Abstract
The fatigue–creep behaviors of 15CrNbTi and 15Cr0.5MoNbTi steels in the simulated automotive exhaust gas and in argon at 800 °C were investigated. The fatigue–creep tests were conducted under stress-controlled mode with a stress ratio of 0.1 at 800 °C in the simulated automotive exhaust gas and in argon on an electrohydraulic servo fatigue testing machine. A trapezoidal waveform was adopted, and when the cyclic stress reached the maximum tension stress, a 10-s hold time was introduced. The tests results show that 15Cr0.5MoNbTi steel has higher cyclic deformation resistance and fatigue life than 15CrNbTi steel in two atmospheres. Compared with the simulated automotive exhaust gas, both experimental materials possess higher cyclic deformation resistance and fatigue life in argon. The cracks of two experimental materials in two atmospheres were both transgranularly initiated from the free surfaces of the steels and propagated in a transgranular manner. The main substructures of two experimental materials after fractured in two atmospheres were sub-grain structures which formed during fatigue–creep tests. The synergistic effects of solid solution strengthening of Mo and precipitation strengthening of fine Fe2Nb phases increased the cyclic deformation resistance and the fatigue life of 15Cr0.5MoNbTi steel at 800 °C in two atmospheres. In addition, Mo element improved the oxidation resistance of 15Cr0.5MoNbTi steel in the simulated automotive exhaust gas, which has a beneficial effect on the prolongation of fatigue life.
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