Effects of roller burnishing parameters on surface roughness of tempered AISI 4340 steel: Evaluation of Ra, Rz, Rsk, and Rku

Surface roughness is a critical surface characteristic that directly affects the functional performance, fatigue behavior, and surface integrity of engineering components. Roller burnishing (RB) is an effective surface enhancement method capable of improving surface quality through the plastic deformation of surface asperities. The aim of this study is to investigate the effects of RB force and number of passes on the surface roughness parameters Ra, Rz, Rsk, and Rku using cylindrical specimens of tempered AISI 4340 steel. RB was performed under different burnishing forces (200 N and 350 N) and number of passes (1 and 3 passes). The results demonstrated that significant improvements in surface roughness were achieved compared to the turned condition. Under appropriate RB parameters, the Ra value was reduced from 3.21 µm to as low as 0.07 µm. The Rz value decreased from 14.07 µm to 0.40 µm. Within the investigated parameter range, according to the ANOVA results, the reducing effect of the number of passes on the Ra and Rz parameters was found to be more pronounced compared to the burnishing force. Furthermore, the shift of the Rsk parameter from positive to negative values indicated that surface asperity peaks were suppressed and a more balanced surface profile was obtained. In terms of the Rku parameter, no significant change in the sharpness of the surface height distribution was observed after RB. The findings indicate that RB is an effective method for improving the surface quality of tempered AISI 4340 steel. In addition, this study contributes to a more comprehensive understanding of surface topography by evaluating not only Ra but also Rz, Rsk, and Rku surface roughness parameters.