Effects Of Environments On Fracture Toughness And Fracture Behavior Of 304 Stainless Steel

Reactor internals of nuclear reactor are said to be the heart of nuclear power plant, which direct contact the reactor core, and are the key components for the life of nuclear power station. The main materials of reactor internals are the nuclear grade 304 stainless steel. The nuclear power application problems of 304 stainless steel have been studied abundantly. Up to now, the strength and corrosion of 304 stainless steel have been studied systematically. For considering microcracks and the servicing safe, the understanding for fracture toughness and fracture mechanism of 304 stainless steel have not only practical significance, but also have academic value. Based on such background, the effects of environmental factors on the fracture toughness and fracture behavior of the nuclear grade 304 stainless steel were carried out in the present research.Three-point bending specimens are used for characterizing the fracture toughness of 304 stainless steel, in this study. The effects of environmental factors on the fracture toughness of 304 stainless steel, such as temperature, water bath, warm water with hydrogen and hydrogen-charged specimens, were discussed.The results show that JQ of 304 stainless steel decreased gradually in air and water bath with the increasing temperature, but JQ don't be influenced significantly by the water bath and warm water of containing hydrogen. JQ of charged 304 stainless steel decreased obviously with the increasing of hydrogen content in the specimens.Fracture behavior of 304 stainless steel in different testing environment was analyzed using Optical Microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD) technology. The morphology of 304 stainless steel appears mainly to be microvoid coalescence at room and elevated temperatures, and the dimples were connected by secondary dimples. The diameter and depth of dimples was decreased with the increasing temperature, and the secondary dimples could not be found at 350℃. For the low concentration of saturated hydrogen in the water bath, hydrogen concentration at the crack tip can't reach a degree that influence crack propagation significantly so that fracture toughness of 304 stainless steel have no influence on warm water of containing hydrogen condition at room temperature. Hydrogen-induced martensite transformation in the matrix varied the fracture mechanism of 304 stainless steel. Hydrogen-induced microcrack occurred in the vicinity of martensite and induced quasi-cleavage brittle fracture. Therefore, fracture toughness of 304 stainless steel decrease greatly.