Further, the creep crack growth rate of the flux cored arc weld metal, which contained large amounts of bismuth, was exceedingly rapid compared with that of SMA and FCA weld metal not containing bismuth. Bismuth that has segregated at the grain boundaries is extremely harmful with respect to creep ductility and creep crack growth properties. Hot cracks are caused by internal stresses that develop after solidification and during cooling from an elevated temperature in the welding process.
Cracks develop as creep fracture elongation decreases during use.
However, assuming that the aforementioned cracking is ascribable to these causes, as the cracks occur along the grain boundaries, detection of such cracks by nondestructive inspection methods while they are still small is, by their very nature, extremely difficult. Namely, to be able to adequately assess equipment safety in cases where the existence of embedded cracks is assumed, research is necessary with regard to creep crack growth properties that will quantitatively clarify how such cracks will propagate during use of the equipment concerned. The welding process and bismuth content of the weld metal were varied to produce three types of samples. Sample NS was added to determine whether the difference in welding process due to the noninclusion of Bi would have any influence on creep properties. Table 1 gives the welding conditions of the NF, NFB and NS materials.