Increasing surface hardness of S45C steel by AIH-FPP/Gaseous nitriding treatment

In this study, in order to increase surface hardness of S45C steel, hybrid surface modification treatment which is a combination of atmospheric-controlled induction-heating fine particle peening (AIH-FPP) and gaseous nitriding, were developed. Surface microstructures of gaseous nitrided specimens pre-treated with AIH-FPP were characterized by a scanning electron microscope (SEM), an energy dispersive X-ray spectrometer (EDX), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS). In the case of the AIH-FPP treated specimens with a Cr transferred layer, the nitrided layer was not formed. This was because diffusion of nitrogen into the surface of the specimens was disturbed by chromium oxide in the transferred layer. On the other hand, the nitrided layer was formed at the surface of the specimen with Cr diffused layer induced by pre-AIH-FPP, and this nitrided layer showed higher hardness than that of the specimen without AIH-FPP. However, the surface hardness decreased near the surface. In order to suppress decreasing of the surface hardness, the specimens with different Cr concentrations were prepared. It was clarified that the surface hardness was affected by Cr concentration in the Cr diffused layer; the lower the Cr concentration, the higher the surface hardness. It was assumed that the pore formation during nitriding process was inhibited by decreasing Cr concentration. These results suggest that surface hardness of S45C steel remarkably increases by hybrid surface treatments of AIH-FPP and gaseous nitriding.