Effect Of Hydrogen On Tensile Deformation Behavior Of GH690Alloy

The steam generator is the most important facilities in the main water-cooled reactor pressure system of the Pressured Water Reactor (PWR). The steam generator tubes are subjected simultaneously to the action of hydrogen environment with high temperature and pressure for long term service. Hydrogen with high temperature and pressure would decrease the tube’s ductility and even cause the hydrogen induced fracture, affects the normal operation of the reactor. It’s important for the research of mechanism of hydrogen embrittlement and the safety of PWR to research the relationship among steam generator tubes material’s microstructure, hydrogen diffusion and fracture behavior.690alloy is a new kind of nickel base austenitic anticorrosive alloy, which consists of30%Cr. The high strength and excellent anticorrosion make this alloy replaces the600alloy that used in the most of PWR, and become the new nuclear material. GH690alloy made in China was employed as the experimental material in the present work. The tensile properties and hydrogen brittleness behavior of GH690alloy, exposed to the simulated service environment of PWR, were investigated by using tensile testing combined with the analysis of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The main contents and results are as follows:GH690alloys were precharged with different hydrogen (H) content by thermal precharging technology, and the tensile strength and fracture behavior of these alloys were investigated. The results show that the tensile strength and elongation at break decreases with increasing H content and the fracture mode changes from dimple ductile fracture to intergranular brittle fracture. It is proposed that the H enrichment at the grain boundary is the main reason for the decrease of ductility.Furthermore, the effect of strain rate sensitivity on the H-embrittlement of GH690alloy was investigated. The results show that the H-embrittlement of GH690 alloy is decided by the interaction of H and dislocations. When the strain rate is lower than3.14×10-4s-1, the Cottrell-H-atmosphere can move along with the slipping of dislocations. Thus H can be transported to the grain boundary, and make the alloy appear H-embrittlement. When the strain rate is higher than3.14×10-4s-1, the Cottrell-H-atmosphere can not keep up with the slipping dislocations. Thus H can not be transported to the interface anymore, so the possibility of H-embrittlement will be considerably reduced, the fracture morphology mainly is ductile fracture.The tensile deformation behavior of GH690alloy were also investigated at different percentage of grain boundary. The results show that the decrease of tensile strength and elongation at break of the alloy increases with the decreasing percentage of grain boundary which means the increasing percentage of carbides on grain boundary. It shows that the enrichment of H will effect the grain boundary instead of the cabides on grain boundary.Then we research the influence of hydrogen to dislocation of GH690alloy. The results show that the hydrogen enriched in grain boundary will enhance the dislocation proliferation. When hydrogen-enhanced dislocation proliferation reached a critical condition, the hydrogen-induced crack nucleation will happen in grain boundary. Crack in the grain boundary will expand rapidly along the grain boundary, which leads to the characteristics of brittle fracture of GH690alloy. It means hydrogen-enhance dislocation proliferation is the main reason of ductility decrease of GH690alloy.