Martempering is a metallurgical production process intended to control martensite characteristics in steel and alloys. Martensite is one of the crystalline states induced in metals by thermal manipulation and one which renders the metal exceptionally hard. Metals consisting of exclusive martensite structures are often too hard and brittle and require a reduction of the martensite characteristics to usable levels. The process of martempering is used to manipulating martensite levels and consists of heating and a sequential series of cooling cycles which gradually reduce the extent of martensite characteristics in the metal. It is beneficial to begin the process with a high level of martensite formation and to reduce the level gradually because the process minimizes distortion and cracking of the metal.
Iron and its alloys experience a variety of crystalline structure changes as they are heated. These state changes are achieved by controlling the temperature to which the metal is heated and its rate of cooling. These crystalline states include austenite, martensite, pearlite, and bainite; each possesses particular working characteristics. Martensite is a crystalline state characterized by extreme hardness which, although a desirable characteristic, is generally accompanied by brittleness, distortion, and the inclination to crack. To offset these negative factors while retaining a degree of the latent hardness of martensite, the process of martempering is applied to manipulate the crystalline structure of the metal.
The martempering process is based on the fact that reheating martensite to above its formation point destroys its structure. Tempering martensite involves heating a pure, complete martensite structure and cooling or quenching it in steps until the desired degree of hardness is reached. In other words, the process destroys the martensite structure by degrees, thereby avoiding brittleness and distortion while maintaining some of the original hardness. Getting the balance correctly is often tricky because too little or too much tempering leaves the metal brittle or soft and would require repeating the whole process.
Tempering martensite is generally carried out in salt, lead, or oil baths at temperatures of 400° to 500°F (200°–260°C). Once the part has reached the bath temperature, it is then allowed to cool to room temperature. The extent to which the part is heated then dictates how much of the original martensite structure is maintained. Martempering heating times differ according to the size of the part, and in most cases are only as long as it takes for the part to reach bath temperature.