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

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>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.</p></div>