Heat Treatment of M2 Steel Cold Heading Die

W6MO5CR4V2 is a representative of general high-speed steel (HSS). It is a widely used high-speed steel tool material internationally, and it is also the steel with the most stable performance and quality of tungsten-molybdenum series high-speed steel. Replace part of W with Mo to refine the cast ledeburite. After rolling, the unevenness of carbides is improved and the grain size is also fine. It is the main material for making general-purpose tools. Because M2 steel has high strength, good wear resistance and moderate toughness, it can be used as wear-resistant parts under high load, such as cold extrusion dies, punches, etc. Due to the rapid development of the mold industry, M2 steel is more and more widely used in the mold industry. The heat treatment of the M2 steel cold heading die is introduced below.

Process 1: Low quenching and low return process

In order to improve the toughness, fracture resistance and service life of high-speed steel cold-deformed molds, the low-temperature quenching process has been successfully adopted in the past 10 years.

According to experiments and production practices, the strength, toughness and life of low-temperature quenching and low-temperature tempering processes are significantly improved compared with conventional quenching.

The w6mo5cr4v2 steel mold adopts 1150oC low temperature quenching and low temperature tempering below 300oC, which is suitable for cold stamping and cold extrusion molds that require high strength, high toughness, and high plasticity, and can solve the problem of breaking. However, low-temperature quenching also has its limitations. Due to the low compressive yield strength and insufficient wear resistance, it is not suitable for high-load die with unit extrusion stress greater than 2500MPa and upsetting as the main failure mode; and; due to wear resistance Insufficient, it is easy to stick to the mold, and it may also cause the die to break during demolding. At the same time, the influence of carbide inhomogeneity on the performance of high-speed steel is more prominent during low temperature quenching.

From the above analysis, it cannot be seen that the refrigerating heading die of M2 steel must be applied with a targeted low-quenching and low-return process, which is not universal.

Process 2: Low quenching delay time tempering

It is reported that Japan uses SKH51 (M2) to make cold heading molds, and adopts low-temperature quenching to extend the special process of tempering time, which can extend and increase the service life of the mold. The process is as follows:

1) Three-stage preheating temperature: 550oC, 850oC, 1050oC.

2) Quenching heating temperature: 1160~1180oC.

3) Cooling medium: 250~280oC nitrate salt graded and slowly cooled.

4) Tempering process: 500oC2h+560~580oC4h.

According to the information, small molds must be tempered three times. Even if the tempering holding time exceeds the optimal time, the material performance will not deteriorate. Therefore, if the effective thickness is below 100mm, the tempering holding time will be 4h; if it is above 100mm, it will be extended by 1h for every 25mm increase.