Environmentally Assisted Cracking Behavior Of Candidate Materials In Ultra-Supercritical Power Plants

The mechanical properties of materials decrease under the combined action of stress and corrosion environment.The phenomenon of premature brittle fracture is called environmental assisted cracking.The damage of materials under the combined action of stress and environment is often more severe than their single action or superposition action of these two.The synergistic action of environment and stress can promote each other and accelerate the damage of materials.A series of experiments were conducted to evaluate the suitability of candidate materials to be used as structural materials in thermal power plants and nuclear power plants.The corrosion cracking behavior and corrosion fatigue behavior of candidate materials for power plants were studied.For this purpose,TP347HFG was tested to evaluate its stress corrosion cracking(SCC) behavior in supercritical water(SCW) environment while Inconel 617 was tested for SCC and corrosion fatigue(CF) behavior in steam and SCW environment.SCC tests of austenitic steel TP347HFG were conducted under<10-8000 ppb dissolved oxygen(DO) in steam,the alternating environment of air and steam and SCW at 600-650?.A constant stress intensity factor(K) method and direct current potential difference technique were used in the tests to evaluate its environmentally assisted cracking(EAC) properties as superheater and reheater boiler tubes in ultra-supercritical(USC) power plants.The experimental results show that CGR increases monotonically with temperature and DO content.CGR in SCW is higher than that in the high-temperature steam which indicates that exposure pressure has a noticeable effect on crack propagation.The varying CGR in different experimental environments how that exposure medium participates in the cracking process.Fracture surface and cross-sectional surface of compact tension specimen were analyzed by scanning electron microscope(SEM).The mechanism of crack propagation in different exposure environment is further discussed.The SCC behavior of Inconel 617 was studied to evaluate its suitability for Generation IV SCW reactor concept.A series of slow strain rate tensile(SSRT) tests at a constant strain rate of 5 × 10-7 s-1 were carried out in superheated steam and SCW over a pressure range of 0.1-25 MPa at 650?.SSRT test in dry N2 gas was also performed to compare the results in a non-corrosive environment with the corrosive medium environment.Intergranular cracks were observed for all specimens regardless of the test environment.Inconel 617 showed susceptibility to SCC at the tested experimental conditions.The dominance of intergranular stress corrosion cracking was observed on the gage surface and fracture surface by using SEM analysis.The effect of varying environmental conditions on SCC susceptibility is further discussed.Inconel 617 is a candidate material to be used as a structural material in USC power plants.In such an extreme corrosive medium environment,metallic materials are susceptible to CF.The effect of maximum stress intensity factor,loading mode,and temperature on the CF cracking behavior of Inconel 617 in a deaerated(<10 ppb DO) SCW environment.For the selected parameters,microscopic investigations and detailed analysis affirmed corrosion-fatigue interaction to be a leading factor for change in CGR.The crack propagation mechanism for the selected parameters in the applied test conditions is further discussed.