A study on the Effect of Hot Isostatic Pressing on Tensile Properties and Creep Behaviors of Fine-grained MAR-M247 Superalloy via Microcast

摘要

The effect of hot isostatic pressing (HIP) on the tensile properties and creep behaviors of fine-grained MAR-M247 superalloy was investigated in this study. A Microcast method was used to cast the test bars with grain sizes of 70 and 90 um, and then part of test bars were proceeded by HIP in order to compare the samples without HIP. The experimental results show that the tensile strength and elongation of the test bars without HIP are significantly poor due to the existence of microporosity resulting from the liquid metal solidification and contraction. It is found that the microporosity in fine grain casting can be eliminated after HIP and the tensile strength and elongation are significantly improved. However, the yield strength of HIPed samples is decreased as the results of coarsening of γ'phase during the cooling processing after HIP. Therefore, the temperatures of solid solution must be higher than that of HIP in order to eliminate the coarseγ'phase during cooling. Meanwhile, the fine γ'phase will be precipitated during aging treatment to enhance yield strength. In the creep test under moderate temperature/high stress (760℃/724 MPa), the creep life of a HIPed sample with 70μm. grain size is drastically enhanced to 10-20 time than that of non-HIPed sample. Also, the creep elongation is greatly increased (3-5 times) and the scattering of properties is remarkably improved. Under high temperature/low stress(982℃/200 MPa) creep where the creep mechanism is changed from dislocation glide to grain boundary sliding, the sample with 90μm grain size still shows superior creep resistance after HIP due to the elimination of microporosity distributed along the grain boundary.