Effect Of Microstructures On The Hydrogen Embrittlement Of Stainless Steels

Stainless steels(SSs)are usually used in many industry fields due to their good mechanical properities,high corrosion resistance and good weld property.However,it is inevitable to introduce hydrogen into SSs during the manufracture and service,leading to hydrogen embrittlement(HE)fracture of SSs.Stable austenitic stainless steels(ASSs)are intrinsically resistant to HE eventhough it is costly.Metastable ASSs with relatively low nickel content can suffer suffer from HE when they are hydrogen charged cathodically or thermally.'Meanwhile,ASSs ususlly possess a low strength which can not satisfy the strength requirements of industry applications.Hence,how to develop a high strength with high HE resistance metastable ASS is still a challenging task.In addition,ASSs can be sensitized when they are heat treated at 500 ?to 850 ?,resulting in carbide precipitation at grain boundaries.Sensitization can not only affect the intergranular corrosion and stress corrison cracking of ASSs,but also have an influence on the HE resistance of ASSs.The strength of martensitic stainless steels(MSSs)are usually higher than that of ASSs,but MSSs are generally more HE sensitive than ASSs.Hence,whether introducing austenite into MSSs can improve the HE resistance of MSSs is worth exploring.In this paper,a commercial 304 ASS and S41500 MSS have been investigated by using microscopic observation,statistic analysis,in-situ hydrogen charged slow strain rate test(SSRT)and hydrogen permeation.The effect of grain size,nanotwinned structure and sensitization on the HE susceptibility of 304 ASS,along with the effect of reversed austenite(RA)on the HE susceptibility of S41500 MSS have been studied.Main research process is as follows:304 ASSs with different grain sizes(in the range from 4 ?m to 12?m)have been made by using cold rolling and subsequent annealing treatment.HE susceptibility experiments show that grain refinement not only increase the strength of 304 ASS,but also improve the HE reisistance of 304 ASS.Electron backscattered diffraction kernel average misorientation(EBSD-KAM)mapping shows that the strain localization can be mitigated by grain refinement.Hence,strain localization sites which act as highways for hydrogen diffusion and preferred crack initiation sites can be reduced along with grain refinement,leading to a high HE resistance.Meanwhile,grain size shows no influence on the strain induced martensite(SIM)transformation during the hydrogen charging SSRT.Hydorgen permeation test indicate that hydrogen diffusion coefficient increases first and then decrease as the grain size decreases.Hydrogen diffusion is supposed to be controlled by a competition between short-circuit diffusion along random grain boundary(RGB)and hydrogen trapping at dislocations.Although grain refinement can improve both the strength and HE resistance of ASS,it can also bring a severe ductility loss when the grain size is decreased to nanoscale.Hence,in this paper,we introduce nanotwinned structure into 304 ASS to improve the strength of 304 ASS.Meanwhile,the effect of nanotwinned structure on the HE resistance of 304 ASS has been studied.Nanotwinned 304 ASS with a mixed structure of nanotwins(41%volume fraction),recrystallization grain(RXG,32%volume fraction)and dislocation structures(27%volume fraction)possesses a high strength and a high HE resistance.Hydrogen causes a 12.5%reduction in yield strength and 5.4%reduction in tensile strength of nanotwinned 304 because hydrogen enhances dislocations/twin boundaries(TBs)interactions.Hydrogen charging also causes a 41%reduction in SIM and has no influence on the strain hardening capacity of nanotwinned 304 ASS.This is mainly because the partial dislocations generated from the dislocations/TBs can provide extra stain hardening capacity for nanotwinned 304 ASS.Strain localization is supposed to be mitigated by the bundle form of TBs and the micro-sized RXGs,accounting for the high HE resistance of nanotwinned 304 ASS.After sensitized at 650? for 24 h,a discontinuous carbide precipitation has been found along the grain boundary in 304 ASS,and the carbide has a cube-cube orientation relationship with the austenite phase.Hydrogen susceptibility experiment shows that sensitization treatment greatly reduce the HE resistance of 304 ASS.TEM observation of the sensitized 304 ASS after 10%strain under hydrogen charging shows that SIM prefer to nucleate around the carbides.This is mainly because hydrogen accumulated at the carbide/matrix interface decreases the stacking fault energy and increases the stress concentration,which is in favor of SIM nucleation and growth.Hydrogen charging leads to an intergranular cracking of 304 ASS.Both slip bands and tear ridges are found on the intergranular fracture facets,which is related to the hydrogen enhanced slip localization around grain boundaries.S41500 MSSs with different fractions of RA have been made throught different heat treatment.HE susceptibility experiment shows that the HE resistance of S41500 MSS increases with the increase in RA.Hydrogen charging leads to a quasi-cleavage fracture of S41500 MSS.Focused ion beam(FIB)was used to prepare sample for TEM observation of fracture path after HE fracture.It clearly shows that RA near the fracture surface has transformed to the newly formed martensite(NFM)and cracking occurs along both the tempered martensite/NFM boundaries and the lath boundaries.Three dimension atom probe(3DAP)confirms that RA is the H trapping site.Thus the beneficial role of RA is that it can act as a stable hydrogen trapping site which can increase the HE resistance by reducing hydrogen content at lath and grain boundaries.But its beneficial effect should not.be overestimated since it can transform to NFM under the attack of stress concentration and hydrogen.