Iron and steel may be classified on the basis of composition, use, shape, method of manufacture, etc. Some of the more important ferrous alloys are described in the sections below.

Ingot iron is commercially pure iron and contains a maximum of 0.15% total impurities. It is very soft and ductile and can undergo severe cold-forming operations. It has a wide variety of applications based on its formability.

Its purity results in good corrosion resistance and electrical properties, and many applications are based on these features. The average tensile properties of Armco ingot iron plates are tensile strength 320 MPa (46,000 lb/in2); yield point 220 MPa (32,000 lb/in2); elongation in 8 in, 30%; Young’s modulus 200 GPa (29 # 106 lb/in2).

Plain carbon steels are alloys of iron and carbon containing small amounts of manganese (up to 1.65%) and silicon (up to 0.50%) in addition to impurities of phosphorus and sulfur. Additions up to 0.30% copper may be made in order to improve corrosion resistance.

The carbon content may range from 0.05% to 2%, although few alloys contain more than 1.0%, and the great bulk of steel tonnage contains from 0.08% to 0.20% and is used for structural applications.

Medium-carbon steels contain around 0.40% carbon and are used for constructional purposes—tools, machine parts, etc. High-carbon steels have 0.75% carbon or more and may be used for wear and abrasion-resistance applications such as tools, dies, and rails.

Strength and hardness increase in proportion to the carbon content while ductility decreases. Phosphorus has a significant hardening effect in low-carbon steels, while the other components have relatively minor effects within the limits they are found.

It is difficult to generalize the properties of steels, however, since they can be greatly modified by cold working or heat treatment.

High-strength low-alloy steels are low-carbon steels (0.10% to 0.15%) to which alloying elements such as phosphorus, nickel, chromium, vanadium, and niobium have been added to obtain higher strength.

This class of steel was developed primarily by the transportation industry to decrease vehicle weight, but the steels are widely used. Since thinner sections are used, corrosion resistance is more important, and copper is added for this purpose.

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