Effect of HIP on Elevated-Temperature Low Cycle Fatigue Properties of an Equiaxed Cast Superalloy

Abstract

A typical high strength equiaxed cast superalloy, B-1900, was tested in the fully heat-treated condition, preceded by an optional hot isostatic pressing (HIP) cycle. Fully reversed constant total strain amplitude testing was carried out at temperatures ranging from 760 to 982°C in air. The HIP cycle successfully eliminated all non-surface connected porosity, and specimens, machined from the center of test bars, were totally pore free. Changes in grain boundary carbide morphology and γ′ size also ensued from the HIP treatment. Fatigue crack initiation sites were identified in scanning electron microscopy (SEM) fractographic analysis along with modes of crack propagation. An enhancement of fatigue life by a factor of 6 to 8 was realized in the HIP alloy at temperatures of 871°C and below. Above this temperature no effect of HIP was observed. Cracks of HIP specimens initiated at grain boundary-surface intersections. Fatigue crack initiation was enhanced by local oxidation of surface-connected carbides, with fatigue crack propagation being predominantly intergranular. The non-HIP specimens typically contained surface cracks which propagated through near surface clusters of microporosity which in turn were formed adjacent to the interdendritic carbide networks. The HIP treating of equiaxed cast superalloys for turbine blade applications completely closed interdendritic and intergranular microporosity and caused grain growth to occur. This led to enhanced fatigue life at low temperatures where oxidation of grain boundary carbides was minimized. Fatigue life enhancement due to HIP was separated into microstructural modifications effecting yield stress and grain boundary structure.