Stress Corrosion Cracking Behavior In Supercritical Water For The Materials Of Boiler Tube In Power Plant

The key technology in development of ultra-supercritical power generation is metal material performance of high temperature mechanical properties and stress corrosion cracking.The materials for superheater and reheater at high temperature mechanical properties and environmental oxidation resistance changed due to interconnection of mechanics and environmental for under extreme conditions.In ASME standard,the criteria of material performance is characterised as that under ordinary steam conditions,which is different from that in real conditions such as in high-pressure steam or supercritical water and ignores the material mechanical properties changed of high temperature and high pressure environment.For study of thermal power unit,people tend to focus on a single factor: the influence of environmental factors or pure mechanical factors and often ignore the damage caused by stress corrosion cracking.Once damage occurs can lead to serious safety accidents,it can affect the safe and reliable operation of the units directly.Therefore,research influence of stress corrosion cracking mechanism for ultra supercritical unit material in extreme environment is necessary.In this paper,the SCC crack growth rate tests on a compact tension specimen of austenitic steel TP347 HFG was conucted under air and water vapor alternating environment,supercritical water environment,0 and 8 ppm dissolved oxygen concentration in 600-650 °C.High precision servo control system was adopted to realize sine wave,trapezoidal wave and up(or down)a constant stress intensity factor(K)loading model.The crack growth rate during the test was measured using direct current potential drop(DCPD)technique,completed the austenitic steel TP347 HFG in stress corrosion cracking performance study in different environments.The results showed that the fractures of all speciments presented typical intergranular stress corrosion cracking morphology.The crack growth rate increased with temperature and dissolved oxygen content continually.Because the oxidation rate in high temperature steam environment is higher than high temperature air,leading to crack growth rate is higher than air environment.In the case of the higher temperature,steam environment compared to air environment in multiples of crack growth rate was smaller.Supercritical water environment in the crack propagation rate is greater than the steam environment,the reason is that supercritical water environment changed material molecules to the rate of diffusion and mass transfer rate of the crack tip,leading to the crack tip region boundary binding energy decrease.Crack growth rate changes influenced by temperature,dissolved oxygen content,chemical environment and pressure,through mechanics and function of environmental factors cause changes in the conditions of crack propagation.Finally,analyzed the reason of the crack closure phenomenon occurring in a test.