The most common metal is steel which is available in many different alloys. The most common type of steels are:
1: Those which may be grouped under the heading "mild steel" and generally contain less than 0.25% carbon.
2: Those classed as medium to high carbon which contain carbon over 0.25%.
The through hardening process is used on medium and high carbon steels. Case hardening is used on mild steels.
medium carb. 0.25% - 0.5%
Hardening occurs during heat treating when the steel (containing sufficient carbon) is cooled rapidly (quenched) from above its critical temperature. This temperature varies for different alloys but generally is in the range
A mild steel treated in this manner would show no appreciable increase in hardness. In order to make this steel react its carbon content must be increased. In the case hardening process the surface layer of the mild steel has its carbon content increased by a prolonged contact at a high temperature with a chemically reactive source of carbon. If this steel is subsequently quenched it will harden the surface layer, also known as the case, -hence "case hardening". The case depth will generally be from two thousandths of an inch up to one hundred and fifty thousandths of an inch. However the most common depth is in the range .020" - .030".
lt is important to choose the correct steel and the appropriate process to achieve the desired end result. For moderate strength with moderate surface hardness choose a medium carbon steel - through hardened.
For a very hard surface where a lower core strength is acceptable choose a low carbon steel - case hardened.
MILD STEEL CANNOT BE THROUGH HARDENED
MED. & HIGH CARBON STEEL CANNOT BE CASE HARDENED
(although other surface treatments are available)