Influence of chromium content on the bainite transformation nucleation mechanism and the properties of 800 MPa grade low carbon bainite weld deposited metal

The strength-toughness trade-off theory is always the focus in the field of high-strength structure materials. Especially for the deposited metals, the matching of high-strength and accepted toughness is difficult to achieve. In this work, the nucleation mechanism of bainite ferrite (BF) and impact toughness were investigated systematically for the deposited metals with different chromium (Cr) contents (0.3 wt %, 0.6 wt % and 1.0 wt %). The microstructure of the deposited metals with different Cr contents was analyzed by a combination of multiple analytical tools: scanning electron microscope, transmission electron microscope and electron backscatter diffraction. The results revealed that autocatalytic nucleation during the bainite transformation had a significant effect on the microstructure evolution and the impact toughness due to the complex crystallographic orientation of BF. As the Cr content increased, the autocatalytic effect became weaker, and the selection tendency of variants enhanced. The autocatalytic nucleation during bainite transformation was primarily controlled by the transformation kinetics of BF. Enough space at the side of the existing BF resulted from the low transformation speed of bainite was necessary for the occurrence of autocatalytic nucleation. In addition, toughness decreased as autocatalytic nucleation weakened due to the coarsening of microstructure and appearance of coalesced bainite in the microstructure of deposited metal. The interlaced BF arrangement could bring a better combination of high strength and toughness for the deposited metals.