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Hardenability and Vacuum Heat Treatment of Carburized Steel

Gear heat treatment distortion is the change in size and shape of the gear during the vacuum heat treatment process due to the different organizational structure of each part of the gear or the different transformation sequence of the organizational structure and the different thermal stress of each part. Among them, the organizational structure is closely related to the specific composition and quality of the material itself. Due to the instability of the material, the out-of-control deformation of the gear during the vacuum heat treatment has become one of the key problems affecting the precision of the gear.

In terms of the influence of raw materials on deformation, the hardenability of materials has the most prominent influence on the distortion of vacuum heat treatment. The hardenability of steel is affected by many factors such as chemical composition, deoxidation operation conditions, grain size, element segregation, and vacuum heat treatment parameters. Among them, the influence of chemical composition is more significant.

The effect of hardenability of carburized steel on dimensional change is obvious: this effect is mainly displayed in the transition from hardenable to hardenable cross-section of parts. When the part has been hardened, the effect of continuing to increase the hardenability on the distortion is not significant. Therefore, the carburized steel for gears should narrow the allowable hardenability bandwidth as much as possible, otherwise it is difficult to avoid fluctuations in the size of different smelting steels.

Vacuum heat treatment process

20CrMnNiMoH steel and 20CrMnTi steel are selected, and the sample is the driven wheel of the drive axle spiral bevel gear of ZL40 loader. After vacuum carburizing at 925C, the furnace is cooled to 840C and then directly vacuum quenched. The infiltration layer of the ring sample is controlled at 0.8mm 2mm。 About, passive bevel gear seepage layer is about 1. 2mm.

Effect of Ni Content on Hardenability and Distortion

The Jominy hardenability curves of steels with different Ni contents measured in experiments. As the Ni content increases, the hardenability increases. As the Ni content increases, the shrinkage of the outer diameter and inner diameter of the sample increases until the Ni content is about 2.25%, at which time the sample can be hardened. When the Ni content is higher (3.25% and 4.25%), the dimensional change decreases because the material is in an overalloyed state. It can be seen that the effect of hardenability of carburized steel on dimensional change is obvious. This effect is mainly displayed in the transition from hardenable to hardenable cross-section of parts. When the part has been hardened. The effect of increasing hardenability on distortion is not significant.

Summary of Vacuum Heat Treatment for Hardenability of Carburized Steel

(1) The chemical composition of carburizing steel has a significant effect on the hardenability of steel, and the hardenability is closely related to the distortion of carburizing steel in vacuum heat treatment.

(2) The precision (contact area) of gears using H steel after vacuum heat treatment is 70%~80% higher than that of ordinary steel, and the service life is also long, so the fluctuation range of hardenability of steel should be strictly controlled.

(3) In order to reduce deformation, H steel with hardenability broadband below 4HRC should be selected.