Experimental characterisation and computational modelling of cyclic viscoplastic behaviour of turbine steel

Abstract

Fully reversed strain controlled low cycle fatigue and creep-fatigue interaction tests have been performed at ±0.7% strain amplitude and at three different temperatures (400°C, 500°C and 600°C) to investigate the cyclic behaviour of a FV566 martensitic turbine steel. From a material point of view, the hysteresis mechanical responses have demonstrated cyclic hardening at the running-in stage and subsequent, hysteresis cyclic softening during the rest of the material life. The relaxation and energy behaviours have shown a rapid decrease at the very beginning of loading followed by quasi-stabilisation throughout the test. A unified, temperature-and rate-dependent visco-plastic model was then developed and implemented into the Abaqus finite element (FE) code through a user defined subroutine (UMAT). The material parameters in the model were determined via an optimisation procedure based on a genetic solver. The multi-axial form of the constitutive model developed was demonstrated by analysing the thermomechanical responses of an industrial gas turbine rotor subjected to in-service conditions. A sub-modelling technique was used to optimise the FEA. A 2D global model of the rotor with a 3D sub-model of the second stage of the low pressure turbine were then analysed in turn. The complex transient stress and accumulated plastic strain fields were investigated under realistic thermo-mechanical fatigue loading (start-up and shutdown power plant loads). The sub-model was then used for local analysis leading to identification of potential crack initiation sites for the presented types of blade roots.

Characterization of cyclic behavior, deformation mechanisms, and microstructural evolution of MarBN steels under high temperature conditions

“…• The hysteresis loops are crescent shaped and shrank with a point tail in the compressive region at the last stages before the macroscopic rupture. This observation has been reported for many high temperature steels (Rae et al, 2019));…”

“…These were monitoblack by three thermocouples attached to the test samples, with temperature variations not exceeding ±1°C. It is worth nothing that for high temperature steels, the creep mechanisms become increasingly significant at temperatures above 300-400°C (Rae et al, 2019). The reason for having the temperature gap between 400°C and 625°C is i) to emphasize the fatigue plasticity effects as dominant mechanisms in the deformation process and ii) to ensure that the interaction between timedependent (creep viscosity) and time-independent deformation (fatigue plasticity) mechanisms is less pronounced.…”

“…Classical strain-controlled tests with and without dwell periods are often conducted under isothermal (and/or anisothermal) conditions to understand and calibrate the "hysteretic" mechanical response of power plant materials (Saad et al, 2013;Benaarbia et al, 2018a;Rae et al, 2019).…”

“…The second is the decelerated stress relaxation behaviour which is ascribed to the blackuction in the viscous stress due to continuous cyclic softening. As interest developed in high temperature fatigue, it was found that in the creep range fatigue endurances are significantly shorter, particularly when hold periods are introduced (Rae et al, 2019). The reason for the blackuced fatigue life is that, in addition to fatigue damage due to cyclic loading, creep damage is also accumulated during hold periods.…”

High temperature strain heterogeneities tracking within hole-specimens of FV566 turbine steel via digital image correlation

“…The material used for the experimental investigations is a FV566 martensitic stainless steel cut from an area close to the centerline of a service-aged gas turbine rotor. The later had previously been subjected to 90,000 operational hours at a maximum speed of 3000rpm and temperature of approximately 420°C [3]. The steel was then tempered at 650℃ for 4 hours after normalizing at 1050℃.…”