Ni基合金Alloy 617の母材および溶接継手の長時間クリープ変形および破断特性

抄録

Long-term creep rupture behaviors of nickel-based Alloy 617 base and similar welded joint were investigated. Creep tests were conducted at 650°C, 700°C and 750°C using smooth bar specimens, and the creep deformation and fracture were discussed in relation to the changing microstructure. It was found that fracture ductility of base metal decreased with decreasing the temperature and was less than 10% at 650°C. There was little difference in time to creep rupture of the base metal due to product geometry. Times to creep rupture of welded joint were shorter than those of base metal at 700°C but were comparable at 750°C, and most of the welded joints were ruptured at weld metal with significant reduction of ductility in comparison with base metal. γ' in grain and carbides (M₂₃C, M₆C) at grain boundaries were formed in both base and weld metals during creep. Especially M₆C precipitated in the grains and coarsened after a long time and the amount in the weld metal was greatly higher than that in the base metal. Strain concentrated near the grain boundary and voids or microcracks were formed at the grain boundaries or interface between the carbides and matrix. The local strain near the boundary at fracture in the weld metal was smaller than that in the base metal observed from EBSD.