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Vacuum carburizing and quenching process of 20Cr2Ni4A thin-walled ring gear

1.Structure and heat treatment requirements of thin-walled ring gear

The material used in the design of the ring gear is 20Cr2Ni4A. The ring gear requires overall carburization, the depth of the carburized layer is 0.9~1.2mm, and the surface hardness is ≥60HRC. From the structural analysis of the ring gear, the spline is an internal spline with a thin wall, which is more prone to distortion during heat treatment, and the distortion control is more difficult.

The main production and testing equipment are: vacuum carburizing furnace, the maximum working temperature is 1350℃, the ultimate vacuum degree is 20Pa, and the maximum furnace load is 1200kg. Straightening machine, metallographic microscope and microhardness tester.

2.Analysis of gear material properties

The total content of alloying elements (mass fraction) of 20Cr2Ni4A steel is more than 5%, which belongs to high alloy carburized steel. The steel has high hardenability and excellent comprehensive mechanical properties, especially the high Ni content, which makes the steel have good strength, toughness and hardenability. The alloying elements Cr and Ni of the 20Cr2Ni4A steel can delay the austenite transformation, so the steel has high stability of the supercooled austenite in the pearlite range, and the bainite transformation occurs even in air cooling. After surface carburization, due to the combined effect of carbon and alloying elements, the bainite transformation curve is greatly shifted to the right, and the incubation period of the “nose tip” of the bainite isothermal transformation diagram is delayed from ten seconds before carburization to twenty minutes. , can be air quenched into martensite or bainite.

After the material is vacuum carburized, a large amount of carbon and other alloying elements in the surface layer dissolve into austenite, which significantly improves the alloying degree of austenite. Ms ) is 271℃, and the martensitic transformation point (Ms) of the carburized layer is reduced to 80~90℃, which greatly improves the stability of the supercooled austenite. After carburizing and air cooling, the surface structure is martensitic body and a large amount of retained austenite, thus affecting the hardness after quenching. To eliminate a large amount of retained austenite, the direct quenching method cannot be used like general low-alloy carburized steel. After secondary quenching, the retained austenite increases and the hardness decreases. There is a unique treatment method for this kind of high-alloy carburized steel, that is, tempering at a high temperature of 650 ° C after carburizing. During tempering, at a certain temperature, the process of carbide precipitation from retained austenite is an atomic diffusion process. The higher the temperature, the more conducive to diffusion, the increase of precipitation carbides, the reduction of the supersaturation of retained austenite, which is conducive to the transformation of retained austenite. However, the tempering temperature should not be too high, because the austenite of the high-carbon surface layer has a temperature of about 700 ° C. Too high tempering temperature is easy to cause phase transformation, but more carbon and alloys are dissolved in the austenite. element, which improves the stability of austenite and increases the amount of retained austenite.

3.Analysis of Distortion Characteristics of Ring Gear and Distortion Control Measures

Heat treatment distortion is a physical phenomenon in which the shape and size of a part change after heat treatment. Three conditions are required to cause heat treatment distortion: stress, plasticity, and action time. The distortion characteristics of the thin-walled ring gear are mainly manifested in taper, roundness, expansion and contraction of inner and outer diameters and end face warping.

To control the distortion amount of the ring gear, take the following measures:

(1) After rough machining, the parts are quenched and tempered to 28~32HRC, which not only helps to improve the machinability, improve the surface machining accuracy of the parts, but also minimize the residual stress generated in the rough machining, which is conducive to the later carburization. , Quenching provides good conditions to control the amount of deformation.

(2) When the ring gear is carburized, it is flat-mounted to reduce the axial circular runout.

(3) Air-cooled by carburizing, measure the outer diameter of the ring gear after high temperature tempering, and increase the pressure rounding treatment if the roundness is large, so that the roundness of the ring gear is controlled within 0.2mm.

(4) The ring gear is quenched with a mandrel.

4.Analysis of production process

The process used in heat treatment production: vacuum carburizing → high temperature tempering → rounding → vacuum quenching with mandrel → two low temperature tempering → shot blasting. Check the sample with a φ32mm round bar sample from the same furnace.

(1) Vacuum carburizing: 930℃ strong infiltration period of 2.5~3h, carbon potential is about 1.18%Cp, diffusion period is 1.5h, carbon potential is about 0.68%Cp; after cooling to 800℃, it is air-cooled.

(2) High temperature tempering: keep at 650℃ for 2h, cool to 550℃ and release.

(3) Perform pressure rounding treatment on the ring gear with large roundness deformation, so that the deformation amount is controlled within 0.2mm.

(4) Vacuum quenching with mandrel: temperature 800℃, carbon potential 0.85%Cp, holding time 1.5h, use graded quenching oil, vacuum quenching oil temperature 65℃, quenching time 1min.

(5) Two low-temperature tempering: according to the measured hardness value, it is determined to be 140 ℃, and the temperature is kept for 4 hours.

(6) Shot blasting.

5.Sample test results

(1) The microstructure of the microstructure of the sample is cryptoneedle martensite + a small amount of ferrite + a small amount of retained austenite.

(2) The surface hardness of the spline tooth surface and the end surface of the gear ring is 60~61HRC, and the hardness of the tooth surface is slightly higher.

(3) The depth of the hardened layer is 1.19mm, and the hardness gradient is shown in the figure.

(4) Mark No. 1~4 on the end faces of the four ring gears respectively. After heat treatment, the measured roundness and the M value of the internal spline meet the requirements. The circular runout varies between 0.08 and 0.15 mm, and the M value is between 0.11 and 0.17. mm.

After testing, various heat treatment technical requirements of the ring gear are up to standard, and the distortion is also controlled within a reasonable range, which provides a theoretical basis for the future production of such thin-walled ring gears.

6.Conclusion

In order to meet the technical conditions of the 20Cr2Ni4A ring gear, it is necessary to conduct a detailed analysis of the material characteristics, machining process and the whole process of vacuum carburizing and vacuum quenching and tempering. By taking the following targeted measures, the ring gear is carburized and quenched Distortion is controlled within the acceptable range.

(1) Increase the quenching and tempering treatment, improve the machinability, improve the surface machining accuracy of the parts, and eliminate the residual stress generated during rough machining.

(2) The ring gear is carburized and air-cooled, and the rounding treatment can be carried out when the roundness is large after high temperature tempering.

(3) Appropriate tooling needs to be designed for pressure quenching for thin-walled internal spline parts to control the deformation of the internal spline.

(4) The deformation can be appropriately reduced by adopting a reasonable furnace loading method.