Microstructural Evolution of Super304H Upon Ultrasonic Shot Peening and Subsequent Annealing.

In this study we analyze the effect of shot peening on the microstructure and mechanical properties of Super304H, which is widely used for boiler tubes. Specimens were shot peened for 5, 10, and 15 minutes, and then heat treated at 600, 700, and 800 °C for 15 minutes. Vickers hardness was measured and the microstructure of cross sectional specimens was analyzed by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Heat treatment led to particle refinement on the upper layer of Super304H. In this region, nanograins, twins, and dislocations were generated to a depth of ~400 μm. The microstructure of the shot peened specimen did not change even after a heat treatment at 700 °C. However, rapid grain growth was observed after rapid annealing at 800 °C. Precipitation hardening occurred after heat treatment at 800 °C.