Steel is improved by purification (reduction of accompanying elements), carburisation (bringing the C content to the right level) and alloying (fusing the steel with alloying elements).
Steel are classified into unalloyed and alloyed types, depending upon the content of alloying components in them. Unalloyed steels are further classified into basic steel, high grade steels and super refined steels. Alloyed steels are classified into high grade and super refined steels only.
Basic steels are types of steel whose guaranteed properties, such as tensile strength, elastic limit and breaking load, lie within specified limits.
High grade steels are carefully produced with respect to surface quality, weld ability, cold and hot forming characteristics. They may be alloyed or unalloyed.
Superrefined steels are alloyed steels and unalloyed steels which are distinguished from high grade steels by virtue or greater homogeneity and near freedom from non metallic impurities (P and S maximum 0.035%). They are smelled with the greatest care and therefore have particularly homogeneous structures.
Depending upon the use, one distinguishes between, case hardening and heat treatment steel, high speed steel, heat acid and rust resistant steels and spring steel. Vacuum steels are particularly pure steels.
Unalloyed steel is steel which does not contain the following elements in proportion higher than those specified (C does not count as an alloying component here):
0.5% Si, 0.8% Mn, 0.1% Al or 0.1% Ti or 0.25% Cu.
Low alloyed steels contain more than 5% alloying component
High alloyed steels contain more than 5% alloying components. They must not, however, contain more than 0.045% of phosphorus (P) and sulphur (S) taken together.
Cast steel is steel which lies been cast in moulds. Its mechanical strength is greater than that of gray cast iron and annealed cast iron. If the steel is to be used for workpieces subject to high stress alloying metals are added to the melting charge. The material cannot be distinguished from forged steel. Cast steel, however, shrinks twice as much as cast iron (2% as against 1%). The casting must therefore be more uniform in cross section and smoother in moulding than cast iron parts.
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