The Hydrogen Embrittlement Behavior Of 304 Austenitic Stainless Steel In Different Microstructure States

Austenitic stainless steel(ASS)is widely used in petrochemical,marine engineering,hydrogen storage and transportation industries because of its low cost and excellent corrosion resistance.However,hydrogen would inevitably be introduced into the material in production process or service process,leading to hydrogen embrittlement(HE)cracking.At the same time,the strength of ASS is needed if it serves as structural parts.However,in general,the higher the strength of the ASS,the worse the HE resistance.Therefore,how to improve the HE resistance of ASS while guarantee its strength is challenging.Different manufacturing processes would result in different microstructure,affecting the mechanical properties and HE resistance of ASS.In order to obtain ASS with good mechanical properties and HE resistance,and to rationally select the microstructure states and processing technologies of the material under specific application requirements,it is necessary to systematically study the HE behavior of ASS in different microstructure states.In this dissertation,the typical 304 ASS was taken as the research object.Different microstructure states were generated by different traditional processes(such as solution treating,cold rolling,annealing,warm rolling)and additive manufacturing process;Scanning electron microscopy(SEM),energy dispersion spectrometer(EDS),electron backscatter diffraction(EBSD),and transmission electron microscopy(TEM)were used to observe the static structures of material;In-situ tensile EBSD tests were used to analyze the changes of dynamic microstructure and properties during tensile process;Thermal desorption analysis system(TDS)and Time-of-flight secondary ion mass spectrometer(TOF-SIMS)equipped on the two-beam focused ion-scanning electron microscope(FIB-SEM)were used to examine the content and distribution of hydrogen;By electrochemical pre-charging H + slow strain rate tensile(SSRT)experiments and dynamic hydrogen charging tensile experiments,The HE behavior of 304 ASS in two different environments was investigated.The results showed that,under the condition of pre-charging H(simulating the service of hydrogen-contained materials,such as pickling,electroplating or welding materials in a non-hydrogen environment),the materials with cold-rolled microstructure in the recovery state after annealing and warmrolled microstructure can obtain the better mechanical properties and HE resistance;under the condition of dynamic hydrogen charging(simulating the material’s service in a hydrogen environment),all the materials with traditional processing microstructure exhibited extremely poor HE resistance.In contrast,additive manufactured 304 ASS exhibited excellent HE resistance and high strength under both pre-charging H and in-situ dynamic hydrogen charging conditions.The specific research contents are as follows:(1)HE behavior of materials with different microstructure states under prehydrogen charging conditionHomogeneous austenite microstructure was obtained by solution treatment,and the solution-treated ASS had good HE resistance but low strength.The results showed that hydrogen tended to concentrate on the grain boundary,which promoted the nucleation of α’-martensite along the twin boundary and slip bands,and promoted the α’-martensite transition.Combined with crack growth and fracture analysis,it was shown that the HE mechanism was mainly hydrogen-induced local plastic deformation mechanism(HELP).The mechanical property and HE resistance of cold-rolled ASS with deformed austenitic structure and deformed α’-martensitic structure were closely related to the amount of deformation.The analysis showed that,with small deformation,the main factor affecting HE sensitivity was dislocations,the HE sensitivity of the material was low,the HE resistance was good,and the HE mechanism was mainly HELP;with large deformation,the main factor affecting HE sensitivity was α’-martensite,the HE sensitivity increased rapidly,and the HE resistance decreased rapidly,and the HE mechanism was mainly hydrogen-induced weak bond mechanism(HEDE),HELP and HEDE mechanism worked together.Microstructure changes such as reverse martensitic transformation,recovery,recrystallization,and grain growth of cold-rolled ASS would occur during different annealing processes,and the comprehensive properties of ASS in the recovery stage were the best.The results showed that,in the stage of the reverse martensitic transformation,martensite content and hydrogen content decreased,and the HE resistance of the materials increased;in the stages of recovery and recrystallization,the dislocation density and hydrogen content decreased,and the HE resistance increased;in the stage of grain growth,the hydrogen content in the grain boundary per unit area increased,and the HE resistance decreased.The sensitization treatment had different effects on the HE resistance of solution-treated and cold-rolled material.After the sensitization of solution-treated material,a small number of carbides precipitated at the grain boundary,which acted as a weak hydrogen trap,resulting in the increasing of hydrogen content and the decreasing of the HE resistance;after the sensitization of cold-rolled material,although a large number of carbides precipitated,the martensite content decreased,then the hydrogen content decreased,and the HE resistance of the material increased.Combined with fracture analysis,it was shown that the sensitized precipitates changed the nucleation position of martensite in the process of tensile,making it preferentially nucleate at the grain boundary,resulting in the enrichment of hydrogen at the grain boundary,reducing the intergranular binding force,and leading to the occurrence of intergranular fracture.HE mechanism was mainly HEDE mechanism,and HELP and HEDE mechanisms worked together.The warm-rolled ASS had good mechanical properties and HE resistance.The results showed that,the higher the temperature,the less the dislocation and deformation twins,the lower the HE susceptibility and the better the HE resistance;the smaller the deformation,the less the dislocation and deformation twins,the lower the HE sensitivity and the better the HE resistance.Combined with fracture analysis,the HE mechanism of warm-rolled ASS was mainly HELP mechanism.Under the condition of pre-hydrogen charging studied in this paper,additive manufactured ASS with dislocation cellular structure,had very low HE sensitivity,excellent HE resistance,and high strength.The analysis showed that the dislocation-cellular structure had a strong hydrogen trap effect,which alleviated the segregation of hydrogen at the grain boundary,slowed down the diffusion of hydrogen,increased the HE resistance of the material while ensuring the strength,then better solved the problem of "the higher the strength,the worse the HE resistance" of the traditional material.(2)Hydrogen embrittlement behavior of materials with different microstructure states under in-situ dynamic hydrogen charging and stretching conditionUnder in-situ dynamic hydrogen charging and stretching condition,materials in solutiontreated state,cold-rolled state,warm-rolled state and other traditional processes states showed very high HE sensitivity and poor HE resistance.The analysis showed that,α’-martensite produced during the tensile process can be used as a hydrogen diffusion channel,further accelerated the diffusion of hydrogen under the stress,so that hydrogen can continuously enter the the material through α’-martensite,then be captured by the grain boundary,reducing the intergranular binding force.When the hydrogen concentration exceeded the critical hydrogen concentration causing brittle fracture,destructive intergranular fracture will be triggered.The HE mechanism was mainly HEDE mechanism,HELP and HEDE mechanism worked together.In contrast,additive manufactured 304 ASS with dislocation-cellular structure had low HE sensitivity and excellent HE resistance under in-situ dynamic hydrogen charging and stretching condition.The analysis showed that there was no martensite formation during the stretching process,and the hydrogen content entering the material was small,which was mainly evenly distributed at the wall of the dislocation cell,alleviating the hydrogen polarization at the grain boundary and reducing the possibility of intergranular fracture.At the same time,hydrogen promoted the local dislocation movement and slip of the crack tip,resulting in local plastic deformation,and the crack propagation was carried out by local micropore consolidation.The mechanism of HE was mainly the co-action of HELP and hydrogen enhanced strain-induced vacancy formation mechanism(HESIV).In addition,the microstructure and properties of additive manufactured material with different building directions were investigated: the microstructure of horizontal-printed 3D-0° material was mainly equiaxed crystal,with slightly lower strength but better HE resistance;the microstructure of 3D-45° material was mainly inclined crystal,with higher strength but low HE resistance;the microstructure of 3D-90° material was mainly columnar crystal,with the too poor the mechanical properties to be used for structural parts.Finally,the effect of annealing treatment on the HE resistance of 3D-0° material was studied: stress relief annealing can reduce the HE sensitivity of the material;however,after annealing at high temperature,the HE sensitivity increased and the HE resistance decreased due to the grain growth and twin increase.