What Are Stainless Steel Grades and categories?

Different grades of stainless steel are composed of carbon, iron, 10.5%-30% chromium, nickel, molybdenum and other alloying elements. It is a popular metal used in a variety of products, tools, equipment and structures serving many industrial, commercial and domestic applications.

Corrosion resistance is the most valuable property of stainless steel. Chromium is a corrosion-resistant alloying element. This element reacts with oxygen in air and water to form a thin, stable film of chromium oxide that protects metal surfaces. Molybdenum improves corrosion resistance by preventing the formation of pits in the protective film, and the protective layer can be regenerated after slight wear. However, rust can still form when the layer is severely damaged after exposure to chlorides, strong cleaning agents, high salinity, high humidity environments, high abrasion.

Besides corrosion resistance, stainless steel is also known for its excellent mechanical properties such as high strength, toughness, ductility, fatigue strength and wear resistance. Stainless steel can withstand high temperature, high pressure and low temperature environments. They are not reactive to most chemicals; therefore, they are commonly used in chemical processing equipment and containers. They also have a nice, shiny and bright finish.

Stainless steel grades are divided into five main categories:

Austenitic Stainless Steels

By further adding nickel, manganese and nitrogen, austenitic stainless steel has a face-centered cubic (FCC) crystal structure. The FCC structure exists at all temperatures. The nickel content in austenitic stainless steel varies from 8-12%. Their high manganese and nitrogen contents stabilize the austenitic structure. However, the addition of these elements makes them more expensive, and carbon is present in low concentrations.

Ferritic Stainless Steels

Ferritic stainless steels are magnetic and have high thermal conductivity; therefore, they are suitable materials for building boilers, heat exchangers, and other applications involving heat transfer. Their coefficient of thermal expansion is very low, so they are stable over a wide temperature range. Due to this construction, they also have excellent resistance to stress corrosion cracking, which allows them to withstand chlorides, high humidity, and high temperatures.

Martensitic Stainless Steels

Martensitic stainless steel has a body-centered tetragonal crystal structure. They consist of 11.5-18% chromium and 0.1-1.2% carbon. Martensitic stainless steels have higher strength and brittleness due to their higher carbon content. However, the nickel content in martensitic stainless steel is lower, which makes it less resistant to corrosion. Martensitic stainless steel is divided into low carbon martensitic steel (carbon content 0.05-0.25%) and high carbon martensitic steel (carbon content 0.61-1.50%) according to carbon content. Low carbon martensitic steels have better corrosion resistance, while high carbon martensitic steels have higher strength and are more brittle.

Duplex Stainless Steels

The proportions of austenite and ferrite in the duplex stainless steel structure are approximately equal. They are twice as strong as ordinary austenitic and ferritic stainless steels. Their toughness, ductility, and formability are all greater than those of ferritic steels, but never to the level of austenitic steels. Duplex stainless steels have good resistance to ferritic side stress corrosion cracking. The corrosion resistance of duplex stainless steels varies widely as this property depends on its composition; with increasing nickel, molybdenum, and nitrogen content, resistance to pitting and crevice corrosion increases. In terms of cost, duplex stainless steels are still cheap alternatives to austenitic stainless steels.

Precipitation-Hardened Stainless Steels

PH stainless steel is divided into three types: martensitic, austenitic and semi-austenitic PH stainless steel. Austenitic PH steels maintain their crystal structure at all temperatures. Semi-austenitic PH steel remains austenite after solution treatment and quenching; after cryogenic treatment or cold working, the austenite structure is transformed into martensite structure.