Surface Hardening and Microstructural Changes in 304 Stainless Steel Resulting from Elevated Temperature Ultrasonic Vibration

Abstract

The influence of 40 kHz ultrasonic vibration at 925 °C on hardness and substructure has been studied for 304 stainless steel. Vibration at 33 MPa and 40 MPa stress amplitude produced an increase in hardness although the degree of hardening was nonuniform throughout the specimens. Hardening was greatest near the surface and decreased with distance from the surface. The amount of hardening and depth of the hardened region increased nonlinearly with the stress amplitude of the vibration. The surface hardening resulted from an increased dislocation density, deformation twinning, and enhanced precipitation of dispersed M₂₃C₆ carbide. The incidence of deformation twinning in the surface region was attributed to the inherently low stacking fault energy of the material, high strain rate, and severe strain localization along slip bands. Although deformation was more uniform at the surface, grain boundary regions in the interior of vibrated specimens showed a hardening compared to unvibrated annealed samples. In this case, the strengthening resulted from a dislocation cell structure primarily near grain boundaries combined with increased carbide precipitation.