| Fatigue failure is a major factor threatening reliability of equipments, and its investigation has always become a hot but difficult topic in both engineering and technical fields. Recently, many process indurstries work at extreme conditions such as high temperature, high pressure, and thus undergo scale enlargerment, service life extension and higher risk. This new developing trend challenges the life and reliability of structures and components, as a result, failure prediction at early stage and long life design become new hot spots for study. In this work, by focusing on effect of microstructure on fatigue behavior, near-threshold fatigue crack growth (FCG) mechanisms of advanced steam turbine rotor steels in fossil power plant and very high cycle fatigue (VHCF) behavior of welded rotors in nuclear power plant were systematically investigated in terms of theoretical analyses, numerical simulations and experiments.The main research contents and conclusions are listed as follows:(1) It is the fact that the near-threshold FCG is strongly influenced by microstructure. For the high-low-pressure integral steam turbine rotor steel 25Cr2NiMolV, the microstructures in high pressure (HP) and low pressure (LP) parts are different. The microstructure dependence of the near-threshold FCG and fracture mode was illustrated by fatigue tests and microscopic analyses. It was found for the first time that the striation and crystallographic mode of crack propagation could be determined from the cyclic plastic zone size at crack tip being equivalent to prior austenitic grain size. Furthermore, the formation mechanism of surface faceted fracture was also correlated to the theory of the cyclic plastic zone. In addition, fatigue resistance of one specific microstructure and its contribution to crack closure were elucidated. These findings lay great foundations for the establishment of FCG models in the near-threshold regime.(2) It is still short of generalized FCG models in the near-threshold regime due to its complex influencing factors. The near-threshold FCG behavior of rotor steel 25Cr2Ni2MoV at different load ratios was investigated experimentally, and the FCG driving mechanism was theoretically analyzed based on equivalent driving force model. It was found for the first time that the crack growth process was determined by combined effects of equvalent driving force at constant amplitude loading and crack closure, whereas the crack closure was a function of both load ratio and FCG rate. Then the mechanism of the effect of load ratio on the crack closure was clarified, and the crack closure phenomenon was further related with the transitioin of driving forces in crack advance. These findings are beneficial to unify crack closure theory and crack growth driving parameters in the near-threshold regime.(3) A series of strength performance tests especially VHCF experiments were conducted by using welding joint in steam turbine rotors, which include:◆Based on the nano-and micro-hardness tests in the welding joint, the relationship between nano-hardness and micro-hardness was established. Lath width of the tempered martensites was selected as the characteristic microstructure size for correlating the micro-hardness, and therefore gave us confidence for the estimation of strength distribution in local zones.◆It was found that the location for the lowest strength in the welding joint was temperature dependent by tensile tests at service temperature range (200-370℃), and the mechanism of improving impact properties by multilayer welding technique was illuminated by impact tests at weld metal. These work can provide important information for evaluating the performance of welding joint and optimization of welding process.◆It was reported for the first time the duplex shape of stress-life (S-N) curves in the lower strength welding joint at service temperature in the VHCF regime. And factors affecting VHCF behavior were comprehensively analyzed by paying special attention to the temperature and specimen location. These findings are valuable for understanding the long life service behavior of welded structures in power plant.(4) It is known that fatigue failure in the very high cycle regime is strongly related to micro-defects. Based on the VHCF results at moderate temperature, the crack initiation transition from surface to interior and fatigue failure mechanisms were systematically investigated, which include:◆It was found that cracks were always initiated from interior inclusions, porosities and discontinuous microstructures by microscopic observations. And then internal crack initiation and propagation behavior was analyzed. These can help understand the mechanism of fatigue failure from subsurface in lower strength steels.◆Stress concentration behavior at micro-defects were performed by finite element analyses, and it was found that interior failure was affected by both defect size and location. Then the relationship between defect size and distribution was illustrated. These work lay great foundations for defining fracture location from subsurface and estimating critical inclusion size in materials. ◆The mechanisms of higher crack initiation potential in internal material at higher temperature were clarified by numerical simulations of matrix softening and surface oxidation, which can provide important information for understanding the role of temperature in promoting interior failure.◆The role of residual stress and its evolution in the transition behavior of crack initiation modes were illuminated by both residual stress test and 3-D finite element modelling, which can provide a basis for evaluating the effect of manufacturing process on fatigue life.◆The VHCF life prediction model involving fatigue load, inclusion size and location was revised, and a new life controlling parameter was proposed. These can lay the foundwork for life prediction of engineering structrures in ultralong life regime.
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