Effect of Ni addition on toughness and microstructure evolution in coarse grain heat affected zone

A Gleeble simulation was performed on low carbon steels to investigate the effect of Ni addition on toughness and microstructure evolution in coarse grain heat affected zone from low heat input welding. Ni addition increased the impact toughness at −20 °C and the prior austenite grain size.Microstructures of the low Ni and high Ni steel show noticeable differences in crystallography. Lath-structures in the two can be defined as crystallographic packet. In low Ni steel the packet consisting of acicular ferrite and lower bainite has all the three Bain groups uniformly distributed; in high Ni steel the packet consisting of only lower bainite has a dominant Bain group within it, making the other two groups subsidiary. The closely linked M/A particles were found only in low Ni steel by tracking the orientation information. It is able to facilitate the fracture through the link-up of the micro-cracks initiated by each M/A particle. In the going direction of the link-up there is no high angle boundary which can deflect the propagation of crack, thereby in this case the high angle boundary in low Ni steel is relatively ineffective. The mechanism of the toughness improvement brought by Ni addition should be ascribed to the relatively uniform transformation to final microstructure, which reduces the appearance of closely linked M/A.