Austenite formation and solute redistribution during double soaking of a 7Mn-steel

Document Type

Article

Publication Date

9-1-2024

Abstract

In this work, the influence of secondary soaking parameters on microstructural evolution in medium manganese steels is evaluated with a specific focus on the mechanisms driving austenite formation and solute partitioning at an intermediate secondary soaking temperature of 750 °C. The secondary soaking heat treatment represents the latter step of the double soaking heat treatment where it is used, after an initial intercritical annealing heat treatment, to form additional, solute-lean austenite. Here, data are obtained on a 0.14C-7.17 Mn steel for secondary soaking temperatures between 700 °C and 800 °C, and for times up to 1200 s. Austenite formation during secondary soaking is characterized using dilatometry and XRD, while Mn redistribution is experimentally observed using STEM-EDS. Manganese partitioning during secondary soaking, as well as calculated changes in carbon partitioning, are further explored through a DICTRATM model. Experimental and simulation results suggest that the ferrite-to-austenite transformation during the secondary soaking step may proceed under a partitioning, diffusional transformation at the 750 °C secondary soaking temperature. Manganese partitioning was demonstrated to primarily occur from ferrite to newly formed secondary austenite, while carbon diffusion principally occurred from primary to secondary austenite. Transformation of secondary austenite to martensite upon quenching along with some fraction of primary austenite, whose transformation was attributed to a reduction in C concentration, was shown demonstrated. The results of this study are expected to be of interest to those seeking to design multi-step heat treatments which produce austenite of variable solute concentrations and stabilities.

Publication Title

Materialia

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