Tempering is a process of annealing Martensite at low temperatures.

Tempering is a heat treatment process that is used to improve the mechanical properties of steel, particularly its toughness and ductility. It is performed after the steel has been hardened through quenching, which involves rapid cooling to form a hard and brittle phase called Martensite. Tempering is necessary because Martensite is extremely brittle and can easily fracture under stress. By tempering the steel, the hardness is reduced, and the toughness and ductility are improved.

Why is tempering necessary?

When steel is quenched to form Martensite, it undergoes a rapid transformation from austenite to a hard and brittle phase. This phase is extremely strong but lacks toughness and ductility. The high internal stresses within the Martensite structure make it susceptible to cracking and failure under impact or heavy loads. Therefore, tempering is necessary to relieve these stresses and improve the steel's mechanical properties.

The tempering process

The tempering process involves heating the hardened steel to a specific temperature below its lower critical temperature and holding it at that temperature for a certain period of time. The exact temperature and time depend on the desired mechanical properties of the steel. During tempering, the Martensite undergoes a transformation, known as tempering transformation, which leads to the formation of small carbide particles within the Martensite matrix.

Tempering of Martensite at low temperatures

The tempering of Martensite at low temperatures, typically between 150°C and 250°C, is commonly referred to as low-temperature tempering. During this process, the small carbide particles precipitate within the Martensite matrix, acting as obstacles to dislocation movement. This impedes the formation and propagation of cracks, thereby increasing the toughness and ductility of the steel.

Effect of tempering temperature on steel properties

The tempering temperature has a significant influence on the mechanical properties of the steel. Higher tempering temperatures result in larger carbide particles and a decrease in hardness, while lower tempering temperatures lead to smaller carbide particles and increased hardness. Therefore, tempering at low temperatures is preferred when the goal is to improve toughness and ductility while maintaining a certain level of hardness.

In conclusion, tempering is a process of annealing Martensite at low temperatures. It is necessary to relieve internal stresses and improve the mechanical properties of steel, particularly its toughness and ductility. Tempering at low temperatures allows the formation of small carbide particles within the Martensite matrix, which enhances the steel's resistance to cracking and improves its overall performance.