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Alloy steel
1-20 usd/kg
The properties of steel are transformed with the addition of alloys – such as chromium (Cr), manganese (Mn) and silicon (Si) – and tempering (controlled heating and cooling cycles in manufacture). While adding no more than 0.05% alloy to a plain carbon steel can almost double its strength, the cost is raised only very slightly. Other alloys, such as copper (Cu) and Cr, are added to improve corrosion resistance and yield materials that can tolerate extremely corrosive environments, or be left outdoors unpainted for more than a century.
Steel is relatively low cost and grades have been developed to suit almost every imaginable application. Its properties are highly tailorable and as a result, it is used in packaging (coated mild steel or naked stainless), automotive (steels with tensile strength of more than 550 MPa are known as advanced high-strength steel, AHSS), furniture, construction, buildings, bridges, heavy duty equipment, manufacturing equipment, laboratory environments and shipbuilding. Its tolerance to low and high temperatures in service depends on the grade, with some tool steels able to withstand extreme loads and shocks, and maintain incredible hardness (equivalent to granite and concrete) at over 500 degC.
Heat treatment (tempering) is a critical step in the production of many high performance steels. It is as important as the ingredients for the mechanical properties of the final part. Typically carried out once forming and welding have been completed, a steel item may be worth many more times the initial cost of the base metal by this point. Therefore, processes have been developed to reduce the risk of distortion, cracking and other defects. It has evolved into a sophisticate and critical step in the production of many types of steel.
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Sustainability concerns
Non-renewable ingredients
Potential conflict commodity (3TG)
Raw material generates polluting by-products
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.