Study On Properties,Microstructural Evolution And Hydrogen Embrittlement Behavior Of Cr-Ni-Mo Martensitic Ultra-High Strength Stainless Steels

Martenisitic ultra-high strength stainless steel（MUHSSS）has been viewed as one of the necessary candidate materials for high-end manufacturing,including aeronautics and astronautics,and maritime engineering,due to its excellent combination of strength and toughness,comparatively higher specific strength and rigidity,simplicity of thermal treatment,good hot workability and prominent corrosion resistance.On the one hand,it is the requirement for weight lightening of components to improve the fuel efficiency and reduce the emission of greenhouse gas that urges the continually breakthrough of the strength-toughness trade-off dilemma of MUHSSS.On the other hand,since the ocean plays an increasingly important role in the economic prosperity and national security,the components of equipment have to serve in the hydrogen-rich corrosive maritime environment for a long duration,facing the severe challenge of stress corrosion cracking（SCC）and hydrogen embrittlement（HE）.Therefore,on the basis of a more profound insight into the strengthening and toughening mechanisms,it is vitally important to unveil the interactions between the hydrogen and the microstructure of MUHSSS.All the efforts have been made to provide theoretical evidence for further devolepment of novel MUHSSS with optimum combination of high strength,toughness and hydrogen resistance serving in attacking environment.Generally,iron-based alloys will be more susceptible to HE with the augment of strength grade.From this perspective,it is reasonable to clarify the correlation among the mechanical properties,microstructure,and HE behavior of MUHSSS to further optimize the multi-dimenshional properties combination of strength,toughness,and HE resistance and guarantee the service security.In the current study,there are three types of Cr-Ni-Mo MUHSSSs strengthened by various precipitates in different strength grades,namely Custom 465（1.8 GPa）,Ferrium S53（1.9 GPa）steels and a newly developed steel（NDS,2.2 GPa）,being selected as the experimental steels.The microstructure of the tested steels is tuned through thermal treatment.The focus of research is primarily placed on the variation of both mechanical properties and HE behavior induced by microstructural evolution of tested steels.And the main conclusions of the present study can be carefully drawn as follows:Compared with the underaged and peakaged Custom 465 steel,the overaged samples exhibited significantly lower HE indexes.It can be ascribed to the number,density and stability of benign hydrogen traps were improved due to the dissolution of nanoscaleη-Ni₃Ti phase into reversed austenite and the independent transformation ofη-Ni₃Ti into reversed austenite after overaging.The large number of fine and dispersive reversed austenite precipitated in the overaged samples could further act as dual traps of both hydrogen-assisted cracks and the hydrogen atoms at the crack tips.Meanwhile,the ripening ofη-Ni₃Ti during overaging reduced the stress concentration sites inside the matrix and resultantly enhanced the cracking resistance of the experimental steel.Thus,the HE susceptibility of overaged samples decreased.Moreover,overaging treatment effectively reduced the fractions of high-angle grain boundaries（HAGBs）andΣ3 coincidence site lattice（CSL）boundaries and decreased the kernel average misorientation（KAM）values as well,which concurrently favored the enhancement of HE resistance.The M₂C carbide and the totally coherentα’_Cr phase originated from the spinodal decomposition were recognized as the primary strengthening agents in Ferrium S53steel.While the NDS steel were precipitation hardened by three categories of secondary phases,namely M₂C carbide,intermetallic Laves phase andα’_Cr phase.Among the precipitates,Laves phase contributed to the strongest age hardening effect.A unique thermal treatment scheme consisted of duplex cold and aging processes（including two steps of cold and aging processses）was adopted for both Ferrium S53 and NDS steels.In comparison with the first aging treated（FAT）samples,the strength and the ductility of secondary aging treated（SAT）Ferrium S53 and NDS steels simultaneously improved.It was attributed to the further precipitation of high-density dispersive secondary phase particles,which postponed the formation of shear bands and instability of necking.As a result,the plastic deformation of SAT samples became more homogeneous.Besides,the coherent nanosized precipitates played a dual role as both the obstacles to dislocation movement and a special“dislocation source”,which could sustainably provide dislocations.The secondary cold treatment conducted in the interval between the two aging processes reduced the content of meta-stable reversed austenite,refined the matrix and precipitates,ameliorated the dislocation recovery and annihilation,and increased the fraction of HAGBs.It resultantly favored the concurrent enhancement of strength and impact toughness of SAT samples.Transformation-induced plasticity（TRIP）effect of the large amount of meta-stable austenite retained in the first cold treated（FAT）in Ferrium S53 and NDS steels would be triggered during tensile test,which increased the ductility of steels.However,in the presence of hydrogen,the transformed retained austenite acted as“hydrogen source”,which increased the content of the diffusible hydrogen in the steels and,thus,caused severe brittle fracture.The varying tendency of HE susceptibility for FAT and SAT samples of Ferrium S53 and NDS steels were quite different.Specifically,as the aging treatments proceeded,the HE susceptibility of Ferrium S53 gradually decreased.In contrast,the HE susceptibility of NDS steel decreased at first,whilst remarkably increased after SAT.It can be attributable to reduction of coherence of M₂C originated from the further precipitation and growth of M₂C,which enhanced the binding energy of the M₂C hydrogen traps and resultantly reinforced the energy barrier for hydrogen desorption.What’s more,the reduction of fractions of HAGBs,Σ3 CSL boundaries as well as KAM value further benefited to the HE susceptibility reduction of Ferrium S53steel.The fractions of HAGBs,Σ3 CSL boundaries,KAM value and the content of meta-stable retained austenite in NDS steel pronouncedly reduced after FAT,which favored the HE resistance.On the other hand,in the SAT samples of NDS steel,the completely hydrogen-assisted brittle fracture occurred owing to the further segregation of hydrogen at the matrix/Laves phase boundaries and precipitation of Laves phase,which undermined the cohesion of phase boundaries.