Heat Treatment

Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. It applies only to metals, and is used to alter their physical and sometimes chemical properties.

Common Processes

1. Annealing

Heating the metal to a specific temperature and then allowing it to cool slowly. This softens the metal, makes it more workable (malleable), and relieves internal stresses.

Use: When a piece becomes too hard to work during forging, it is annealed to prevent cracking.

2. Normalizing

Heating the metal above a critical temperature and then cooling it in open air. This refines the grain structure and improves uniformity.

Use: To return the metal to a “normal” state after forging but before hardening.

3. Hardening (Quenching)

Heating the metal to a critical temperature (where it becomes non-magnetic for steel) and then cooling it rapidly by plunging it into water, oil, or brine. This makes the metal extremely hard but also brittle.

Critical: Only steel with sufficient carbon can be hardened. Iron cannot.

4. Tempering

Performed after hardening. The metal is reheated to a lower temperature (often judged by the color of the oxide layer formed, e.g., straw, purple, blue) and then cooled. This sacrifices some hardness to increase toughness and reduce brittleness.

Essential: A hardened blade without tempering will shatter like glass upon impact.

5. Case Hardening

Used for low-carbon iron or steel. The outer surface is infused with carbon (using bone, leather, or charcoal) and then hardened, leaving a tough, ductile core.

Quenching Media

Water: Fast cooling. Maximum hardness, high risk of cracking.
Vegetable Oil: Slower cooling. Good hardness, lower risk of cracking.
Brine (Salt Water): Very fast cooling. Used for specific steels.
Air: Slow cooling. Used for normalizing or air-hardening steels.

Forging
Metallurgy