We studied the effect of robotic hammer peening (RHP) on plastic deformation, martensite transformation, and electrochemical process of austenitic stainless steels (SS304L) by employing microscopy and X-ray diffraction (XRD) characterizations. By changing the stepover distance dStep in a range of 0.1-2.0 mm we found that onset of surface plastic deformation at the center areas between adjacent grooves (i.e., those induced by RHP along the longitudinal feeding direction) occurs at dStep < 0.8 mm. XRD mapping along the transverse direction across the RHP-induced grooves revealed remarkable undulations in the distribution of martensite; the undulations decrease with the decrease in dStep and are eventually undetectable when dStep is reduced to smaller than 0.4 mm. These observations indicate that the martensite transformation occurs only at the peened areas, i.e., in the individual grooves rather than in their surrounding pileup materials. Morphological comparisons of the surface in the vicinity of the RHP-induced grooves before and after an electrochemical process were further carried out. They revealed pitting corrosions of the surface and the pitting is significantly reduced by the RHP process, which is most likely due to the RHP-induced smoothening. These results shed light on cold-working, especially when designing RHP process, for advanced surface enhancements of stainless steels.
This research is supported by A*STAR RIE2020 advanced manufacturing and engineering (AME) programmatic grant through the structural metal alloys program (SMAP, Grant no. A18B1b0061).