Abstract
The simulation welding thermal cycle technique was employed to generate different sizes of prior austenite grains. Dilatometry tests, in situ laser scanning confocal microscopy, and transmission electron microscopy were used to investigate the role of prior austenite grain size on bainite transformation in low carbon steel. The bainite start transformation (Bs) temperature was reduced by fine austenite grains (lowered by about 30 °C under the experimental conditions). Through careful microstructural observation, it can be found that, besides the Hall–Petch strengthening effect, the carbon segregation at the fine austenite grain boundaries is probably another factor that decreases the Bs temperature as a result of the increase in interfacial energy of nucleation. At the early stage of the transformation, the bainite laths nucleate near to the grain boundaries and grow in a “side-by-side” mode in fine austenite grains, whereas in coarse austenite grains, the sympathetic nucleation at the broad side of the pre-existing laths causes the distribution of bainitic ferrite packets to be interlocked.
Highlights
Prior austenite grain size is a vitally important parameter to influence phase transformation behavior in steels, because to a great extent, it determines the number of nucleation sites and the space of the growth of products
The bainite start transformation (Bs) temperature is found to be lowered with decreasing austenite grain size under the continuous cooling condition, a very similar trend to that found with the martensite start transformation (Ms) temperature
On the basis of the dilatation curves, the transformation start/finish temperatures could be confirmed with a normal extrapolation method [15]
Summary
Prior austenite grain size is a vitally important parameter to influence phase transformation behavior in steels, because to a great extent, it determines the number of nucleation sites and the space of the growth of products. It is well recognized that fine prior austenite grain size decreases the martensite start transformation (Ms) temperature, as the. Hall–Petch strengthening effect for fine austenite grains improves resistance to the plastic deformation of martensite transformation [1]. It further refines the martensitic block width and packet size, or likely forms a phenomenon of variant selection in the crystallography, which plays a critical role in tailoring the final mechanical properties [2,3,4,5]. The bainite start transformation (Bs) temperature is found to be lowered with decreasing austenite grain size under the continuous cooling condition, a very similar trend to that found with the Ms temperature. It is taken for granted that they have a similar mechanism for the decrease in Metals 2018, 8, 988; doi:10.3390/met8120988 www.mdpi.com/journal/metals
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