Alloy 625 Forgings: Thermo-Metallurgical Model of Solution-Annealing Treatment

Metallurgical optimization of engineering alloys is traditionally addressed to improve the overall performance from a mechanical point of view. Grain size is one of the most influential and critical parameters to be controlled in nickel alloys, especially in the high-temperature shaping process and final solution treatment, since it can irremediably damage the alloy performance. For this reason, grain coarsening of alloy 625 was investigated in the temperature interval from 980 to 1150 °C ranging from 0.5 to 6 h. The grain-coarsening data were fitted via regression analysis as a function of time and temperature to develop a predictive model. Grain boundary strengthening was studied by hardness and tensile tests, and the relationships between the grain size and the mechanical properties were finally determined by regression analysis. Such equations were included in a thermo-metallurgical model able to predict the mechanical properties after annealing treatment. This predictive model was validated on a forged tube subjected to solution annealing at 1150 °C for 90 min. Then, it was finally used to compare different microstructural conditions in terms of the alloy impact on the environment.