A cyclic quenching treatment(CQT)succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments.The...A cyclic quenching treatment(CQT)succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments.The improvement of mechanical properties is attributed to the refinement and increased chemical heterogeneity of the martensitic substructure,rather than the refinement of prior austenite grain(PAG),as well as the Transformation-Induced Plasticity(TRIP)effect facilitated by small austenite grains.The role of local segregation of Ni during CQT in the formation of Ni-rich austenite grains,Ni-rich martensite laths and Ni-poor martensite laths,was investigated and verified by DICTRA simulations.This study highlights the important influence of Ni partitioning behavior during CQT,providing insights into microstructural evolution and mechanical properties.展开更多
The effect of aging precipitation on the stress corrosion cracking(SCC)mechanism of Ni(Fe,Al)-maraging steel was studied through the comparative characterization and analyses of the microstructures and fracture featur...The effect of aging precipitation on the stress corrosion cracking(SCC)mechanism of Ni(Fe,Al)-maraging steel was studied through the comparative characterization and analyses of the microstructures and fracture features of solid–solution and peak-aged steels.Aging precipitation exerts a chain of impacts on the deformative compatibility and electrochemical difference between the matrix and oth-er phases or interfaces.The strength of the martensite matrix is enhanced by abundant and evenly dispersed Ni(Fe,Al)precipitates,thereby reducing the possibility of splitting across martensite laths.Meanwhile,the Volta potential difference(VPD)between the matrix and primary NbC particles increases from 11.43 to 18.60 mV.Given that most of the primary NbC particles tend to be distributed along high-angle grain boundaries(HAGBs),anodic dissolution along HAGBs accelerates.Therefore,mechanical and electrochemical factors triggered by aging precipitation are involved in the variation in SCC behavior and mechanism.The SCC susceptibility of the steel in-creases along with the increasing tendency for intergranular cracking.展开更多
Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility.Here,we report an intermittent printing strategy to tailor the microstructure and mechanical properties of ...Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility.Here,we report an intermittent printing strategy to tailor the microstructure and mechanical properties of maraging 250 steel via tuning the thermal history during wire-arc directed energy deposition.By introducing a dwell time between adjacent layers,the maraging 250 steel is cooled below the martensite start temperature,triggering thermally-driven martensitic transformation during the printing process.Thermal cycling during subsequent layer deposition results in the formation of reverted austenite which shows a refined microstructure and induces elemental segregation between martensite and reverted austenite.The Ni enrichment in the austenite promotes stabilization of the reverted austenite upon cooling to room temperature.The reverted austenite is metastable during deformation,leading to strain-induced martensitic transformation under loading.Specifically,a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% reverted austenite,resulting in improved work hardening via martensitic transformation induced plasticity during deformation.Meanwhile,the higher cooling rate and refined prior austenite grains lead to substantially refined martensitic grains(by approximately fivefold)together with an increased dislocation density.With 3 min interlayer dwell time,the yield strength of the printed maraging 250 steel increases from 836 MPa to 990 MPa,and the uniform elongation is doubled from 3.2% to 6.5%.This intermittent deposition strategy demonstrates the potential to tune the microstructure of maraging steels for achieving strength-ductility synergy by engineering the thermal history during additive manufacturing.展开更多
The strip casts of cobalt-free maraging steel were fabricated using a twin-roll strip casting simulator,and its characteristics of sub-rapid solidification were studied.Subsequently,the confocal laser scanning microsc...The strip casts of cobalt-free maraging steel were fabricated using a twin-roll strip casting simulator,and its characteristics of sub-rapid solidification were studied.Subsequently,the confocal laser scanning microscope(CLSM)was employed to in situ observe the phase transformation during the heat treatment of maraging steel strip cast such as austenitization,solution treatment,and aging processes.It was found that due to the high cooling rate during the twin-roll strip casting process,the sub-rapid solidified strip cast possessed a full lath martensitic structure,weak macrosegregation,and evident microsegregation with a dendritic morphology.During austenitization of strip cast,the austenite grain size increased with the austenitization temperature.After holding at 1250℃for 250 s,the austenite grain size at the high temperature owned a high similarity to the prior austenite grain size of the strip cast,which effectively duplicates the microstructure of the strip cast after sub-rapid solidification.During the solution treatment process,the martensitic structure of the strip cast also underwent austenitic transformation,subsequently transformed into martensite again after quenching.Due to the low reheating temperature during solution treatment,the austenite grain size was refined,resulting in the fine martensitic microstructure after quenching.During the aging process of strip cast,some of martensite transformed into fine austenite,which was located in the interdendritic region and remained stable after air cooling,resulting in the dual-phase microstructure of martensite and austenite.The solute segregation of Ni and Mo elements during the sub-rapid solidification of strip cast caused the enrichment of Ni and Mo elements in the interdendritic region,which can expand the austenite phase region and thus enhance the stability of austenite,leading to the formation of austenite in the interdendritic region after aging treatment.展开更多
Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mech...Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances,while its potential role in microstructure architecting during post-heat treatment is rarely explored.In this work,we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300)steel and revealed how this microstructure influences austenite reversion upon aging.Segregation of Ti and Mo is experimentally detected at cell boundaries.It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging,whereas much less austenite is found in conventionally treated 18Ni(300)steels.The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible.Phase-field simulations suggest austenite grows in a fast,partitionless manner along cell boundaries where the chemical driving force for austen-ite reversion is substantially enhanced by Ti and Mo segregations,but in a sluggish,partitioning manner towards cell interiors.Contrary to conventional views that austenite fraction should be confined to avoid strength reduction,current SLM-fabricated 18Ni(300)steel containing∼13%cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite.The design of austen-ite also shows its potential to enhance fracture toughness.The current study demonstrates that cellular structures could substantially alter austenite reversion behavior,providing a new route for microstructure architecting in additively manufactured steels.展开更多
The microstructure evolution and micromechanical behaviors of additively manufactured 18Ni_(3)00 marag-ing steel for both as-printed and aged one were investigated using the in situ high-energy X-ray diffrac-tion(HE-X...The microstructure evolution and micromechanical behaviors of additively manufactured 18Ni_(3)00 marag-ing steel for both as-printed and aged one were investigated using the in situ high-energy X-ray diffrac-tion(HE-XRD)technique with uniaxial tensile tests.The investigations revealed that the volume frac-tion of reversed austenite increased as the annealing temperature rose.The maraging steel was strength-ened byη-Ni_(3)Ti precipitates,where the aged maraging steel had a higher UTS value of∼1860 MPa than∼1135 MPa in the as-printed one,but sacrificed more than half of ductility(from∼8.6%to∼4.0%).The austenite in aged steel presents more stability induced by the aging process than that in as-printed counterpart,which has a higher critical martensitic transformation stress of∼1200 MPa than that of∼780 MPa in as-printed steel.The austenite grains orientated with[200]//LD yield before the macro-yielding and preferential martensite transformation occurs.This study provides further insight into the intricated micromechanical responses of additively manufactured 18Ni_(3)00 maraging steel,enlarging the scope of its adaptation and application.展开更多
Microstructure of two different 18Ni Co-free maraging specimens and their electron beam weld joints were investigated comparatively by optical microscopy and SEM. It is showing that both of the steels are typical lath...Microstructure of two different 18Ni Co-free maraging specimens and their electron beam weld joints were investigated comparatively by optical microscopy and SEM. It is showing that both of the steels are typical lath martensite, however, one grain size is about three times as another one, and XRD reveals that the amount of the retained austenitic phase in the former is less then the latter. The austenite distributes in plate form along granular and lath boundaries while some in fine particle within the matrix. The microstructural difference between two specimens led to diverse behaviors in electron beam welding. The first specimen is weldable well but the second shows obvious welding defects of pits and burn-through holes in weld face. The welding microstructure exhibits a typical dendritic morphology, and the grains in the heat-affected zone recrystallized and grew up obviously for high temperature heated by welding electron beam. The weldablity is relative to the thermal conduction performance of the base materials,which is contributed greatly for grain size and austenite content.展开更多
Hot processing behavior of an ultra-high-strength Fe–Ni–Co-based maraging steel was studied in temperature range of 900–1200℃and strain rate range of 0.001–10 s^-1.Deformation processing parameters and optimum ho...Hot processing behavior of an ultra-high-strength Fe–Ni–Co-based maraging steel was studied in temperature range of 900–1200℃and strain rate range of 0.001–10 s^-1.Deformation processing parameters and optimum hot working window were characterized via flow stress analysis,constitutive equation construction,hot processing map calculation and microstructure evolution,respectively.Critical strain value for dynamic recrystallization was determined through theoretical mathematical differential method:the inflection point ofθ–σand-αθ/ασ-σcurves.It was found that the flow stress increased with the decrease in deformation temperature and increase in the strain rate.The power dissipation maps in the strain range of 0.1–0.6 were entirely similar with the tendency of contour lines which implied that strain had no strong effect on the dissipation maps.Nevertheless,the instability maps showed obvious strain sensitivity with increasing strain,which was ascribed to the flow localization and instability.The optimized hot processing window of the experimental steel was obtained as 1100–1200℃/0.001–1 s^-1 and 1000–1100℃/0.001–0.1 s^-1,with the efficiency range of 20–40%.Owing to high Mo content in the experimental steel,high dynamic activation energy,Q=439.311 kJ mol^-1,was achieved,indicating that dynamic recrystallization was difficult to occur in the hot deformation process,which was proved via microstructure analysis under different hot deformation conditions.展开更多
Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength,ductility,and toughness in high-strength steels.In this work,the effects of Cu precipitation on...Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength,ductility,and toughness in high-strength steels.In this work,the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography,transmission electron microscopy,and mechanical tests.Our results indicate that Cu accelerates the austenite reversion kinetics in two manners:first,Cu,as an austenite stabilizer,increases the equilibrium austenite fraction and hence enhances the chemical driving force for the austenite formation,and second,Cu-rich nanoprecipitates promote the austenite reversion by serving as heterogeneous nucleation sites and providing Ni-enriched chemical conditions through interfacial segregation.In addition,the Cu precipitation hardening compensates the strength drop induced by the formation of soft reverted austenite.During tensile deformation,the metastable reverted austenite transforms to martensite,which substantially improves the ductility and toughness through a transformation-induced plasticity(TRIP)effect.The Cu-added maraging stainless steel exhibits a superior combination of a yield strength of~1.3 GPa,an elongation of~15%,and an impact toughness of~58 J.展开更多
The 18%Ni alloy steels provide high strength and toughness, while age-hardenable or PH stainless steels also have good corrosion resistance. This paper focuses on an investigation of the heat treatment, mechanical pro...The 18%Ni alloy steels provide high strength and toughness, while age-hardenable or PH stainless steels also have good corrosion resistance. This paper focuses on an investigation of the heat treatment, mechanical properties and microstructural development of a new maraging stainless steel. It is reported that the heat treatment process should consist of solution treatment and cryogenic cooling to attain a fully martensitic structure, followed by aging at 813 K. This heat treatment resulted in an ultimate tensile strength of over 1900 MPa combined with good impact toughness. Transmission electron microscopy is used to show that, for the peak-aged condition （813 K/4 h）, nano-sized precipitates, e.g. Ni3Mo and/or R-phase, and a high density of dislocations were uniformly dispersed in the lath martensite matrix. The calculated yield strength, based on a revised Orowan mechanism, is in good agreement with the test data. The steel studied has an ultimate tensile strength over 1900 MPa, excellent fracture toughness, and good resistance against over-aging and relatively good corrosion resistance as well.展开更多
This paper reviews the latest research progress in the additive manufacturing(AM)process of 18%nickel maraging steels,which involves laser-based powder bed fusion(L-PBF),laser-based directed energy deposition(L-DED),a...This paper reviews the latest research progress in the additive manufacturing(AM)process of 18%nickel maraging steels,which involves laser-based powder bed fusion(L-PBF),laser-based directed energy deposition(L-DED),and wire arc additive manufacturing(WAAM).The emphasis is on the defects,structure,and mechanical properties of the additive manufactured 18%nickel maraging steels.At first,this article outlines the influences of the processing parameters of AM techniques on the defects formed in the fabricated parts,in terms of the L-PBF and WAAM.The macrostructure and microstructure characteristics of as-built and heat-treated 18%nickel maraging steel are then described in detail.Later,the mechanical properties of as-built and heat-treated 18%nickel maraging steel are assessed,such as their tensile,hardness,impact toughness,and fatigue performances.Finally,future directions for work on the AM of 18%nickel maraging steel are provided.展开更多
Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of...Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle. During assembly preparations, one of the shear screws which connected the separation band and band end block has failed at the first thread. Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode. The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking.展开更多
The effect of Co addition on the formation of Ni-Ti clusters in maraging stainless steel was studied by three dimensional atom probe（3 DAP） and first-principles calculation. The cluster analysis based on the maximum...The effect of Co addition on the formation of Ni-Ti clusters in maraging stainless steel was studied by three dimensional atom probe（3 DAP） and first-principles calculation. The cluster analysis based on the maximum separation approach showed an increase in size but a decrease in density of Ni-Ti clusters with increasing the Co content. The first-principles calculation indicated weaker Co-Ni（Co-Ti） interactions than Co-Ti（Fe-Ti） interactions, which should be the essential reason for the change of distribution characteristics of Ni-Ti clusters in bcc Fe caused by Co addition.展开更多
Electron-beam （EB） welding was used in T250 maraging steel, microstructures of both base material and heat affected zone （HAZ） were investigated by optical microscopy （OM）, scanning electron microscopy （SEM） a...Electron-beam （EB） welding was used in T250 maraging steel, microstructures of both base material and heat affected zone （HAZ） were investigated by optical microscopy （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）, and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstrueture of the entire HAZ, from fusion line, could be divided into four zones： fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material.展开更多
The development of ultrastrong maraging stainless steels(MSSs)is always in high demand.However,traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitate...The development of ultrastrong maraging stainless steels(MSSs)is always in high demand.However,traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic(BCC)lath martensitic matrix.Here,we design a novel ultrahigh strength MSS(Fe-5.30 Cr-13.47 Ni-3.10 Al-1.22 Mo-0.50 W-0.23 Nb-0.03 C-0.005 B,wt.%)using a cluster formula approach.A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-Ni Al nanoprecipitates(3-5 nm)in BCC martensitic matrix was successfully obtained.This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa,but also a decent uniform elongation of 4.2%-5.1%,which is almost triple of the value observed in existing MSSs.We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit highperformance MSSs.展开更多
Maraging steel (250) and 13-8 Mo stainless steel plates were joined by gas tungsten constricted arc welding(GTCAW) process in similar and dissimilar metal combinations using 13-8 Mo stainless steel filler wire. The si...Maraging steel (250) and 13-8 Mo stainless steel plates were joined by gas tungsten constricted arc welding(GTCAW) process in similar and dissimilar metal combinations using 13-8 Mo stainless steel filler wire. The similar and dissimilar metal welds made in solutionized condition were subjected to standard post weld hardening treatments direct ageing at 485 ℃, soaking for 31/2 hours followed by air cooling(ageing treatment of maraging steel) and direct ageing at 510 ℃, soaking for 4 h followed by air cooling(ageing treatment of 13-8 Mo stainless steel). The joint characterization studies include microstructure examination, microhardness survey across the weldments and transverse weld tensile test.Similar and dissimilar metal weldments responded to both the post weld ageing treatment. After post weld aging, increase in yield strength, UTS and slight reduction in % elongation of similar and dissimilar metal were observed. The observed tensile properties were correlated with microstructure and hardness distribution across the welds.展开更多
This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures(900 e1200℃) and strain rates(0.001 e100 s^(-1)) to examine hot deformability and concurr...This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures(900 e1200℃) and strain rates(0.001 e100 s^(-1)) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125℃-1200℃ and strain rate range of 0.001 e0.1 s^(-1). High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling.展开更多
基金sponsored by the National Natural Science Foun-dation of China(Grant Nos.52271122,52203384).
文摘A cyclic quenching treatment(CQT)succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments.The improvement of mechanical properties is attributed to the refinement and increased chemical heterogeneity of the martensitic substructure,rather than the refinement of prior austenite grain(PAG),as well as the Transformation-Induced Plasticity(TRIP)effect facilitated by small austenite grains.The role of local segregation of Ni during CQT in the formation of Ni-rich austenite grains,Ni-rich martensite laths and Ni-poor martensite laths,was investigated and verified by DICTRA simulations.This study highlights the important influence of Ni partitioning behavior during CQT,providing insights into microstructural evolution and mechanical properties.
基金support from the National Key Research and Development Program of China(No.2023YFB3710300)National Natural Science Foundation of China(Nos.12174296 and 52101088)+1 种基金Major Program(JD)of Hubei Province,China(No.2023BAA019-5)Numerical calculation is supported by the High-Performance Computing Center of Wuhan University of Science and Technology,China.
文摘The effect of aging precipitation on the stress corrosion cracking(SCC)mechanism of Ni(Fe,Al)-maraging steel was studied through the comparative characterization and analyses of the microstructures and fracture features of solid–solution and peak-aged steels.Aging precipitation exerts a chain of impacts on the deformative compatibility and electrochemical difference between the matrix and oth-er phases or interfaces.The strength of the martensite matrix is enhanced by abundant and evenly dispersed Ni(Fe,Al)precipitates,thereby reducing the possibility of splitting across martensite laths.Meanwhile,the Volta potential difference(VPD)between the matrix and primary NbC particles increases from 11.43 to 18.60 mV.Given that most of the primary NbC particles tend to be distributed along high-angle grain boundaries(HAGBs),anodic dissolution along HAGBs accelerates.Therefore,mechanical and electrochemical factors triggered by aging precipitation are involved in the variation in SCC behavior and mechanism.The SCC susceptibility of the steel in-creases along with the increasing tendency for intergranular cracking.
基金the U.S.Army Research Laboratory under Cooperative Agreement Award No.HQ0034-15-2-0007the U.S.National Science Foundation(DMR-2207965).
文摘Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility.Here,we report an intermittent printing strategy to tailor the microstructure and mechanical properties of maraging 250 steel via tuning the thermal history during wire-arc directed energy deposition.By introducing a dwell time between adjacent layers,the maraging 250 steel is cooled below the martensite start temperature,triggering thermally-driven martensitic transformation during the printing process.Thermal cycling during subsequent layer deposition results in the formation of reverted austenite which shows a refined microstructure and induces elemental segregation between martensite and reverted austenite.The Ni enrichment in the austenite promotes stabilization of the reverted austenite upon cooling to room temperature.The reverted austenite is metastable during deformation,leading to strain-induced martensitic transformation under loading.Specifically,a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% reverted austenite,resulting in improved work hardening via martensitic transformation induced plasticity during deformation.Meanwhile,the higher cooling rate and refined prior austenite grains lead to substantially refined martensitic grains(by approximately fivefold)together with an increased dislocation density.With 3 min interlayer dwell time,the yield strength of the printed maraging 250 steel increases from 836 MPa to 990 MPa,and the uniform elongation is doubled from 3.2% to 6.5%.This intermittent deposition strategy demonstrates the potential to tune the microstructure of maraging steels for achieving strength-ductility synergy by engineering the thermal history during additive manufacturing.
基金supported by National Natural Science Foundation of China(Nos.52130408 and 52304361)Natural Science Foundation of Hunan Province(No.2023JJ40737)the Open Project Program of Anhui Province Key Laboratory of Metallurgical Engineering&Resources Recycling(No.SKF23-02).
文摘The strip casts of cobalt-free maraging steel were fabricated using a twin-roll strip casting simulator,and its characteristics of sub-rapid solidification were studied.Subsequently,the confocal laser scanning microscope(CLSM)was employed to in situ observe the phase transformation during the heat treatment of maraging steel strip cast such as austenitization,solution treatment,and aging processes.It was found that due to the high cooling rate during the twin-roll strip casting process,the sub-rapid solidified strip cast possessed a full lath martensitic structure,weak macrosegregation,and evident microsegregation with a dendritic morphology.During austenitization of strip cast,the austenite grain size increased with the austenitization temperature.After holding at 1250℃for 250 s,the austenite grain size at the high temperature owned a high similarity to the prior austenite grain size of the strip cast,which effectively duplicates the microstructure of the strip cast after sub-rapid solidification.During the solution treatment process,the martensitic structure of the strip cast also underwent austenitic transformation,subsequently transformed into martensite again after quenching.Due to the low reheating temperature during solution treatment,the austenite grain size was refined,resulting in the fine martensitic microstructure after quenching.During the aging process of strip cast,some of martensite transformed into fine austenite,which was located in the interdendritic region and remained stable after air cooling,resulting in the dual-phase microstructure of martensite and austenite.The solute segregation of Ni and Mo elements during the sub-rapid solidification of strip cast caused the enrichment of Ni and Mo elements in the interdendritic region,which can expand the austenite phase region and thus enhance the stability of austenite,leading to the formation of austenite in the interdendritic region after aging treatment.
基金the National Key R&D program of China(Grant Nos.2022YFB3705200 and 2021YFB3702301)the National Natural Science Foundation of China(Grant No.52171008)+2 种基金the National Key R&D program of China(Grant No.2022YFE0110800)the National Natural Science Foundation of China(Grants Nos.51922054 and U1808208)the Mobility Programme from the Sino-German Center(Grant No.M-0319).
文摘Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances,while its potential role in microstructure architecting during post-heat treatment is rarely explored.In this work,we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300)steel and revealed how this microstructure influences austenite reversion upon aging.Segregation of Ti and Mo is experimentally detected at cell boundaries.It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging,whereas much less austenite is found in conventionally treated 18Ni(300)steels.The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible.Phase-field simulations suggest austenite grows in a fast,partitionless manner along cell boundaries where the chemical driving force for austen-ite reversion is substantially enhanced by Ti and Mo segregations,but in a sluggish,partitioning manner towards cell interiors.Contrary to conventional views that austenite fraction should be confined to avoid strength reduction,current SLM-fabricated 18Ni(300)steel containing∼13%cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite.The design of austen-ite also shows its potential to enhance fracture toughness.The current study demonstrates that cellular structures could substantially alter austenite reversion behavior,providing a new route for microstructure architecting in additively manufactured steels.
基金supported by the National Key Research and De-velopment Program of China(No.2023YFB3711901)the National Natural Science Foundation of China(NSFC)(Nos.52171098 and 51921001)+2 种基金the State Key Laboratory for Advanced Metals and Ma-terials(Grant No.2022Z-02)the Fundamental Research Funds for the Central Universities(No.FRF-TP-20-03C2)supported by the U.S.Department of En-ergy,Office of Science,Office of Basic Energy Sciences,under Con-tract No.DE-AC02-06CH11357.
文摘The microstructure evolution and micromechanical behaviors of additively manufactured 18Ni_(3)00 marag-ing steel for both as-printed and aged one were investigated using the in situ high-energy X-ray diffrac-tion(HE-XRD)technique with uniaxial tensile tests.The investigations revealed that the volume frac-tion of reversed austenite increased as the annealing temperature rose.The maraging steel was strength-ened byη-Ni_(3)Ti precipitates,where the aged maraging steel had a higher UTS value of∼1860 MPa than∼1135 MPa in the as-printed one,but sacrificed more than half of ductility(from∼8.6%to∼4.0%).The austenite in aged steel presents more stability induced by the aging process than that in as-printed counterpart,which has a higher critical martensitic transformation stress of∼1200 MPa than that of∼780 MPa in as-printed steel.The austenite grains orientated with[200]//LD yield before the macro-yielding and preferential martensite transformation occurs.This study provides further insight into the intricated micromechanical responses of additively manufactured 18Ni_(3)00 maraging steel,enlarging the scope of its adaptation and application.
文摘Microstructure of two different 18Ni Co-free maraging specimens and their electron beam weld joints were investigated comparatively by optical microscopy and SEM. It is showing that both of the steels are typical lath martensite, however, one grain size is about three times as another one, and XRD reveals that the amount of the retained austenitic phase in the former is less then the latter. The austenite distributes in plate form along granular and lath boundaries while some in fine particle within the matrix. The microstructural difference between two specimens led to diverse behaviors in electron beam welding. The first specimen is weldable well but the second shows obvious welding defects of pits and burn-through holes in weld face. The welding microstructure exhibits a typical dendritic morphology, and the grains in the heat-affected zone recrystallized and grew up obviously for high temperature heated by welding electron beam. The weldablity is relative to the thermal conduction performance of the base materials,which is contributed greatly for grain size and austenite content.
基金sponsored by the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2017233)the National Natural Science Foundation of China (No. 51472249)+1 种基金the Innovation Project of Institute of Metal Research (2015-ZD04)the National Natural Science Foundation of China Research Fund for International Young Scientists (No. 51750110515)
文摘Hot processing behavior of an ultra-high-strength Fe–Ni–Co-based maraging steel was studied in temperature range of 900–1200℃and strain rate range of 0.001–10 s^-1.Deformation processing parameters and optimum hot working window were characterized via flow stress analysis,constitutive equation construction,hot processing map calculation and microstructure evolution,respectively.Critical strain value for dynamic recrystallization was determined through theoretical mathematical differential method:the inflection point ofθ–σand-αθ/ασ-σcurves.It was found that the flow stress increased with the decrease in deformation temperature and increase in the strain rate.The power dissipation maps in the strain range of 0.1–0.6 were entirely similar with the tendency of contour lines which implied that strain had no strong effect on the dissipation maps.Nevertheless,the instability maps showed obvious strain sensitivity with increasing strain,which was ascribed to the flow localization and instability.The optimized hot processing window of the experimental steel was obtained as 1100–1200℃/0.001–1 s^-1 and 1000–1100℃/0.001–0.1 s^-1,with the efficiency range of 20–40%.Owing to high Mo content in the experimental steel,high dynamic activation energy,Q=439.311 kJ mol^-1,was achieved,indicating that dynamic recrystallization was difficult to occur in the hot deformation process,which was proved via microstructure analysis under different hot deformation conditions.
基金financial support from the National Natural Science Foundation of China(51801169)State Key Laboratory for Advanced Metals and Materials Open Fund(2017-ZD01)+5 种基金Chinese National Engineering Research Centre for Steel Construction(Hong Kong Branch)at PolyU(P0013862)Guangzhou International Science&Technology Cooperation Program(201907010026)financial support from the Youth Innovation Promotion Association of Chinese Academy of Sciences(2017233)the Innovation Project of Institute of Metal Research(2015-ZD04)the National Natural Science Foundation of China Research Fund for International Young Scientists(No.51750110515)the National Natural Science Foundation of China(No.51472249)。
文摘Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength,ductility,and toughness in high-strength steels.In this work,the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography,transmission electron microscopy,and mechanical tests.Our results indicate that Cu accelerates the austenite reversion kinetics in two manners:first,Cu,as an austenite stabilizer,increases the equilibrium austenite fraction and hence enhances the chemical driving force for the austenite formation,and second,Cu-rich nanoprecipitates promote the austenite reversion by serving as heterogeneous nucleation sites and providing Ni-enriched chemical conditions through interfacial segregation.In addition,the Cu precipitation hardening compensates the strength drop induced by the formation of soft reverted austenite.During tensile deformation,the metastable reverted austenite transforms to martensite,which substantially improves the ductility and toughness through a transformation-induced plasticity(TRIP)effect.The Cu-added maraging stainless steel exhibits a superior combination of a yield strength of~1.3 GPa,an elongation of~15%,and an impact toughness of~58 J.
文摘The 18%Ni alloy steels provide high strength and toughness, while age-hardenable or PH stainless steels also have good corrosion resistance. This paper focuses on an investigation of the heat treatment, mechanical properties and microstructural development of a new maraging stainless steel. It is reported that the heat treatment process should consist of solution treatment and cryogenic cooling to attain a fully martensitic structure, followed by aging at 813 K. This heat treatment resulted in an ultimate tensile strength of over 1900 MPa combined with good impact toughness. Transmission electron microscopy is used to show that, for the peak-aged condition （813 K/4 h）, nano-sized precipitates, e.g. Ni3Mo and/or R-phase, and a high density of dislocations were uniformly dispersed in the lath martensite matrix. The calculated yield strength, based on a revised Orowan mechanism, is in good agreement with the test data. The steel studied has an ultimate tensile strength over 1900 MPa, excellent fracture toughness, and good resistance against over-aging and relatively good corrosion resistance as well.
基金supported by the Natural Sciences and Engineering Research Council(NSERC)of Canada。
文摘This paper reviews the latest research progress in the additive manufacturing(AM)process of 18%nickel maraging steels,which involves laser-based powder bed fusion(L-PBF),laser-based directed energy deposition(L-DED),and wire arc additive manufacturing(WAAM).The emphasis is on the defects,structure,and mechanical properties of the additive manufactured 18%nickel maraging steels.At first,this article outlines the influences of the processing parameters of AM techniques on the defects formed in the fabricated parts,in terms of the L-PBF and WAAM.The macrostructure and microstructure characteristics of as-built and heat-treated 18%nickel maraging steel are then described in detail.Later,the mechanical properties of as-built and heat-treated 18%nickel maraging steel are assessed,such as their tensile,hardness,impact toughness,and fatigue performances.Finally,future directions for work on the AM of 18%nickel maraging steel are provided.
文摘Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle. During assembly preparations, one of the shear screws which connected the separation band and band end block has failed at the first thread. Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode. The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking.
基金sponsored by Youth Innovation Promotion Association of Chinese Academy of Sciences (2017233)National Natural Science Foundation of C hina (No. 51472249)+2 种基金Innovation Project of Institute of Metal Research (2015-ZD04)National Natural Science Foundation of China Research Fund for International Young Scientists (No. 51750110515)the Special Program for Applied Research on Super Computation of the NSFCGuangdong Joint Fund (second phase) under Grant No. U1501501
文摘The effect of Co addition on the formation of Ni-Ti clusters in maraging stainless steel was studied by three dimensional atom probe（3 DAP） and first-principles calculation. The cluster analysis based on the maximum separation approach showed an increase in size but a decrease in density of Ni-Ti clusters with increasing the Co content. The first-principles calculation indicated weaker Co-Ni（Co-Ti） interactions than Co-Ti（Fe-Ti） interactions, which should be the essential reason for the change of distribution characteristics of Ni-Ti clusters in bcc Fe caused by Co addition.
基金Item Sponsored by National Natural Science Foundation of China (50771073)
文摘Electron-beam （EB） welding was used in T250 maraging steel, microstructures of both base material and heat affected zone （HAZ） were investigated by optical microscopy （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）, and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstrueture of the entire HAZ, from fusion line, could be divided into four zones： fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material.
基金supported by the National Natural Science Foundation of China[grant numbers 91860108,U1867201]Natural Science Foundation of Liaoning Province of China[grant number 2019-KF-05-01]the Fundamental Research Funds for the Central Universities[grant number DUT19LAB01]。
文摘The development of ultrastrong maraging stainless steels(MSSs)is always in high demand.However,traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic(BCC)lath martensitic matrix.Here,we design a novel ultrahigh strength MSS(Fe-5.30 Cr-13.47 Ni-3.10 Al-1.22 Mo-0.50 W-0.23 Nb-0.03 C-0.005 B,wt.%)using a cluster formula approach.A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-Ni Al nanoprecipitates(3-5 nm)in BCC martensitic matrix was successfully obtained.This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa,but also a decent uniform elongation of 4.2%-5.1%,which is almost triple of the value observed in existing MSSs.We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit highperformance MSSs.
基金Financial assistance from Defence Research and Development Organisation
文摘Maraging steel (250) and 13-8 Mo stainless steel plates were joined by gas tungsten constricted arc welding(GTCAW) process in similar and dissimilar metal combinations using 13-8 Mo stainless steel filler wire. The similar and dissimilar metal welds made in solutionized condition were subjected to standard post weld hardening treatments direct ageing at 485 ℃, soaking for 31/2 hours followed by air cooling(ageing treatment of maraging steel) and direct ageing at 510 ℃, soaking for 4 h followed by air cooling(ageing treatment of 13-8 Mo stainless steel). The joint characterization studies include microstructure examination, microhardness survey across the weldments and transverse weld tensile test.Similar and dissimilar metal weldments responded to both the post weld ageing treatment. After post weld aging, increase in yield strength, UTS and slight reduction in % elongation of similar and dissimilar metal were observed. The observed tensile properties were correlated with microstructure and hardness distribution across the welds.
文摘This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures(900 e1200℃) and strain rates(0.001 e100 s^(-1)) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125℃-1200℃ and strain rate range of 0.001 e0.1 s^(-1). High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling.
