Microstructural characterization of Haynes 282 after heat treatment and forging

Licentiatavhandling, 2015

Over the past 75 years, solid-solution strengthened superalloys have been one among the most widely used materials for long term applications at elevated temperatures. The combination of properties such as high temperature strength, resistance to oxidation and corrosion, fabricability, and creep strength make them an unusual class of materials, attracting researchers and scientists to explore its full potential. Among this group, the nickel superalloys find wide applications in aero engines and land-based gas turbines. They are still being modified in chemical composition to meet the increasing demands of aircraft and energy producing industry. One such newly developed Ni-base alloy is Haynes 282. Haynes 282 showed sensitivity to heat treatment temperatures. The heat treatment temperatures were varied around the conventional heat treatment and within the typical tolerance limits. The microstructural development was systematically studied at intermediate stages through microscopy. To understand the influence of microstructural change on mechanical properties tensile testing was performed at room temperature. The gamma prime (γ׳) morphological change was observed to change from cuboidal to spherical to bimodal in three different heat treatment conditions. The carbide morphology changes from interconnected to discrete morphology. The strength of the material is affected by the size and shape of the cuboidal γ׳ precipitates, while the ductility at room temperature seem to be affected by interconnected morphology of the carbides at the grain boundaries. Haynes 282 are used in different forms such as forgings, bars, sheets in component applications. The important aspect of such alloy is to understand the structure/property relations at in-service conditions. Haynes 282 in form of forgings showed ductility variations in short transverse direction (ST) from 24% to 12% as compared to its longitudinal transversal (LT) direction. The lower limit of ductility is close to the design tolerance and thus creates a need to understand the variation in ductility. In this part, the study is focused to understand ductility variation by microscopic investigations. The influence of carbide segregation and banding is seen to influence the ductility when oriented perpendicular to the tensile axis. This influence is also qualitatively captured through micromechanical modelling.