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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