According to metallurgy, stainless steel can be explained as a metal alloy that contains about 30% of chromium (Cr) and approximately 50% of iron (Fe). It is named as 'stainless' due to its highly stain resistant properties. The formation of an invisible chromium-rich oxide surface film gives stainless characteristics to steel. Other alloying substances like copper, nickel, aluminum, molybdenum, nitrogen, silicon and titanium are added to enhance stainless properties of steel. The amount of carbon in stainless steel ranges from 0.2% to 2.14%. Mechanical properties and corrosion resistance are the major criteria to decide the grade of stainless steel. Note that, there are more than 150 grades of stainless steel.
Types and Properties
There are five major types of stainless steel such as Ferritic, Martensitic, Precipitation hardening, Austenitic, and Duplex (ferritic-austenitic). Ferritic stainless steel contains chromium with a crystal structure (body centered cubic, bcc) called ferritic. It consists of about 30% of chromium. It is known to have ferromagnetic properties as well as formability and ductility. But, it lacks in high-temperature mechanical properties. Martensitic stainless steel is a mixture of carbon and chromium. It can be hardened by the heat treatments. It is known to have ferromagnetic properties and less corrosion resistance. Martensitic stainless steel consists of about 18% of chromium and about 1.0% of carbon. It is used for razor blades, knives and corrosion-resistant bearings.
Precipitation hardening stainless steel contains a mixture of nickel and chromium. This type of steel has the properties of austenitic or martensitic steel in the normalized condition. Precipitation hardening of the martensitic structure gives high strength to stainless steel. Austenitic stainless steel consists of face centered cubic (fcc) crystal structure. It is formed by using nitrogen, manganese and nickel. It contains about 16-26% of chromium and less than 35% of nickel. Austenitic stainless steel is tough, ductile with cryogenic and high temperature strength properties. This type of steel is used for a wide range of applications such as kitchen sinks, commercial food processing equipment, building facades and chemical plant piping. Duplex stainless steel constitutes a combination of fcc austenite and bcc ferrite crystal structures. Most duplex stainless steels consist of equal amounts of austenite and ferrite. Chromium and nickel are the main alloying elements in duplex stainless steel. It is corrosion resistance as well as stress corrosion cracking resistance.
Corrosion resistance is a peculiar feature of stainless steel. Chromium is the main element that prevents corrosion of steel. It reacts with oxygen and forms an invisible, adherent layer of chromium oxide on the surface of stainless steel. Increased content of chromium in stainless steel improves corrosion resistance properties of steel. The addition of nickel enhances corrosion resistance properties in case of aggressive usage. The addition of molybdenum gives localized corrosion resistance against scarring. Other alloying metals like copper, titanium and vanadium are also added in order to improve the properties and structure of stainless steel.
Ductility is another important property of stainless steel. It is measured by percentage elongation. Austenitic stainless steel possesses high elongation, about 60-70%. Hardness of this steel is measured by Rockwell, Brinell, or Vickers hardness testing machines. Hardness shows resistance to penetration. Heat resistance of stainless steel varies depending upon the type of steel. Grade 316H is the most resistant to high temperatures.
While selecting stainless steel, the following criteria should be considered. It should be tough,ductile, corrosion resistant, resistant to sulfidation and oxidation. It should have cryogenic strength, magnetic properties, elevated temperature strength, thermal conductivity, electrical resistivity, surface finishing, resistance to erosions and/or abrasion, resistance to seizing and/or galling and suitability to intended cleaning procedures and fabrication techniques.
Stainless steel is used to manufacture cookware, surgical instruments, plates, sheets, bars, wire, kitchen cutlery, industrial equipment, building construction materials and hardware. It is also used in commercial kitchens and food processing plants.