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I. Test conditions The four material labels and chemical components used in the test are 25kW well gas nitriding furnaces equipped with KRN-3 microcomputer nitrogen potential control cabinet. The nitriding medium is NH3. By adjusting the ammonia supply, the ammonia decomposition rate can be changed at 30e. If an ammonia pyrolysis furnace is used, the ammonia decomposition rate can be increased to 80.
Second, the preparatory heat treatment Due to the limitation of the gear processing tool, the traditional preparatory heat treatment process is to control the quenching and tempering hardness of the gear below 290HBS. In recent years, with the development of gear processing tools, it has been able to cut the medium hard gears in the 300400HBS hardness range, thereby developing the nitriding gear of the medium hard matrix, further improving the bearing capacity of the gear. The preliminary heat treatment process used in this test is a medium hard tempering process, and the process parameters are listed in 2.
III. Gas nitriding test results and analysis A gas nitriding process test of four furnaces was carried out. The process parameters are listed in 3, the test results are listed in the No. 4.1 process is constant temperature nitridation, the nitridation cycle is 72h; the No. 3 process is variable temperature nitridation, the nitridation cycle is 50h. Under the premise of keeping the nitridation cycle unchanged, appropriate Increasing the seepage/expansion ratio and lowering the ammonia decomposition rate in the second stage formed the No. 2 and No. 4 processes. The purpose is two: one is to strengthen the nitriding process, it is expected to obtain a thicker nitride layer in the same nitriding time; the second is to simplify the process, cancel the diffusion phase to achieve high decomposition rate and set the ammonia gas pyrolysis furnace. The effective hardened layer depth in the middle was measured by the hardness gradient method, and the load for measuring the Vickers hardness was 0.98 N (100 g). According to the relevant standards, the effective hardened layer depth is from the surface to a distance of 50 HV above the hardness of the substrate.
It can be seen from 4 that whether it is constant temperature nitridation with a period of 72h or variable temperature nitridation with a period of 50h, the effective hardened layer depth and white bright layer thickness of the nitrided layer are improved by adjusting the process parameters and strengthening the nitridation process. Increased, while other indicators did not change much. The effective hardened layer of constant temperature nitriding is deep at 0.650.75 mm, and the effective hardened layer of variable temperature nitriding is 0.600.65 mm deep. The brittleness of the nitrided surface is basically one level.
It can be seen from the test results of 4 that the surface hardness of 40CrNiMo, 42CrMo and 34CrNi3Mo steel after nitriding is not much different, and the surface hardness of 25Cr2MoV steel after nitriding is obviously higher than that of the other three kinds of steel. It is generally believed that the high hardness of the nitrided layer is related to the precipitation of the dispersed alloy nitride from the alpha solid solution. It can be seen that the reason why the 25Cr2MoV steel obtains higher hardness after nitriding may be the precipitation of the compound formed by the elements V and N in the steel in the layer, which causes a greater degree of distortion of the matrix lattice.
And a photograph of the nitriding layer of a typical nitriding gear product and a 25Cr2MoV furnace sample, respectively.
And is the microhardness distribution curve of four kinds of steel. It can be seen from the figure that the microhardness distribution curve of the nitrided layer of 25Cr2MoV steel is significantly higher than that of the other three kinds of steel. And near the surface, the microhardness curve distribution of 25Cr2MoV steel is convex, while the other three steels are concave, which further shows that 25Cr2MoV steel has stronger tendency of nitriding hardening than other three kinds of steel, which can be used Gears with higher strength requirements.
IV. Conclusion Through the deep gas nitriding test on four kinds of gear steels, the following conclusions can be drawn: (1) The recommended No. 2 and No. 4 processes can be used without ammonia gas pyrolysis furnace. The deep gas nitriding treatment is performed. The effective hardened layers of the nitrided layer are 0.650.75 mm and 0.600.65 mm, respectively, and the brittleness of the nitrided layer is substantially controllable to the first order.
(2) There is no significant difference in surface hardness and hardness distribution curve of 40CrNiMo, 42CrMo and 34CrNi3Mo steel after nitriding; 25Cr2MoV steel has strong tendency of nitriding hardening, and the surface hardness and hardness distribution curve after nitriding are obviously higher than before. Three steels, similar to the results under ion nitriding conditions. 25Cr2MoV steel is the steel of choice for the manufacture of high-strength nitriding gears.
