Rapid heating and quenching involves preheating the furnace temperature to a temperature higher than the required quenching temperature. The workpiece is then loaded into the furnace and the heat (or electricity) supply is turned off. When the furnace temperature drops to the quenching temperature, the heat (or electricity) supply is resumed and controlled. After the workpiece is fully heated, it is removed and quenched.
During rapid heating and quenching, the furnace temperature is approximately 100–200°C higher than the quenching temperature, so strict heating time control is required to prevent overheating of the workpiece.
Production practice has shown that simply raising the quenching temperature by several tens of degrees above the typical temperature can significantly shorten the heating time. Because rapid heating promotes more nucleation and the short heating time prevents grain growth, a finer grain size is achieved.
The rapid heating and quenching method is suitable for low- and medium-carbon non-alloy steels and low-alloy steels. For example, the Q345 (formerly designated 16Mn) steel chain hoist hook utilizes a rapid heating and quenching process (heating at 960°C to 980°C for 3.5 to 5 seconds per mm, quenching in 10% NaCl salt water, and tempering at 160°C for 90 minutes). This overcomes the long cycle times and low workpiece hardness associated with the original process (heating at 920°C for 30 seconds per mm, quenching in 10% NaCl salt water, and tempering at 160°C for 90 minutes). The hardness is increased from 42 to 44 HRC with the original process to 42 to 48 HRC with the new process. Simultaneously, the lath martensite grains are refined, the hardened layer depth, and overall mechanical properties are enhanced.
