Abstract
Low cycle fatigue behavior of two Ni-Cr-W alloys (Ni-15Cr-25W (KSN) and Ni-23Cr-18W (113MA)) was investigated at 1 273 K in the simulated High Temperature Gas-cooled Reactor (HTGR) helium, He-2. Both alloys showed little cyclic hardening in fatigue tests and failed in a completely intergranular manner. Although the fatigue life (Nf) of 113MA was a little longer than that of KSN, observed intergranular fractures indicated that the fatigue properties of both alloys were severely influenced by impurities in He-2.
As well as in He-2, fatigue tests of KSN and 113MA were carried out in pure He, air and vacuum. The results clearly showed that, regardless of the concentration of reactive gases, both alloys failed intergranularly with short Nfs in gaseous environments, as compared with the transgranular fracture with long Nf in vacuum. A minor effect of impurity contents was shown as an increase in Nf with substituting the environment from air then He-2 to pure He. It is considered from these behaviors that the fatigue fractures were affected by residual oxygen gas in the environments penetrating into specimens along the grain boundaries.
