Austenite Reconstruction Elucidates Prior Grain Size Dependence of Toughness in a Low Alloy Steel

Texture and grain size distribution of the austenite phase at high temperature have distinct effects on the subsequent transformed microstructure and mechanical properties of steel alloys at room temperature. Measurement of austenite at high temperature is not, however, a trivial task especially if orientation maps are desired. Therefore, a technique for determining austenite texture and grain size that was present at high temperature is highly desirable. In this work, we review several pipe samples intended for use as well casing with large variations in toughness and martensitic microstructures at room temperature. Microstructural analysis of the martensite structures could not easily explain these differences due to how martensite forms from austenite. An algorithm was developed to reconstruct the parent austenite at high temperature from martensite microstructure at room temperature. This technique successfully reconstructed orientation maps for the prior austenite in these samples, demonstrating differences in texture, grain orientation spread, and austenite grain size that could account for the differences in mechanical properties. (C) The Minerals, Metals & Materials Society and ASM International 2018