Research And Numerical Simulation On Vacuum Carburizing Process

Carburizing of steel is a widely employed chemical surface heat treatment in which a carburized layer with high hardness is formed, while the core of steel keeps the strength and toughness. Vacuum carburizing has attracted much more attention as an alternative new technology to traditional atmosphere gas carburizing, the process can be conducted at the temperatures higher than the temperatures applied in traditional atmosphere gas carburizing which allows reducing the processing time. Compared with other carburizing methods, vacuum carburizing can meet the requirements of cost and environment-friendly operation, can gain clean workpiece surface and excellent uniformity of carburized layer, simultaneously, it has the advantages that include small deformation, high carburizing efficiency, and preventing oxidation at the grain boundaries. In this paper, different vacuum carburizing processes which are based on 16 Mn steel, 20CrNi2 Mo steel and 20 CrMnMo steel are designed, the metallograph, microhardness distribution, surface carbon concentration, scanning electron micrograph and X-ray diffraction results of different heat treatment are analyzed, as well as comparing with the difference between the numerical simulation results and practical results on carbon concentration, all of the research work is aimed at providing data and theoretical support for practical production practice.The main research work inthis paper are as follows:1. The carburizing layer surface of 16 Mn steel after vacuum carburizing and quenching is in good condition, the surface microstructure are consist of acicular martensite, retained austenite and tiny carbides, the core microstructure is low-carbon lath martensite and ferrite; The surface hardness of carburized layer is the highest, and with the increasing distance from the surface, the hardness decreases slowly at first, and then it shows at a parabolic decline, after this it shows at a linear decrease, eventually to the core it has little change; The appropriate ratio of carburizing time and diffusion time is 1:6 when the vacuum carburizing layer depth of 16 Mn is less than or equal2mm; The vacuum carburizing layer depth at 960℃ is double than that at 900℃ under the same other conditions.2. The highest hardness of 20CrNi2 Mo steel sample is 880HV1 at surface and the core hardness is about 410HV1 after vacuum carburizing and quenching process; The appropriate ratio of carburizing time and diffusion time is 1:6 when the vacuum carburizing layer depth of20CrNi2 Mo steel is less than or equal 2mm, while the appropriate ratio is 1:10 when the vacuum carburizing layer depth is between 2 and 3mm; The cold treatment on 20CrNi2 Mo steel sample after vacuum carburizing and quenching can increase its hardness, but before and after the cold treatment the vacuum carburizing layer depth remains unchanged; The data of mumerical simulation of carbon concentration on 20CrNi2 Mo steel surface of vacuum carburizing is a little less than the experimental data, so it has a certain reference significance tothe practical production.3. When 20 CrMnMo steel is carburized 75 minutes at deep vacuum carburizing process, the appropriate ratio of carburizing time and diffusion time should be controlled between 1:20 and1:22, and the vacuum carburizing layer depth is between 4.2 and 4.4mm, and the carbide level is between 1 and 2 level; The data of mumerical simulation of carbon concentration on 20 CrMnMo steel surface of vacuum carburizing is lower than the experimental data of 0.035%, the data of mumerical simulation on the depth of vacuum carburizing layer is less than the experimental data of 0.22 mm.4. When 20 CrMnMo steel large heavy-duty gear(diameter Φ=1435mm) is carburized at the ratio of 1:22 carburizing time and diffusion time under deep vacuum carburizing process, the results show that the gear workpiece is metallic silver, the surface carbon concentration is 0.86%,the vacuum carburizing layer depth is 4.4mm, and the carbide level is between 1 and 2 level; The average rockwell hardness of 20 CrMnMo steel furnace sample after vacuum carburizing and quenching is 62.8HRC, and the average rockwell hardness of gear tooth surface after vacuum carburizing and quenching is 61.3HRC. The radial deformation increment of 20 CrMnMo steel large heavy-duty gear after deep vacuum carburizing and quenching is 1.66 mm, and it becomes1.28 mm after tempering process.