Modelling of Strengthening Mechanisms in Wrought Nickel-Based 825 Alloy Subjected to Solution Annealing

Abstract

Wrought nickel-based Alloy 825 is widely used in the oil and gas industries, attributed to its high strength at temperatures up to 540 °C. However, differences in mechanical properties arise in finished components due to variations in both grain size and dislocation density. Numerous experimental studies of the strengthening mechanisms have been reported and many models have been developed to predict strengthening under thermomechanical processing. However, there are debates surrounding some fundamental issues in modeling and the interpretation of experimental observations. Therefore, it is important to understand the evolution of strain within the material during the hot-forging process. In addition, there is a lack of research around the behavior during hot deformation and subsequent stabilization of Alloy 825. This article investigates the origin of this strength and considers a variety of strengthening mechanisms, resulting in a quantitative prediction of the contribution of each mechanism. The alloy is processed with a total forging strain of 0.45, 0.65, or 0.9, and subsequent annealing at a temperature of 950 °C, reflecting commercial practice. The microstructure after annealing is similar to that before annealing, suggesting that static recovery is dominant at this temperature. The maximum yield strength and ultimate tensile strength were 348 MPa and 618 MPa, respectively, obtained after forging to a true strain of 0.9, with a ductility of 40%. The majority of strengthening was attributed to grain refinement, the dislocation densities that arise due to the large forging strain deformation, and solid solution strengthening. Precipitate strengthening was also quantified using the Brown and Ham modification of the Orowan bowing model. The results of yield strength calculations are in excellent agreement with experimental data, with less than 1% difference. The interfacial energy of Ti(C,N) in the face-centered cubic matrix of the current alloy has been assessed for the first time, with a value of 0.8 mJm−2. These results can be used by future researchers and industry to predict the strength of Alloy 825 and similar alloys, especially after hot-forging.