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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
Raw material generates polluting by-products
Weathering steel is classed as high strength low alloy (HSLA) steel. It contains up to 0.2% C and small amounts of alloy – importantly copper (Cu), nickel (Ni), manganese (Mn) and silicon (Si) – totalling no more than about 3-5%. The unique property of these steels is the formation of an iron oxide (rust) layer on the surface when exposed to weathering, which provides protection for the underlying material. The Cu migrates to the surface to form an orange-brown oxide layer. After which the Mn, Si and Ni help to bind the layer and for a protective barrier. Left unpainted, weathering steel structures will last more than a century with minimal maintenance. Compared to painted steel, they can offer a cost effective solution, with lower embodied energy and CO2. However, it is important that the structure is designed to allow surfaces to be both wet and dry, otherwise there is a risk that the oxide layer will not develop and the base metal will be unprotected. For example, if puddles are allowed to form in crevices or on the surface, the oxide layer cannot develop. It is also important to consider run-off, because the rust can discolour the surrounding area.
These steels can be formed, welded (the colour harmonises with time) and machined, similar to plain carbon steel. They have higher strength-to-weight than plain carbon steel and are used in structural and demanding applications including buildings, bridges, sculpture, rail cars and heavy-duty machinery and vehicles.
Some notable grades and specifications:
– A242 (UNS K11510, DIN 1.8946), the regional ASTM standard for Cor-Ten (Corten), also known as Cor-Ten A, is used for thin gauge applications, such as coil, sheet and wire. It is available in thickness up to 12.7 mm and has slightly different ingredients, compared to A588, to promote additional corrosion resistance. Otherwise, the two are very similar.
– A606 is HSLA sheet or strip that has been hot or cold rolled. It is used for corrugated roofs and fabricated structures, for example.
– A588 Grade A (UNS K11430), also known as Cor-Ten B, is available as plates, shapes, beams, channels and bars – thicker sections than A242 and A606, from approximately 5 to 130 mm. It is used to make welded bridges and buildings, and train cars.
– A709 (S355J2W, DIN 1.8965), also known as Patinax 355, is another type of weather steel used in similar applications. And S355J2G2W, also known as Patinax 355P, which contains more phosphorous for greater resistance to corrosion – similar to A242.
– A847 is a cold-formed HSLA steel formed into square, rectangular or circular pipe and tube. It is cold formed to ensure chemical consistency throughout the tube.
