抄録
Effects of morphology of grain boundary carbides and grain boundary configuration on low-cycle fatigue properties were investigated at 700°C with various strain wave-forms, using an austenitic heat resisting steel SUH 38 (JIS) precipitation-hardened by M23C6 carbides. In ordinary straight grain boudaries, fatigue life decreased remarkably with decreasing strain rate in triangular or sawtooth strain wave-forms and increasing hold time in trapezoidal one, due to intergranular cracking induced by creep deformation during strain cycling. In particular, the notable decrease in fatigue life due to creep damage occurred in unsymmetrical strain wave-forms where the creep deformation accumulated only during tensile straining. The zigzag grain boundaries with coarse carbides could prevent an intergranular cracking by virtue of the retardation of grain boundary sliding and the good coherency between coarse grain boundary carbides and matrix. Consequently, grain boundary strengthening by zigzag boundaries could improve remarkably the fatigue life even in unsymmetrical strain wave-forms where the ordinary straight boundaries caused a drastic creep damage.
