Study On Hydrogen Diffusion And Its Effect On Hydrogen Embrittlement Of High Strength DP Steels

Dual phase（DP）steel composed of ferrite and martensite is an ideal lightweight material for automobile due to its good strength and ductility.However,during production,processing and servicing of DP steel,hydrogen may permeate into the steel and lead to security problems.In this paper,the effects of service condition and heat treatment microstructure on hydrogen diffusion and hydrogen embrittlement susceptibility of DP steels were studied by a combination of hydrogen permeation,slow strain rate tensile test,scanning electron microscopy,transmission electron microscopy and other experimental methods.Meanwhile,the hydrogen embrittlement mechanism was discussed from the aspect of hydrogen diffusion.The main results were as follows:（1）The influence of strain rates(10^-4～10^-6s^-1)on the interaction between hydrogen and dislocation was analyzed through the fracture morphology observation on static and dynamic hydrogen charging,and the calculation of dislocation migration velocity and hydrogen segregation velocity.It was found that effect of strain rate on hydrogen embrittlement susceptibility I_HEwas influenced by hydrogen charging condition.The decreasing of strain rate can prolong migration time of hydrogen with dislocation,which increases the width of hydrogen embrittlement region and I_HE.In addition,the initial hydrogen induced cracks,which exist in a sample after serious hydrogen charging,will affect the diffusion of hydrogen and slow the increasing amplitude of I_HE.（2）By observation of dislocation structures,hydrogen permeation test and calculation of hydrogen binding energy of dislocation,it was found that hydrogen embrittlement susceptibility I_HEvaried with applied pre-stress in stages.When stress was less than 60%YS,I_HEalmost unchanged.When stress increased to 80%YS,dislocation proliferated in ferrite and these dislocations has high strong mobility and with hydrogen binding energy of 38.5 kJ/mol,which could promote the diffusion of more hydrogen atoms,thus I_HEincreased.However,when stress increased to 120%YS,hydrogen binding energy of dislocation increased to 76.3 kJ/mol due to increasing of dislocation density and entanglement degree.Part of the dislocations can act as irreversible hydrogen trap to pin hydrogen atoms,resulting in decreasing of I_HE.（3）Based on observation of martensite morphology in DP steels with different quenching temperatures and calculation of effective hydrogen diffusion coefficient in ferrite and martensite,it was found that isolated martensite was more beneficial to the diffusion of hydrogen and the reduction of I_HE.When martensite content was low and isolated,hydrogen preferred diffuse continuous along ferrite with higher hydrogen diffusion coefficient.With increasing of martensite content,effective hydrogen diffusion coefficient D_effin DP steel decreased and I_HEincreased.However,when martensite content increased to a certain amount（about 68.3%）with continuously distributed,continuous hydrogen diffusion channel along ferrite was blocked by martensite,which resulting in decreasing of D_effand continuous increasing of I_HE.In addition,a method which evaluating I_HEin DP steel with different martensite content was proposed by established the function relationshipbetween D_effand I_HE,log（I_{H E}）（28）3.13-3.54?log(D _eff).（4）By analysis of microstructure evolution during tempering and quantitatively characterization of hydrogen trapping ability in various microstructures,it showed that with increasing of tempering temperatures,martensite decomposed with dipersion carbide precipitation,which could act as irreversible hydrogen traps,leading to an increasing of hydrogen trap concentration up to 16.81×10^-7mol/cm³,accounting for more than 80%of total hydrogen trap concentration,thus the hydrogen embrittlement resistance improved.