Stainless steel is usually classified into ferrite, martensite, austenite, duplex, and precipitation hardening types. More than 100 types of stainless steel are classified into these five categories, each with unique composition, structure, and organization.

Stainless steel is an alloy primarily composed of iron, at least 10.5% chromium, ≤ 1.2% carbon, and other alloying elements. The argon oxygen decarburization (AOD) process is widely regarded as the most effective process for producing different types of stainless steel. In contrast, the vacuum deoxidation (VOD) process can produce ultra-low carbon (=0.02%) stainless steel with almost no additional cost. Alloy elements such as nickel, molybdenum, nitrogen, titanium, niobium, and 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 a continuous and 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.

ferritic

Ferritic stainless steel is composed of a microstructure called ferrite phase, with a body centered cubic lattice. Ferritic stainless steel has a chromium content exceeding 12% and a carbon content below 0.20%. They cannot be hardened by heat treatment, only slightly hardened by cold rolling. The body centered cubic lattice is the reason why ferritic steel has magnetism, which sets it apart from all other types of stainless steel.

Although ferritic stainless steel does not have the same strength as martensitic stainless steel, it has high corrosion resistance. Usually used in kitchenware, industrial machinery, and automotive industries. Ferritic stainless steel is a substitute for austenitic stainless steel and can be used to manufacture thin plate products such as automotive exhaust pipes, washing machine drums, containers, buses, train carriages, LCD displays, microwave ovens, and solar water heaters.

Compared with austenitic stainless steel, ferritic stainless steel does not add expensive nickel and is inexpensive. Due to its good cold formability, ferritic stainless steel is easy to design and process. Rich chromium ferritic stainless steel has 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 type of steel is quenched in oil or air at 1050 ℃ (when fully austenitized) and then tempered. Low temperature tempering can achieve low toughness, high tensile strength, and high yield strength, while high temperature tempering can achieve higher toughness. In general, martensitic stainless steel can achieve a yield strength of 550-1860 MPa after quenching and tempering treatment, and its hardenability increases with the increase of chromium content. These steels are generally air hardened steels with large cross-sections. According to the carbon content, martensitic stainless steel can be divided into two categories:

1) Low carbon high-strength martensitic stainless steel: with low carbon content (about 0.10%) and 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 petrochemical construction, gas turbine engines, turbine blades, power plants, compressors, discs and aircraft structural components and engines, as well as freshwater propeller shafts. They are also widely used in components such as bolts, valves, tableware, 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 cost of sacrificing weldability, toughness, and even corrosion resistance. It also increases the amount of carbides that require higher austenitization temperatures to dissolve, thereby reducing impact performance.

Common applications include cutting tools (containing 0.3% C and 12% Cr, with a hardness of 400 VPN after quenching and tempering); Gears, bearings, needle valves, and components for high-temperature applications; Shaver blades, surgical instruments, coal hammers, and ball bearings for high-temperature applications (0.95%~1.20% C and 16%~17% Cr, with a hardness of 600-700 VPN after tempering). Martensitic stainless steel is highly suitable for aerospace, defense, and electric manual tools due to its durability, strength, and corrosion resistance.

austenitic stainless steel 

Austenitic stainless steel is the most popular stainless steel, recognized for its high corrosion resistance and temperature stability, known for its unparalleled strength and formability. Austenitic stainless steel cannot be hardened by heat treatment and is expensive due to 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 stable elements for austenite; Generate "austenite" by adding nickel or nitrogen.

Austenitic stainless steel is usually non-magnetic and has good weldability and formability. It is used in various industries, including medical, automotive, industrial, consumer, and aerospace.

High chromium austenitic stainless steel has excellent oxidation resistance and anti scaling properties, and is suitable for steam pipelines, boiler tubes, heating furnace components, etc. High molybdenum austenitic stainless steel is used for ocean platform pipelines, saltwater or seawater cooling heat exchanger/condenser tubes in power plants, as well as pulp and paper industries. They continuously provide seawater for pumps, propellers, valves, and other marine equipment. Austenitic stainless steel is used for reactor process related pipelines in nuclear power plants. It can withstand higher temperatures than ferritic stainless steel, even though both may exist in a typical reactor.

Austenitic stainless steel has a wide range of properties and can also be used at low temperatures. Although its toughness slightly decreases in the low temperature zone, it still maintains high tensile strength. They are widely used in liquefied natural gas (LNG) at -161 ℃ and in factories producing liquefied gases.

300 series and 200 series austenitic stainless steel are widely used in the following fields:

300 series - mining and chemical equipment, pharmaceutical equipment, storage tanks, catalytic converter components, food and beverage, aerospace piping.

200 series - cookware and tableware, household water tanks, dishwashers, indoor buildings, automotive parts, washing machines, etc.

Due to its many beneficial properties, austenitic stainless steel has become a popular material, accounting for approximately three-quarters of the global stainless steel market.

Duplex stainless steel

Dual phase stainless steel combines the toughness and weldability of austenitic stainless steel with the strength and localized corrosion resistance of ferritic stainless steel. The dual microstructure of ferrite and austenite is obtained by balancing chromium and nickel equivalent elements. In general, the Cr content in duplex steel is 23% to 30%, The Ni content is 2.5% to 7%, and it contains a certain amount of titanium or molybdenum. Dual phase stainless steel has strong corrosion resistance and high work hardening rate. Its strength is approximately twice that of ordinary austenitic or ferritic stainless steel. Super duplex stainless steel has higher molybdenum and chromium content and better corrosion resistance.

Dual phase stainless steel is used in chemical, 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 steamers, bleaching equipment, food processing, biofuel plants, marine and high chloride environments, etc.

Precipitation Hardening Stainless Steel

These steels are a combination of austenitic steel and martensitic steel, either alone or rich in copper, molybdenum, aluminum, and titanium. Precipitation hardened (PH) stainless steel has a high strength to weight ratio and is particularly suitable for high-temperature environments such as power plants. Through heat treatment, the tensile strength can reach 850-1700MPa and the yield strength can reach 520-1500MPa, which is approximately 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 power, and aerospace industries, which require high strength, corrosion resistance, and low but acceptable toughness, with special applications including high-speed applications such as turbine blades.

17-4 PH stainless steel plate is easy to produce and process, greatly saving costs. Engineers around the world rely on this PH stainless steel to solve urgent problems in product design, manufacturing, and processing. Castings with high strength and moderate corrosion resistance are also typical applications of 17-4 stainless steel. The required strength and toughness can be controlled by the temperature range during the heat treatment process.

Impressive series of products

The stainless steel series products are impressive and constantly expanding, mainly due to their wide range of applications, even under extreme environmental conditions. From 2019 to 2027, the global market is expected to have a compound annual growth rate of 6.3%, reaching $182.1 billion by 2027. With the growth of per capita stainless steel consumption, the increase in demand and production will greatly benefit it. Stainless steel has truly become the metal of the future due to its iconic applications.