Microstructure and Toughness in Weld Metal and Its Reheated Zone of High Strength Steel

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The multipass welding are carried out for high strength low alloy steels such as 80 kgf/ <mm^2> class steel in order to mitigate the heat affected zone (HAZ) embrittlement. In that case weld pass is reheated to various temperature by welding heats of the subsequent ones. This effect will naturally lead to the microstructural change of weld metal, and as a result, to the change of fracture toughness.The objectives of this study were to provide a detailed analysis of microstructure that can be found in 80 kgf/ <mm^2> class tensile strength weld metal as a function of heat input, chemical compositions and reheating temperature which was decided by imagining multipass welding, and then to discuss the relationship between microstructure and toughness. The major findings of this work are as follows.(1) In the weld metal of high strength steel the degradation of toughness was found at high heat input welding conditions as well as in the HAZ of base metal. (2) Such an embrittlement seemed to be caused by both the increasing in the area fraction of M-A constituent and the coarsening of microstructure. (3) In the reheated zone of weld metal the degradation of toughness was found in the peak temperature range between 750 and 950゜C at the multi-thermal cycles in spaite of welding conditions. The degradation of toughness was remarkable at high heat input welding conditions. (4) This embrittlement also seemed to be mainly caused by the increasing in the area fraction of M-A constituent. (5) Therefore, it implied that the decreasing in the area fraction of M-A constituent was effective as a method of improvement of toughness in both the weld metal and its reheated zone.