Thermomechanical creep and environmental effects on hot gas path in gas turbines MAR-M-247 stationary components

Abstract

Industrial Gas Turbines (GTs) have been mostly used in mechanical drive applications to run compressor lines in Oil & Gas pipelines. In the last decade the operability of GTs has moved towards power generation applications and as a back-up solution for recently developed renewable energy plants to allow for grid fluctuations. Further to this, the need to improve GTs efficiency has pushed the designers to increase the firing temperatures, to optimize the cooling of the hot gas path components and to deepen our knowledge on nickel and cobalt base superalloys. The end effect of reshaping GTs operability profile and of optimizing the design for allowing higher temperatures in the combustion chamber, is that hot gas path components are now subjected to more aggressive thermomechanical cycles. This is mainly due to the grid fluctuations, which are dramatically higher than what is expected in Oil & Gas pipelines. Furthermore the higher firing temperatures coupled with the improvements in cooling practices lead to more aggressive thermal gradients within the components. Further to this, the coupling effects of cyclic thermomechanical fatigue (TMF) and steady state creep will need to be better understood as up to now they have been considered separately. The aim of this dissertation is to define a methodology for coupling two models, within a single transient analysis in a commercial finite elements analysis (FEA) program. One model for assessing plastic strains, the Lemaitre-Chaboche (LC) viscoplastic model, and one for assessing creep strains, the theta projection model. This coupled analysis had not been conducted to date for this application over a large temperature range (870, 980 and 1038°C). The first step in the model definition was to enlarge the temperature range of previous low cycle fatigue (LCF) and creep studies for understanding MAR-M-247 behaviour and getting the parameters to be implemented in a mathematical model, focusing on temperatures and strain ranges, namely 0.4, 0.6 and 0.8%, which are relevant to GT operation. The microstructural MAR-M-247 response was investigated and was correlated to the mathematical parameters. The Lemaitre-Chaboche model was fitted to the data collected (cyclic stress vs strain curves), implementing both the multi-linear kinematic hardening as well as isotropic hardening models, through a Levenberg-Marquardt (LM) algorithm within a Matlab routine. Microstructural characteristics and fractographic observations were correlated to the coefficients mathematically interpolated. A correlation was found between the pure mathematical model and the material results. Complementarily, the theta projection creep model was investigated. The parameters for primary, secondary and tertiary creep were fitted against the results from constant stress testing, conducted by Baker Hughes, at the three temperatures of interest for LCF and 12 stress levels. Transfer functions correlating the temperature and the stress level to the creep parameters were developed. Finally, a demonstration of the coupling of the viscoplastic Lemaitre-Chaboche model and the theta projection creep model as applied to a nozzle transient FEA analysis, by Ansys parametric design language (APDL) and user programmable features (UPF), was verified against the results from a GT test performed in Florence at Baker Hughes. The FEA transient boundary conditions reflected the operating conditions of the tested gas turbine as per previous Computational Fluid Dynamics (CFD) validation. Besides extending the literature on available LCF and creep data for the MAR-M-247 up to 1038°C, a higher temperature than what already exploitable, a novel approach considering the mean compressive effects has been employed to define the LC model parameters. The parameters for both kinematic and isotropic hardening models have been identified and optimized for GTs representative strain ranges, thus extending the available literature data. It has been demonstrated that through a specific optimization process, the LC model defined at a specific strain range can be applied to predict the cyclic response of different strain ranges, thus simplifying the model implementation. The theta projection model methodology has been applied for the first time to MAR-M-247 and to a spread range of stress and temperature. Finally the two models have been verified for the first time in a coupled analysis of a full transient mission of a GT nozzle, leading to results in agreement with the nozzle data from the test bench. These results demonstrate a first improvement in GTs hot gas path components TMF analysis, leading to more reliable assessments of inelastic strains and then to a more reliable design. Future research may focus on the effects of considering more than one cycle in the analysis, focusing on creep relaxation and residual stress state on the cyclic response. Further to this a damage model to accurately predict the crack initiation as well as propagation may be developed. Finally, in order to improve the accuracy of the models, more experimental data over a larger range of temperature, would be required.

Date of Award	16 Sept 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Matthew Roy (Supervisor)