Study Of Hydrogen Embrittlement Resistance For Hydrogen Traps In Steels Based On Microstructure Design

The increasing strength of metals will inevitably aggravate the hydrogen embrittlement(HE)susceptibility.With the development of high-strength steel,the design of metal with high HE resistance is particularly important.Generally,there are a lot of defects which can be used as hydrogen traps in practical steel,such as dislocations,grain boundary(GB),retained austenite(RA),etc.In the process of smelting,processing and service,the hydrogen entering the steel tends to accumulate at defects such as dislocation and GB,resulting in final fracture.Moreover,the RA in martensitic steel is prone to martensitic transformation during deformation.Because the hydrogen solubility of martensite is much lower than that.of austenite,the excess hydrogen after transformation will lead to material failure.Therefore,improving the HE resistance of materials through the microstructure design of materials and design of the structures is of vital importance.In this work,the effects of dislocations on the hydrogen adsorption properties,hydrogen trap behaviors and HE resistances of Armco iron with various amounts of cold rolling(CR)deformation were firstly studied.Subsequently,the effect of residual compressive stress(?H),which was generated by phase transformation,on the RA morphologies,mechanical properties and HE resistance were subsequently investigated.Finally,two high-Mn steels(Fe-30Mn-0.6C and Fe-30Mn-3A13Si in wt.%)with similar SFEs were used,thereby avoiding any influence from the deformation mechanism.This study focuses on clarifying the difference in the HE extent of the two high-Mn steels and demonstrating the important role of compositional differences on the initiation and propagation of hydrogen induced crackings(HICs).The major conclusions are listed below:(1)The various dislocation morphologies caused by various CR deformation degrees in pure iron were characterized by transmission electron microscopy(TEM)and X-ray diffraction(XRD).Subsequently,an electrochemical hydrogen permeation test,slow rate tensile test(SSRT)and thermal desorption spectroscopy test combined with the Iyer-Pickering-Zamenzadeh(IPZ)model were used to investigate the effects of dislocation morphologies on the hydrogen adsorption properties,hydrogen trap behaviors and HE resistances of Armco iron with various amounts of CR deformation.The total dislocation density increases as the deformation increases.When the deformation reaches 50%,dislocation cell(DC)walls are formed.When the deformation continues to increase,the size of the DC walls and the thickness of the DC walls decrease,concurrently,the dislocation density of the DC walls increases,and the dislocation density of DC walls decreases.These structural changes first lead to the decrease of the effective hydrogen diffusion coefficient.Moreover,the surface hydrogen coverage first increases and then decreases with increasing deformation,and reaches a maximum at 50%deformation.Furthermore,the HE resistance is improved when the deformation reaches 80%.Because the DC walls can be used as effective hydrogen traps when the dislocation density reaches a sufficiently high value.(2)The effect of CR deformation on the hydrogen permeation and trapping behavior of pure iron was systematically investigated.Increased deformation resulted in a decreased effective hydrogen diffusion coefficient,an extended effective lattice interstitial hydrogen diffusion path length and increased hydrogen trap densities.However,the proportion of irreversible hydrogen traps to all traps was constant irrespective of deformation level in the range of 30%to 70%CR,and increased when the deformation reached 80%.This is because the DC wall is composed of a center layer with high trap binding capacity and an outer layer with low trap binding capacity.More importantly,once the dislocation density in the center layer of the DC walls reached a relatively high value due to the severe plastic deformation,these regions become effective irreversible hydrogen traps.(3)XRD combined with TEM tset were used to investigated the ?H and RA morphologies caused by martensitic transformation via sub-zero treatment.Subsequently,the HE resistance of steel under various ?H was investigated by SSRT.The results are as follows:The tensile curves without hydrogen revealed that the ?H,which stabilized the RA and suppressed the deformation-induced martensite transformation,can delay of the start of necking and the improvement of ductility.However,the nanotwins RA formed due to the high ?H(231 MPa)and sub-zero treatment(77 K),resulting in hydrogen-induced cracks formation at the boundary of RA and martensite matrix and thereby induce high HE sensitivity.(4)In order to eliminate the influence of SFE on the deformation mechanism,two steels with similar SFEs(36.3 mJ/m2 and 37.3 mJ/m2 for Fe-30Mn-0.6C and Fe-30Mn-3Al3Si,respectively)were used.Electron back-scattered diffraction(EBSD)was used to investigated the initiation and propagation of HICs.The results of the SSRT under various in-situ hydrogen charging conditions demonstrated that the HE resistance of 3A13Si-1100 was much higher than that of 0.6C-1100,this was due to the different composition of the two steels.The interstitial C atoms were more disadvantageous to the mechanical properties in the presence of hydrogen,compared to substitutional atoms(Al and Si).Because in the presence of hydrogen,interstitial C atoms promoted more RGBs cracking.This also leads to a significant difference in the correlation between random grain boundaries(RGBs)and Schmidt factor difference(??)between the two steels.The RGB cracks of 3A13Si-1100 steel were related to the ?Q of two adjoining crystals.High stress and a huge number of dislocations could accumulate the RGB between two adjacent grains which had high ??,simultaneously,hydrogen could accumulate in these GBs and promote the GB cracking.Nevertheless,the RGB cracking of 0.6C-1100 was uncorrelated with ??,due to the high HE susceptibility of RGBs.The dominant effect of hydrogen in cracking result in the premature fracture at low strain.Moreover,the crack mode of Fe-30Mn-3A13Si steel was a combination of transgranular and intergranular cracks.However,the micro-cracks inside the grains did not have enough time to propagate even if they were formed,because the RGBs of 0.6C-1100 were extremely sensitive to hydrogen.