Surface modification of ultrahigh strength 300M steel under supercritical carbon dioxide (scCO2)-assisted grinding process

Abstract

300M steel has been widely utilized in the manufacturing processes of functional parts served in harsh environments. The properties of the parts’ surface layer induced by the grinding process are of great importance for the service performance, which can be determined by the input parameters of processing. This paper carried out evaluation of surface modification of 300M steel material under the scCO2-assisted grinding process. To further enhance the understanding of this processing design, the generation of surface modification with dry and flood techniques were also compared. The surface morphology, surface roughness, microstructure evolution, work hardening and surface residual stresses were recorded. The mechanism of grain refinement and the substructure of the martensite lath has been revealed. It was found that the scCO2-assisted grinding processes are responsible for smoother surface profile compared to the dry and flood techniques. Meanwhile, the machined surface roughness was maintained at a stable lower level as the grinding depth increases, proving the great superiority of scCO2 medium for improving the productivity and process sustainability. The grains in the superficial layer have been noticeably refined after grinding operation while it presents deeper refinement with scCO2-assisted grinding process. Meanwhile, nanocrystalline and carbide precipitation were clearly found on the refined layer with scCO2 medium, the crystal defects are mainly the combination of dislocation tangles (DTs), dislocation walls (DWs) and dislocation cells (DCs), and the stacking fault was observed to be the substructure for the lath martensite except the dislocation with TEM analysis. The designed scCO2-assisted grinding contributes to higher work hardening and compressive stresses compared to dry and flood methods.