| A certain advanced cryogenic engineering is a strategic and fundamental instrument supporting independent development of aircrafts and leading the investigation of aerodynamics and its related disciplines.Severing as key component of the cryogenic engineering,the properties of 00Cr12Ni10MoTi maraging stainless steel heavy forgings mostly determines the service safety of the system.Until now,there is still lack of domestic manufacturing experience of hundred-ton stainless steel heavy forgings.Chemical composition segregation as well as the content excess of gas elements will easily occurs inside the conventional electro-slag remelted(ESR)steel ingot,which seriously limits the overall quality of heavy forgings.To resolve the problem,we firstly develop metal additive forging(MAF)technology,adopting high-quality slabs to process larger homogenized base materials.MAF has been successively applied in manufacturing crucial components,such as the support ring in 4th generation nuclear fast reactor container and large pressure vessel forgings.However,the healing mechanism of the interface in stainless steel by MAF,especially for the dissolution theoretics of interfacial oxides,is still not completely clear at present.Besides,extreme low-temperature environment and alternating impact load require the heavy forgings to possess high-strength and good cryogenic impact toughness.However,the size effect in practical heat treatment process leads to the variation of microstructure and properties at different thickness sites in the section,which seriously deteriorate the service safety of the product-level heavy forgings.Thus,it is necessary to investigate the material heat treatment and microstructure-properties,providing theoretical guidance for manufacturing high strength-toughness stainless steel heavy forgings.In this thesis,with respect to 00Crl2Nil0MoTi maraging stainless steel,we systematically investigate the relationship of heat treatment process-microstructure-properties,and reveal the underlying mechanisms of the thermodynamic dissociation and diffusion of interfacial oxides by first-principles calculations methods.The main investigation content and conclusions are as following:(1)The effect of aging temperature on microstructure and mechanical properties of 00Cr12Ni10MoTi maraging stainless steel was studied.By multi-scale microstructure characterizations and thermodynamic calculations,it was found that with increase of aging temperature,the content of reversed austenite continuously increases with the recovery of martensite matrix.When the aging temperature is up to 560?,there forms martensite and austenite dual-phase microstructure.The precipitation of ?-Ni3(Ti,Al)and austenite reversion can occur independently of each other though there is competition of Ni between them.It leads to the coarsening and dissolution of ?-precipitates as the aging temperature increases over 500?.It emphasized that aging at intermediate temperature of 500? is beneficial to obtain a desirable heterogeneous microstructure of the aged martensite matrix,soft reversed austenite and stable ?-precipitates,which contributes to a better combination of high yield strength(?1 GPa,25 ?;?1.4 GPa,?196?)and reasonable cryogenic impact toughness(?60 J,-196?).The precipitation strengthening effect from high density of ?-precipitates is estimated to be 443 MPa based on Orowan bypassing model,while the increased content of reversed austenite(?21 vol.%)does not lead to the sacrifice of strength.Its cryogenic toughness mechanisms include TRIP toughening effect from the lath-like reversed austenite and the increased misorientation of matrix.(2)The microstructure evolution and strengthening-toughening mechanisms of ultrafine-grained dual-phase 00Cr12Ni10MoTi maraging stainless steel was studied.Adopting new heat treatment of double low-temperature solution treatment followed by over-aging treatment at intermediate temperature of 500?,it can develop a high-strength,high-cryogenic-toughness maraging stainless steel with ultrafine-grained martensite and austenite dual-phase microstructure.Compared to the conventional heat treatment of high-temperature solution treatment followed by peak-aging,the new heat treatment enables cryogenic impact toughness(?140 J at 77 K,V-notched)remarkably increase by 12 times without an obviously reduced yield strength.Microstructure characterizations verified that,the growth kinetics of?)-Ni3(Ti,Al)is limited by the low partitioning level of Al and Mo in oc' matrix.It contributes to the good aging stability of ?-precipitates to avoid coarsening after over-aging treatment.Intensive nanoprecipitates are found not only in a' but also in y phases.Such intensive nanoprecipitation in the dual-phase contributes to the high yield strength of the steel.The excellent cryogenic toughness of the steel by new heat treatment mainly origins from the pronounced amount of FCC austenite,the TRIP-toughening effect during impact test,the ultrafine-grained structure of both ?'and y phases.(3)The thermodynamic dissociation mechanism and kinetic diffusion behavior of interfacial oxides in stainless steel by MAF was studied.By combination of classical thermodynamics and first-principles calculations,the thermodynamic stability region of MnCr2O4 can be determined by constructing oxygen chemical potential related Mn-Cr-O phase map using GGA + U method.The region is wider than the experimental result due to the overestimation of formation energy and the complexity of oxides,while the predicted transition of oxides is basically consistent.The range of ??O for MnCr2O4 stably existing is-28.95 kJ·mol-1 to-386.97 kJ·mol-1.Various phases of oxides or metals coexist with it depending on the chemical potentials of O and Mn.The calculated results indicated there exists a sequent process of oxidation,reduction,and decomposition at the interface of hot compression bonding(HCB);all oxides can decompose into metal phases and dissociated oxygen[O]from thermodynamics when the oxygen chemical potential near the interface is below-386.97 kJ·mol-1.Besides,based on characterization of HCB interfacial oxides,the Cr2O3-type oxides were identified at the MAF interface of 00Cr12Ni10MoTi steel,and it can spontaneously dissociate and dissolve into matrix during the post-holding treatment.Interface model of Cr2O3/FCC-Fe at certain ambient condition was established,and the diffusion driving force of oxygen from oxides migrating to matrix was calculated by adopting transition theoretics.The calculated results illustrated that,at the condition of 1200? and oxygen partial pressure of e-10 atm,the O-terminated interface is thermodynamically favorable and beneficial to the formation of oxygen vacancy.The diffusion barrier of oxygen in Cr2O3 side is path-dependent:oxygen prefers to diffuse through the facet of coordination tetrahedron of Cr atoms than their bridge-site.The dissolution of Cr2O3 is controlled by the diffusion process of dissociated oxygen atoms towards interface due to a high activation of 720 kJ·mol-1.Post-holding treatment at high temperature would be beneficial to the above process.We emphasized the Cr2O3/FCC-Fe interface as a energetically favorable path promoting oxygen diffuse into iron matrix. |
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