| Factors that influencing crack growth rate of typical pressure vessel materials are studied.Based on fracture mechanics and taking 4130 X steel as the research object,the variation rule of fatigue crack growth rate of 4130 X steel is explored from three aspects,namely corrosion environment,stress ratio and fatigue load frequency.It provides a theoretical basis for life prediction and safety detection of gas cylinders under complicated conditions.Firstly,under the high frequency(f=100Hz)fatigue load in the air,tests were carried out under the conditions of different stress ratios(R=0.1,0.3,0.5)to obtain the effect of the stress ratio on the fatigue crack growth rate of 4130 X material.Secondly,the specimens were immersed in a concentration of 400 ppm hydrogen sulfide environment for 48 hours for pre-corrosion treatment.After being taken out,high-frequency fatigue crack propagation tests under three stress ratios were carried out in the air.The experimental results were compared with those of non-corroded specimens to analyze and obtain the influence of hydrogen corrosion on the fatigue crack propagation rate of materials.Thirdly,under low frequency(f= 0.01Hz)fatigue load,fatigue crack propagation tests of 4130 X material were carried out in air and in hydrogen sulfide corrosion environment with a concentration of 400 ppm,crack propagation performance of the material under simulated working condition was obtained,and the influences of frequency and corrosion environment on crack propagation rate were studied.Finally,based on the fatigue crack growth rate data of 4130 X material obtained under simulated real working conditions,the safety assessment and life prediction of the defective gas cylinder were carried out according to the relevant ASME standards,and safety work suggestions were given for the gas cylinder.The main research results:(1)8 groups of crack growth rate curves of 4130 X steel were obtained under different frequencies(100Hz,0.01Hz),different stress ratios(0.1,0.3,0.5)and different environments(air,400 ppm pre-corrosion,400 ppm corrosion environment),and the effect of each factor on FCGR were discussed.(2)Under the same environment and frequency,the fatigue crack propagation rate increases with the increase of stress ratio.When R=0.3,the crack growth rate is about 3 times that of R=0.1,and when R=0.5,the crack growth rate is about 10 times that of R=0.1.This is because alone with the increase of the stress ratio,the average stress of crack tip also increases,which reduces the crack closure effect.When the frequency decreased from 100 Hz to 0.01 Hz,while other conditions remained unchanged,the fatigue crack growth rate increased by 1.5 times.The main reason is that with the decrease of loading frequency,the strain rate of crack tip decreases and the plastic deformation of crack tip becomes more complete.The pre-corrosion treatment makes atomic hydrogen infiltrate into the material and increases the brittle fracture sensitivity of the steel.As a result,the fatigue crack growth rate of the material under three stress ratios is about 1.3 times higher than that without corrosion.The results of the simulated real working condition showed that during the crack propagation process,hydrogen atoms continuously penetrated into the material with the dislocation and accumulated at the crack tip,resulting in a 7.3-fold increase in the corrosion fatigue crack propagation rate compared with that in air.(3)Based on the analysis of the cracked cylinders,the service life of the cylinders under safe working conditions was predicted,and the scheme of using the cylinders from safe use to scrap was put forward.The obtained research results theoretically illustrate the crack propagation rules of 4130 X steel under different environments,providing a theoretical basis for the subsequent study of the material.It provides the basis for life prediction,safety evaluation and accident investigation of 4130 X steel in actual engineering. |
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