In this study, the correlation between HAZ microstructure and low temperature impact toughness of bainitic steel plates was investigated. The Steel with high carbon content and finish cooling temperature has a higher volume fraction and large packet size of granular bainite compared to the steel with low carbon content and finish cooling temperature. The room temperature tensile properties of the two steels are similar, mostly because the microstructure is composed of acicular ferrite having fine grains. On the other hand, the low temperature Charpy absorbed energy of two steels showed a big difference. As the test temperature decreased, the Charpy absorbed energy of the steel with high carbon content and finish cooling temperature decreased more rapidly than the steel with low carbon content and finish cooling temperature. This is due to the high volume fraction of granular bainite and large packet size of granular bainite in the steel with high content and finish cooling temperature. HAZ specimens have a very complex microstructure with a mixture of acicular ferrite, granular bainite, and bainitic ferrite through the rapid cooling process. In the HAZ specimen with high carbon content, more volume fraction of granular bainite + bainitic ferrite was formed, and the packet size of granular bainite + bainitic ferrite was also coarse. Because of this result, the HAZ specimen with high carbon content has low Charpy absorbed energy compared to the HAZ specimen with low carbon content.

In this study, the effects of Mo and Nb on the microstructure and Charpy impact properties of coarse-grain heat affected zone (CGHAZ) in high-strength low-alloy (HSLA) steels were investigated. The high-Mo steel had a lower volume fraction of acicular ferrite (AF) and higher volume fraction of granular bainite (GB) than those of the low Mo steel. The high-Mo CGHAZ specimen had a lower volume fraction of AF and higher volume fraction of GB compared to the low-Mo CGHAZ specimen. This is because, an increase in Mo lowers the bainite transformation temperature, induces the formation of GB and bainitic ferrite (BF), and reduces the diffusion velocity of carbon, thereby inhibiting the formation of ferrite. The Nb-added steel had a volume fraction similar to that of AF but smaller packet sizes of GB compared to the non-Nb steel. Compared to the non-Nb CGHAZ specimens, the Nb-added CGHAZ specimen had a lower volume fraction of AF, a higher volume fraction of GB, smaller packet sizes of GB and BF, and a smaller prior austenite grain size. This is because, Nb forms Nb(C,N) to inhibit the grain growth, thus inhibiting the formation of GB and BF. The Nb-added low-Mo CGHAZ specimen with the highest AF fraction and smallest average grain size had the highest Charpy-absorbed energy. However, articles in the Journal of Welding and Joining reported that the other CGHAZ specimens with high volume fractions of GB and BF with coarse packets showed low Charpy-absorbed energy.