The effect of fine particle peening (FPP) on the gas nitriding of AISI 4135 chromium-molybdenum steel was investigated. Surface microstructures of nitrided specimens pre-treated with FPP were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GDOES). The nitrided layer formed on the fine grained specimen treated with FPP was thicker than that on the nitrided-only specimen, because the fine grains created by FPP accelerate the formation of iron nitride during the nitriding process. Furthermore, FPP reduces the concentration of chromium near the nitrided surface, which suppresses the formation of cracks and pores in the compound layer. Therefore, a nitrided specimen pre-treated with FPP has higher hardness than a nitrided-only specimen. Reciprocating ball-on-disk friction tests were performed at room temperature to investigate the tribological properties of the nitrided AISI 4135 steel. The nitrided specimen pre-treated with FPP had a lower friction coefficient and exhibited less wear loss due to the presence of a thick compound layer without cracks and pores. Therefore, a hybrid surface modification that employs FPP as a processing step prior to gas nitriding is effective for improving the tribological properties of AISI 4135 steel.
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