Stainless steel
4-20 usd/kgStainless steels have very good corrosion resistance compared to carbon steels, coupled with impressive mechanical properties. This combination is the result of the addition of a relatively high proportion of alloys, in particular chromium (Cr). They tend to be more expensive, and so reserved for applications that demand their superior properties.
Based on their crystalline structure, they are grouped in five families: ferritic, austenitic, martensitic, precipitation-hardening (PH) and duplex. Ferritic stainless steel is not seen much outside industrial applications, automotive and white goods, for which it is well suited. The lower alloy content makes it less expensive than the more common austenitic types.
Austenitic stainless steel, which includes the very popular 300 series, is a good all-rounder with high resistance to corrosion. It is non-magnetic (which makes it easy to distinguish from other steels), with excellent formability, machining properties and weldability.
Martensitic stainless steel has high hardness and strength, good for industrial applications, household appliances and some knife blades, with moderate resistance to corrosion. Its high hardness makes it relatively more difficult to form, cut and weld.
Duplex types are a modification on austenitic grades, designed to provide higher strength with comparable or improved corrosion resistance. PH stainless steels are a family of corrosion resistant alloys, which can be heat treated to tensile strength several times greater than austenitic types. They are used in very demanding applications in aerospace, off-shore and nuclear energy, for example.
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Super duplex stainless steels contain around 25% chromium (Cr) and 7% nickel (Ni). This extends the range of possible applications compared to lean and standard types of duplex steels. Example grades include LDX2101 (UNS S32101), F255 (UNS S32550), 2507 (UNS S32750) and Z100 (UNS S32760).
The high level of Cr provides excellent corrosion resistance, which is why these alloys are used in place of the 300 series austenitic grades for some demanding industrial, marine and subsea applications, such as in oil and gas, chemical processing and manufacturing. While the price per kg may be similar to 300 series austenitic grades, the higher mechanical properties mean that it can potentially save cost over large projects. However, production quantities and distribution is not so high, which limits potential applications.