Modeling the Relative Contribution of Matrix Dislocations and M23C6 Coarsening to Acoustic Nonlinearity in 9Cr-1Mo Stainless Steel

9Cr-1Mo tempered martensite stainless steels are highly desirable for their corrosion resistance and creep strength. Standard austenitizing followed by thermal aging of the material results in several microstructural processes that can affect the classical nonlinearity beta. These include dislocation density decreases over time and carbide precipitation and growth in the material. This research explores contributions to beta by combined processes of dislocation reduction and grain boundary precipitate M23C6 growth in a 9Cr-1Mo steel. The Thermo-Calc DICTRA module is used to predict the coarsening of grain boundary M23C6 and this is verified through experimental measurement via SEM. Dislocation density reduction models are based on experimental data from previous measurements of dislocation density in heat-treated 9Cr-1Mo. The predictions of radius and dislocation density are then applied to relationships established by simplified physics-based models. The model shows good qualitative agreement with experimental results, though further exploration of modeling assumptions is needed before definitive quantitative modeling can be done.

Automated summary

9Cr-1Mo tempered martensite stainless steels are highly desirable for their corrosion resistance and creep strength. This research explores the contributions to nonlinearity beta by dislocation reduction and grain boundary precipitate M23C6 growth in a 9Cr-1Mo steel. The predictions of grain boundary M23C6 coarsening and dislocation density reduction are validated through experimental measurement.