Steel is the most extensively used material for machine components. By suitably
varying the composition, thermal treatment, and mechanical treatment, manufacturers
can obtain a tremendous range of mechanical properties. Three basic relationships
are fundamental to the appropriate selection of steel composition.
1. All steels have essentially the same moduli of elasticity. Thus, if rigidity is the
critical requirement of the part, all steels perform equally and the least costly
(including fabricating costs) should be selected.
2. Carbon content, almost alone, determines the maximum hardness that can be
developed in steel. Maximum potential hardness increases with carbon content
up to about 0.7 percent. This means that relatively small, regularly shaped parts
can be heat-treated to give essentially the same hardness and strength with plain
carbon steel as with more costly alloy steels.
3. Alloying elements (manganese, molybdenum, chromium, nickel, and others)
improve the ease with which steel can be hardened. Thus, the potential hardness
and strength (which is controlled by carbon content) can be realized with less
drastic heat treatments when these alloying elements are used. This means that
with an alloy steel (a) parts with large sections can achieve higher hardnesses inthe center or core of the section, and (b) irregularly shaped parts, subject to
warpage during a drastic quench, can achieve the desired hardness from a more
moderate heat treatment.