4.5 Article

The In Situ Observation of Phase Transformations During Intercritical Annealing of a Medium Manganese Advanced High Strength Steel by High Energy X-Ray Diffraction

Journal

FRONTIERS IN MATERIALS
Volume 8, Issue -, Pages -

Publisher

FRONTIERS MEDIA SA
DOI: 10.3389/fmats.2021.621784

Keywords

in situ HEXRD; phase transformation; lattice constant; austenite stability; medium Mn steels

Funding

  1. Department of Energy Office of FreedomCAR and Vehicle Technologies under the Automotive Light-Weighting Materials Program

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The microstructural changes in a medium manganese steel during thermal processing were evaluated using HEXRD, showing that the 30-min hold time was insufficient to reach equilibrium conditions and the austenite fraction increased during isothermal holding. The observed changes in lattice parameters were interpreted to be caused by the redistribution of C and Mn between austenite and ferrite.
Microstructural changes during thermal processing of a medium manganese steel containing (in wt%) 0.19C and 4.39 Mn were evaluated in situ with a high energy X-ray diffraction system (HEXRD). Samples with an initial fully martensitic microstructure were heated to intercritical annealing (IA) temperatures of 600 or 650 degrees C, held for 30 min, and cooled to room temperature. Diffraction data were analyzed to determine the variations in austenite and ferrite phase fractions and phase lattice constants throughout the ICA cycles. On heating, the 2 vol. pct of austenite present in the starting microstructure decomposed, and cementite precipitation then occurred. During isothermal holding, the austenite fraction increased, up to 20% for the sample annealed at 650 degrees C. The measured austenite fractions were less than those calculated by Thermo-Calc for equilibrium conditions, indicating that the 30-min hold time was insufficient to achieve near-equilibrium conditions. Observed changes in lattice parameters during isothermal holding were interpreted to reflect composition changes due to redistribution of the C and Mn between austenite and ferrite. The results are discussed in relation to the potential for controlling austenite stability during ambient temperature deformation.

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