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Mine pressure plate vacuum hardening

The pressure plate is the key part of the scraper conveyor. During use, it forms a scraper chain with the scraper, bolts, and chains. Driven by the power unit, the coal is transported to the ground. The pressure plate has been operated in the harsh environment of the underground for a long time. With the prolongation of the service time, fatigue wear will occur on the part meshing with the traction chain, which will eventually lead to failure. According to the research, it is known that the cracking of the pressure plate is related to the temperature of the vacuum hardening cooling medium. When the temperature of the vacuum hardening cooling medium changes to 20C, quenching cracking of the pressure plate will occur, indicating that the safety factor of the original process to prevent vacuum hardening cracks is already very small, and the quenched workpiece is already on the edge of cracking. It is necessary to further analyze the factors causing quenching cracking and improve Safety factor against quench cracking.

1 Influence of platen structure on cracks

The structural shape of the workpiece is one of the important factors affecting the vacuum hardening cracks. According to statistics on the cracked parts of the pressure plate on site, 90% of the cracks occurred in the holes, and 10% of the cracks were caused by forging defects. Analysis of the structure of the pressure plate shows that there are obvious thickness inconsistencies between the hole and other parts. During the vacuum hardening and cooling process, the weak parts of the workpiece are all hardened, and the center and surface of the workpiece are martensitic. The inner and outer hardness are similar, but the surface of the workpiece The transformation process of the structure and the center is not carried out at the same time. Due to the fast cooling rate of the surface, the martensite transformation occurs first. When the surface martensite transformation is completed, the subsurface layer and the core begin to undergo martensite transformation. Due to the large specific volume of martensite, the final structural stress forms tensile stress on the surface and compressive stress at the center. When the structural stress exceeds the strength limit of the material, it will lead to cracks.

2 Improving measures for platen quenching and tempering process

Through analysis, it is considered that the reason for the cracking of the pressure plate may be related to the improper vacuum heat treatment process, the structure of the pressure plate and the banded structure of the workpiece. For these factors, the following improvement measures are used to verify.

2.1 Optimizing quenching and tempering process

Without changing the process flow of the pressing plate (drilling before quenching and tempering), the vacuum hardening process is mainly improved from the following two aspects.

1) Pre-cooling vacuum hardening process is adopted for the pressing plate

In actual production, a continuous quenching and tempering production line is used for quenching and tempering treatment. The vacuum hardening temperature used in the original process is 810C, and the temperature is controlled in 3 zones. The temperatures in zone 1, zone 2 and zone 3 are all set to 810C. After the improved process, the vacuum hardening temperature in zone 1 and zone 2 is set to 820C, and the temperature in zone 3 (vacuum hardening exit zone) is set to 790C, which is equivalent to pre-cooling the hole part when it comes out of the furnace. Pre-cooling vacuum hardening can effectively reduce vacuum hardening. cracking.

2) Reduce the cooling intensity of the vacuum hardening cooling stage.

vacuum hardening cracks generally occur in the vacuum hardening cooling stage, that is, after the martensitic transformation is basically completed or after complete cooling, the cracking is caused by the tensile stress on the surface of the workpiece exceeding the fracture strength of the steel. Comparing and analyzing the cracking of the pressure plate before and after the replacement of the air cooler and the subsequent analysis of the material and the crack shape of the vacuum hardening, the essential cause of the vacuum hardening cracking of the pressure plate is the excessive cooling speed of the martensitic transformation zone in the later stage of vacuum hardening. Therefore, the cooling rate should be slowed down during vacuum hardening, considering that the high temperature of the vacuum hardening cooling medium will affect the depth of the hardened layer of the workpiece, reduce the mechanical properties of the workpiece, balance the vacuum hardening cracking and mechanical properties of the workpiece, and control the vacuum hardening cooling medium Temperature to 40~45C. At the same time, in the later stage of vacuum hardening, the final return temperature of the hole after vacuum hardening of the pressure plate is controlled, and the return temperature is generally controlled at 200~250C.

The above two methods were used to improve the quenching and tempering process of the press plate, and 100 press plates after forging were randomly selected for a small batch test, and no cracking of the hole occurred, indicating that the improvement measures were effective.

3 vacuum hardening process improvement

Through the comprehensive analysis of the pressure plate vacuum hardening process, equipment, materials, structure and other aspects, the reasons for the cracking of the pressure plate are as follows.

1) The temperature of the vacuum hardening cooling medium of the pressure plate is too low, which is the factor that causes cracking in the hole.

2) The cracking ratio of the pressure plate is relatively large, which has a certain relationship with the upper limit of the carbon content of the material composition of this batch. For water quenching products, strict control of material composition can greatly reduce the risk of vacuum hardening cracking.

3) In the case of controllable processing costs, in view of the 40Mn2 steel material and the uneven wall thickness of the press plate structure, the production process of the press plate is optimized, and the original drilling before quenching and tempering is changed to drilling after quenching and tempering, which can improve Safety factor against quench cracking.