Alloy steel
1-20 usd/kgThe 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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Cold work tools steels includes oil-hardening (O), air-hardening (A) and high-carbon, high-chromium (D). Like water-hardening tool steel (W), they are similar to high-carbon steels – 0.9-1.45% carbon (C) – with the addition of small amounts of tungsten (W), manganese (Mn), chromium (Cr) and molybdenum (Mo). Compared to W-series steels, these alloys enable oil-hardening, which reduces distortion and so increases dimensional accuracy. Many different formulations are possible, with each offering a unique balance of properties tailored to an application. They are used for short-run tooling applications that require very high wear resistance, such as blanking and forming dies, gauges, drawing and piercing dies, reamers, taps, plastic moulds and mandrels. They are suitable for intricate tooling and gauges, that require hardening with minimal dimensional change and without cracking.
The O-series is the least expensive of this group. They form graphite and martensite in the hardened structure. The graphite acts as a lubricant and creates good machining properties. The addition of tungsten (W) forms tungsten carbide (WC), which results in a harder surface, improving wear resistance and edge retention. The different grades are as follows:
– O1 (UNS T31501, DIN 1.2510) contains Mn, W and Cr. They are relatively inexpensive. In the annealed condition, they can be formed by conventional methods, welded with care, and have very good machining properties.
– O2 (UNS T31502, DIN 1.2842) contains Mn.
– O6 contains (UNS T31506) Mn, Mo, and Si. The presence of graphite in the hardened microstructure means it has the best machining properties of this type.
– O7 (UNS T31507) contains Cr, Mn and slightly more W than O1. It has the highest wear resistance, but hardenability is low.