Study On Low-cycle Fatigue Property Of 30Cr1Mo1V Steam Turbine Rotor Steel

With the frequent start-stop or the great load variation, the steam turbine rotor often bears the drastic changing of temperature and tremendous thermal stresses, plastic deformation will be occurred in local area, which lead to low-cycle fatigue damage, it will affect the safe operation of steam turbine. The influence factors of the property of high temperature low-cycle fatigue for steam turbine rotor steel material which including loading waveform, temperature, etc. have been investigated by domestic and foreign scholars. In the paper, the investigations have been carried out based on experiments which considering the effect of loading rate on the high temperature low cycle fatigue, it will provide a reference for the research of property and designing of steam turbine rotor.The experiments were carried under stain control using extensometer mounted on the specimen, the total stain amplitudes was varied between 0.4% and 1.0%, and the ratio R of the strains was -1, the waveforms were tension-compression symmetrical triangular, the loading rates were 0.1%/s, 0.3%/s, and 0.5%/s. The temperature was 538℃. Based on the experiments, the effects of loading rate on low cycle fatigue characteristics were analyzed. The results showed that the cyclic stress increases with the increasing of loading rate, the cyclic stress shows softening properties with the cycles increasing. The high temperature low-cycle fatigue damage evolution equation for 30Cr1Mo1V rotor steel based on the continuum damage mechanics was derived, and stress amplitude method was selected to calculate actual damage. The damage exponentαincreases linearly with the increasing of loading rate. When the loading rate is from 10-3/s to 0.2/s, the low-cycle fatigue damage of 30Cr1Mo1V steel decreases with the loading rate increasing under the given strain amplitude at 538℃, exceeding the range, the effect of the loading rate needn't considering. And at the same loading rate, the low-cycle fatigue damage of 30Cr1Mo1V increases with the strain amplitudes increasing.The effect of the loading rates on the low cycle fatigue life of 30Cr1Mo1V rotor steel was researched at 538℃. The result showed that at the same strain amplitude, with the loading rate increasing, the fatigue life increases, and with the increasing of the loading rate, the effect on the life shows a weakening trend. At the large strain amplitude, the loading rates have less effect on the life. With the strain decreasing, the effect of loading rate on the life increase. In addition, with the strain increasing, the fatigue life decreases. The low cycle fatigue life was predicted by the amending frequency correction formula method, Masson-Coffin formula and tensile hysteresis energy damage function method. The results of low cycle fatigue life prediction shows that the life prediction capability of the frequency correction formula which considerate the effect of loading rate is better than those of Manson-Coffin equation and tensile hysteresis energy damage function method.The fracture morphology and the fracture longitudinal morphology of 30Cr1Mo1V rotor steel were analyzed by the SEM microscope. The fracture zone is divided into crack origin area, crack propagation area and fast fracture area. Crack origin area is smoother, more flat. The numbers of the crack are more in the crack propagation region, and the cracks extend towards one direction. This stage of crack growth rate is slower, but it occupies most of the life. The fast fracture area is rough, and it shows a certain angle shear lip with the other areas. It can be seen that the cracks originate from the defect of the sample surface and the oxidation of the fracture is more serious due to lower loading rate, it is mainly because that the sample at high temperature stayed relatively longer at the lower rates, the oxidation is more sufficient, and the oxidation of crack tip is more severe. The crack growth rate was accelerated by these oxides, so the fatigue life was reduced. It can be seen that the oxidation of crack were more serious by the analysis of the longitudinal section of the fracture, the surface were covered with oxide layer. There were more melted beads near the crack surface, while few in the more distant regions.