Study On The Kinetics Of Hydrogen Diffusion And Control Technology Of Hydrogen-induced Delayed Fracture Of High Strength Bolt Steels

High strength bolts are usually used in marine engineering equipments under the bad environment of seawater,which is inclined to fracture induced by hydrogen resulting in huge risk of safety for application.The microstructure,hydrogen and stress state are the main factors for leading to hydrogen embrittlement.Especially when hydrogen atoms entry into the thread in which exists with stress concentration,their accumulation will induce the delayed fracture.Therefore,the control of hydrogen induced delayed fracture must commences studying the state of stress,kinetics of hydrogen diffusion,and control method.The subject is very significance for theory and engineering.The 40CrNiMoA and 0Cr16Ni5Mo and 12CrNi9MoV steels are selected in this dissertation.The methods of finite element analysis,tensile test,microstructure observation and electrochemical technique are used for analyzing the endogenous and external factors of hydrogen induced delayed fracture so as to reduce the probability of the phenomenon.In order to investigate the mechanical and chemical behaviors of experimental steels with hydrogen charged,it is necessary to study the technology of cathodic hydrogen charging for preparing the specimens.The results of cathodic hydrogen charging of experimental steels show that the principle of hydrogen charging for tensile specimens is small charging current density and long charging time.It can avoid the damage in the surface of specimen,and make the hydrogen distribution uniformly in the steel.The hydrogen concentration increases with the increased charging time and charging current density,and reaches steady state finally.The work is aim to prepare test verification for calculating the time and hydrogen concentration under the condition of hydrogen charging.Hydrogen diffusion coefficient is one of hydrogen diffusion ability parameters.The apparent hydrogen diffusion coefficients of experimental steels are determined by means of hydrogen diffusion theory of time to breakthrough method and time lag method.As for one steel,hydrogen diffusion coefficient of experimental steel increases with the increased strength,whereas it is different with different types of steels due to various microstructure.With the hydrogen diffusion coefficient,it is easy to evaluate the hydrogen embrittlement susceptility of experimental steels.Meanwhile,the hydrogen trapping behaviours and activation energies of hydrogen desorption are also investigated by means of thermal desorption spectroscopy technique.The potentiodynamic polarization curves of two experimental steels are also measured in order to evaluate the ability of release hydrogen.Making use of this technology,the optimal bolt materials will be choosed so that the hydrogen induced delayed fracture can be avoided.The effect of hydrogen on notch strength of notched specimens is investigated by means of slow strain rate test,which can evaluate hydrogen embrittlement susceptibility of bolt steels.With the increase in hydrogen concentration,the notch strength decreases.And the hydrogen embrittlement susceptibility is different with the strength grade.The smooth specimens are used for evaluating the loss of plasticity which is important to reduce stress concentration in the notch root.Moreover,with the increase in hydrogen concentration,the fracture features transfer from ductile to a mixed morphology of dimples,quasi-cleavage and intergranular features.The stress distribution near the notch root of bolt is obtained by means of finite element analysis method.It exists with triaxial stress concentration ahead of notch tip.And the location is defined as the boundary between plastic and elastic regions.By designing different notch root radius,the stress concentration of various notch models is determined,which is necessary to analyse the problem about hydrogen induced fracture.A sequential coupling calculation method on stress analysis and hydrogen diffusion are studied that the trend of hydrogen distribution is similar to the stress distribution near the notch root.What important is that the plasticity of bolt steels are used to control the problem of hydrogen induced delayed fracture.The plastic deformation in the notch root can release stress concentration.The relationship among hydrogen concentration,notch root radius and critical strain is obtained by theory calculating and finite element analysis methods.With the relationship,it can provide the consideration for bolt material selection and structure designing.Electrodeposited coatings for high strength bolts are helpful for improving the service life.The corrosion resistance and corrosion procedure of various coatings are evaluated by means of electrochemical method.In contrast with various coatings systematically,it is easy to find the appropriate coating for bolt.In the light of environment friendliness and economics,the Zn-Ni coating is likely to replace Cd-Ti coating in marine atmospheric environment.However,in the environment of seawater,the Ni-W-P coating is the best alternative with lower susceptibility of hydrogen embrittlement for protecting the high strength bolts.