Final heat treatment process of high carbon and high chromium cold work die steel

Taking Cr12MoV steel as an example, three commonly used quenching and tempering processes are introduced.

　　Cr12MoV steel is one of the widely used cold work die steels in China. The steel has the advantages of good hardenability, high hardness, wear resistance, and small heat treatment deformation. It is often used to make cold work molds that bear heavy loads, large production batches, and complex shapes, such as cold punching, embossing, cold heading, and cold extrusion Die etc. However, the significant disadvantage of this steel is its brittleness, which often leads to early failure of the mold. Therefore, how to improve its strength and toughness and prevent premature failure of the mold is a problem frequently encountered and urgently needed to be solved.


　　 Three quenching and tempering processes:

1) Low-quenching and low-return process (one-time hardening method): the quenching heating temperature is 950~1000℃, and the tempering temperature is 180~200℃, which can make Cr12MoV steel obtain high hardness (61~63HRC) and toughness, but the compressive strength is relatively high. low. Suitable for cold punching convex and concave dies, cold shearing knives, etc. with complex shapes.

　　 2) Middle quenching and middle-returning process: the quenching heating temperature is 1030~1050℃, and the tempering temperature is 380~400℃, which can make Cr12MoV steel obtain the best strength and toughness and higher fracture resistance. Suitable for cold heading dies, cold extrusion dies, etc.

3) High-quenching and high-returning process (secondary hardening method): the quenching heating temperature is 1080~1100℃, and the tempering temperature is 490~520℃, which can make Cr12MoV steel obtain good thermal hardness, higher hardness and wear resistance, but Its fracture toughness is low. It is suitable for molds with high working temperature (400~500℃) and low load or the surface needs to be nitrided after quenching.

　　 When the quenching temperature is 1050℃, the hardness reaches the highest value; when it is 1100℃, the retained austenite can reach 80%. With the increase of heating temperature, the amount of carbide dissolved increases, the degree of austenite alloy increases, the amount of retained austenite increases after quenching, the hardness decreases, and the volume change decreases; increasing the amount of retained austenite to a certain extent will offset the The swelling caused by the transformation of magnesia causes the volume change to be zero or even negative, and the volume shrinks instead. The fluctuation of the quenching temperature can cause large changes in the structure and properties. According to this characteristic of this type of steel, the quenching temperature and tempering temperature can be selected to control the deformation of the mold. When the tempering temperature is lower than 300~400℃, the size is reduced due to the decomposition of martensite. When tempering at more than 450℃, the retained austenite will be transformed into martensite, which can cause the increase in volume. In order to eliminate the retained austenite after high temperature quenching, it is necessary to use 490~520℃ multiple tempering to make the hardness rise to 60~62HRC.