Stainless Steel Properties

Stainless steel key

Stainless steel is a type of alloy steel that resists rust and other forms of corrosion and has an attractive appearance.

Elements

Stainless steel contains chromium which provides the unique stainless and corrosion resisting properties.

Self Protective

Stainless steel has a unique self-healing property. Due to the alloying elements used, a thin, transparent layer is formed on the surface. In case the surface is scratched or damaged otherwise, this thin layer, which is only a few atoms thick, immediately rebuilds with the assistance of oxygen from air or water. This is the reason why stainless steel does not require any coating or other corrosion protection to remain bright and shiny even after decades of use.

Alloys

For a list of common alloys click here

Types

There are five major types of stainless steel.

Austenitic stainless steels are 200 series or 300 series. They are non magnetic and have high corrosion resistance. They can be hardened only by cold working - not by heat treatment. They are highly formable. But, they are prone to stress corrosion cracking. There are three subtypes: straight, L and H. Popular straight types are grades are 201, 202, 301, 302, 303, 304, 305, 308, 309, 310, 314, 316, 317, 321, 347, 348 and 384. L types have higher corrosion resistance than the straight types. Examples include 304L and 316L. H types are suitable for use in high temperature environments. Austenitic stainless steels are used in shafts, valves, bolts, bushings, nuts, aircraft fittings, chemical equipment, food processing equipment, brewing equipment, cryogenic vessels etc.

Ferritic stainless steels are 400 series. They are magnetic. They have lower ductility and lower corrosion resistance than the austenitic grades. But, they offer high resistance to stress corrosion cracking. They are hardened by cold working and are not heat treatable. Some of the popular grades are 405, 409, 430, 434, 442, 436 and 446. Typical applications are heat exchangers, automotive fasteners, furnace parts, heater parts.

Martensitic stainless steels are 400 series and 500 series. They are magnetic. They have higher strength, higher wear resistance and higher fatigue resistance than the austenitic and ferritic grades. They can also be heat treated. But, corrosion resistance is moderate and lower than the austenitic and ferritic grades. They are highly machinable. Some of the popular grades are 410, 414, 416, 420, 440 and 431. Major applications are machine parts, pump shafts, bolts, bushings, coal chutes, cutlery, hardware, jet engine parts, mining machinery, rifle barrels, screws, valves, aircraft fittings, fire extinguisher inserts, rivets.

Precipitation Hardening Grade Stainless Steels are also called PH types. Their corrosion resistance is equivalent to that of austenitic grades and strength is generally higher than that of martensitic grades. They also retain high strength at elevated temperatures. They are heat treatable. They are mainly used in aerospace industry for aerospace structural components. Popular grades include 17-4PH and 15-5PH.

Duplex stainless steels are mixture of austenitic and ferritic. They provide higher corrosion resistance than the austenitic stainless steels. They are also more resistant to stress corrosion cracking than the austenitic stainless steels. They have higher strength than the austenitic grades. Popular UNS duplex grades include S32101, S32304, S32003, S31803, S32205, S32760, S32750, S32550, S32707 and S33207. Typical applications are water treatment plants and heat exchanger components. 

Advantages

The lack of need for a finish reduces delivery time of stainless parts. However parts cost more to machine due to their hardness. Stainless parts offer higher strength and corrosion resistance. The metal usually remains bright and shiny even after decades of use. Stainless steel also retains strength and shock resistance even at high temperatures. It is hygienic due to the smooth and minimally porous surface. Most alloys are non-magnetic. Stainless steel has excellent fatigue and impact resistance. Products made with the metal generally last a long time. 

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