Deformation-induced phase transition and nanotwins in SS 304 steel during cryogenic laser shock peening without coating

The influence of Laser shock peening without coating at a cryogenic temperature (CLPwC) on SS 304 alloy was investigated. Three different laser energies, such as 150 mJ, 300 mJ, and 450 mJ were chosen to investigate the effects of CLPwC in SS 304 steel. The impact of CLPwC on the residual stress, surface roughness, hardness, phase transformation and microstructural changes was studied. The XRD studies show the peak broadening and strain-induced martensite (γ→α′) of 36.3% after CLPwC at 9 GW cm−2. The work-hardening effect was observed up to 300–400 μm depth from the peened surface. The microhardness could effectively increase to 309.43 HV at 9 GW cm−2 when it compared to unpeened hardness (192.1 HV). Relaxation of compressive residual stress at cryogenic temperature was distinguished, and the maximum compressive stress of −138 MPa was obtained at 50 μm from the surface. Microstructural studies were carried out by different techniques such as EBSD and TEM. An increase in low angle grain boundary fraction (43%) and grain size reduction was observed using EBSD after CLPwC. The TEM investigations revealed deformation twins (width of 6–70 nm), shear bands, stacking faults, and martensitic phase. Strain induced martensitic phase is witnessed in XRD, EBSD, TEM studies.