Numerical Simulation Of Hydrogen Induced Cracking Of 35CrMo High Strength Steel In High Pressure Hydrogen Environment

Low-alloy high-strength steel can be applied in many ways because of its good obdurability.However,due to the fact that high-strength steel has high sensitivity to hydrogen,there may be hydrogen induced fracture in the application of this steel,which is one of the most frequent failures of application in this field.It is generally acknowledged that hydrogen brittleness sensitivity will become higher with the strength and toughness of the high-strength steel increasing.The author of this thesis took the 35 Cr Mo as the research subject and compared the differences of its mechanical properties under both high pressure hydrogen environment and nitrogen environment,and then carried out analysis concerning the specimen fracture with the help of scanning electron microscope(SEM).Meanwhile,the author combined the ABAQUS finite element software and the theory of stress inducing the diffusion of hydrogen and simulates the hydrogen diffusion under the influence of stress using the model of different radius of curvature.Based on the hydrogen diffusion simulation results of the process,the author conducted the simulation of hydrogen induced crack extension and fracture of 35 Cr Mo high-strength steel.The main research findings can be listed as follows:(1)After comparing the experiment results of slow strain rate tension under 5MPa high pressure hydrogen environment and nitrogen environment,the author finds that the strength of the notch specimen of different radius of curvature under hydrogen environment is reduced and the time for the notch specimen in hydrogen environment to completely break is shorter than the time in the nitrogen environment.At the same time,it is found that the stress concentration factors of the notch specimen of different radius of curvature are quite different from one to another.After investigating into the displacement--the load curve and the SEM of the notch specimen of different radius of curvature in the hydrogen environment,the author finds that the brittleness of the materials that show higher levels of stress concentration is more obvious to see,which leads to the conclusion that stress induces hydrogen diffusion.(2)The author has set up the analysis model of sequential coupling based on the theory of the stress inducing hydrogen diffusion and conducts investigation into the regularities of distribution of hydrogen under the influence of stress.According to the stress simulation results,the authors finds that the notch specimen of different radius of curvature have shown different degrees of stress centralization with the stress concentration decreasing with the notch increases.Under the relevant hypothesis conditions,the author could simulate the stress inducing hydrogen diffusion of the notch specimen of different radius of curvature through the establishing of the model of sequential coupling and come to the conclusion that the hydro-static stress distribution and the distribution of hydrogen concentration of all the notch specimen are relatively the same,that is there is more hydrogen close to area of the concentration of stress and there is lower degrees of hydrogen away from the concentration of stress.Compare the simulation results of the notch specimen of different radius of curvature,the author concludes that there is higher levels of hydrogen accumulation where there is higher levels of stress concentration.With the above model,it could be derived that the steady-state hydrogen concentration distribution of the notch tip is correlated to the hydro-static stress,which is consistent with the theoretical analysis results of the stress inducing hydrogen diffusion.(3)The author establishes the analytic model of the crack propagation and fracture of the notch specimen of different radius of curvature of the built-in tension units and carries out research regarding hydrogen induced fracture of 35 Cr Mo high-strength steel.With constant displacement,the notch model of different radius of curvature of the built-in tension units first experiences unit damage,that is the dissipated energy increases to a certain degree(where the internal energy is maximized)to make the unit become damaged and ineffective(also known as crack initiation)before crack propagation.However,during the process of crack propagation,the internal energy is becoming less and less,which makes it easier for the notch specimen to show crack propagation under the same stress condition.When the dissipated energy is accumulated to the highest level,the whole model becomes completely broken.By comparing the internal energy,the dissipated energy and the work of the external force of the notch specimen of different radius of curvature of the built-in tension units,the author comes to the conclusion that the time of the crack of the notch specimen to break around the area where the hydrogen is more tense under the hydrogen environment is relatively shorter.The author simulates the hydrogen induced fracture of the high-strength steel with the ABAQUS finite element software built-in tension simulation model and finds that the simulation results and experiment results coincide quite well,which also verifies that the hydrogen which could reduce the linkage force plays a major role in the hydrogen induced fracture of the high-strength steel.