Heat treatment is the process of heating metal below its molten stage, and then cooling the metal in a controlled way to select desired mechanical properties. PURPOSES (OR) OBJECTIVES OF HEAT TREATMENT:

• To harden and strengthen metals.
• To relieve internal stress.
• To improve machinability.
• To improve ductility and toughness.
• To increase wear and corrosion resistance of metals.
• To improve electrical and magnetic properties.
• To refine grain size.

Hardening or (Quenching) treatment consists of heating the steel to a hardening temperature, holding at that temperature for a particular time followed by rapid cooling in water, oil or brine solution.

Tempering is a heat treatment followed after hardening and involves heating the hardened steel to some temperature below the lower critical temperature soaking at this temperature for sufficient time followed by slow cooling in air. The purpose of tempering is to relieve the residual stresses and improve ductility and toughness of the hardened steel. Types of tempering based on heating temperature:
A. Low temperature tempering (1-2 Hours at a Temperature up to 250°C)
– This treatment is given normally to tools of plain carbon and low alloy steels, to develop high cutting-ability, wear, and abrasion resistance with some toughness.
– Reduces the brittleness of steel and increases the toughness.
B. Medium temperature tempering: (350 C to 500°C)
– Increase of ductility and toughness.
– Decrease in hardness and strength.
C. High temperature tempering(500-650°C)
– This treated steel has better tensile yield and impact strength and is free from internal stresses.
– This structure is very soft and tough.

Heat treating is a part of the knife making process that is designed to help harden the steel of the blade for use. The purpose of heat treating a knife is to harden the steel sufficiently for use.

The correct degree of hardness will depend on the blade’s intended purpose. It needs to be hard enough to maintain its edge yet flexible enough to endure regular and sometimes intense use.

Heat treatment for knife makers is done with a forge, furnace, or in a heat-treating oven. They all basically serve the same function: heating a blade to a critical temperature. The better the forge, the faster and more controllable this can be done.

Electric heat-treating ovens are available for knifemakers that want a little more control over the heat-treating process. For materials like stainless steel, they are almost mandatory. The size of the heat-treating oven and type of controllers are determined by the size of the blades you want to heat treat and the knifemakers budget. Keep in mind that a good quality, heat-treating oven will give you more control and better temperature accuracy which results in more consistent blade hardness.