Quenching is the core link of stamping die heat treatment. Its principle is to heat the die to above the critical temperature, keep it warm for a certain period of time and then cool it rapidly. During the heating process, the ferrite and cementite in the die steel dissolve to form austenite. When cooling rapidly, the austenite cannot be fully transformed into pearlite and other structures, and is forced to transform into a martensite structure with extremely high hardness. Taking the commonly used Cr12MoV die steel as an example, the hardness of the martensite formed after quenching can reach HRC60-62, which significantly improves the surface hardness of the die. The high-hardness martensite structure enables the die to effectively resist the extrusion and friction of the metal sheet during the stamping process, reduce surface wear, and extend the service life of the die. However, if the quenching cooling speed is too fast or the process is improper, excessive internal stress may be generated, causing deformation or even cracking of the die, so the quenching process parameters need to be strictly controlled.
The wear resistance of the die is closely related to the organizational morphology formed after quenching. In addition to the martensitic structure, fine and dispersed carbide particles are also produced during the quenching process. The hardness of these carbides is much higher than that of the matrix structure, like a "hard shield" embedded in the mold surface, which effectively hinders the cutting effect of abrasive particles on the mold surface. When the mold contacts the metal sheet, the carbide bears the friction first, slowing down the wear rate of the matrix. In addition, quenching forms a dense microstructure on the mold surface, reduces grain boundary defects and loose tissue areas, reduces the invasion path of the wear medium, and further improves the wear resistance. However, if the quenching heating temperature is too high, it will cause coarse grains, which will reduce the wear resistance and toughness of the mold.
Tempering is a heat treatment process after quenching. Its main function is to eliminate the internal stress of quenching and adjust the balance between the hardness, strength, toughness and wear resistance of the mold. According to the tempering temperature, it can be divided into low-temperature tempering, medium-temperature tempering and high-temperature tempering. Low-temperature tempering (150 - 250℃) is mainly used to maintain the high hardness of the mold. It is suitable for stamping dies that require extremely high wear resistance. Through tempering, the supersaturated carbon in the martensite is precipitated to form dispersed ε-carbides, which can reduce internal stress and slightly improve toughness, avoiding brittle cracking of the mold due to excessive hardness. Medium-temperature tempering (350 - 500℃) can obtain tempered troostite structure, which significantly improves the elasticity and strength of the mold and is suitable for molds that bear large impact loads. High-temperature tempering (500 - 650℃) produces tempered troostite structure, which gives the mold good comprehensive mechanical properties and is often used for complex molds with high toughness requirements.
The effect of tempering process on mold hardness and wear resistance presents a complex change pattern. As the tempering temperature increases, the mold hardness will gradually decrease. This is because martensite decomposes during the tempering process, carbides aggregate and grow, and the strengthening effect on the matrix is weakened. However, within a certain temperature range, tempering can improve the wear resistance of the mold. For example, although low-temperature tempering slightly reduces the hardness, it eliminates internal stress, reduces the risk of microcrack initiation, and the uniform distribution of carbides enhances the ability to resist abrasive wear. Excessive tempering will cause a large amount of carbides to aggregate and coarsen, and the hardness and wear resistance will drop sharply. Therefore, it is necessary to accurately control the tempering temperature and time according to the specific use conditions and performance requirements of the mold to achieve the best balance between hardness and wear resistance.
In addition to the process parameters such as quenching and tempering temperature and time, factors such as the chemical composition, original organizational state, and quenching cooling medium of the mold steel will also significantly affect the hardness and wear resistance after heat treatment. The content and distribution of alloying elements (such as Cr, Mo, V, etc.) in mold steel will change the stability of austenite, the type and size of carbides, and thus affect the organizational changes during martensite transformation and tempering. Mold steel with uniform and fine original organization is more likely to obtain fine martensitic organization and dispersed carbides after quenching, which is conducive to improving hardness and wear resistance. The cooling rate and cooling uniformity of the quenching cooling medium are equally critical. For example, oil-cooled quenching can achieve a gentler cooling rate, reducing the risk of deformation and cracking, but it may affect the amount of martensite formed and the hardness; while water-cooled quenching has a fast cooling rate and can achieve high hardness, but the internal stress is large, and a suitable tempering process is required to eliminate the stress.
If the heat treatment process is not properly controlled, the mold may have problems such as insufficient hardness, uneven hardness, and poor wear resistance. Insufficient hardness may be caused by too low quenching temperature, insufficient holding time, or too high tempering temperature. The quenching and tempering parameters need to be readjusted, and secondary quenching is performed if necessary. Uneven hardness is usually caused by uneven heating or cooling, which can be solved by optimizing heating equipment, improving furnace loading methods, or using appropriate cooling medium flow methods. For the problem of poor wear resistance, if it is caused by carbide aggregation and coarsening, high-temperature quenching can be used to refine the grains and adjust the tempering process; if it is caused by too much residual austenite in the structure, deep cryogenic treatment can be used to reduce the amount of residual austenite and improve hardness and wear resistance.
With the development of mold manufacturing technology, advanced technologies such as vacuum heat treatment, controlled atmosphere heat treatment, and induction heating quenching have gradually become popular. Vacuum heat treatment can avoid oxidation and decarburization on the mold surface and obtain more uniform structure and performance; controlled atmosphere heat treatment can accurately control the carbon content on the mold surface and improve surface hardness and wear resistance. Induction heating quenching has the advantages of fast heating speed, small quenching deformation, energy saving and environmental protection, and is suitable for local hardening treatment. In the future, heat treatment technology will develop in the direction of precision, intelligence, and greening. Through computer simulation technology to optimize heat treatment process parameters, combined with artificial intelligence to achieve real-time monitoring and automatic adjustment of the heat treatment process, the hardness, wear resistance and comprehensive performance of the stamping die will be further improved.
