PSI - Issue 23

Jaroslav Polák et al. / Procedia Structural Integrity 23 (2019) 275–280 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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stainless steel grade UNS S31035 Sanicro 25 has been developed by Sandvik for the next generation of A-USC power plants mainly for use in superheaters and reheaters in advanced coal fired power boilers which should resist temperatures up to 700 °C . Low cycle fatigue properties of this material were studied both at room temperature and at temperature 700 °C , Polák et al. (2014). Also initiation of fatigue cracks has been studied at room (Po lák et al. (2016)) and at elevated te mperatures (Mazánová et al. (2018), (Li et al. (2019)) as well as during thermomechanical fatigue (Petráš et al.( 2016)). Since time and varying temperatures represent important factors for life of components it is necessary to study the effect of time and temperature history. Standard procedures which allow characterizing the resistance to component failure are cycling with dwells and thermomechanical fatigue. In this contribution low cycle fatigue testing and in-phase thermomechanical fatigue testing of Sanicro 25 specimens is reported simultaneously with the analysis of the damage introduced to the material. The austenitic heat resistant stainless steel Sanicro 25 was supplied by Sandvik, Sweden in the form of cylindrical rod of 150 mm in diameter. The chemical composition of material in wt% is 0.1 C, 22.5 Cr, 25.0 Ni, 3.6 W, 1.5 Co, 3.0 Cu, 0.5 Mn, 0.5 Nb, 0.23 N, 0.2 Si and the rest Fe. Half-products were annealed at 1200 °C for one hour and ccoled in air. Cylindrical specimens had the diameter of 6 mm and the gauge length of 15 mm. The gauge length was mechanically and electrolytically polished. Servohydraulic testing machine was used to apply constant total strain rate 2x10 -3 s -1 at 700 °C and various strain amplitudes. 10 min tensile dwell was applied at maximum strain in the loading cycle. Thermomechanical testing was performed on cylindrical specimens having 7 mm diameter and gauge length. Temperature interval was 250 °C to 700 °C with various constant mechanical strain amplitudes . The change of temperature with the rate 4 K/s during in-phase cycling resulted in the strain rate 5.3× 10 -5 s -1 for the lowest strain amplitude 3 × 10 -3 and 1.07 × 10 -4 s -1 for the highest strain amplitude 6 × 10 -3 . The strain was in both cases measured and controlled by the high temperature longitudinal extensometer with ceramic bars spaced apart 12 mm. The mechanical strain was calculated as total strain minus thermal strain. The surface evolution of the specimens subjected to cycling was investigated by means of the scanning electron microscope (SEM) Tescan Lyra3 XMU FESEM equipped with focused ion beam (FIB) and with X-Max80 energy dispersive X-ray spectroscopy (EDS) detector for X-ray microanalysis and electron back-scattered detector (EBSD) by Oxford Instruments with Aztec control system. The profiles of the secondary cracks were exposed using FIB cutting. The internal damage of the cycled specimens and crack paths of growing cracks were studied on the longitudinal cross-sections parallel to the loading axis. The crack paths were revealed using electron backscatter diffraction (EBSD) technique. Fig. 1 shows fatigue hardening/softening curves in cycling with constant strain rate and with the same constant strain rate but 10 min dwell period at maximum strain. In both cases the materials cyclically hardens from initial stress amplitude around 200 MPa to 300 up to 400 MPa. The hardening is slower in cycling without dwells but in both cases there is a tendency to saturation. Rapid hardening in case of cycling with dwells result in early saturation and pronounced saturation region. However, the fatigue life of specimens cycled with dwells is substantially shorter. Fig. 2 shows the Manson-Coffin plot for both sets of specimens. Experimental data were fitted with the Manson Coffin law in the form = ′ (2 ) (1) where ′ is the fatigue ductility coefficient and c is the fatigue ductility exponent. Table 1 gives the values of both parameters evaluated using least square method. 2. Experimental 3. Results and discussion 3.1. Cycling with dwells