Dynamics of phase transformations and microstructure evolution in carbon-manganese steel arc welds using time-resolved synchrotron X-ray diffraction

Phase transformations that occur in both the heat-affected zone (HAZ) and the fusion zone (FZ) of a carbon-manganese steel spot weld have been investigated using time-resolved X-ray diffraction (TRXRD) with time resolutions down to 50 ms. It is found that in both zones the gamma(f.c.c.) --> alpha(b.c.c.) transformation on cooling is twice as fast as the forward transformation of alpha --> gamma on heating. Profile analysis of the major Bragg reflections recorded in the TRXRD patterns reveals similarities and differences in the microstructural evolution with time in the HAZ and in the FZ. The latter undergoes melting and solidification in addition to solid-state transformations. With increasing temperature, the (110) d-spacing of the alpha phase prior to and during the alpha --> gamma transformation and the (111) d-spacing of the phase just after the same transformation exhibit a decrease. The observed (and unusual) lattice contraction with temperature rise may be attributed to chemical effects, such as carbide precipitation in the alpha matrix, and/or mechanical effects due to stress relief. In the FZ, the gamma-Fe that forms has a preferential (200) texture on solidification of the liquid, whereas, on cooling in the HAZ, the gamma-Fe retains largely a (111) texture that is induced in the alpha --> gamma transformation on heating. On cooling in the HAZ, the width of the gamma(111) reflection increases initially, which is indicative of microstrain developing in the f.c.c. lattice, but decreases as expected, with a reduction of thermal disorder, on further cooling until the completion of the gamma --> alpha transformation. In the FZ, however, the microstrain in the phase increases steadily on solidification and more rapidly for the duration of the gamma --> alpha transformation on further cooling. The final microstructure of the FZ is likely to consist of a single alpha phase dispersed in two morphological entities, whereas in the HAZ the alpha phase persists in one morphological entity in the final microstructure.