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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A range of stainless and alloy steels are converted into powder suitable for metal additive manufacturing (MAM) – such as direct metal laser sintering (DMLS) and selective laser melting (SLM) – and metal injection moulding (MIM). These processes are used to make prototypes, as well as short to medium production runs of functional parts. The properties of 3D printed and MIM parts are impressively close to regular metals – up to around 95% after thermal treatments – although performance is greatly dependent on the density of the build, with higher density parts achieving much better mechanical properties. It is possible to produce parts with very high surface quality. In addition, the surface may be blasted, polished or coated with physical vapour deposition (PVD) – all typical steel finishing processes may be used.
Steel powder is produced by atomisation, whereby refined and degassed molten alloy is poured into a gas nozzle and disintegrated into metal powder by a high-pressure gas stream. This adds an estimated 15-20 MJ/kg of energy to the raw material production process.
Some notable types and grades:
– 316L is a low carbon version of austenitic type 316 (UNS S31600, DIN 1.4401, V4A). The reduced carbon is important for the sintering process and improves pitting resistance. 316L is also known as A4, or marine grade, it is marginally more expensive than 304L. It has excellent resistance to corrosion (although not to warm salty water) and combines high strength with high ductility (toughness) and good thermal properties. It is suitable for a wide range of applications, including food contact, jewellery, dental, watches, machine components, automotive, aerospace and medical instruments.
– 304L is a low carbon version of austenitic type 304 (UNS S30400, DIN 1.4301). The reduced carbon is important for the sintering process and improves pitting resistance. Slightly less expensive than 316L powder, 304L is a versatile metal with a combination of good mechanical properties, corrosion resistance and heat resistance. It is resistant to most chemicals and is durable in typical food processing environments; qualities utilised in commercial and domestic kitchens, industrial food production, cookware, tableware and packaging. Other applications include parts for products, buildings (interior and exterior), automotive, machinery, furniture and lighting.
– 17-4PH is a chromium-nickel-copper, martensitic precipitation hardening stainless steel, also known as alloy 693 (A693 Tp630, FV 520, UNS S17400, DIN 1.4542). It can be heat treated to high levels of strength and hardness, and its machinability and corrosion resistance are equivalent to type 304. It is used in demanding applications in aerospace, jet engines, rockets, missiles, chemical, petrochemical, food processing, paper and general metalworking industries.
– 8620 (UNS G86200, DIN 1.6523) is a popular carburising steel – during heat treatment it absorbs carbon, which increases hardness in the surface layer. It is suited to applications that require a combination of toughness and wear resistance. It is used in demanding automotive, aerospace and construction applications where it is helpful to have a steel that can be readily machined and hardened to controlled case depth.
– Tool steel powder, or high speed tool steel (HSS) powder, is based on high carbon steel with tungsten (W), molybdenum (Mo), chromium (Cr), vanadium (V) and for specific application cobalt (Co), which boost the steels capacity for retaining a high level of hardness while cutting metal at high speed and elevated temperatures. It is used for industrial cutting tools, such as drills, tapers and reamers. It is specialist and expensive.