Hydrogen Diffusion Behaviors In The Axle Steels

As one of the key parts of the train,the quality of the axle is directly related to the safe and effective operation of the train.The bubbles formed by hydrogen on the surface of the axle steel are one of the reasons for the failure of the axle.And the formation of hydrogen bolistering is related to the diffusion of hydrogen in the steel.In order to explore the hydrogen diffusion behavior and the formation conditions of hydrogen bubbles in axle steel,the hydrogen permeation characteristics of three kinds of high-speed axle steel30NiCrMoV12,DZ1,DZ2 and heavy duty axle steel LZ45CrV were studied by electrochemical hydrogen charging method.Based on the mechanical property test,the diffusion mechanism of hydrogen in axle steel is analyzed by means of metallographic microscope,scanning electron microscope?SEM?and transmission electron microscope/energy spectrum?TEM/EDS?.The results are included as following:?1?Hydrogen permeation characteristics of the axle steels show the same variation,namely,with increasing of hydrogen charging current density,the test steel apparent hydrogen diffusion coefficient D_app increases.When I_?increases to a certain value,D_app reaches a stable value.The stable D_app can use to the hydrogen diffusion coefficient D₀.By comparing and analyzing the D₀ and the critical hydrogen charging current density I₀*and C*of hydrogen bubbles of the axle steels,it is fand that the D₀ of four specimens shows the opposite rule with D₀ and C*,that is,decrease of D₀,both I₀*and C*increase.Among them,the D₀ of 30NiCrMoV12 sample steel is the smallest,only 0.288×10^-66 cm²/s.But its C*is maximum,up to 41.93 wppm.The D₀ of DZ2 is maximum,the value is 3.100×10^-6cm²/s;it has a smallest C*?11.01wppm?.Compared with DZ2,the hydrogen diffusion coefficient of 30NiCrMoV12 sample steel is less than one order of magnitude.The critical hydrogen concentration for producing hydrogen bubble is 30.92 wppm.These resusts imply that the 30NiCrMoV12 steel has a highest resistance to hydrogen embrittlement.?2?The mechanical properties have an important influence on the hydrogen diffusion behavior of steel.In general,with the increase of tensile strength,the resistance to hydrogen embrittlement of steel tends to decrease.The mechanical test results at room temperature indicate that all test steels achieve a good comprehensive mechanical properties.Based on the observation of microstructure,three kinds of high-speed axle steel consist of tempered sorbite,and the grain sizes are higher than 8 grades,the heavy axle steel group by the fine and uniform distribution of ferrite and pearlite.By comparing the relationship between mechanical properties and hydrogen diffusion behavior,it is found that the yield strength and section shrinkage of axle steel are not necessarily related to hydrogen diffusion behavior.However,increasing of tensile strength and decreasing of elongation,D₀ decreases,while C*increases.And the tensile strength has an obvious effect on the hydrogen diffusion behavior.Although the tensile strength of 30Ni CrMoV12specimen steel is up to 1038 MPa,it shows the highest resistance to hydrogen embrittlement due to its chemical composition,microstructure and high plasticity and toughness.?3?The fundamental reason for the difference of hydrogen diffusion behavior between the test steels is due to the difference in the type and quantity of the hydrogen traps produced by different components and different heat treatment conditions.The LZ45CrV axle steel processed by two normalizing and one tempering has the characteristics of uniform structure,fine grain size,small P spacing and large VC particles even precipitation in F.The grain and phase boundaries and VC particles as hydrogen traps reduce the capacity of hydrogen permeation and diffusion,and increase the solid solution of hydrogen.Therefore,the LZ45CrV axle steel has a lower hydrogen diffusion coefficient and a high critical hydrogen concentration for producing hydrogen bubble.The hydrogen traps in three kinds of high-speed axle steel after tempering and quenching and tempering are mainly Fe₃C with rod like and equiaxed Fe₃C,and 30NiCrMoV12 test steels still contain a small amount of spherical Mo₂C type carbides.A large number of Fe3C carbides,which are uniformly precipitated along the direction of martensite,as irreversible strong traps of hydrogen and Ni as reversible hydrogen traps in the F base,make the solid solution of hydrogen in the 30Ni CrMoV12 sample high,and the hydrogen atom is evenly distributed,which makes the hydrogen diffusion coefficient low and the critical hydrogen content of the hydrogen bubble is high.For DZ1 and DZ2 test steel,although the number of carbides is basically the same,the size of the Fe₃C carbide precipitated on the F matrix in the DZ1 test steel is larger,and the partially coarse-shaped carbide is precipitated in irregular multilateral shape.Compared with DZ2 steel,the number of solid hydrogen atoms is large and the hydrogen diffusion coefficient is small.The results of SEM and TEM test show that the overall distribution of carbides on the F matrix in the DZ1 test steel is not uniform,and the precipitates in the DZ2 test steel are mainly concentrated along the recrystallized grain boundaries.This is the main reason for the low concentration of hydrogen atoms in the DZ1 and DZ2 test steels due to the enrichment of hydrogen atoms at the precipitate aggregates.