Influence of Surface Mechanical Attrition Treatment Parameters on the Residual Stress of EN8 Steel

Abstract

The present article addresses the influence of surface mechanical attrition treatment (SMAT) parameters such as the size of the balls (2, 5, and 8 mm circle divide 316L stainless steel (SS) balls) and treatment time (15, 30, 45 and 60 min) on the microstructural features, deformation characteristics, surface roughness and compressive residual stress induced on EN 8 steel. The effective deformation is similar to 25 mu m from the top surface along with elongated grains in the near subsurface region. The decrease in crystallite size and an increase in micro-strain become more pronounced during SMAT using balls with a larger diameter and for a longer treatment time. A similar trend is observed in surface roughness. This is mainly attributed to the involvement of higher kinetic energy imposed by SMAT on EN 8 steel. The residual stress induced during SMAT of EN 8 steel measured by x-ray diffraction (XRD) measurements is higher when treated using 2 mm circle divide SS balls when compared to those treated using 5 and 8 mm circle divide SS balls for a similar time duration. Irrespective of the size of the balls, an increase in treatment time has led to a decrease in the residual stress of EN8 steel. Overworking of EN8 steel that creates surface cracking has been considered responsible for the decrease in residual stress of EN8 steel treated using 5 and 8 mm circle divide SS balls for any given time duration as well as with an increase in treatment time for a given size of the balls. The development of surface cone cracks and material removal from the surface evidenced by the microstructure validates such an occurrence. When the duration of SMAT is limited to 15 min, a large difference in residual stress values is observed for the EN8 steel treated using 2, 5, and 8 mm circle divide balls, whereas, with a further increase in treatment time, the difference between them becomes marginal. Hence, to achieve better mechanical properties, the treatment time employed for SMAT of carbon steels should be carefully optimized. The other option available is to choose lower-size balls for SMAT. The change in residual stress of EN8 steel as a function of process parameters employed for SMAT and the correlation between the ball size and treatment time with the residual stress are discussed.