Quench & Temper

Achieve specific mechanical properties
When a part is expected to provide specific levels of strength and toughness, it is usually put through a quench-and-temper process, which by industry convention is usually simply referred to as heat treating.

The first step in the quench-and-temper is to heat the parts to a uniform temperature above the transformation range, usually about 1650º F. The hot parts are then quenched in a liquid, usually oil (for medium carbon plate or alloy plate like 1045, 4140, or 4340) or water (for low carbon alloy grades like A514). The rapid cooling imparts a very high degree of hardness, often 600 Bhn or more. Some grades, such as 4140, are often hardened by quenching in rapidly moving air; this process is also known as normalize-and-temper.

As-quenched, the parts are too brittle to be used, and are returned to the furnace for another cycle, this time at a lower temperature that will reduce or temper the hardness and restore some ductility.

The final surface hardness that can be achieved in this process is dependent on the carbon content of the steel. The alloy content dictates the depth of that hardness and the ultimate mechanical properties that can be expected. A 1045 part can have the same surface hardness as one cut from 4340, but the core hardness and tensile strength of the two parts will be quite different.