Fig. 1 (a) Metallographic structure of the bainite 2.25Cr-1Mo steel under OP and (b) the dimension specification of test specimen (in mm).
2.1 Cyclic test program
Uniaxial static creep and cyclic creep tests are carried out on an electronic creep testing machine RD-50, as shown in Fig. 2 (a). Three K-type thermocouples are uniformly distributed along the gauge section of the tested specimen, which is heated to 455 °C in air by a high-temperature heating furnace. A period of 30 minutes is held before loading to obtain the target temperature, within a fluctuation of ±1 °C. The deformations recorded on the left and right sides of the gauge section (denoted by dl anddr ) are measured by two sides of heat resistant rail-type extensometers correspondingly, with an accuracy of 1 μm. Hence, the actual deformation \(\overset{\overline{}}{d}\) can be determined by the mean value of dl anddr to reduce the error, and the engineering strain ε is obtained by the ratio of the actual deformation\(\overset{\overline{}}{d}\) and the initial gauge length L as:
\(\overset{\overline{}}{d}=\frac{d_{l}+d_{r}}{2}\)
\(\varepsilon=\frac{\overset{\overline{}}{d}}{L}\times 100\%\)
For cyclic creep tests, a uniaxial pulsating tension-tension wave is applied under stress control, as shown in Fig. 2 (b). Thetp and tv are the durations imposed at peak and valley stresses respectively, with\(\dot{\text{\ σ}}\) and \({\dot{\sigma}}^{{}^{\prime}}\) being the stress rates under loading and unloading. The peak stress \(\sigma_{\max}\) is 400 MPa and the stress ratio R is 0.375. The loading condition is fixed at a loading rate of 12 MPa/s with a durationtp of 60 min, but various unloading conditions are applied, with \({\dot{\sigma}}^{{}^{\prime}}\) being 0.6 MPa/s, 12 MPa/s, 19 MPa/s, 39 MPa/s and tv being 0 min, 10 min and 30 min. The static creep test under 400 MPa is also performed as a comparison. All specimens are tested till fracture, and test results are summarized in Table 2. The endurance life is defined as total fracture time deducting loading, unloading and valley stress duration time.