Vacuum Heat Treatment of LD Steel Hexagonal Cold Heading Die

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When cold die steel is working, due to the relatively large deformation resistance of the processed material, the working part of the die, especially the cutting edge, is subjected to strong friction and extrusion, so the die should have high hardness, strength and wear resistance. In addition, the mold is affected by the impact force during the working process, so the mold is required to have good toughness. Through the research on the vacuum heat treatment process of LD steel (7Cr7Mo2V2Si steel) cold heading hexagonal mold, the performance of LD steel after vacuum heat treatment is analyzed, and more experience in heat treatment of tool and die steel is accumulated, so as to give full play to the potential of the material and improve the service performance of the mold.

LD steel hexagon die is a cold heading die commonly used in the standard parts industry, which is used for cold heading forming of standard bolt hexagon heads of different specifications. Since the square size S of the hexagonal head of the bolt formed by cold heading varies from 10mm to 40mm, the size and specification of the hexagonal die are different, and its shape is shown in the figure. The size of the LD steel hexagonal mold with the furnace sample is d30mm × 13mm. The manufacturing process of LD steel hexagonal die is raw material spheroidizing annealing → blanking → semi-finishing turning → cold extrusion hexagonal die inside hexagonal → finishing turning → milling hexagonal cutting → vacuum heat treatment → grinding edge.

LD steel is a cold work die steel with high strength, toughness and high wear resistance. The C content is higher than that of 65Nb steel, and the content of alloying element V is also higher. Therefore, under the condition of maintaining strength and toughness, its compressive strength, bending strength and resistance The abrasiveness is better than that of 65Nb steel; the C content is about half of that of Crl2MoV steel, so its eutectic carbide is much lower than that of Cr12MoV steel. Compared with Cr12 type cold work die steel and W6Mo5Cr4V2 high speed steel, LD steel has higher strength and toughness, and has better wear resistance, which is suitable for the manufacture of cold extrusion, cold heading, cold stamping and other molds that bear high loads.

The cold heading frequency of the LD steel hexagonal die during operation is 60~100 times/min, and the bolts to be processed are low and medium carbon steels such as Q235 and ML35. Therefore, the hexagonal die is subjected to multiple impact and wear, and the working conditions are harsh. There must be enough compressive yield strength, flexural fatigue strength and wear resistance. The hardness of LD steel hexagonal mold after heat treatment should be controlled at 59~61HRC. If the hardness is low, the inner wall of the mold cavity is prone to roughening (scratch or wear) and impact fatigue failure; while the hardness is slightly higher, it is easy to appear die cracking and expansion. or subsidence.

Vacuum heat treatment process:

The quenching test is carried out in a vacuum quenching furnace. The size of the effective heating zone of the furnace is 900mm × 600mm × 600mm, the hexagonal mold is 120kg, and the workpiece is quenched in vacuum quenching oil with the furnace. The vacuum heat treatment process curve is shown in the figure, and the quenching temperature is 1100 At ~1150℃, the hardness of the sample after quenching is 64.0-64.5HRC, and the retained austenite after quenching is about 34% (volume fraction), so the distortion after quenching is small.

After the first tempering at 520~530℃, the hardness of the LD steel hexagonal die is 65.0~65.5HRC, showing the phenomenon of secondary hardening. After 4 times of tempering, the retained austenite is fully transformed into tempered martensite, and the hardness is 59.5~60.0HRC, which meets the technical requirements.

Equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.