Abstract:

The crystallography of bainite, transformed isothermally at 450 °C in 0.2C–2.0Mn–1.5Si–0.6Cr steel, was investigated by electron backscatter diffraction (EBSD) analysis. The orientation relationship (OR) was found to be closer to Nishiyama–Wassermann (N–W) than Kurdjumov–Sachs orientation relationship. Bainite microstructure consisted of parallel laths forming a morphological packet structure. Typically, there were three different lath orientations in a morphological packet. These orientations were dictated by a three specific N–W OR variants sharing the same {111} austenite plane. A packet of bainite laths with common {111} austenite plane was termed as crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size. Locally, crystallographic packets with only two dominant orientations were observed. This indicates strong local variant selection during isothermal bainite transformation. The relative orientation between the variants in crystallographic packets was found to be near 60°/<110>. This appears to explain the strong peak observed in the grain boundary misorientation distribution near 60°. Bainite also contained pronounced fraction of boundaries with their misorientation in the range of 2.5°–8° with quite widely dispersed rotation angles. Spatially these boundaries were found to locate inside the bainite laths, forming lath-like sub-grains.

Key words: Bainite, Austenite, NishiyamaeWassermann orientation relationship, Electron backscatter diffraction (EBSD), Crystallography, Packet