| The microstructures and mechanical properties of three states(The aging temperature was 540?,620? and 640?,respectively.)of FV520B materials were studied by means of optical microscopy(OM),scanning electron microscopy(SEM)and mechanical properties testing in this paper.The corrosion behavior and mechanism of materials in simulated service environment were studied and analyzed by means of electrochemical testing.The fatigue behavior of the materials were studied by fatigue test,and fatigue life prediction models were established.The stress corrosion cracking behavior of materials in simulated service environment were studied by slow strain rate tensile test(SSRT),and the mechanisms of stress corrosion cracking were analyzed.The results showed that the microstructure of the material was tempered martensite when the aging temperature was 540?.The microstructures of the materials were tempered martensite and retained austenite when the aging temperatures were 620? and 640?.A large number of fine strengthening phases appeared after aged at 640?.When the aging temperature ranged from 540? to 640?,with the increase of aging temperature,the strength and hardness of the materials decrease,the plasticity and toughness increase,and the strain hardening index increase.The micro-morphology of tensile and impact fracture were dimples.The dimple size of the fiber zone was larger,and the dimple size of the radiation zone was smaller.The dimples size of the fiber zone and radiation zone increase with the increase of aging temperatre.With the increase of aging temperature,the open-circuit potential of the materials shifted positively,the tightness of corrosion product film increased,the corrosion rate decreased and the corrosion resistance increased.The corrosion rate increased,and the corrosion became more serious when there was CO2 or H2S in the environment.The corrosion rate of the material got to the maximum state,and the corrosion got to the most terrible extent when there were CO2 and H2S coexisted in the environment.The order of corrosion resistance of FV520B materials in test environments was simulated solution>simulated solution+CO2>simulated solution+H2S>simulated solution+H2S+CO2.The FV520B materials of three states all showed their own fatigue limit in laboratory atmospheric environment.The fatigue limits of materials decreased with the increase of aging temperature.The fatigue limits of materials after ageing at 540? was 695.2 MPa,603.4 MPa after aging at 620?,and 565.7 MPa after aging at 640?.The fatigue life prediction formulas of FV520B materials of three states were respectively Nf=2.13×109(Sa-695.2)-2,Nf=5.40×109(Sa-603.4)-2,Nf=2.34×109(Sa-565.7)-2.The fatigue cracks of FV520B materials of three states originated on the surface or edge of the specimens in laboratory atmospheric environment.The micro-morphology of fatigue crack propagation zone was mainly shown as cleavage fracture,and accompanied by a small amount of fatigue striations after aging at 540?.The micro-morphology of fatigue crack propagation zone were dominated by fatigue striations,accompanied by a small amount of cleavage characteristics after aging at 620?.The micro-morphology of fatigue crack propagation zone were mainly shown as fatigue striations after aging at 640?.There was no obvious tendency of stress corrosion cracking in the environments of simulated solution,simulated solution+CO2.Materials showed obvious tendency of stress corrosion cracking in simulated solution+H2S,simulated solution+CO2+H2S environments,and the order of stress corrosion sensitivity was simulated solution+C02+H2S>simulated solution+H2S>simulated solution+CO2>simulated solution.The stress corrosion susceptibility of materials in various test environments decreased with the increase of aging temperature.The tensile stress and Cl'led to the rupture of passive film on the surface of FV520B materials,resulted in pitting corrosion,and the stress corrosion cracking of materials were promoted under the action of tensile stress in the environments of simulated solution and simulated solution+C02.On one hand,tensile stress and Cl-led to the rupture of passive film on the surface of FV520B materials,pitting pits appeared on materials surface,cracks initiated at pitting pits,and cracks grew and propagated under tensile stress,and ultimately lead to stress corrosion cracking of materials in the environments of simulated solution+H2S and simulated solution+CO2+H2S.On the other hand,the hydrogen ion,which produced by ionization of H2S,was reduced to hydrogen atom,and the hydrogen atom diffused and gathered to discontinuities(such as grain boundary)or defects led to hydrogen-induced cracking,which accelerated the stress corrosion cracking of the materials. |
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