Effect of Carbon Partitioning and Residual Compressive Stresses on the Lattice Strains of Retained Austenite During Quenching and Isothermal Bainitic Holding in a High‐Silicon Medium‐Carbon Steel

Abstract

The residual compressive stresses and dimensional changes related to the lattice strains of retained austenite (RA) phase in a high‐Si, medium‐carbon steel (Fe‐0.53C‐1.67Si‐0.72Mn‐0.12Cr) are investigated for samples austenitized and quenched for isothermal bainitic transformation (Q&B) in the range 5 s to 1 h at 350 °C. Also, samples are directly quenched in water (DWQ) from the austenitization temperature for comparison with Q&B samples. Field emission scanning electron microscopy (FE‐SEM) combined with electron backscatter diffraction (EBSD) analyses, and X‐ray diffraction are used to investigate the microstructural evolution, phase distribution, and lattice parameters of RA phase. While the Q&B samples showed formation of bainite and high‐carbon fresh martensite in conjunction with stabilization of various fractions of RA, the DWQ samples displayed nearly complete martensitic microstructure. For short holding durations (≪200 s), there was limited formation of bainite and the inadequate carbon partitioning to the adjacent untransformed austenite areas resulted in significant martensite formation and the associated c/a ratio of martensite resulted in high compressive residual stresses within the RA phase. While, at long isothermal holding times (≫ 200 s), there was a significant formation of bainite. The DWQ samples displayed maximum lattice strain in a small fraction of untransformed RA phase.