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How is steel quenched using a high-frequency quenching machine? What is the effect of different original structures on quenching?
Release time:
2024-01-25
When steel is quenched using a high-frequency quenching machine, there are two requirements: first, the temperature of the steel must be heated to the austenitizing temperature; second, rapid cooling must be carried out, and the cooling speed must be greater than the critical cooling speed of the steel.
When steel is quenched using a high-frequency quenching machine, there are two requirements: first, the temperature of the steel must be heated to the austenitizing temperature; second, rapid cooling must be carried out, and the cooling speed must be greater than the critical cooling speed of the steel. The characteristic of induction hardening is that only the surface of the steel is heated. When the surface undergoes austenitization, heating should be immediately stopped, and adjacent unheated metal parts can quickly take away the heat in the heating area. If the cooling rate exceeds the cooling rate, the surface of the steel becomes a hardened layer. From this perspective, the surface of steel is not cooled by external coolant, but by adjacent metals. However, this situation can only occur when the density of steel is very high when heated. Using quenching heating is just one of the methods. Due to the high power density and short quenching time, this method is also called pulse heating.
The heat generated by heating steel during quenching is mainly used to heat the surface layer of the steel. The composition, original structure, and heating temperature of steel all affect the rate at which its ferrite and cementite transform into austenite. The original structure mainly determines the rate of generation of new austenite phase centers. As the original structure becomes more dispersed, the distance between the ferrite and cementite in the steel becomes smaller. Therefore, during high-frequency quenching by a quenching machine, the rate of generation and growth of austenite crystal nuclei also increases. Due to the fact that the austenite formed by the mixture of ferrite and cementite occurs at the boundary of the structure, the finer the original structure, the larger the effective reaction surface of the steel. Conversely, the smaller the reaction surface, and the shorter the heating time required. So the changes in the state of the original organization have a significant impact on the induction hardening of steel.
When steel is in a normalized or annealed state, its original structure is pearlite and free ferrite. The rate of austenitization is much slower than that of martensite in a quenched and tempered state, so the quenching temperature is also much higher than that of quenched and tempered steel. Due to the fact that the quenching temperature of the high-frequency quenching machine directly affects the residual stress of the steel during quenching, the higher the quenching temperature, the greater the residual stress of the steel. Therefore, in order to obtain a martensitic structure, it is necessary to prevent the steel from generating significant residual stress during induction quenching. During quenching and tempering, the quenching temperature of the steel is very low, and the residual stress is also small, which reduces the quenching cracking and peeling on the surface of the steel and improves its core strength.
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