| In comparison with carbon steels and low-alloy steels,grade 300 austenitic stainless steels(ASSs)possess higher hydrogen embrittlement(HE)resistance.However,due to lower strength,ASSs are not suitable for the fabrications of high loading/high pressure equipment exposed to hydrogen-containing environments,such as high-pressure gaseous hydrogen storge vessles and transmission piplines.Grain refinement can not only elevate the yield strength but also improve the HE resistace of ASSs.Combined severe cold-rolling deformation and annealing treatment is one of the promising methods of reducing the grain size of ASSs.Although several studies have focused on the effect of grain size on HE of ASSs,the effect of annealing condition,the combined effect of strain rate/hydrogen-charging method and grain size are not reported,and the role of grain boundaries in HE process is not well-understood.Therefore,in thsis thesis,the HE behaviors of 304/304L ASSs with different grain size produced by combined severe cold-rolling and annealing treatment are investigated from three aspects and the conclusions are summarized as follows:(1)A wider range of grain size(0.4～35 μm)was produced for the 304 steel by severe cold-rolling and annealing treatment.The effect of annealing temperature and duration on the HE susceptibility was investigated,and the role of grain boundary increase in the HE process was analyzed.The results indicate that the HE susceptibility not only depends on the grain size,it also depends on the annealing condition.When the annealing duration and temperature is not enough to reverse 90%α’ martensite to γ austensite,the HE can not be suppressed significantly,even if the grain size is relatively low,because of the accelerated effect of α’ martensite on hydrogen transport.However,if the annealing duration is extended to induce severe carbide precipitation,the HE can also not be suppressed effectively,because the carbide particles can offset the beneficial effect of grain refinement on HE by providing stress concentrations.(2)The role of grain boundaries is to play as hydrogen trapping sites,therefore the hydrogen trapping site density increases with decreasing grain size.On one hand,the increase of hydrogen trapping sites suppresses the hydrogen transport,i.e.the effect of grain size d on apparent hydrogen diffusion coefficient Dapp can be expressed as:DL/Dapp=1+2n/dNL exp(Eb/RT)where DL is the hydrogen diffusion coefficient without the effect of trapping sites,NL is the number of normal interstitial lattice sites,Eb is the binding energy,and n means that one grain boundary area provides n hydrogen trap sites.On the other hand,due to the huge quantity of grain boundaries in the fine-grained specimens,the hydrogen concentration trapped by each grain boundary is low.These two reasons contribute to the suppression of HE by grain refinement.(3)The effect of tensile deformation rate on HE of the 304L steel with different grain size was investigated by in-situ hydrogen charging.With the decrease of tensile rate the HE susceptibility increases.Grain refinement does not modify the trend,but the effect degree of tensile deformation rate on the HE becomes reduced.(3)The effect of hydrogen-chargin condition on HE susceptibility and fracture morphylogies of 304L steel was investigated.It is found that in the pre-charging condition the HE susceptibility is increased with decreasing grain size,which is contrary to the case of in-situ hydrogen-charging.Therefore,whether or not the grain refinement will suppress the HE depends on also the hydrogen-charging condition.This artcle concludes 31 pictures,8 tables,and 172 references... |
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