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Stainless steel
4-20 usd/kg
Stainless 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.
Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Type 316 (UNS S31600, DIN 1.4401, V4A) is an austenitic stainless steel with added molybdenum (Mo), which enhances corrosion resistance. It is sometimes referred to as ‘marine grade’ stainless, although it is not resistant to warm salty water. The mechanical properties of the alloy are similar to type 304 except that this grade is stronger at elevated temperatures. It contains 16.5-18.5% chromium (Cr), 10-13% nickel (N), 2-2.5% Mo, 2% manganese (Mn), 1% silicon (S), 0.1% nitrogen (N), 0.07% carbon (C), 0.045% phosphorous (P) and <0.015% sulphur (S). It has very good cold forming and machining properties. Its excellent corrosion resistance is utilised in industrial applications, such as paper making, textile production, furnaces, chemical processing and food processing. It is used in architectural facades and panelling, including in marine and seaside environments; cookware, kitchen utensils, vacuum flasks and cups (thermos), saucepans, sinks and cutlery; sanitary ware and troughs. Medical (biocompatible) grades, such as used for implants, typically go through a secondary melting step to improve cleanliness, which helps maximise corrosion resistance and provide a more controlled microstructure. Modified grades: - Type 316L is a low carbon version of type 316 which minimises carbide precipitation, making it more suited to welding applications. - Type 316Ti contains a small amount of titanium (Ti), typically around 0.5%. The titanium atoms help to stabilise the structure of the 316 at temperatures over 800 degC, making it suitable for higher temperature applications. 1.4435 is an austenitic-ferritic chromium-nickel stainless steel that contains molybdenum and nitrogen as additional alloying elements. It is similar to 316L, with slightly higher strength and corrosion resistance, making it more suitable for demanding applications in harsh environments, such as marine and chemical applications.
