A line from classical poetry refers to the "Hero's well annealed blade".
Clearly the meaning of "annealed" has changed over the years. Our hero would have expected his blade to be hard and resilient, while today an annealed steel is soft and ductile.

Other terms are sometimes misunderstood and may lead to confusion between a heat treater and a client. Terms such as "hard as possible'', ''maximum toughness'', "case harden” have different meanings to different people. In particular the terms "hard" and "tough" which are sometimes used interchangeably may have opposite meanings.

In the hope that this difficulty can be alleviated, we offer the following definitions which refer to plain carbon and low alloy steels: -

Annealing is a general term which refers to a variety of processes, whose aim is to produce a softer metal. An annealed microstructure may be produced by:

1. Heating above, and then slowly cooling through, the transformation temperature (full anneal)
2. Heating to above 1150oF followed by slow cooling - A process used n low carbon steels (process or stress relief anneal)
3. Holding in the range l240o-I300oF
   - for high carbon steels to produce a spheroidized microstructure (spheroidize anneal)
4. The rapid cooling (water quenching) of the 300 series stainless steels from around 2000oF. (solution annealing)

Hardening - Produced in most steels by rapid cooling from above their respective transformation temperatures.
Steel is rarely used in its fully hardened condition because its associated brittleness makes it unsuitable for most purposes. Hardening is almost always followed by tempering - an operation which restores some toughness at a cost of some loss of hardness. Any tempering operation involves a degree of stress relieving - the residual stresses produced during hardening are relieved as the tempering temperature increases the mobility of the microconstituents The higher the temperature, the greater the stress relief, and generally speaking, the softer the steel.

Case Hardening - is the process which gives a hard surface to mild (low carbon) steels. It is done by maintaining a carbon-rich atmosphere in contact with the mild steel while maintaining the temperature generally in the 1550o-1700oF range, followed by rapid cooling. The carbon combines with the mild steel surface to produce a thin high carbon layer which hardens when quenched.
Case hardening should not be used on steels which are already high in carbon as the surface stress pattern is adversely affected. Case hardened mild steels have compressive residual surface stresses, while higher carbon steels may have tensile residual surface stresses. (i.e. under no-load conditions, they are already trying to tear apart.)

Hardness is a measure of the ability of a metal to withstand wear.

Toughness indicates resistance to failure under conditions of shock loading.

A correctly hardened and tempered component will have the best combination of both properties for its particular application.