High-speed steel cold work die cryogenic treatment and application

Abstract : It is pointed out that the use of -196 °C liquid nitrogen cryogenic treatment for high-speed steel can significantly change the microstructure, effectively promote the transformation of retained austenite to martensite and the precipitation of ultrafine carbides, so that the mold can obtain better comprehensive mechanical properties. After the cryogenic treatment, the service life of the high-speed steel mold is more than three times higher than that of the conventional heat treatment, and it has very important use value.
Key words high speed steel mold retention austenite ultrafine carbide service life

1 Introduction Since the advent of high-speed steel in 1903, it has been known for the manufacture of metal cutting tools. With the rapid development of science and technology, the application range of high-speed steel has been expanding. Since the 1960s, Japan has succeeded in testing high-speed steel as a center for the automotive and bicycle industries. About 15% of the high-speed steel produced today is used to make molds. High-speed steel is mainly used to manufacture cold extrusion die and cold pierdie die. In particular, Mo-based high-speed steel is superior to W-series high-speed steel. The main technical difficulty of high-speed steel for molds lies in the mastery of heat treatment technology. At present, the most widely used high-speed steel in China is tungsten W18Cr4V (abbreviated as 18-4-1) steel and tungsten-molybdenum W6Mo5Cr4V2 (6-5-4-2) steel [1]. The traditional quenching and tempering process of these two steels is characterized by tempering three times in a hardening range after high temperature quenching to obtain high hardness and hot hardness. The process specifications are shown in Table 1. The main disadvantage is the lack of hardness in some locations. In order to improve the toughness of the mold, the traditional quenching and tempering process of high speed steel has also undergone changes in recent years.

Table 1 Common heat treatment specifications for high speed steel

2 Cryogenic treatment principle and process The cold treatment of high speed steel was proposed in the late 1930s. According to the traditional concept, the purpose of cold treatment is to cool the hardened steel parts to below zero (generally -60 ° C - 70 ° C). The retained austenite in the steel is transformed into martensite. In the past, the use of high-speed steel cold treatment in the industry was mainly used to shorten the heat treatment production cycle, that is, to replace the treatment method with quenching + cold treatment + one tempering [2], that is, to treat the quenched parts at -100 ° C - -196 ° C (liquid nitrogen), After tempering at 400 ° C, it is not necessary to repeat tempering 2-3 times. After the cryogenic treatment, the hardness and wear resistance of the parts are further improved, and the wear resistance can be improved by 40%, which shortens the tempering time, saves energy, and significantly improves the service life of the mold. Since the 1970s, the research work on cryogenic treatment at home and abroad has been fruitful. The former Soviet Union, the United States, Japan and other countries have successfully used cryogenic treatment to improve the service life of the tool, the wear resistance and dimensional stability of the workpiece.
(1) Tissue transformation after cryogenic treatment.
The cryogenically treated quenched high-speed steel not only causes austenite transformation, but also causes martensite transformation. What has been emphasized in the past few decades is the remnant austenite transformation. The new discovery of martensite decomposition can be seen as a new development in the research of high-speed steel cryogenic treatment in recent years.
The martensite final transformation point Mf of high-speed steel is very low. For example, the Mf point of W18Cr4V steel is about -100 °C. Therefore, a large amount of austenite remains after quenching and cooling to room temperature. It is generally believed that austenite remains in steel. Harmful, it will reduce the hardness, wear resistance and service life of steel, and also reduce many physical properties, especially thermal properties and magnetic properties. The test proves that the use of cryogenic treatment can reduce the retained austenite in the steel to the minimum limit. It can be seen from Table 2 that after quenching and tempering of W18Cr4V high-speed steel, the cryogenic treatment can reduce the amount of retained austenite after tempering 24 %.

Table 2 Effect of different treatment processes on retained austenite of W18Cr4V steel (% by volume)

The Leningrad University of Technology in the former Soviet Union studied the effect of 15 min cryogenic treatment in liquid nitrogen at -196 °C on the transformation of high-speed steel. The test results showed that -70 °C - -75 °C to -130 °C - -140 °C Martensite transformation occurred during cryogenic treatment, and the transition stagnated when cooled to -196 °C. In the temperature range of -90 ° C to -120 ° C, the effect of the sample volume appears, which proves that the martensite has partially decomposed and precipitated carbon atoms on the dislocation surface and formed ultra-microscopic carbides. It can be seen that the cold treatment of the high-speed steel precipitates the carbide particles and the dispersion is uniform. After the cryogenic treatment of the W18Cr4V steel, the carbide particles increase by about 8%, and the carbide particles precipitated by the W6Mo5Cr4V2 steel increase by about 76%, and the matrix structure is also fine. Turn.
(2) The effect of cryogenic treatment on the performance of high speed steel.
During the cryogenic treatment, a large amount of retained austenite is transformed into martensite, especially the supersaturated metastable martensite will reduce the supersaturation during the process from -196 ° C to room temperature, and the dispersion is only 20 - 60A and ultrafine carbides in a coherent relationship with the matrix can reduce the martensite lattice distortion and reduce the microscopic stress, while the finely dispersed carbides can hinder the dislocation motion during the plastic deformation of the material, thereby strengthening the matrix structure. . At the same time, due to the precipitation of ultrafine carbide particles, it is uniformly distributed on the martensite matrix, which weakens the grain boundary catalysis, and the refinement of the matrix structure not only weakens the degree of segregation of impurity elements at the grain boundaries, but also exerts grain boundary strengthening. The effect is to improve the performance of high-speed steel, so that hardness, impact toughness and wear resistance are significantly improved [3]. The hardness of the mold is high, and the wear resistance is also good. For example, the hardness is increased from 60HRC to 62-63HRC, and the wear resistance of the mold is increased by 30%-40%.
It can be seen that the relative wear resistance of the mold after the cryogenic treatment is increased by 40%. After the deep cooling treatment time is extended, the relative wear resistance is increased under the condition that the hardness does not change much [4].
(3) High-speed steel mold cryogenic treatment process To prevent high-speed steel molds (especially complex shapes) from breaking and becoming brittle during cryogenic treatment, it is recommended that the high-speed steel mold after quenching be tempered at 560 °C for 1 h before liquid nitrogen Cryogenic treatment, and then final tempering at 400 ° C for 30-60min, this heat treatment process can not only prevent mold breakage and embrittlement, but also increase mold life by 1.5 to 2 times.
The process of deep-cooling treatment of high-speed steel mold is “, the mold removes grease → put into the heat preservation tank → a small amount of liquid nitrogen is injected multiple times → heat preservation for 4 hours → the mold is taken out → 400 ° C tempering for 45 minutes.
3 High-speed steel mold cryogenic treatment application examples (1) Punch: The high-speed steel punch of the automobile factory can only be used 100,000 times without cryogenic treatment, and 400 times after liquid nitrogen is cooled by -196 °C × 4h. Fire, the service life has increased to 1.3 million times.
(2) Stamping die: The production and use results show that the output after the cryogenic treatment is more than doubled.
(3) Silicon steel sheet cold die: In order to reduce the brittleness and internal stress of the mold after cryogenic treatment, the combination of cryogenic treatment and medium temperature tempering can improve the mold resistance and other comprehensive performance, and the grinding life of the mold is increased by more than 3 times. It is stable at 5-7 thousand times.
4 Conclusion (1) During the cryogenic treatment of high-speed steel, hardness, wear resistance, impact toughness, and red hardness are improved due to precipitation of retained austenite to martensite and ultrafine carbide.
(2) As a new process, cryogenic treatment is applied in the heat treatment of high-speed die steel, which can significantly improve the service life of the mold and has great practical value.

references
1 Chen Jingwei. High-speed steel cold work die[J].Mechanical Industry Materials,1994,(8)
2 Zhongshan Jiuyan. Liquid supercooling temperature with liquid nitrogen [J]. Foreign metal heat treatment, 1987, (1)
3 Cong Jiyuan et al. Tissue transformation and wear resistance of high-speed steel cryogenic treatment [J]. Thermal Processing Technology, 1998, (3)
4 Chen Changfeng et al. Effect of cryogenic treatment on abrasive wear properties of T12 steel[J].Heat Treatment of Metals,2000,(10)
Author: Air Force Institute of Mechanical and Electrical Engineering, Aviation Maintenance Technology (Changsha 410124) Liu Jinsong

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