Characterization of coarse bainite transformation in low carbon steel during simulated welding thermal cycles

• Main microstructure changes and the width of lath structure increases with cooling time.
• Variant grouping changes from Bain zone to close-packed plane grouping with the transformation.
• The change of variant grouping results in uneven distribution of high angle grain boundary.
• Bain grouping is main mode for large heat input, which lowers the density of high angle boundary.

Coarse austenite to bainite transformation in low carbon steel under simulated welding thermal cycles was morphologically and crystallographically characterized by means of optical microscope, transmission electron microscope and electron backscattered diffraction technology. The results showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to granular bainite with the increase in cooling time. The width of bainitic laths also increases gradually with the cooling time. For a welding thermal cycle with relatively short cooling time (e.g. t8/5 is 30 s), the main mode of variant grouping at the scale of individual prior austenite grains changes from Bain grouping to close-packed plane grouping with the progress of phase transformation, which results in inhomogeneous distribution of high angle boundaries. As the cooling time is increased, the Bain grouping of variants becomes predominant mode, which enlarges the effective grain size of product phase.