調質高張力鋼潜弧溶接熱影響部の切欠靱性と溶接軟化に関する研究(第2報)

Abstract

Of the metallurgical factors influencing the notch toughness of submerged-arc weld heat-affected zone, study was made on the effect of pre-treatment of base metal on notch toughness of coarse grained heat-affected zone, especially for steel containing strong carbide or nitride forming elements.Also the effect of cooling rate and microstructure on the notch toughness of duplicated weld heat-affected zone was investigated with maximum heating temperature 1350°C.Notch toughness of duplicated weld heat-affected zone in solution-treated steel is lower in general. In steels containing niobium, vanadium and titanium, notch toughness of duplicated weld heat-affected zone is higher in rapidly cooled cycles, when the base metal is normalized or quenched, both followed by spheroidizing treatment. Austenite grain size in heat-affected zone is smaller, comparing to that of steel free from such elements.However, the effect was hardly observed in duplicated weld heat-affected zone cooled with slower rate of cooling, 7°C/sec at 540°C.In ferritic steels such as Steel AtoH, slower the cooling rate of duplicated weld heat-affected zone, the lower the notch toughness greatly. While low alloy high tensile steel such as Steel ItoL, cooling rate dependency of notch toughness is smaller, i.e., low alloy high strength steel with higher hardenability has superior notch toughness in weld heat-affected zone in case of large heat input welding.According to the results for quenched and tempered Steel I highest notch toughness is obtained, when the weld heat-affected zone is cooled, keeping up with the Cf' time of CCT diagram. Especially the peak of notch toughness is brought by mixed structure of lower bainite and martensite.