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The microhardness profiles of the nitrided specimens are shown in Fig. 7. The highest values of surface hardness in the profiles were in the range of 635−700 HV0.05; the higher nitriding temperature the higher the microhardness of the nitrided layer. This clearly shows that the hardness of the nitrided specimens is higher by a factor of approximately 1.6 to 1.8 over that of the substrate material. The depth of the hardened layers extended nearly 100, 160, and 200 μm belowthe surface for the specimens nitrided at 475 °C, 500 °C, and 525 °C 更多：https://www.bmcx.com/ , respectively, in accordance with morphological observations. The progressive reduction of hardness from surface to the core of thespecimens suggests the presence of a diffusion layer which contains precipitates of nitrides, of irons, and other metals are formed at the grain boundaries as well as within the grains. These precipitates distort the lattice- and pin crystal dislocations and thereby raise the hardness of the surface layers of the nitrided specimens [25].