Nonlinear ultrasonic parameter in quenched martensitic steels

We have determined the nonlinear ultrasonic parameter β and the ultrasonic longitudinal phase velocity vL for a series of martensitic steel specimens which varied in carbon content. The specimens were measured in the as-quenched state to ensure that the carbon was present primarily as an interstitial in the martensite. β increased monotonically with carbon content and hardness over the range 0.10–0.40 mass % C (39.0–57.5 Rockwell C hardness). However, vL remained virtually the same for all specimens. Therefore we conclude that β is sensitive to microstructural variations between the specimens, but vL is not. X-ray diffraction experiments indicate that the dislocation density in the specimens is large (∼1011–1012/cm2) and increases with increasing carbon content. These results support the hypothesis that the observed increase in β can be attributed to dislocation-related effects in the specimens.