Surface modification and tribological performance of AISI 4140 steel via shot peening and plasma nitriding: A systematic evaluation of conventional, repeated and severe plastic deformation

Despite its widespread industrial use, AISI 4140 steel exhibits limited resistance to wear and surface damage. This study investigates the effects of vibratory shot peening and plasma nitriding processes, applied individually and in combination, on the tribological performance of AISI 4140 steel. Shot peening was performed under conventional (CSP), re-shot peening (RSP) and severe shot peening (SSP) conditions, while plasma nitriding was carried out at temperatures of 460 °C and 535 °C. The severe plastic deformation created by shot peening increased dislocation density, significantly improving nitrogen diffusion during plasma nitriding. The SSP condition resulted in the highest surface hardness, increasing from 248 HV0.1 to 533 HV0.1, while the combined SSP and plasma nitriding treatment produced a maximum hardness of 920 HV0.1. In dry wear tests, the combined application of shot peening and plasma nitriding significantly reduced the wear rate, achieving a reduction of approximately 94% compared to the untreated sample. In saltwater environments, wear behaviour was controlled by the continuity of the nitride layer and the diffusion depth, resulting in an 82% reduction in wear rate for the SSP and plasma nitriding condition. Overall, it has been observed that applying vibratory shot peening prior to plasma nitriding significantly improves the tribological performance of AISI 4140 steel.