| Due to their excellent corrosion resistance and good mechanical properties,duplex stainless steels have been widely used in offshore oil and gas development.However,several failure events have been reported in marine duplex stainless steel components due to hydrogen embrittlement.So far,there are still many issues that need to be further clarified regarding the hydrogen diffusion,hydrogen trapping,and hydrogen embrittlement(HE)mechanisms of duplex stainless steels.Therefore,this paper carries out a systematic and in-depth study on the hydrogen transport and hydrogen-induced fracture mechanism of duplex stainless steels,providing theoretical support for their application and further development.Taking 2205 duplex stainless steel as the research object,this doctoral dissertation comprehensively uses electrochemical hydrogen permeation,multiphysics finite element method,X-ray diffraction(XRD),slow strain rate tensile(SSRT),scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscope(TEM),hydrogen microprinting,scanning Kelvin probe force microscope(SKPFM),mass transfer theory,and continuum mechanics theory to systematically investigate the hydrogen transport,hydrogen trapping,and hydrogen-induced crack(HIC)initiation and propagation mechanisms of duplex stainless steels.The main results are as follows:(1)Thin 2205 stainless steel sheets with different plastic deformation amounts were prepared,and hydrogen permeation and hydrogen microprinting tests were performed on them.Based on their microstructure,a finite element model was established to simulate the hydrogen permeation process.Further,by treating the phase boundary as a thin film material,a new effective hydrogen diffusion coefficient model for duplex stainless steels was established.The results show that the hydrogen diffusion coefficient in the rolling direction of the 2205 steel plate is 1.07×10-14 m2/s,and in the thickness direction,it is 1.62×10-15 m2/s,and its value is almost constant within 18%plastic deformation amount.Both hydrogen microprinting and SKPFM confirm that the α/γ phase boundary is a hydrogen trap.The effective hydrogen diffusion coefficient of duplex stainless steels can be expressed as De=λgDα/ηe,where the hydrogen diffusion geometric factor λgis related to the microstructure and the hydrogen diffusion barrier at the phase boundary,Da is the hydrogen diffusion coefficient of ferrite,and the trap factorηe is related to the hydrogen inside the two phases and at the phase boundary.The binding energy of hydrogen traps at the α/γ phase boundary relative to ferrite is about 43.6 kJ/mol,while the energy barrier for hydrogen diffusion from ferrite to adjacent austenite can be as high as 96.7～102.4 kJ/mol.(2)The hydrogen embrittlement sensitivity of DSS annealed at different temperatures after cold rolling was evaluated using pre-charging hydrogen and SSRT testing,and the length and quantity of HICs were quantified.Microstructural deformation-induced hydrogen redistribution states were also simulated.The results showed that the HEsensitivity of the DSS annealed at 1000℃(16%)was significantly lower than that annealed at 1090℃(39%)and 1180℃(37%).The expansion of hydrogen-induced microcracks determined the HE sensitivity of DSS,and the DSS annealed at 1180℃ began to exhibit rapid expansion of single HICs at 7%tensile strain,resulting in the minimum fracture strain.The austenite phase effectively hindered the expansion of HICs,while the ferrite phase acted as the preferred crack propagation path.Therefore,the width and connectivity of ferrite blocks were identified as critical microstructural factors affecting the HEof DSS.The ferrite width was the smallest for the DSS annealed at 1000℃,and the intermittent gaps between ferrite blocks made it difficult for HICs to propagate,resulting in the lowest HE sensitivity.Microstructure characterization revealed that the HICs tended to propagate along {100} cleavage planes when passing through ferrite,while they propagated along {111} slip planes when passing through austenite.(3)The effects of anisotropy and slip transfer on HE sensitivity were investigated.Firstly,three types of samples with rolling directions of 0°,45°,and 90° were prepared,and the HE sensitivity of each sample was studied by SSRT.The number and length of HICs were counted and characterized.Secondly,the distribution of strain in two phases during deformation was measured,and the distribution of stress was calculated.Then,the hydrogen distribution after deformation was characterized using hydrogen microprinting,and the dislocation configuration of the deformed sample was observed.Subsequently,based on the observed hydrogen-induced austenitic transgranular cracking phenomenon,statistical analysis of slip transfer was carried out.Finally,an energy model was established to provide or consume key physical processes in slip transfer.The results show that the HE sensitivity index of the 45° rolling direction is 24%,significantly lower than that of the 0° and 90° rolling directions of 39%and 49%,respectively.The anisotropy of duplex stainless steel affects the initiation of HICs by affecting the distribution of stress and strain in two phases.The Schmid factor of ferrite in the 45° rolling direction is the smallest and the strengthening effect of the phase boundary on ferrite blocks is the weakest,which makes the deformation of the two phases more coordinated and the stress on the ferrite phase smaller,effectively delaying the initiation and propagation of HICs.In the early stage of tensile deformation,most(>62%)HICs initiate at austenite grain boundaries.The statistical results show that the critical M value and residual Burgers vector br of slip transfer and hydrogen-induced austenitic transgranular cracking are related to the type of grain boundary,where M is the product of the Luster-Morris m’parameter and Schmid factors(Sin+Sout).For random high-angle grain boundaries,slip transfer requires M>0.3 and br<0.6b<110>,while no hydrogen-induced transgranular cracking occurs when M>0.88 and br<0.15b<110>.For Σ3 twin boundaries,slip transfer requires M>0.54 and br<0.34b<110>,while no hydrogen-induced transgranular cracking occurs when M>0.65 and br<0.30b<110>.Hydrogen-enchanced dislocation planar slip can accelerate the accumulation of residual dislocations at grain boundaries,and hydrogen at grain boundaries can increase slip transfer resistance,resulting in a smaller range of M and br values where no hydrogen-induced transgranular cracking occurs.(4)The HE sensitivity of 2205 duplex stainless steel was tested at three temperatures,22℃,32℃,and 42℃,and the hydrogen-induced cracking was statistically analyzed and characterized by EBSD.The effect of hydrogen on the austenite stacking fault energy was also calculated.The results showed that within the temperature range of 22-42℃,with the increase in temperature,the HE sensitivity of pre-charged 2205 duplex stainless steel decreased from 43%to 10%,and the fracture brittleness layer thickness decreased from 56 μm to 37 μm.The hydrogen desorption during the tensile process accelerated sharply,and the austenite phase transformed from strain-induced martensite to deformation twinning.Calculation showed that only a few hundred ppm of hydrogen can significantly reduce the austenite stacking fault energy within 22-42℃,which is consistent with the deformation behavior of austenite after hydrogen charging.The effect of temperature on stacking fault energy was only a secondary factor,attributed to the recovery of the stacking fault energy of austenite phase lowered by hydrogen during high-temperature stretching,which relieves the plane slip of dislocations,and thus the anti-hydrogen-induced cracking effect of austenite phase is restored,resulting in a lower HE sensitivity at higher environmental temperatures. |
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