Understanding the effect of temperature and time on Gamma prime coarsening for Nickel-base superalloy Haynes 282
Year: 2019
Type: dissertation
Abstract: Haynes 282 is a gamma prime (𝛾′)-strengthened nickel base superalloy developed in 2005, exhibiting a good combination of high temperature properties and fabricability. Microstructural features such as 𝛾′ and carbides play an important role in deriving the mechanical properties of the alloy during heat treatment. As Haynes 282 is a relatively new alloy with insufficient literature availability, the present thesis is aimed at studying the evolution of microstructure for different heat treatment times and temperature with a special focus on 𝛾′ phase precipitation kinetics with different initial conditions for the material. The study is divided into two sections with objectives which are focused on the different ends to the heat-treatment time scales. The first objective of this study was to investigate γ' precipitation at short heat treatment times and develop Time-Temperature Precipitation (TTP) and Hardness (TTH) diagrams for Haynes 282 using a novel arc heat treatment. In this technique, a steady state temperature gradient, covering room temperature to liquidus, was created using stationary TIG arc on a disc mounted on a water-cooled chamber. Aged and solutionized samples were arc heat treated for 1.5 minutes, 30 minutes and 4 hours. The study was complemented with temperature modelling, thermodynamic calculations, and 𝛾′ precipitation simulation. A unique graded microstructure formed, consisting of dendritic region in fusion zone; dissolution area of all phases including MC carbides, grain boundary carbides, and 𝛾′; grain boundary carbide zone, 𝛾′ band; and base metal. 𝛾′ precipitate size increased with increasing time and temperature. 𝛾′ precipitation simulation model was developed, and it showed very good agreement with experimental results. Finally, the results were summarized in TTH and TTP diagrams. The second objective in this work was to study understand the coarsening behaviour of 𝛾′ phase with an initial pre-heat-treated GKN heat treatment using furnace heat treatment. Isothermal heat treatments for temperatures from 800°C to 1120°C and times from 30 seconds to 96 hours were performed. Morphological changes in 𝛾′ phase, particle size distribution, grain sizes and hardness on these isothermal heat-treated states are presented in this work. Additionally, A TC PRISMA precipitation model was evaluated to predict 𝛾′sizes and compare it with the measurements. It was concluded that complex initial microstructure, containing bimodal distribution of 𝛾′ precipitates, caused deviations between predicted and measured values, while the model, in the previous objective, predicted the sizes in close approximation to the experimental values. Therefore, further understanding and development of precipitation kinetics with the software should be done to achieve closer results to the experiment.
Author: Kevin Vattappara
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3135083170