Fracture Mechanics Approach to Creep Growth in Welded IN738LC Gas Turbine Blades

Microcracks caused by hot cracking or strain age cracking mechanisms are very likely to be discovered in the weld repair zone of precision-cast IN738LC gas turbine blades. The possibility of crack propagation under the operating conditions of the gas turbine thereby becomes a crucial issue for gas turbine designers. The creep crack growth rate in air of the hipped and fully heat-treated IN738LC was measured at the service temperature experienced by the first-stage turbine blade tip. The corresponding growth behavior was also studied. The creep crack growth rate, da/dt, versus crack tip stress intensity factor, K_I, a relation that exhibits the typical primary, secondary, and tertiary behavior, supports the applicability of K_I, as an appropriate correlating parameter for the creep crack growth of this Ni-based superalloy under the loading conditions used in this study. Microstructural examination illustrated that the creep crack growth of IN738LC principally takes place by the nucleation, growth, coalescence, and link-up of grain boundary microvoids and microcracks. An excellent approximation of the stress intensity factor under service loading conditions in the vicinity of the crack tip was obtained by using the Westinghouse WECAN finite element analysis. It is shown that the crack tip stress intensity factor under normal loading conditions will not be able to drive the transverse through-the-wall-thickness blade tip crack in this study.