural steels with good weldability, formability and impact toughness. They are used in (petroleum) chemical construction, gas turbine engines, turbine blades, power plants, compressors, dishes and aircraft structural parts and engines, as well as freshwater propeller shafts, and are also widely used in bolts, valves, cutlery, pump shafts and bearings.
2) High-carbon and high-hardness martensitic stainless steel: by increasing the carbon content to improve strength and hardness, but at the expense of weldability, toughness and even corrosion resistance. It also increases the amount of carbides that require higher austenitizing temperatures to dissolve, thereby reducing impact properties.
Common applications include cutting tools (0.3% C and 12%Cr, hardness of 400 VPN after quenching and tempering); Gears, bearings, needle valves and components for high temperature applications; Razor blades, surgical instruments, coal hammers and ball bearings for high temperature applications (0.95% ~ 1.20%C and 16% ~ 17%Cr, tempered hardness of 600 ~ 700 VPN). Due to its durability, strength and corrosion resistance, martensitic stainless steel is ideal for aerospace, defense and power hand tools.
Austenitic stainless steel
Austenitic stainless steel is the most popular stainless steel, recognized for its high corrosion resistance and temperature stability, known for its unmatched strength and formability, austenitic stainless steel cannot be hardened by heat treatment, and is expensive because of its high nickel content, at least 10.5% chromium and 8% to 12% nickel, plus nitrogen and other elements.
Austenitic stainless steel has an austenitic crystal structure and a face-centered cubic lattice at both high and low temperatures. Magnesium, nickel and nitrogen are austenitic stable elements. “Austenite” is formed by adding nickel or nitrogen.
Austenitic stainless steel is usually non-magnetic, weldable and well-formed, and is used in a variety of industries, including medical, automotive, industrial, consumer and aerospace.
High chromium grade austenitic stainless steel has excellent oxidation resistance and scaling resistance, suitable for steam pipes, boiler tubes, heating furnace parts and so on. High molybdenum austenitic stainless steels are used in offshore platform pipes, heat exchangers/condenser tubes for brackish or seawater cooling in power stations, and in pulp and paper industries. They continuously supply water for pumps, propellers, valves and other Marine equipment. In nuclear power plants, austenitic stainless steel is used for piping related to reactor processes. It can withstand higher temperatures than ferritic stainless steel, even though both may be present in a typical reactor.
Austenitic stainless steel has a wide range of properties, can also be used at low temperatures, in the low temperature zone although the toughness is slightly reduced but still maintain high tensile strength, they are widely used in liquefied natural gas (LNG) at -161 ° C temperature and the production of liquefied gas plants.
300 series and 200 series austenitic stainless steels are widely used in the following fields:
300 Series – Mining and chemical equipment, pharmaceutical equipment, storage tanks, catalytic converter components, food and beverage, aerospace pipes.
200 series – cookware and tableware, household water tank, dishwasher, interior construction, auto parts, washing machine, etc.
Because austenitic stainless steel has many beneficial properties, it has become a popular material, accounting for about three quarters of the global stainless steel market.
Duplex stainless steel
Duplex stainless steel combines the toughness and weldability of austenitic stainless steel with the strength and local corrosion resistance of ferritic stainless steel. The double microstructure of ferrite and austenite is obtained by balancing chromium and nickel equivalent elements. Under normal circumstances, the Cr content in two-phase steel is 23% to 30%, the Ni content is 2.5% to 7%, and contains a certain amount of titanium or molybdenum. Duplex stainless steel has strong corrosion resistance and high work hardening rate. Its strength is about twice that of ordinary austenitic or ferritic stainless steel. Super duplex stainless steel has higher molybdenum and chromium content and has better corrosion resistance.
Duplex stainless steel is used in chemical industry, transportation and storage, oil and gas production and transportation pipelines, oil and gas exploration and offshore drilling platforms. Other common applications include pressure vessels and heat exchangers, paper and pulp digesters, bleaching equipment, food processing, biofuel plants, Marine and highly chlorinated environments, among others.
Precipitation hardened stainless steel
These steels are either alone or a combination of austenitic and martensitic steels rich in copper, molybdenum, aluminum and titanium, and precipitation-hardened (PH) stainless steels have a high strength-to-weight ratio, especially suitable for high temperature environments such as power plants. Through heat treatment, the tensile strength can reach 850 ~ 1700M Pa, and the yield strength can reach 520 ~ 1500MPa, which is about three to four times the strength of 304 or 316 austenitic stainless steel.
The PH stainless steel family can be divided into three main types – low carbon martensite, semi-austenite and austenite. They are used in the oil & gas, nuclear and aerospace industries where high strength, corrosion resistance and low but acceptable toughness are required, and special uses include high-speed applications such as turbine blades.
17-4 PH stainless steel plate is easy to produce and process, which greatly saves costs. Engineers around the world rely on this PH stainless steel to solve pressing problems in product design, manufacturing and processing. High strength and medium corrosion resistance castings are also typical uses of 17-4 stainless steel. The required strength and toughness can be controlled by the temperature range during the heat treatment process.
Impressive range of products
The stainless steel range is impressive and expanding, mainly due to its wide range of uses, even under extreme environmental conditions. From 2019 to 2027, the global market is expected to grow at a CAGR of 6.3% and reach $182.1 billion by 2027. With the growth of per capita stainless steel consumption, the increase in demand and production will benefit it greatly. Stainless steel is truly the metal of the future because of its iconic applications.