Martempering is a metallurgical process used to control the characteristics of martensite in steels and alloys. Martensite is a crystalline state that makes the metal hard but brittle. The process involves heating and cooling cycles to gradually reduce the extent of martensite characteristics while minimizing distortion and cracking. Tempering martensite involves heating and cooling it in stages to retain some of the original hardness while avoiding brittleness and distortion. The process is done in salt, lead, or oil baths at temperatures from 400° to 500°F (200°-260°C).
Martempering is a metallurgical production process intended to control the characteristics of martensite in steels and alloys. Martensite is one of the crystalline states induced in metals by thermal manipulation and which makes the metal exceptionally hard. Metals consisting of unique martensite structures are often too hard and brittle and require the martensite characteristics to be reduced to usable levels. The martensite process is used to manipulate 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 advantageous to start the process with a high level of martensite formation and gradually reduce the level as the process minimizes distortion and cracking of the metal.
Iron and its alloys undergo a variety of changes in crystal structure when heated. These changes of state are achieved by controlling the temperature to which the metal is heated and its cooling rate. These crystalline states include austenite, martensite, pearlite and bainite; each has particular working characteristics. Martensite is a crystalline state characterized by extreme hardness which, while a desirable characteristic, is generally accompanied by brittleness, distortion, and a tendency to crack. To compensate for these negative factors while maintaining some degree of latent hardness of martensite, the quenching process is applied to manipulate the crystalline structure of the metal.
The martempering process is based on the fact that heating martensite above its point of formation destroys its structure. Tempering martensite involves heating a complete, pure martensite structure and cooling or quenching it in stages until it reaches the desired degree of hardness. In other words, the process gradually destroys the structure of the martensite, thus avoiding brittleness and distortion while retaining some of the original hardness. Getting the balance right is often difficult because too little or too much quenching makes the metal brittle or soft and would require the entire process to be repeated.
Tempering of martensite is generally done in salt, lead or oil baths at temperatures from 400° to 500°F (200°-260°C). Once the part has reached bath temperature, it is then allowed to cool to room temperature. The extent to which the part is heated then determines how much of the original martensite structure is retained. Heating times for martempering vary according to the size of the piece and in most cases are only the time necessary for the piece to reach the temperature of the bath.
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