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

摘要

Tensile-torsional-combined biaxial low-cycle fatigue tests on welded tubular specimens of the Ni-base superalloy Hastelloy-X were carried out and the micro-crack growth behavior in the weldments was investigated with the aim of improving life assessment methods for high-temperature components. Welded hollow cylindrical specimens of two types were prepared, one welded in the axial direction and the other welded in the circumferential direction. Strain-controlled low-cycle fatigue tests were carried out at 700 deg C. Fatigue lives of the welded specimens were about one-half that of the base metal specimens and the corresponding fatigue strength reduction factor was about 1.4. Micro-cracks also observed by using a replication technique during occasional interruptions of the tests. In both weld and base metal, the initiation of micro-cracks was observed in the early stage of life, but the initiated lengths of the micro-cracks in the weld metal was about 0.5 mm while the equivalent figure for base metal was about 0.1 mm. The Grain boundary oxidation would strongly affect these crack initiations. The crack growth life from 0.5 mm to failure in the base metal specimen almost coincided with the failure life in the welded specimen. Therefore, the reduction of the fatigue life of welded specimen is because the initiated lengths of micro-cracks in the weldment are greater the lengths in the base metal. The maximum principal strain was confirmed as a good parameter for the evaluation of crack growth rates for both weldments and base metal. This result shows that the fatigue strength reduction is not due to the strain concentration at the weld. The initiated lengths of micro-cracks affect the fatigue life of weldments of Hastelloy-X.