Carburizing and quenching and cooling
For the cooling of carburized isothermal quenching, the martensite structure of the workpiece should be obtained by rapid cooling first, and the deformation should be reduced to prevent the generation of cracks. Therefore, cooling is the key operation related to quenching quality.
(1) the ideal quenching cooling rate is derived from the isothermal transformation curve of austenite in eutectic steel. To obtain martensite, the quenching cooling rate must be greater than the critical cooling rate. However, quenching steel does not require rapid cooling during the entire cooling process. The key is in the supercooled austenite most unstable C curve near the tip, namely in 650 ~ 400 ℃ temperature range to rapid cooling. From the quenching temperature below 650 ℃ and 400 ℃, especially don't want to be under 300 ~ 200 ℃ cold fast. Thermal stress is caused by the difference of internal and external temperature of the workpiece during quenching and cooling. In addition, due to the volume of steel (the volume of unit mass material), one of the martensite volume the biggest, the minimum volume of austenite, as a result, the transformation of martensite will make the volume swell of artifacts, such as cooling rate is bigger, the internal and external temperature difference on the workpiece cross section will increase, make the martensite transformation cannot be caused by the phase transformation stress at the same time. The higher the cooling rate, the greater the thermal stress and phase change stress, and the deformation and cracking of steel during martensitic transformation are easy to be caused.
(2) when the workpiece of commonly used quenching medium is cooled by quenching, the proper quenching medium must be selected in order to obtain a reasonable quenching cooling rate. The cooling medium used in production is water and oil. When the water cooling medium is 20 ℃, workpiece temperature during 200 ~ 300 ℃, the average cooling speed is 450 ℃ / s; Workpiece temperature at 340 ℃, the average cooling speed was 775 ℃ / s; The workpiece temperature during 500 ~ 650 ℃, the average cooling speed is 135 ℃ / s. Therefore, water cooling characteristics is not ideal, in need of quick cold 500 ~ 650 ℃ temperature range, the cooling speed of it is very small, and in 200 ~ 300 ℃ need slow cooling, the cooling speed of it instead.