Stainless Steels
Stainless steels are iron alloys with a minimum of 10.5% chromium. Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness. These include metals such as:
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Nickel
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Molybdenum
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Titanium
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Copper
Non-metal additions are also made, the main ones being:
- Carbon
- Nitrogen
Stainless steel does not readily corrode, rust or stain with water as ordinary steel does, but despite the name it is not fully stain-proof, most notably under low oxygen, high salinity, or poor circulation environments. There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and resistance to corrosion are required.
Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide, and due to the greater volume of the iron oxide this tends to flake and fall away. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure, and due to the similar size of the steel and oxide ions they bond very strongly and remain attached to the surface.
Stainless Steel - Comparison of Grade Specifications
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Stainless Steel Properties
| Precipitation Hardening Stainless |
The main characteristics of precipitation hardening stainless specifications are the combination of the corrosion resistance of an austenitic stainless with the hardenability of a martensitic stainless.
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| High Alloyed Austenitic Stainless |
High contents of chromium, nickel, molybdenum and nitrogen give high alloyed austenitic stainless steel grades different characteristics to those of standard austenitic specifications. 904L has similar mechanical properties to the more common austenitic grades but has superior resistance to uniform corrosion. UNS S31254 provides exceptionally good resistance to uniform corrosion combined with higher strength and mechanical properties than the more common austenitic grades
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| Austenitic Stainless |
With the average chromium content of 18% and nickel content of 8% austenitic stainless steels are sometimes referred to as 18/8 (although the actual chromium and nickel content can vary from these figures). Austenitic stainless steels are non magnetic and cannot be hardened by heat treatment, though they can strain harden rapidly when cold worked.
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| Martensitic Stainless |
Heat treated martensitic stainless steel grades offer high strength and hardness. Martensitic grades can be heat treated due to having higher carbon content than other types of stainless steel. Martensitic grades are less ductile than other types of stainless steel. These grades are magnetic and are best suited for applications which require good corrosion resistance with good mechanical properties.
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| Duplex & Super Duplex Stainless |
Duplex and Super Duplex stainless steel grades offer resistance to stress corrosion and crevice corrosion. They are higher strength grades than standard austenitic stainless steel specifications.
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| Heat Treatment |
Heat treatment is only suitable for specific grades of stainless. When heat treating a stainless the hardening temperatures, including rate of heating, cooling and soaking times will vary due to factors such as the shape and size of each component. Other considerations during the heat treatment process include the type of furnace, quenching medium and work piece transfer facilities. Please consult your heat treatment provider for full guidance on heat treatment of stainless steel grades. |
