2.1 Material and Test Program
The material used for both MTP and SSFS specimen tests is a martensitic FV566 steel extracted from an area close to the centerline of a gas turbine rotor, where it was service-aged for approximately 90,000 h at ~ 420 °C at a maximum speed of 3000 rpm. The chemical compositions of the material are (wt.%): C-0.6; Si-0.038; Mn-0.668; Cr-11.9; Mo-1.68; Ni-2.52; V-0.298; S-0.006 and Fe-remainder.
Uniaxial tensile, fully reversed saw-tooth, and hold-dwell LCF tests with a strain-rate of 0.01%s−1 until failure were carried out for SSFS specimens previously1. The geometry and dimension of the SSFS specimen with a 5 mm gauge diameter and a 10 mm gauge length is shown in Fig.1a. All the strain-controlled LCF tests and CF tests were conducted at 600 °C under a total strain-range of ± 0.7% using the two prescribed loading conditions, i.e., saw-tooth (Fig.1b) and dwell-type (Fig.1c) waveforms. A symmetrical triangular waveform (push-pull load ratioRe = -1) was employed for the saw-tooth loading, and a trapezoidal waveform for the dwell-type loading. For dwell-type tests, the dwell period duration was imposed at the maximum tensile strain for a period of 2.5 hours for the first cycle and 5 mins for all the subsequent cycles. The dwell duration was chosen after careful investigation of the stress relaxation times (i.e., the 5 mins hold period was enough to reach the quasi-equilibrium state of the viscous stress). The high temperature LCF and CF tests for SSFS specimens were carried out on a Tinius Olsen H25KS electromechanical testing machine. More information about the testing can be found in the refs1,3,49.