Correlation Between HAZ Microstructure and Low Temperature Impact Toughness of Bainitic Steel Plates

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.