Effect of the cooling rate during heat treatment and hot isostatic pressing on the microstructure of a SX Ni-superalloy
Lehrstuhl Werkstofftechnik, Ruhr-Universität Bochum, 44801 Bochum, Germany
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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.
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