Effect Of Pre-deformation On Hydrogen Embrittlement And Corrosion Resistance Of Stainless Steels

More and more attention has been paid to steels with high-strength and excellent resistance to corrosion,and they are in urgent demand especially for the high strength fasteners that are used in the fields of wind power and nuclear power.However,the higher the strength of steels,the more susceptible they are to hydrogen embrittlement.How to solve this inverted relation between high-strength and hydrogen embrittlement is the challenge for many researchers.Austenitic stainless steel possesses excellent corrosion resistance and hydrogen embrittlement resistance,but its relatively low strength limits its application in the field of highstrength bolts.It is necessary to investigate how to improve its strength without losing its corrosion resistance and hydrogen embrittlement resistance.Stainless steels were chosen as the subject of this research and they were conducted to different pre-deformation to generate large number of dislocation tangles inside these materials.Dislocation tangles can not only improve their strength,but also can serve as hydrogen traps to improve their resistance to hydrogen embrittlement.This research has significant application meaning for developing corrosionresistant and high-strength bolt fasteners.In this study,duplex stainless steel 2205(2205DSS)and two austenitic stainless steels 316L and 304 were investigated due to their features of dual phase,stable austenitic phase and deformation induced martensite,and the effects of predeformation on microstructure evolution,mechanical properties,corrosion resistance and hydrogen embrittlement resistance were studied.Hydrogen embrittlement behavior of these stainless steels are also tested after hydrogen charging and the relation among microstructure change,hydrogen distribution change and hydrogen embrittlement susceptibility were established.The effect of pre-deformation and phase boundary on hydrogen embrittlement behavior of 2205DSS.316L and 304 were chosen to explore the effect of deformation-induced phase transformation on hydrogen embrittlement behavior.By comparing the hydrogen embrittlement behavior of undeformed and heavily deformed specimens of three stainless steels,the hydrogen embrittlement mechanisms after predeformation were investigated.Furthermore,the austenitic stainless steels were heat treated after pre-deformation to investigate the effect of the combined predeformation and heat treatment process on the hydrogen embrittlement behavior and mechanical properties of austenite stainless steels.In addition,the effect of predeformation on the corrosion resistance of stainless steels,especially the pitting behaviour,is also investigated.The obtained conclusions are as follows:After rolling to a large thickness reduction at 150℃,2205DSS possessed a good combination of high-strength and low hydrogen embrittlement susceptibility.Phase boundaries are prone to accumulate hydrogen in.Moreover,the hydrogen content at phase boundary is more likely to increase during tensile test due to the pile-up of dislocation around it.Thus,undeformed 2205DSS specimen exhibits high susceptibility to hydrogen embrittlement.After heavy deformation,the dislocation density inside the austenite phase increased,and dislocation tangles formed into a uniform structure.After heavy deformation,sites where hydrogen enrichment happens has changed from phase boundary to austenite phase.The stress/strain distribution behaviour of 2205DSS reduces the stress concentration at the phase boundary,and formed the uniformly distributed dislocation tangles within the austenitic phase.Hydrogen was trapped at dislocation tangles,which reduced the enrichment of hydrogen at phase boundaries and grain boundaries.Thus,2205DSS-80%CW specimen exhibited a lower susceptibility to hydrogen embrittlement.The network of dislocation tangles formed after pre-deformation in 316L and 304 contributes to improve their resistance to hydrogen embrittlement.The coupling effect of the formation of tangled dislocation structure and the hydrogen transportation controlled by the movement of dislocation makes hydrogen welldispersed and reduces the localized enrichment of hydrogen.Compared with the grain boundary,hydrogen distributed inside grains more evenly after heavy predeformation.Under this condition,the hydrogen concentration at grain boundary reduced and intergranular crack can be avoided,showing low susceptibility to hydrogen embrittlement.Due to its high austenite stability,316L generated a homogeneous tangled dislocation structure after heavy pre-deformation,thus the hydrogen was also dispersed,showing better hydrogen embrittlement resistance.While the austenite stability of 304 is lower,strain-induced martensite(α’martensite)is transformed after heavy deformation.The generation of α"-martensite leads to an uneven distribution of dislocations,which can easily cause local enrichment of hydrogen.At the same time,α’-martensite can act as highway for hydrogen diffusion in the surrounding austenite,which promote the hydrogen accumulation process and cause premature fracture,showing high susceptibility to hydrogen embrittlement even under the effect of dislocation structure.High-strength stainless steel with the yield strength of 1002.9 MPa,tensile strength of 1132.7 MPa and elongation of 14.3%was prepared by heat treatment under 600℃ for 2 h after heavy pre-deformation for 316L.After heat treatment,many well-dispersed carbides were introduced inside the grains of heavily deformed specimens.Compared with 316L-70%CW specimen,the strength of 316L-70%CW specimen slightly decreased but its ductility recovered after heat treatment.Meanwhile,the hydrogen embrittlement resistance of heat-treated 316L70%CW specimen was improved.The dislocation-carbide structure formed during heat treatment was found to be beneficial to improve hydrogen embrittlement.After the same heat treatment,recrystallisation has taken place for 304-70%CW specimen,resulting in a sharp decrease in strength and a significant recovery in ductility.However,the heat treated 304-70%CW specimen turned out to be highly susceptible to hydrogen embrittlement.Hydrogen was prone to be enriched at grain boundaries,leading to the appearance of intergranular morphology on its fracture surface.It was found that pitting mainly initiated at the deformation zones of deformed stainless steels,and the number of pits increased as the deformation level increased.Deformation has a negative effect on pitting corrosion of 316L and 304 austenite stainless steels.Deformation significantly increased the donor density for the semiconductor properties of the passive film,reduced the stability of the passive film,and made pitting corrosion more easily to nuclear.For 304 stainless steels,although a large amount of α’-martensite was induced after heavy deformation,pits still mainly initiated at the deformation zones.Although the corrosion resistance of stainless steels is reduced after pre-deformation,it can still meet the requirements for corrosion resistance in atmosphere and even coastal atmosphere.The pitting corrosion resistance of 2205DSS does not change too much after deformation.