Development of Forgeable Ni-Base Alloys for USC Steam Turbine Applications by use of Microstructure Simulation and Formability Testing

Abstract

For the industrial production of large safety components such as turbine discs and turbine shafts, FEM coupled microstructure modeling offers a possibility to describe the microstructural changes such as recrystallization and grain growth during forming. Therefore, the material performance during the production process can be judged by understanding of the microstructure information. In this project, the microstructure evolution and the formability during the industrial processes were investigated with the help of FEM simulations. For numerical investigation of a forging process, compression tests, stress relaxation tests and annealing tests were carried out. After the analysis of the experimental results and the metallographic research on the immediately quenched specimens, microstructure models for simulating recrystalization and grain growth on the basis of empirical-phenomenological equations were developed for each alloy. The models were verified and fitted by means of 3-step compression tests. Finally, a sequence of upsetting and hammer forging operations were simulated via FEM coupled microstructure simulation. To determine and compare the formability of the investigated Ni-alloys, compression tests were performed using specimens with flange geometry. The research was firstly focused on the commercial alloys Inconel 706, Inconel 617 and Waspaloy, to identify the best material candidate for the industrial processing and application of stationary steam turbine components. Based on these results, two novel alloy variants, "DT 706" and "DT 750", were developed and studied again with the above described approaches.