クリープ破断試験による溶接金属の高温延性の評価

ステンレス鋼FCAW溶接金属における再熱割れに関する研究(第2報)

抄録

To evaluate the high temperature ductility, which is considered to affect the reheat cracking sensitivity, creep rupture tests with Type 308 stainless steel weld metals made from various commercial welding materials at 823 K-1023 K have been performed. To clarify the effect of bismuth on creep embrittlement, creep rupture test with FCAW weld metal including Type 308, Type 316 and Type 347 containing various bismuth contents have been carried out at 823 K and 973 K.
The experimental results indicate that with an increasing Larson-Miller parameter value, the rupture strength of the Type 308 weld metal decreased linearly with decreasing rupture ductility. Among the welding processes using commercial welding materials, the FCAW weld metal showed the poorest rupture ductility. However the bismuth-free FCAW weld metal showed a better ductility compared to the bismuth containing FCAW weld metal. It is concluded that the main factor which causes creep embrittlement in the FCAW weld metal is bismuth which is usually added to the flux for improvement of slag detachability. Also, the Type 316 and Type 347 weld metals were embrittled by bismuth as in the case of the Type 308 weld metal.
The characterization of cracks in the creep rupture test specimen indicated that regardless of the bismuth content in the weld metal, the fracture mechanism can be categorized into two types, the transgranular type and the intergranular type. At the higher test temperature and/or the prolonged rupture time, the intergranular type fracture becomes dominant and the rupture ductility decreases. Bismuth is considered to severely affect the grain boundary embrittlement and enhances the degradation of rupture ductility.