Cross Scale Study On Hydrogen Behavior In Hot Forming Process And Teh Mechanism Of Hydrogen Induced Damage Of Heavy Forgings

Heavy forgings play an important role in all fields of national economy and people’s livelihood.With the development of material science and forging technology,the production level and ultimate manufacturing capacity of heavy forgings in China have achieved a leap.As a long-standing problem that restricts the service safety of heavy forgings,hydrogen induced damage would not disappear with the development of forging technology,but becomes much more serious with the improvement of material strength level.In order to ensure the service safety of heavy forgings,escort the safety of these national critical equipment,clarify the hydrogen embrittlement mechanism of heavy forgings,and find the prevention method of hydrogen embrittlement of heavy forgings is imminent.Heavy forgings are not only troubled by endogenous hydrogen embrittlement and environmental hydrogen embrittlement,but also bring great challenges to the research of hydrogen embrittlement due to its huge size characteristics.Therefore,aiming at the huge scale and time span of hydrogen embrittlement of heavy forgings,a macro and micro scale research method integrating finite element,crystal plasticity theory,molecular dynamics,first principle and experimental materials was proposed on the basis of computational materials science and electron microscopy.A high-throughput electrolytic hydrogen charging experimental platform was built to study the mechanism of hydrogen embrittlement of heavy forgings.Aiming at the endogenous hydrogen embrittlement in the hot forming process of heavy forgings,the interaction between the hot forming process of heavy forgings and hydrogen was studied.The physical simulation experiment of thermoplastic deformation was carried out with the pre-charged hydrogen sample under the thermophysical simulation machine.The new phenomena of "annular hydrogen affected zone" and "central hydrogen affected zone" were found through the change of micro zone hardness.Using the ingenious combination of cross-scale calculation model and characterization technology,the behavior of hydrogen in hot forming process was reduced from multiple dimensions of multi-scale three-dimensional space and time,and the generation mechanism of hydrogen affected zone was deeply revealed.That is,hydrogen will form a low-energy stable structure with defects,so as to reduce the driving force of dynamic recovery and produce the effect of inhibiting dynamic recovery.In addition,under the condition of high temperature and large deformation,hydrogen forms carbohydride at the interface with carbide,which will lead to interface cracking and high-temperature hydrogen attack.Aiming at the environmental hydrogen embrittlement mechanism of heavy forgings,the interaction between hydrogen and dislocation slip deformation mechanism was studied.In order to eliminate the interference of other factors,annealed pure iron was used as the research material.The microstructure evolution under uniaxial stress before and after hydrogen charging was analyzed by in-situ EBSD technology.A molecular dynamics model was established to analyze the effect of different strain rates.The experimental and simulation results shown that hydrogen would change the slip law of dislocation.For the environmental hydrogen embrittlement mechanism of heavy forgings,firstly,the interaction between hydrogen and dislocation slip deformation mechanism was studied.The microstructure evolution of annealed pure iron under uniaxial stress before and after hydrogen charging was analyzed by in-situ backscattered electron diffraction and crystal plastic finite element model.It is found that hydrogen affects the dislocation slip direction.In order to analyze its atomic mechanism,a molecular dynamics model was established to clarify the hydrogen movement law and microstructure evolution mechanism under different strain rates when the stress is oriented along different grains.After that,the relationship between multiphase structure and hydrogen embrittlement sensitivity was studied.The distribution law of the second phase was counted by stereology,and the multiphase finite element model was established from the random aggregate.The mechanical properties of each phase before and after hydrogen doping were predicted by first principles.The results were substituted into the finite element model to predict its hydrogen embrittlement sensitivity.After that,experiments were designed to verify the simulation results.A group of steels with different Cr content were used for hydrogen embrittlement sensitivity test.A scientific method for establishing the relationship between specific element content and hydrogen embrittlement sensitivity was given.Finally,in order to predict the radial gradient change of hydrogen embrittlement sensitivity of heavy forgings.The physical simulation experiment was designed.Firstly,the changes of temperature field and microstructure field during heat treatment of heavy forgings were obtained through simulation.Based on the temperature field in the model,samples similar to the core microstructure of heavy forgings were obtained through heat treatment,which provides a characterization method of hydrogen embrittlement sensitivity of heavy forgings.This study not only clarified the interaction relationship between hydrogen and hot forming process of heavy forgings,but also deeply analyzed the scientific mechanism of hydrogen induced damage,which provides a theoretical basis for the prediction and control of hydrogen embrittlement of heavy forgings.