Experimental and numerical investigation of martensite phase transformation in austenitic stainless steel subjected to ultrasonic nanocrystal surface modification at room and cryogenic temperatures

This study aims to investigate the evolution of martensite volume fraction (xi) following ultrasonic nanocrystal surface modification (UNSM) on metastable austenitic stainless steel 304L specimens at room and cryogenic temperatures using both numerical and experimental methods. A user subroutine (VUMAT) was utilized to implement a constitutive model that describes the phase transformation occurring in the target during the UNSM process. An alternative method was employed to simulate UNSM. The numerical predictions of residual stress (sigma res) and xi exhibited good agreement with those measured experimentally. Finite element models were employed to undertake a comprehensive study of the impact of UNSM parameters on sigma res and xi. The findings demonstrated that UNSM at cryogenic temperature produced significantly higher levels of compressive sigma res in comparison with UNSM at room temperature, while the depth of the layer with compressive sigma res and the extent of plastic deformation decreased. Based on the experimental and numerical findings, a significant value of xi was observed to form within the subsurface region subsequent to UNSM at both temperature conditions. The value of xi on the surface of UNSMed specimens at room and cryogenic temperatures was quantified using X-ray diffraction, resulting in respective values of 82 % and 88 %. Moreover, a significant increase in near-surface layer hardness for the UNSMed specimen at cryogenic temperature was observed, with a value of approximately 600 Vickers. This value is notably higher than the hardness of 480 Vickers obtained for the UNSMed specimens at room temperature.