Multiple-stripe pattern formation of Cr-Rich and CrN-rich alternative layers in the plasma nitriding of Fe-14Cr alloys

Formation of multi-stripe pattern with Cr-rich and CrN-rich alternative layers can be synthesized via the plasma nitriding for high-chromium steels. Corresponding to this multi-stripe formation in microstructure, the hardness value fluctuates even in the inside of plasma-nitrided layer. Key process parameter is holding temperature: this multi-stripe formation can be reproduced only in the middle range of temperature for 873 < T < 973 K. In the lower temperature, normal nitrided layer with constant CrN volume fraction and hardness value plateau, was observed even by using the same plasma-nitriding process. The number of stripes increases monotonically with the nitriding time. CrN-rich and Cr-rich layers reflect on the black - white sublayer in optical microscopy. The growth rate of nitriding front was far lower than that calculated from the diffusion coefficient of nitrogen; the growth of nitrided layer cannot be explained only by the nitrogen diffusion or by the Wagner's model. Precise microstructure analyses were made by XRD, EDX and XPS to investigate the solute Cr and CrN concentration distributions. They became periodic in the nitrided layer, corresponding to the multi-stripe formation. On the basis of these analyses, the difference of nitriding process is discussed with aid of numerical simulation.