Micro-crack Growth Behavior in Weldments of the Ni-Base Superalloy Hastelloy-X under Biaxial Low-Cycle Fatigue at High Temperature

摘要

Tensile and torsional fatigue tests on weldments of the Ni-base superalloy Hastelloy-X were carried out and micro-crack growth behavior in the weldment were investigated. Welded hollow cylindrical specimens of two types were prepared. One was welded in the axial direction while the other was welded in the circumferential direction. Strain-controlled low-cycle fatigue tests were carried out at 700℃. Fatigue life of welded specimens decreased to about one-half those for base metal specimen and the fatigue strength reduction factor was about 1.4. The micro-cracks were also observed by replication. In both weld and base metal, micro-crack initiation was observed in the early stage of life. The initiated micro-crack length in weld metal was, however, about 0.5 mm while the equivalent figure for base metal was about 0.1 mm. The crack-growth life from 0.5 mm to failure of the base metal specimen almost coincided with the failure life of the welded specimen. It has also been confirmed that the principal strain range gives a good correlation with crack-growth rates in biaxial stress states. The crack-growth rates in both weldment and base metal were correlated in the same scatter band. This result shows that the fatigue strength reduction is not due to the strain concentration in the weldment. The micro-crack length at initiation affects the fatigue life of weldments of Hastelloy-X.