Modification Of The Microstructure And Mechanical Properties Of Gas Nitrided Layer On Pure Iron

Nitriding can significantly improve the surface hardness, wear resistance, corrosion resistance and fatigue strength of workpiece, so nitriding has been widely applied in industry application. A nitrided layer is usually composed of a compound layer?CL? and a diffusion layer?DL?. A CL, consisting of the ?-Fe2³N phase and the ??-Fe₄ N phase, has high hardness, such that it contributes to increase the surface hardness, wear resistance, corrosion resistance of the nitrided layer. Compared with the CL, the DL has lower hardness and better toughness. The thickness of a DL is much thicker than that of the CL, therefore the DL has great effect on the properties of the nitrided layer. The microstructure of the DL is affected by the nitriding process, cooling rate and subsequent heat treatment. However, the microstructure and its modification of the DL have not yet been investigated in detail. The raw material used in this study is a commercial pure iron plate with the thickness of 1mm. After different gas nitriding process, the pure iron samples were transformed into thoroughly nitrided samples. The effects of nitrided time, temperature, cooling rate and subsequent ageing on the microstructure and mechanical properties of nitrided samples were systematically studied by means of different characterization measures. And more, the effect of the CL on the fatigue property was also studied. The followings can be concluded:?1? The nitrided sample is composed of a compound outer layer on each side and a N-rich inner? also called as DL? after nitriding at 580 ?. A transition layer?TL? can be formed between the CL and the DL when the nitriding temperature increases to 630?, and the TL should be a Fe-N austenite at 630?.?2? The CL usually consists of the ?-Fe2³N phase and the ??-Fe₄ N phase. The elevated temperature decreases the nitrogen potential, increasing th e ??-Fe₄ N phase in the CL. But the ??-Fe₄ N phase will thoroughly transform into the ?-Fe2³N phase when the nitriding time is long enough. The thickness of the CL increases with the nitriding time and temperature. With the nitriding time prolonging, t he surface hardness first increases and then decreases, but the tensile strength of nitrided layer continuously increases, companied with the decrease of the plasticity.?3? The cooling rate has a great influence on the micro structure of DL. The main precipitates in the DL are ???-Fe₁₆N₂ phase and ??-Fe₄ N phase in the cases of furnace or air cooling after gas nitriding because the rate of furnace or air cooling is very slow. The rate of water or oil cooling is too fast to suppress the precipitation of phase in the DL, so there are no precipitates in the DLs with water or oil quenching. However, the oil-quenched and water-quenched DLs have greatly natural ageing strengthening phenomenon. The faster a cooling rate of nitrided samples, th e higher the peak hardness of natural aged DLs. Such that, with the increase of cooling rate the tensile strength of nitrided layer gradually increases, but its elongation gradually decreases.?4? During artificial ageing?AA?, the structure and the number of precipitates in the DL of oil-quenched samples depend on the ageing temperature and ageing time. The precipitates belong mainly to the ???-Fe₁₆N₂ phase during lower temperature ageing while that is mainly the ??-Fe₄ N phase at higher temperature ageing. The fine ???-Fe₁₆N₂ precipitates greatly enhance the strength of the nitrided layer. The precipitation of the ??-Fe₄ N phase is in favor of improving the elongation of nitrided layer.?5? The CL has a little contribution to the tensile properties of the nitrided samples, but it greatly lowers the fatigue strength of the nitrided samples under the tensile. Therefore, the DL is the main factor to improve the fatigue properties.