PNNL

Abstract

Nanoscale solute partitioning across multiple constituent phases in a 980 grade quenched and partitioned (Q&P) steel was analyzed using atom probe tomography (APT). The Q&P process was used to increase the C content in the retained austenite phase thereby improve its stability under plastic straining. Significant carbon enrichment of austenite was measured with decreased levels of C in martensite, and almost depleted C content in ferrite, supporting the C partitioning mechanism proposed in the literature. The APT analysis of retained austenite surrounded by martensite demonstrated a higher amount of C content in comparison to retained austenite surrounded by the ferrite phase. Lath and discrete carbide particle precipitation was also observed inside martensite colonies, tying up C and reducing the total amount of C available for austenite stabilization. In addition, the partitioning of Mn and other minor elements was quantitatively investigated by correlating APT and SEM-EBSD. These techniques provide a robust methodology for analyzing nanoscale compositional partitioning in multiphase steels, TRIP steels in particular, which can be used to better explain their microstructure-mechanical property relationships.