Tempering as a category of heat treatment

Thread Content

The concept of tempering: A heat treatment process in which a steel part that has been quenched is reheated to a temperature below Ac1, held at that temperature for a certain period of time, and then cooled back to room temperature. Purpose of tempering: To modify the quenched microstructure, which has high strength and hardness but poor plasticity and toughness.         It transforms the unstable quenched structure M and residual A into stable structures, ensuring that no further changes in shape or size occur in the workpiece.         Eliminate quenching internal stresses to prevent further deformation and cracking. Steel must be tempered after quenching, as tempering determines the microstructure and service life of the steel in use. Changes in microstructure and properties of quenched steel during tempering: 1. Decomposition of martensite (100–350°C); 2. Decomposition of residual austenite (200–300°C); 3. Transformation of carbides (250–400°C); 4. Aggregation and growth of cementite and recrystallization of the α-phase (>400°C). Temper brittleness: Type I temper brittleness refers to the decrease in impact toughness that occurs in structural steel during tempering at temperatures between 250–400°C. This type of temper brittleness occurs in both carbon steel and alloy steel; it is independent of the steel’s composition or cooling rate. It cannot be avoided even by adding alloying elements, cooling rapidly after tempering, or reheating to a temperature within the tempering range, which is why it is also known as \"irreversible temper brittleness\". However, alloying elements can shift the temperature range of type I temper brittleness to higher temperatures. It is generally believed that the occurrence of this type of temper embrittlement is related to the decomposition of martensite and residual austenite, as well as the precipitation of Fe3C. The preventive measure is to avoid tempering within this temperature range. Type II temper embrittlement: a decrease in impact toughness that occurs upon slow cooling after tempering at 500–650°C. This type of temper brittleness does not occur if rapid cooling is applied during tempering. Furthermore, once such brittleness has appeared, it can be completely eliminated by heating back to the original tempering temperature, performing tempering again, and then using rapid cooling; this is why this type of temper brittleness is also known as “reversible temper brittleness”. Methods for eliminating type 2 temper embrittlement: Not all steels exhibit type 2 temper embrittlement; it occurs only in alloy steels containing Cr, Mn, or Cr-Ni, Cr-Si. Steels that suffer from this type of temper embrittlement have low impact toughness at room temperature as well as a high temperature at which ductility gives way to brittleness; therefore, it is necessary to find ways to prevent or avoid it. The reason for this is the segregation of elements such as P, Mn, S, and Si at the grain boundaries. Removal method: (1) Rapid cooling from the self-tempering temperature to eliminate the segregation of P, MnS, and Si elements. (2) Adding 0.2–0.3% Mo or 0.4–0.8% W to the steel slows down the process of segregation, thereby eliminating or reducing temper brittleness. Type II temper embrittlement: a decrease in impact toughness that occurs upon slow cooling after tempering at 500–650°C. This type of temper brittleness does not occur if rapid cooling is applied during tempering. Furthermore, once such brittleness has appeared, it can be completely eliminated by heating back to the original tempering temperature, performing tempering again, and then using rapid cooling; this is why this type of temper brittleness is also known as “reversible temper brittleness”. Methods for eliminating type 2 temper embrittlement: Not all steels exhibit type 2 temper embrittlement; it occurs only in alloy steels containing Cr, Mn, or Cr-Ni, Cr-Si. Steels that suffer from this type of temper embrittlement have low impact toughness at room temperature as well as a high temperature at which ductility gives way to brittleness; therefore, it is necessary to find ways to prevent or avoid it. The reason for this is the segregation of elements such as P, Mn, S, and Si at the grain boundaries. Removal method: (1) Rapid cooling from the self-tempering temperature to eliminate the segregation of P, MnS, and Si elements. (2) Adding 0.2–0.3% Mo or 0.4–0.8% W to the steel slows down the process of segregation, thereby eliminating or reducing temper brittleness.