Effect of the cooling rate during heat treatment and hot isostatic pressing on the microstructure of a SX Ni-superalloy

Abstract

Single crystal Ni-based gas turbine blades show a combination of large casting pores and pores from the homogenization heat treatment. Both kinds of pores can only be reduced by HIP; however, HIP not only reduces porosity but also affects the size, number and morphology of γ ′ particles. From the HIP parameters, pressure, temperature, holding time and cooling speed, the main effect on both, the porosity and the γ / γ ′ microstructure is due to temperature and cooling rate. HIP temperatures above the γ ′ solvus temperature allow the fastest and most effective reduction of the porosity, because only the soft γ phase is present. The recent and novel possibility of cooling the samples from the maximum HIP temperature with a fast cooling of about 200 K/min, results in a fine and homogeneous distribution of γ ′ particles, which requires no additional solution annealing treatment to dissolve the developed γ ′ particles during the extremely short cooling time. Therefore, the application of HIP at super γ ′ solvus temperature followed by fast cooling on homogenized samples seems to have the most promising results: no porosity and fine γ / γ ′ microstructure.