Quenching and tempering

Quenching

The choice of the most adequate quenching medium basicaly depends on the type of steel, and the mechanical properties that are being aimed at. The following are some of the treatment that use salt baths as quenching medium.

Martempering: martempering is indicated when there is the need of obtaining parts with minor dimensional variations after the heat treatment. This treatment consists in the heating of the parts until its complete austenitization, quick quenching until the temperature just above the martensintic transformation zone (Ms), keeping this temperature for a period of time that allows the equalization of the temperatures in the core and the surface, followed by an air cooling. This way, the post hardening distortions are minimized and the properties achieved are those obtained from the martensitic structure. The temperature used in the first quenching (salt bath) depends directly on the material to be treated and the geometry of the parts to be martempered. The choice of the adequate martempering salt depends mainly on the type of steel treated, the austenitization temperature and the heating medium used. After the martempering, the parts must go through the tempering treatment.

Austempering: austempering is an isothermal treatment used when tenacity, ductility and mechanical resistance are aimed, properties obtained from the bainitic structure. This treatment consists of a heating for complete austenitization and a following quenching done in two stages, the first usually a salt bath until the temperature of isothermal transformation of austenite into bainite, followed by a posterior air cooling. The isothermal transformation temperature depends of the chemical composition of the steel and on the final hardness desired, and the time in this temperature must be enough to ensure a complete bainitic transformation. The austempering salt choice usually depends on the lowest temperature necessary at the quenching salt bath.

Tempering

Tempering is a heat treatment conducted after hardening with the purpose to reduce the fragility of the material and tensions of the martensitic transformation due to the quick quenching at the hardening. This treatment is based on the heating of the hardened material until a certain temperature, keeping at this temperature for a determined period of time, and posterior cooling which is usually done in air, but sometimes also in water. The tempering in salt baths has been chosen in several occasions due to the great homogeneity of the temperatures.