Life Assessment Methods for Inconel 600 as an Essential Material in Micro Gas Turbines

Abstract

In this research, a comprehensive investigation was conducted to understand the mechanical behavior and fatigue characteristics of Inconel 600. The study involved a series of experimental tests and Finite Element simulations, which yielded useful information about the material's properties, fatigue life, and fracture behavior of Inconel 600. The experiments provided crucial data on the suitability of Inconel 600 for various applications. The mechanical properties, including yield strength, ultimate tensile strength, elongation, and modulus of elasticity, were determined through quasi-static tests under uniaxial loading conditions. The stress-strain relationship beyond the elastic limits was described using the Ramberg-Osgood equation. Furthermore, microstructural analysis revealed variations in grain size and microstructures in different regions of the welded joints, with the Heat-Affected Zone (HAZ) showing distinct microstructural changes due to thermal cycling during welding. These changes significantly influenced the mechanical properties and fracture behavior of the material. The rapid heating and cooling cycles experienced by the HAZ lead to alterations in the material's microstructure, including grain growth, formation of new phases, and changes in grain boundary structures. These transformations directly impact the material's strength, ductility, and susceptibility to fracture, highlighting the intricate interplay between welding processes and microstructural evolution. Fatigue tests were performed under both smooth and notched conditions to investigate fatigue life variations based on different loading conditions. The FE simulations, along with empirical fatigue damage models, provided accurate predictions of fatigue life and identified critical loading conditions that could lead to fatigue failure. While the FE analysis showed a good correlation with experimental fatigue data, some discrepancies were observed, likely due to internal defects associated with the welding process. The study emphasized the importance of considering notch strength reduction factors and mixed mode loading to enhance the precision of fatigue predictions. A case study involving finite element analysis (FEA) was conducted on a KJ66 micro-turbine blade to evaluate the effects of laser shock peening (LSP). The analysis revealed that LSP significantly reduced von-Mises stress and enhanced the fatigue life of the turbine blade by more than ten times, shifting the critical crack-prone areas away from the blade root. The investigation into fracture behavior under tensile and fatigue loading conditions was conducted through fractographic analysis using SEM images. It was found that the presence of welding influenced the fracture mode, with welded samples exhibiting a propensity towards brittle-type fractures, in contrast to the more ductile fracture behavior observed in un-welded samples. This shift towards a brittle fracture mode in welded samples is indicative of altered material properties resulting from the welding process, such as increased susceptibility to crack initiation and propagation under applied loads. Stress concentration points introduced by welding led to preferential crack initiation along grain boundaries, contributing to brittle intergranular features.