Improving the resistance to high-temperature oxidation of 254SMo super-austenitic stainless steel welds using ultrasonic shot peening-induced gradient nanostructures

This paper reported the strengthening of the surface of 254SMo stainless steel welds by the ultrasonic shot peening process (USP) to prepare gradient nanostructures, and their surface properties and high-temperature oxidation performance at different temperatures were explored. After investigating the surface morphology, microstructure, residual stress, and microhardness of the welds after USP, the resistance towards high-temperature oxidation of the samples processed by USP at 700, 800, and 900°C was studied. Also discussed were the possible influence mechanisms of the high-temperature oxidation properties of 254SMo stainless steel welds enhanced by USP processing. The results showed the preparation of gradient nanostructures on the surface of 254SMo stainless steel welds using USP and the residual compressive stress and maximum microhardness at the weld were −496.37 MPa, and 357.5 HV respectively. At 700°C, 800°C welds show better oxidation resistance, while at 900°C due to a large number of MoO₃ generation, oxidation is intense. At 700°C, 800°C, and 900°C, the oxidation reaction rate constants decreased by 61.3 %, 55 %, and 63.8 % respectively, and a thick protective oxide film was formed. This is because a large number of grain boundaries generated by grain refinement promoted the formation of Cr₂O₃ and inhibited the generation of MoO₃. The study shows that the gradient nanostructures prepared by USP are an effective method for improved oxidation resistance of materials at high temperatures.