An Investigation into the Hydrogen Embrittlement Mechanism in the 3rd Generation Advanced High Strength Automotive Steels Employing Trip Effect

Author:Zhu Xu

Supervisor:jin xue jun


Degree Year:2016





The development and application of the 3rd generation advanced high strength automotive steels with superior PSE(≥30 GPa·%),i.e.product of ultimate tensile strength and total elongation,is an effective measure in automobiles industry to simultaneously meet the goals of the weight reduction,the passenger safety,formability and cost efficiency.Introducing metastable austenite into the hard matrix by suitable alloy design and appropriate heat treatment is proved to be promising to the exploration of high PSE steels.To this end,quenching & partitioning(Q&P)and medium manganese TRIP steels are proposed to retain considerable amount of metastable austenite at ambient temperature based on the mechanisms of austenite forward transformation and reverted transformation.Unfortunately,susceptibility to hydrogen embrittlement(HE)has become the critical issue impeding the industrialization of the 3rd generation advanced high strength automotive steels due to the unpredictable catastrophic failure during the manufacturing processes as well as in service.It is therefore of urgency to reveal the HE mechanism and to find the possible solutions to improve the resistance to HE.In this study,HE behavior and the hydrogen induced cracking mechanism in the multiphase microstructure have been investigated in the Q&P and medium manganese steels.Moreover,the effect of retained austenite(RA)stability,morphology and the transition ε carbides on the HE susceptibility were evaluated in the Q&P treated steels.Furthermore,the hydrogen effect on the work hardening behavior of the medium manganese TRIP steel was emphasized and the origin of the reduced work hardening was additionally discussed.The main results are summarised as follows:1.Desipte of the respectively noticeable increases of 280 MPa and 200 MPa in the yield strength and ultimate tensile strength by quenching & partitioning treatment,the susceptibility to HE in Q&P 980 steel is higher than that in TRIP 780 steel with the same chemical composition.The presence of hydrogen causes dramatical degradations in the ultimate tensile strength,uniform elongation and total elongtion,while almost no deterioration in the yield strength and work hardening rate.With the increased amount of hydrogen,the macroscopic fracture gradually changes from noticeable necking around the fracture area to relatively smooth “brittle” fracture and the microscopic fracture surface displays a transition from ductile microvoid coalescence to a morphology of partial dimples mixed with hydrogen-induced “brittle” features,such as “quasi-cleavage” regions,intergranular facets and microcracks.2.TRIP effect is detrimental to the HE resistance.The combined effects of locally high internal stress,supersaturated hydrogen concentration and high hydrogen diffusion coefficient due to the martensitic transformation cause hydrogen segregation at the maternsite/austenite interface,giving rise to the initiation and propogation of hydrogen induced cracking(HIC).Thus,stable RA is a type of effective hydrogen trap,providing that no transformation occurs during the deformation.In addition,it is deduced that filmy RA is less susceptible to HE than blocky RA,given the same extent of TRIP effect upon straining.The possible explanation lies in the following.The enhanced stability of filmy RA retards the martensitic transformation to high strain due to the strain shielding effect of lath martensite,minimizing the tendency for the initiation of HIC.Moreover,the resulting microstructure of high carbon blocky RA is brittle twin martensite,while that of low carbon filmy RA is ductile lath martensite.In addition,even if some voids are formed in the filmy RA,they are hardly to link together to form main crack possibly due to the inhibition effect of marteniste blocks.Furthermore,the isolate voids may act as beneficial hydrogen traps that lower the diffusive hydrogen content.3.An improved resistance to hydrogen embrittlement in a high strength steel is obtained by quenching-partitioning-tempering(Q-P-T)treatment.Especially under the low hydrogen content,total elongation loss drops from 42.7% to 0.6% after the tempering treatment.No remarkable distinction in microstructure could be detected with the only exception of needle-shaped ε carbides within the lath martensite before and after the low temperature tempering.It is thus deduced that the improved resistance to HE is mainly ascribed to the ε carbides precipitation,considering that the variations in the RA content and the dislocation density in martensite are negligible for Q&P and Q-P-T steels.By 3DAP observation,it is demonstrated that ε carbide formed in this steel is effective in trapping hydrogen and its effect on the alleviation of HE is considered as a result of the reduction in the diffusible hydrogen content.On the one hand,ε carbide partially immobilize diffusible hydrogen atoms due to the trapping effect,reducing the diffusible hydrogen content in matrix.On the other hand,the diffusion coefficient of hydrogen is presumed to be decreased by these hydrogen traps.Consequently,the tendency to HIC initiation and propagation are decreased due to the mitigation of hydrogen segregation ahead of the crack tip as a result of the hindrance of diffusive hydrogen movement to the regions with concentrated hydrostatic stress.4.The susceptibility to HE in medium manganese TRIP steel is much higher than that in Q&P steel with the same strength level.Apparent drops in ultimate tensile strength and elongation are observed by introducing extremely small amount of hydrogen.It is discovered in the “in-situ” electrochemical hydrogen charging and slow strain rate tensile tests that the initiation and propagation of HIC could be easily triggered in the hydrgen charging area once the Lüders bands propagate across that local charging area.In addition,the charging current density,i.e.the hydrogen concentration has negligible effect on the cracking position,indicating that the stress intensity factor plays the vital role in the HIC initiation during the process of Lüders bands propagation.5.Apart from the expected deteriorations in elongation and ultimate tensile strength,the work hardening rate(Θ)is dramatically reduced by hydrogen introduction under the specific strain rate and moderate hydrogen content.It is found that hydrogen has negligible effect on the strain-induced martensitic transformation kinetics from both experimental evidence(through XRD and magnetic measurements)and theoretical calculation based on the BMT model.The austenite undergoes the same extent of transformation with straining regardless of hydrogen effect,indicating that not only the phase transformation is accounted for the extraordinarily high work hardening rate of this steel.6.The origin of work hardening in the UFG Mn TRIP steel is due to the collaborative effect of dislocation accumulation in ferrite and the austenite transformation.Significant work hardening rate could be generated in the deformation process under suitable grain size or hydrogen concentration,providing that dislocation accumulation in the interior of ferrite can be intensively promoted as a result of the occurrence of TRIP effect.The effect of austenite transformation is to strengthen the soft austenite by the replacement of hard marteniste and to introduce dislocation sources to the surrounding ferrite due to the volume expansion caused by the phase transformation.To get optimal combination of strength and ductility as well as the superior resistance to hydrogen-induced cracking,much more attentions should be focused on the effect of ferrite grain size on the enhanced work hardening rate,rather than the significance of austenite stability merely.7.It is rather difficult to clearly address the HE mechanism in the UFG medium manganese TRIP steel,considering its complicated deformation mechanisms,which involves the deformation process featured by Lüders bands propagation and martensitic transformation.Considering the grain size effect of ferrite and TRIP effect of austenite,two mechanisms are related to the HE mechanism with respect to the different deformation stages.At the Lüders bands deformation stage,the initiation of HIC is closely related to the formaton and propagation of Lüders bands.During the martensitic transformation stage,HIC initiates at the ferrite/austenite interface under the specific strain rate and moderate hydrogen content under the condition that hydrogen dramatically lowers the work hardening rate.