| With the increase of energy shortage and environment pollution,the development and utilization of hydrogen energy,which is a kind of clean and efficient secondary energy source,has become one of the important directions of China's energy transition.Because hydrogen can easily penetrate into the materials of hydrogen storage containers,causing hydrogen embrittlement(HE),the requirement for HE resistance of the materials keeps increasing.Austenitic stainless steels(ASS)are the preferred materials for the manufacture of hydrogen storage containers.The effects of prior plastic deformation processing and annealing,sensitization(welding)and other heat treatments on HE resistance of ASS in the actual production processes are worth exploring.In this thesis,a relatively inexpensive type 304 metastable ASS is used to study the effects of combined prior plastic deformation and annealing(sensitization)treatment and grain refinement based on severe plastic deformation and annealing treatment on its HE resistance.The research methods include ??-phase content analysis,metallographic observation,electrochemical hydrogen charging,slow strain rate tensile tests and fracture morphology analysis.This thesis provides some theoretical and experimental guidance for the actual productions of hydrogen storage containers and pipelines.The main contents and conclusions are as follows:(1)The effects of combined tensile plastic pre-strain and 650 °C sensitization(simulated by annealing)on the HE susceptibility of the 304 steel were investigated by hydrogen pre-charging and tensile testing.It is found that pre-strains above 10%will cause the steel to produce a large amount of ?? martensite prior to hydrogen exposure,thereby degrading the HE resistance significantly.Sensitization treatment also degrades the HE resistance,but the short-time sensitization of pre-strained and??-containing 304 steel will lead to the reversion of ?? martensite to austenite,and hence retrieve the HE resistance curtailed by the pre-strain.In contrast,steel specimens that were firstly sensitized and then deformed produce more ?? than specimens that were pre-strained alone,because of the precipitation of carbides which decreases the stability of austenite,consequently degrading the HE resistance more significantly.Therefore,in actual productions,it is not recommended to firstly sensitize(welding)and then deform.(2)It is revealed that the content of pre-existing ?? martensite is the key factorcontrolling the HE of metastable ASS.The quantitative relationship between the hydrogen transport parameters(hydrogen diffusion coefficient and solubility)and the pre-existing ?? content is analyzed.It is proved that ?? martensite platelets are the“highways” for hydrogen diffusion in the ASSs,thus increasing the hydrogen entry.In addition,a relation between apparent hydrogen diffusion coefficient and ?? martensite content is derived,which is also applicable to other typical body-centered cubic and face-centered cubic duplex materials.The ?? itself is not the key reason for the increase of HE.It is the increase of hydrogen entry that makes the ??-containing 304 steel been more prone to HE.(3)Using 90% heavy cold rolling and annealing treatment with different parameters,the grains of the 304 steel were changed and the effect of grain refinement on the HE susceptibility was studied by dynamic hydrogen charging.It is found that with the increase of annealing temperature and annealing time,the ?? phase induced previously by cold rolling gradually reversed to austenite,the average grain size gradually increased,and the HE resistance gradually decreased.It is indicated that the304 steel could have high strength,high ductility and low HE susceptibility by using appropriate treatment parameters. |
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