Multi-step phase transformations in a medium carbon steel with large prior austenite grain size

The influence of different prior austenite grain sizes (PAGS) on phase transformation during slow continuous cooling was investigated by high-resolution dilatometry in a medium-carbon low-alloy steel. The selection of different PAGS values was motivated by an industrial case study of a large steel block, in which significant grain size variation occurs between the center and the surface. PAGS of 101 μm, 202 and 304 μm, were achieved through austenitization at 1150 ˚C for 10 seconds, 10 and 35 minutes followed by continuous cooling with a rate of 0.05 ˚C/s to room temperature, representative of the cooling of the large size steel block. Analysis of the dilatometry results combined with microstructure investigation using scanning electron microscopy revealed a two-stage phase transformation process. The first one occurred at high temperature and led to the formation of granular bainite and plate-like bainite. The second stage proceeded slowly over an extended duration, leading to the tempering of the bainite formed in the first stage and the formation of martensite from the residual austenite, accompanied by the auto-tempering. X-ray diffraction analysis indicated that, an increase in PAGS led to a rise in the volume fraction of retained austenite from 10% to 24.4%. Average microhardness values remained very similar despite variations in PAGS and volume fraction of phases. In contrast, nanohardness measurements revealed that bainitic ferrite in granular bainite exhibits the lowest hardness, whereas plate-like morphology shows intermediate hardness, and the (M-A) constituents display the highest hardness.