Analysis of the cold treatment process of gears after heat treatment with medium frequency induction heating equipment

The process in which the workpiece is quenched and cooled to room temperature by intermediate frequency heating equipment and then continues to cool in general refrigeration equipment or low-temperature media (so that the retained austenite that has not yet transformed at room temperature continues to transform into martensite) is called cold treatment.

(1) Purpose

Minimize the residual austenite in the quenched steel, improve the hardness and wear resistance of the steel, and stabilize the size of the workpiece. Mainly used for precision parts such as rolling bearings, tools, measuring tools, oil pump nozzles and some carburized parts.

(2) Selection principle

The cold treatment temperature is near Mf or far lower than Mf point; the cooling and heating speed is generally ≤40℃/h, and segmented cooling and heating can also be used; cooling media include dry ice, ammonia, methanol, and Freon. Liquid nitrogen, etc.

(3) Process points

① The cold treatment operation should be carried out immediately after the workpiece is cooled to room temperature, because staying at room temperature for too long will cause the stabilization of retained austenite. The longer the stay time at room temperature, the more serious the austenite stabilization and the worse the cold treatment effect.

②The temperature of cold treatment should be determined according to the Mf point of the steel. The higher the carbon content and alloying element content of the steel, the lower the Mf point and the lower the cold treatment temperature. The cold treatment temperature is generally -80~-60℃.

③When performing cold treatment, the workpiece cannot be placed directly into the isothermal coolant, but should be placed in a low temperature box filled with air for cooling. To prevent austenite stabilization, low-temperature tempering should not be performed before cold treatment, and parts cannot be cleaned with hot water.

④Cold treatment should not be carried out before the workpiece is cooled to room temperature. For workpieces with complex shapes and large sizes, they should be lowered from room temperature to the processing temperature along with the fixture to prevent cracking of the workpiece.

⑤Martensite transformation mainly occurs during the continuous cooling process, so after the workpiece reaches the cold treatment temperature, the transformation is completed and no heat preservation is required. When there is a large amount of furnace loading, in order to ensure that all workpieces can reach the processing temperature, they can be kept warm for 1 to 3 hours.

⑥After the cold treatment, the workpiece is taken out of the cold treatment equipment and slowly returns to room temperature in the air. The frost on the surface of the workpiece should be wiped off in time. Tempering cannot be carried out before it returns to room temperature. After reaching room temperature, low temperature tempering (160-1800C) should be carried out in time.

(4) Cryogenic treatment

Cryogenic treatment refers to a method of treating workpieces at temperatures below -130°C using liquid nitrogen as the refrigerant. Cryogenic treatment can significantly improve the toughness of the workpiece without reducing its strength and hardness. Cryogenic treatment methods are divided into two types: liquid method and gas method. The liquid method is to put the workpiece directly into liquid nitrogen, and the processing temperature is -1500C. The disadvantage is that the thermal shock is large, sometimes even causing the workpiece to crack; the gas method is to use the latent heat of vaporization of liquid nitrogen and low-temperature nitrogen to absorb heat and refrigeration, and the processing temperature is up to -196℃, the treatment effect is better.