Analysis of the application of various series of stainless steel

Stainless steel is usually classified as ferritic, martensitic, austenitic, duplex and precipitation hardening. More than 100 types of stainless steel are classified into these five categories, each of which is unique in 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 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 at almost no additional cost. Alloying 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. The continuous and dense passivation film prevents further reaction between the steel and the surrounding atmosphere and protects the steel from further oxidation. Other alloying elements affect formability, weldability, strength, oxidation resistance and cold rolling deformation rate.

Ferritic stainless steel

Ferritic stainless steel consists of a microstructure called ferrite phase, with a body-centered cubic lattice. Ferritic stainless steels have a chromium content of more than 12% and a carbon content of less than 0.20%. They cannot be hardened by heat treatment and can only be hardened slightly by cold rolling. The body-centered cubic lattice is the reason why ferritic steels are magnetic, unlike all other types of stainless steel.

Although not as strong as martensitic stainless steels, ferritic stainless steels have high corrosion resistance. They are commonly used in kitchenware, industrial machinery and the automotive industry. Ferritic stainless steels are an alternative to austenitic stainless steels and can be used to make thin sheet products such as car exhaust pipes, washing machine drums, containers, buses, train carriages, LCD monitors, microwave ovens and solar water heaters.

Compared with austenitic stainless steels, ferritic stainless steels do not have expensive nickel added, are inexpensive, and are easy to design and process due to their good cold formability. Chromium-rich ferritic stainless steels have good oxidation resistance at high temperatures and can be used to make 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 austenitized) 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. In general, martensitic stainless steel can obtain a yield strength of 550 to 1860MPa after quenching and tempering, and its hardenability increases with the increase of chromium content. These steels are generally large-section air-hardening steels. According to the carbon content, martensitic stainless steels can be divided into two major categories:

1) Low-carbon high-strength martensitic stainless steels: The carbon content is low (about 0.10%), and the chromium content ranges from 11.50% to 18.00%. These are mainly high-strength structural steels with good weldability, formability and impact toughness. They are used in (petro)chemical construction, gas turbine engines, turbine blades, power plants, compressors, discs and aircraft structures and engines, as well as freshwater propeller shafts, and are also widely used in parts such as bolts, valves, cutlery, pump shafts and bearings.

2) High carbon and high hardness martensitic stainless steel: The strength and hardness are increased by increasing the carbon content, but at the expense of weldability, toughness and even corrosion resistance. It also increases the number of carbides that require higher austenitizing temperatures to dissolve, thereby reducing impact properties.

Common applications include knives (containing 0.3% C and 12% Cr, with a hardness of 400 VPN after quenching and tempering); gears, bearings, needle valves and parts 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, with a hardness of 600~700 VPN after tempering). Due to its durability, strength and corrosion resistance, martensitic stainless steel is very suitable for aerospace, defense and electric hand tools.

Austenitic stainless steel

Austenitic stainless steel is the most popular stainless steel, recognized for its high corrosion resistance and temperature stability. It is 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. It has at least 10.5% chromium and 8% to 12% nickel, plus nitrogen and other elements.

Austenitic stainless steel has an austenitic crystal structure, which is a face-centered cubic lattice at both high and low temperatures. Magnesium, nickel and nitrogen are austenite stabilizing elements; "austenite" is generated by adding nickel or nitrogen.

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

Chromium-grade austenitic stainless steel has excellent oxidation resistance and anti-scaling properties, suitable for steam pipes, boiler tubes, heating furnace parts, etc. High-molybdenum austenitic stainless steel is used for offshore platform pipelines, salt water or seawater cooling heat exchanger/condenser tubes in power stations, and pulp and paper industries. They continuously provide seawater for pumps, propellers, valves and other marine equipment. In nuclear power plants, austenitic stainless steel is used for reactor process-related piping. 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 be used at low temperatures. Although its toughness decreases slightly in the low temperature zone, it still maintains high tensile strength. They are widely used in liquefied natural gas (LNG) at a temperature of -161°C and plants that produce liquefied gas.

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 parts, food and beverage, aerospace pipes.

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

Austenitic stainless steel has many beneficial properties, making it 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. Its dual microstructure of ferrite and austenite is obtained by balancing the chromium and nickel equivalent elements. Generally, the Cr content in duplex 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, etc.

Precipitation Hardening Stainless Steels

These steels are a combination of austenitic and martensitic steels, either alone or enriched with copper, molybdenum, aluminum and titanium. Precipitation hardening (PH) stainless steels have a high strength-to-weight ratio and are 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 about three to four times the strength of 304 or 316 austenitic stainless steels.

The PH stainless steel family can be divided into three main types - low-carbon martensitic, semi-austenitic and austenitic. They are used in the oil & gas, nuclear power and aerospace industries, which require high strength, corrosion resistance and lower but acceptable toughness. Special uses include high-speed applications such as turbine blades.

17-4PH stainless steel plate is easy to produce and process, resulting in significant cost savings. Engineers around the world rely on this PH stainless steel to solve pressing problems in product design, manufacturing and processing. Castings with high strength and medium corrosion resistance are also typical uses for 17-4 stainless steel. The required strength and toughness can be controlled by the temperature range during heat treatment.