Research Effect Of Pre-strain On Hydrogen Embrittlement Of 304 Austenitic Stainless Steel Under Different Hydrogen Conditions

Austenitic stainless steels(ASSs)have been widely used in high-pressure hydrogen applications,such as pipelines and valves,for fuel cell storage systems or hydrogen refueling stations,owing to its excellent resistance to hydrogen embrittlement(HE).However,when the stability of the austenitic phase is low,material directly exposed to hydrogen environment will be suffered the deterioration of the mechanical properties caused by external hydrogen,and then HE of the material may occur,which is related to strain-induced ?? martensite transformation.Cold work hardening by pre-straining is used to improve the mechanical properties of ASSs prior to service.For ASSs,it was found that pre-deformation can cause work hardening and phase or microstructure changes,i.e.?? martensite formation,the diffusion and multiplication of dislocations,or the formation of deformation twins;they could change the HE susceptibility of the ASSs.It is well known that the HE type of austenitic stainless steel can be divided into internal hydrogen embrittlement and hydrogen environment embrittlement.Many researchers have studied the effects of pre-strain on internal hydrogen embrittlement or hydrogen environment embrittlement of austenitic stainless steels.However,due to the complex nature of the hydrogen embrittlement mechanism,no decisive conclusions have been reached.Therefore,it is necessary to further study the effect of pre-strain on hydrogen embrittlement of 304 austenitic stainless steel under different hydrogen conditions.In this paper,304 stainless steel was used as the research object.The effects of pre-strain on the hydrogen embrittlement of 304 stainless steel investigated by a tensile test under cathodic hydrogen-charged,gaseous hydrogen,hydrogen-charged and gaseous hydrogen combined,and thermal hydrogen-charged conditions.The relationship between microstructure and two types of hydrogen embrittlement mechanisms was studied by characterization methods such as SEM,EBSD and TEM.The conclusions are as follows:(1)For 304 ASSs with a hydrogen-charged electrochemical surface,the HE susceptibility increased slowly and then increased rapidly with increasing pre-strain Hydrogen-induced cracking were initiated in the interior of martensite,at the interface between martensite and austenite.The fracture behavior was related to hydrogen's elastic shielding effect.Hydrogen enhanced the local plastic deformation of the martensite and austenite interface due to the increase of dislocation mobility,thus causing cracking.(2)In the gaseous hydrogen environment,hydrogen-induced cracking was initiated at the grain boundary.The HE susceptibility increased and then decreased with increasing pre-strain,in which pre-strain of 10% showed the highest HE susceptibility.This was related to the interfacial stress,dynamic martensite and dislocation density.(3)Under the coupling of hydrogen-charged and gaseous hydrogen,the HE susceptibility gradually increased with increasing pre-strain.The environmental hydrogen played a dominant role during the fracture process for low levels of pre-deformed specimens.The internal hydrogen played a dominant role during the fracture process for high levels of pre-deformed specimens,because the environmental hydrogen could promote crack propagation for low levels of pre-deformed specimens,and the internal hydrogen could promote crack initiation for high levels of pre-deformed specimens.(4)For thermal hydrogen-charged 304 stainless steel,the HE of the specimen decreased with increasing pre-strain,which may be related to the dynamic martensite content.As the pre-strain increased,the content of dynamic martensite gradually decreased,and there was not enough dynamic martensite to provide a channel for hydrogen to diffuse to the crack tip,thus reducing HE.(5)After annealing treatment at 200°C for 96 h,the elongation of the pre-deformed specimen can be increased,as well as that of the pre-deformed specimen charged with hydrogen,but the latter increases the elongation more than the former.This indicates that mechanical Properties of pre-deformed specimens can be improved by annealing at 200°C regardless of whether the specimens are hydrogen charged or not.