Abstract：The determination of depth and content of nitriding layer in gear steel is very important for judging whether the nitriding process is appropriate. 20MnCrS5 gear steel was selected as the test material. The best test parameters for the determination of nitriding layer by electron probe micro analyzer (EPMA) were obtained: acceleration voltage of 10 kV, beam current of 100 nA, beam size of 1 μm, and step size of 1 μm. Firstly, the surface scanning function of EPMA was used for two-dimensional and three-dimensional surface-distribution quantitative analysis of nitriding layer from surface to substrate. The results showed that there was an enriched nitride layer on the surface with a thickness of about 10 μm. The highest nitrogen content (mass fraction, similarly hereinafter) reached 8.46%. The nitrogen content within the nitriding layer was about 0.84%, and the total depth of the nitriding layer was about 600 μm. Secondly, the results of EPMA and hardness method were compared. It was found that the total depth of the nitriding layer measured by EPMA was consistent with the depth of the matrix hardness measured by the hardness method, i.e., about 600 μm. Meanwhile, the relationship curve between depth of nitriding layer and content of nitrogen could be provided. In summary, as a supplementary method to the hardness method, EPMA could obtain accurate depth and nitrogen content of nitriding layer at the same time, which could be used as a basis for judging whether the nitriding process was appropriate or not.
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