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

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 degrees 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 (MUCH LESS-THAN200 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.

Automated summary

The study investigated residual compressive stresses and dimensional changes in the lattice strains of retained austenite (RA) phase in a high-Si, medium-carbon steel. Results showed that samples treated with Q&B exhibited formation of bainite and high-carbon fresh martensite, while samples treated with DWQ displayed nearly complete martensitic microstructure.