Hydrogen Diffusion Behaviors And Hydrogen Diffusion Paths Of Hot Stamping Steels

Hot-stamped steel has become an important material to achieve automobiles lightweight,due to high strength.However,hydrogen atom could permeate easily into the hot stamping steel during manufacturing processes as well as in service,which posing a risk to automotive safety.In this paper,the hydrogen diffusion behaviors and hydrogen diffusion paths of 1200 steel,A-1500 steel,A-1800 steel are investigated,respectively.First,the hydrogen diffusion behavior was investigated from both kinetic and thermodynamic aspects by using electrochemical technique of hydrogen permeation,and the parameters such as hydrogen diffusion coefficient and hydrogen trapping sites were calculated.Then,silver bromide emulsions which used to characterize hydrogen diffusion were investigated,and the optimal synthesis conditions were obtained.Finally,based on the synthesized silver bromide,the hydrogen diffusion paths and accumulation sites were characterized by using hydrogen microprint technique（HMT）.Moreover,the microstructure of three hot-stamped steels was analyzed by optical microscopy（OM）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.And the influence of microstructure on the hydrogen diffusion paths was further discussed.The results are shown as follows.（1）The results of hydrogen permeation experiments showed that the addition of alloying elements Ti,Nb and V changed the hydrogen diffusion behavior at different degrees.The hydrogen diffusion coefficient decreased from 5.37×10^-7cm²/s of A-1200steel to 4.86×10^-7 cm²/s of A-1500 steel and 4.54×10^-7 cm²/s of A-1800 steel,respectively;the hydrogen diffusion activation energy increased from 29.02 k J/mol of A-1200 steel to 30.10 k J/mol of A-1500 steel and 30.42 k J/mol of A-1800 steel,respectively.Additionally,the percentage of irreversible hydrogen trapping site increased from 18.60%of A-1200 steel to 28.15%of A-1500 steel and 42.77%of A-1800 steel,respectively.The hydrogen trapping binding energy increased from 27.19k J/mol of A-1200 steel to 28.35 k J/mol of A-1500 steel and 28.68 k J/mol of A-1800steel,respectively.Due to the addition of alloying elements,a certain amount of carbides are precipitated in the matrix of A-1500 and A-1800 steels,which act as“good hydrogen traps”,effectively hinder the hydrogen diffusion.Compared with A-1500steel,the（Ti,Nb）C precipitates in A-1800 steel are smaller size and more uniform distribution,thus providing higher irreversible hydrogen trap density(N_ir=0.68×10²¹cm^-3)and larger hydrogen trapping binding energy（28.68 k J/mol）.（2）The silver bromide emulsion was synthesized by the complexation precipitation method,which used to characterize hydrogen diffusion.The silver bromide average particle size was approximately 252 nm and had a morphology which close to spherical shape with little accumulation phenomen when synthesized at a reaction solution of 7<p H<11,reaction temperature was 40°C and reaction time was60 min.In this condition,the silver bromide was used to characterize hydrogen diffusion in A-1200 steel with the best results,i.e.the hydrogen distribution in martensitic laths could be clearly characterized.（3）The results of HMT showed that hydrogen diffusion paths are similar in A-1200 steel,A-1500 steel and A-1800 steel.The hydrogen diffusion along the grain boundaries priority.Then the hydrogen diffusion channel is changed as the number of hydrogen atoms increased,and diffuses along the martensitic lath.Noticeably,the hydrogen diffusion along the grain boundaries decreases sequentially in A-1200 steel,A-1500 steel and A-1800 steel under the same hydrogen charging time（20 min）.The hydrogen accumulates heavily along the martensitic laths in A-1200 steel,while in A-1500 and A-1800 steels,the hydrogen accumulation at triple junction and grain boundary,which hydrogen accumulation phenomenon decreases at the same hydrogen charging time（120 min）.With the addition of alloying elements Ti,Nb and V,not only generated a large number of nano-scale carbides precipitate,but also the average grain size is reduced from 23.65±0.32μm in A-1200 steel to 11.69±0.41μm in A-1500steel and 7.59±0.37μm in A-1800 steel,respectively.The refined grains increase the grain boundary area and provide uniformly distributed hydrogen traps,thus hindering the hydrogen diffusion and delaying hydrogen accumulation.Additionally,due to improper heat treatment process,Ti C inclusions of 4-7μm and Al₂O₃ inclusions of 3μm present in A-1200 and A-1800 steels,respectively,which acted as“bad hydrogen traps”capturing a large number of hydrogen atoms and increasing the possibility of hydrogen embrittlement.