Influence of the quenching rate and step-wise cooling temperatures on microstructural and tensile properties of PER72® Ni-based superalloy

The PER72 (R) grade is used as a wrought engine turbine disk, which is a critical high temperature component. During the heat treatment process, residual stresses are generated during the quench, which may lead to irreversible damages on the workpiece. The aim of this study is to better understand the mechanisms involved in the residual stress generation. Therefore, the influence of quenching conditions on the high temperature tensile properties and the multi-scale microstructure evolutions are investigated after cooling. PER72 (R) specimens are annealed above the solvus temperature, directly on the servo-hydraulic testing machine. Three quenching rates are used: 30 degrees C/min, 120 degrees C/min, and 300 degrees C/min. For each condition, the cooling is interrupted at 1000 degrees C, 850 degrees C, 600 degrees C and 20 degrees C to perform isothermal tensile test. Specimens are post-mortem analysed. On one hand the fracture surface is investigated using SEM. On the other hand the microstructure evolution was observed and quantified at different scales using SEM directly on the bulk or after the chemical extraction of precipitation. The precipitation size and volume fraction statistics, X-Ray diffraction for the crystallography and composition of the different phases are investigated. It was shown that the testing temperature does not significantly influence the gamma' distribution of particles. Conversely, the gamma' precipitation is strongly influenced by the cooling rate. Notably, the average size, the distance between particles as well as the number density of gamma' precipitates are significantly modified by the cooling rate. Changes in tensile properties are related to microstructural.