| Nuclear power is an efficient, non-polluting, and renewable energy source. With increasing electricity consumption in our Country in recent years, nuclear power has played a more and more important role in the energy supply. Low alloy SA508-III steel is high strength steel, and is used for safety end material of reactor pressure vessel (RPV) in nuclear pressurized water reactor (PWR) because of its excellent comprehensive mechanical properties. RPV is the non-replaceable large and critical component for the PWR in the nuclear power plant. Rupture in the RPV that occurs in the high temperature and high pressure operation environment, especially in consideration of potential risk of earthquake and seaquake, will greatly affect the safety of the nuclear power plant. Based on the requirements of high reliability and high security for the nuclear power plant, it has great significance to investigate the life prediction of materials used on nuclear power equipment in PWR service environment. With the development of Chinese nuclear power station and the localization of the equipment, it is urgent to conduct the fracture toughness evaluation and establish the objective toughness index of the safety end materials used in the PWR. SA508-III steel made in China was adopted as the experimental material in the present work. The fracture toughness and fracture behavior of SA508-III steel, exposed to the simulated service environment in one loop of PWR (hydrogen, strain rate and temperatures), were investigated with the.J-integral method. The fracture mechanism and H embrittlement were analyzed with optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to provide a theoretical basis for the safe operation of nuclear power plants.The fracture toughness change regulations with the H contents and the relationship between fracture mechanism and the H contents of SA508-III steel were investigated. The results show that the fracture toughness decreases with increasing H contents. When H content exceeds 5.6 ppm, the fracture toughness decreases obviously, and the fracture mode changes from microvoid coalescence to a mixture of similar patterns with quasi-cleavage and dimples. When the H content reaches 8.7 ppm in steel, the fracture toughness is reduced to 311.70 kJ/m2, with the relative fracture toughness loss of 31.80%. As carbides are strong traps for H, the H is induced by the three-dimensional stress and accumulates at the interface between the carbides and the matrix. When the H pressure at the interface reaches the critical value, the voids are formed, which accelerates the crack propagation, thus reduces the fracture toughness. The relationship between the fracture toughness (JQ, kJ/m2) of the steel and the H content (CH, ppm) was put forward in this paper as following equation:JQ=-88.6 exp x/8+ 560.2.With different strain rates, the change regulations of fracture toughness of uncharged and H-charged SA508-III steel were investigated. The effect of strain rate on the H embrittlement sensitivity was also investigated. The results show that with decreasing loading rate, the fracture toughness of SA508-III steel decreases, and both of the fracture toughness loss and H embrittlement sensitivity of H-charged SA508-III steel increase. With decreasing loading rate, the time for the main crack to initiate and propagate along the interface between the carbides and the matrix becomes longer, which results in more cracks and decrease of the fracture toughness. For the H-charged SA508-III steel, H can move along with the moving dislocations and accumulates at the interface between the carbides and the matrix. Thus the H concentration and H pressure increase at the interface and accelerates the propagation of the cracks, which is the main reason for the increase of fracture toughness loss. Besides, H embrittlement sensitivity of the SA508-III steel is decided by the interaction of H and dislocations. When the strain rate is lower than 5.21×10-3 s-1, the Cottrell-H-atmosphere can move along with the slipping of dislocations. Thus H can be transported to the interface between the carbides and the matrix and causes local high H concentration, which can form H induced cracks. The cracks propagate into the ferrite matrix to form the similar patterns with quasi-cleavage fracture and results in the increase of H embrittlement sensitivity. When the strain rate is higher than 5.21×10-4 s-1, the Cottrell-H-atmosphere can not keep up with the slipping dislocations and H carried to the carbides by dislocations is gradually reduced. Consequently, the H embrittlement sensitivity is reduced.The fracture toughness change regulations of SA508-III steel with various temperatures were studied. The temperature effect on the tensile behavior and the mechanism of the steel was also investigated. The results show that the fracture toughness of the steel first decreases and then increases with increasing testing temperature. When the temperature exceeds 260?, the microstructure evolution is found to change from dynamic recovery to dynamic strain aging (DSA) with a certain strain rate. The DSA effectively improve the dislocation density and reduce the size of the dislocation cells. As a consequence, the deformation resistance increases, and crack tip propagation is more strongly inhibited and the fracture toughness of the steel increases. Simultaneously, fine carbides precipitate under the combine action of stress and temperature.Thus, the deformation resistance of the steel is improved obviously and the fracture toughness is enhanced. |
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