A new method was used to analyze the factors affecting the precipitation of reversed austenite during tempering. The samples were kept at various tempering temperatures for 10 min and their length changes were recorde...A new method was used to analyze the factors affecting the precipitation of reversed austenite during tempering. The samples were kept at various tempering temperatures for 10 min and their length changes were recorded. Then, the precipitation of reversed austenite which led to the length reduction was shown by thermal expansion curves. The results show that the effects of process parameters on the precipitation of reversed austenite can be determined more accurately by this method than by X-ray diffraction. When the quenching and tempering process is adopted, both the lower quenching temperature and higher tempering temperature can promote the precipitation of reversed austenite during tempering; and when the quenching, lamellarizing, and tempering process is used, intercritical quenching is considered beneficial to the precipitation of reversed austenite in the subsequent tempering because of Ni segregation during holding at the intercritical temperature.展开更多
It has been widely demonstrated that addition of Ni in low-carbon steels can effectively improve the cryogenic toughness, but the mechanism behind it has yet to be clarified. In the present work, the evolutions of mic...It has been widely demonstrated that addition of Ni in low-carbon steels can effectively improve the cryogenic toughness, but the mechanism behind it has yet to be clarified. In the present work, the evolutions of microstructure and mechanical properties after quenching and tempering for Ni-containing cryogenic steels with different Ni contents （3.5-9 wt%） were investigated. The results showed that after quenching and tempering, the Ni-containing cryogenic steels were composed of tempered martensite and reversed austenite. The volume fraction of reversed austenite has increased from 0 up to 6.3% when the Ni content increases from 3.5% to 9%. The Charpy impact tests indicated that the low- temperature toughness was markedly improved with the increase in Ni content, which can be correlated with the increase in reversed austenite amount. The main contribution of reversed austenite to the toughness lies in： （1） the elimination of cementite precipitates improved the plastic deformation capacity of matrix, and （2） the crack propagation is hindered through plastic deformation.展开更多
The effect of different heat treatments on the reversed austenite in Cr15 super martensitic stainless steel was investigated. The experimental results indicate that the microstructure of the steel is composed of tempe...The effect of different heat treatments on the reversed austenite in Cr15 super martensitic stainless steel was investigated. The experimental results indicate that the microstructure of the steel is composed of tempered martensite and diffused reversed austenite after quenching at 1 050 ℃ and tempering from 550 to 750 ℃. The volume fraction and size of reversed austenite increase with increasing tempering temperature and both of them reach the maximum value at 700 ℃. The volume fraction and size of reversed austenite decrease when the temperature is above 700 ℃. The transmission electron microscope （TEM） results indicate that the orientation relationship between tempered martensite and reversed austenite belongs to Kurdjmov-Sach （K-S） relationship.展开更多
The effect of compressive stress on the stability of reversed austenite in gNi steel was investigated by uni- axial and hydrostatic compression. It was found that the uniaxial compressive pressure promoted the Υ-α t...The effect of compressive stress on the stability of reversed austenite in gNi steel was investigated by uni- axial and hydrostatic compression. It was found that the uniaxial compressive pressure promoted the Υ-α transformation, while the hydrostatic pressure suppressed the -Υ-α transformation. The pressure dependent transformation behavior can be explained according to thermodynamic analysis.展开更多
High-resolution transmission electron microscopy(HRTEM) and X-ray diffraction(XRD) were used to investigate Ni segregation and thermal stability of reversed austenite(RA) in a Fe–Ni alloy processed by quench–lamella...High-resolution transmission electron microscopy(HRTEM) and X-ray diffraction(XRD) were used to investigate Ni segregation and thermal stability of reversed austenite(RA) in a Fe–Ni alloy processed by quench–lamellarize–temper(QLT) heat treatment. The results show that the 77 K impact energy of the alloy increases with RA content increasing. As an austenite-stabilizing element, Ni is found to segregate in RA, though Ni is not evenly distributed within RA. The amount of segregations increases near the boundary(twice as high as the balanced content)and decreases to some extent in the center of the RA regions. Ni concentration in matrix near the boundary is lower than that in matrix far from the boundary because of Ni atom transportation from a to c near the boundary. RA in this alloy has high heat and mechanical stability but is likely to lose its stability and transform to martensite when a mechanical load is applied at ultralow temperatures(77 K), which induces plasticity.展开更多
The effect of deep cryogenic treatment on the formation of reversed austenite （RA） in super martensitic stainless steel was investigated. RA was found to form in steels without （A） and with （B） deep cryogenic tr...The effect of deep cryogenic treatment on the formation of reversed austenite （RA） in super martensitic stainless steel was investigated. RA was found to form in steels without （A） and with （B） deep cryogenic treatment. The volume fraction of RA initially increased and then decreased with increasing tempering temperature over 550-- 750 ℃ for the two steels, which were quenched at 1050 ℃. In addition, for both with and without deep cryogenic treatment, the RA content reached a maximum value at 650 ℃ although the RA content in steel B was greater than that in steel A over the entire range of tempering temperatures. Furthermore, the hardness （HRC） of steel B was greater than that of steel A at tempering temperatures of 550--750 ℃. From these results, the basic mechanism for the formation of RA in steels A and B was determined to be Ni diffusion. However, there were more Ni enriched points, a lower degree of enrichment, and a shorter diffusion path in steel B. It needed to be noted that the shapes of the RA consisted of blocks and stripes in both steels. These shapes resulted because the RA redissolved and trans- formed to martensite along the martensitic lath boundaries when the tempering temperature was 650--750 ℃, and a portion of RA in the martensitie lath divided the originally wider martensitic laths into a number of thinner ones. In- terestingly, the RA redissolved more rapidly in steel B and consequently resulted in a stronger refining effect.展开更多
The alloy was reheated to 580℃for tempering at rates of 2,5,10,20,and 40℃/s,respectively,after quenching.The amount,distribution,and stability of reversed austenite were investigated by X-ray diffraction(XRD)and ele...The alloy was reheated to 580℃for tempering at rates of 2,5,10,20,and 40℃/s,respectively,after quenching.The amount,distribution,and stability of reversed austenite were investigated by X-ray diffraction(XRD)and electron back scatter diffraction(EBSD).The microstructure and cryogenic impact energy were studied by scanning electron microscope(SEM),transmission electron microscope(TEM)and Charpy V-notch(CVN)tests.The results showed that when the sample was heated at 10℃/s,the volume fraction of reversed austenite exhibited maximum of 8%;the reversed austenite was uniform along all kinds of boundaries;the reversed austenite contained higher concentration of carbon which enabled it to be more stable.The cryogenic toughness of the alloy was greatly improved when heated at 10℃/s,as the fracture surface observation showed that it mainly fractured in ductile rupture mode,which was consistent with the results of cryogenic impact energy.展开更多
The austenite(γ)reversely transformed from lath martensite(LM),lath bainite(LB),granular bainite(GB)and pearlite+ferrite(P+F)in a high-strength steel was studied at high temperatures using in-situ electron backscatte...The austenite(γ)reversely transformed from lath martensite(LM),lath bainite(LB),granular bainite(GB)and pearlite+ferrite(P+F)in a high-strength steel was studied at high temperatures using in-situ electron backscatter diffraction(EBSD).The memory effect of initial γ significantly affects the nucleation of the reverted γ in LM and GB structures,while a weak influence on that of LB and P+F structures.This results in a significant difference in γ grain size after complete austenitization,with the first two obtaining larger γ grains while the latter two are relatively small.Crystallographic analysis revealed that the reverted γ with acicular morphology(γA),most of which maintained the same orientation with the prior γ,dominated the reaustenitization behavior of LM and GB structures through preferential nucleation within γ grains and coalesced growth modes.Although globular reverted γ(γ_(G))with random orientation or large deviation from the prior γ can nucleate at the grain boundaries or within the grains,it is difficult for it to grow and play a role in segmenting and refining the prior γ due to the inhibition of γ_(A) coalescing.For LB and P+F structures,the nucleation rate of intragranular γ_(G) increases with increasing temperature,and always shows a random orientation.These γ_(G) grains can coarsen simultaneously with the intergranular γ_(G),ultimately playing a role in jointly dividing and refining the finalγgrains.Research also found that the differences in the effects of four different microstructures on revertedγnucleation are closely related to the variant selection of the matrix structure,as well as the content and size of cementite(θ).High density of block boundaries induced by weakening of variant selection and many fineθformed in the lath are the key to promoting LB structure to obtain more intragranular γ_(G) formation,as well as the important role of the large-sized θ in P+F structure.展开更多
In the present study,a simple but effective two-step annealing processing strategy via manipulating the austenite reversion path is proposed to obtain a large fraction of retained austenite in low-Mn medium-Mn steels....In the present study,a simple but effective two-step annealing processing strategy via manipulating the austenite reversion path is proposed to obtain a large fraction of retained austenite in low-Mn medium-Mn steels.Initially,the Fe-3Mn-0.2C-1.5Si(wt%)steel is intercritically annealed to form Mn-enriched lamellar martensite precursors.Subsequently,the austenite reversion transformation is manipulated to occur within the martensite lamellae during the second annealing process,resulting in an ultra-fine duplex microstructure of laminated austenite and ferrite.This process can not only allow a large fraction of austenite to be retained in low-Mn medium-Mn steels,but also increase the elongation by up to 41%without sacrificing the strength level compared to the conventional annealing.展开更多
How to describe the austenite reverse transformation(ART)has always been considered as a key problem of controlling microstructures and mechanical properties in high-strength steels.So far,numerous studies have been c...How to describe the austenite reverse transformation(ART)has always been considered as a key problem of controlling microstructures and mechanical properties in high-strength steels.So far,numerous studies have been conducted,unfortunately,without fully considering diffusion of elements,interface migration,and interaction between trans-interface diffusion and interface migration,as well as synergy of thermodynamic and kinetic for interfacial migration.A more flexible modeling for the ART is herein developed using thermodynamic extremal principle,where the concept of trans-interface diffusion in two steps,i.e.,from the parent phase to the interface and from the interface to the product phase,as well as the Gibbs energy balance approach,was introduced to predict the behavior of interface migration and element trans-interface diffusion within the migrating interface.Subsequently,the thermodynamic driving force ΔG and the effective kinetic energy barrier Q_(eff) for the ART were also analytically performed,as well as a unified expression for so-called generalized stability(GS).It is demonstrated that the higher driving force in the ART generally results in the increased yield strength,while the larger GS tends to yield improved uniform elongation,thus forming a correspondence between the thermo-kinetics trade-off and the strength-ductility trade-off.Applying a proposed criterion of high ΔG-high GS,the present model can be adopted to design the ART,which will produce the austenite microstructure with high strength and high plasticity,as evidenced by the current experiments.展开更多
Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）...Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.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
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
Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-...Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite.Such a unique microstructure is processed by intercritical annealing,where austenite reversion occurs in a fine martensitic matrix.In the present study,austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS®coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2.In particular,a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite.The simulated microstructural morphology and phase transformation kinetics(indicated by the amount of phase)concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction,respectively.展开更多
In this work,we proposed a novel Cu/θdual nanoparticles strategy to tailor the austenite characteris-tics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties.We explored the e...In this work,we proposed a novel Cu/θdual nanoparticles strategy to tailor the austenite characteris-tics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties.We explored the effects of Cu-rich particles and cementite precipitated in the tempering process on the austenite reversion during the subsequent annealing process.Both experiments and numerical simula-tions verified that the austenite inherited from cementite had a finer size and a higher Mn enrichment compared with the austenite originating from the tempered martensite matrix.In addition,quantitative evaluations revealed that the pinning effect exerted by the Cu-rich particles could significantly hinder theα/γinterface migration and the recrystallized grain growth,thereby further refining the final mi-crostructure.With contributions from the effects of dual nanoprecipitates on the austenite reversion,the heterogeneous austenite grains inherited from varying nucleation sites ensured the sustained and gradual deformation-induced martensite and twinning formation.Therefore,the Cu-added steels subjected to a tempering-annealing process achieved synergetic enhancement of the tensile strength from 1055 MPa to 1250 MPa and elongation from 33%to 45%.This strategy may provide new guidance for the development and alloy design of high-performance medium Mn steels.展开更多
Multistage heat treatment involving quenching(Q),lamellarizing(L),and tempering(T)is applied to marine 10Ni5CrMoV steel.The microstructure and mechanical properties were studied by multiscale characterizations,and the...Multistage heat treatment involving quenching(Q),lamellarizing(L),and tempering(T)is applied to marine 10Ni5CrMoV steel.The microstructure and mechanical properties were studied by multiscale characterizations,and the kinetics of reverse austenite transformation,strain hardening behavior,and toughening mechanism were further investigated.The lamellarized specimens possess low yield strength but high toughness,especially cryogenic toughness.Lamellarization leads to the development of film-like reversed austenite at the martensite block and lath boundaries,refining the martensite structure and lowering the equivalent grain size.Kinetic analysis of austenite reversion based on the JMAK model shows that the isothermal transformation is dominated by the growth of reversed austenite,and the maximum transformation of reversed austenite is reached at the peak temperature(750℃).The strain hardening behavior based on the modified Crussard-Jaoul analysis indicates that the reversed austenite obtained from lamellarization reduces the proportion of martensite,significantly hindering crack propagation via martensitic transformation during the deformation.As a consequence,the QLT specimens exhibit high machinability and low yield strength.Compared with the QT specimen,the ductile-brittle transition temperature of the QLT specimens decreases from-116 to-130℃due to the low equivalent grain size and reversed austenite,which increases the cleavage force required for crack propagation and absorbs the energy of external load,respectively.This work provides an idea to improve the cryogenic toughness of marine 10Ni5CrMoV steel and lays a theoretical foundation for its industrial application and comprehensive performance improvement.展开更多
The rapid development in the aerospace industry requires the service performance of high-strength and heat-resistant bearing steel,especially in terms of strength and ductility synergy.A modified CSS-42L steel was obt...The rapid development in the aerospace industry requires the service performance of high-strength and heat-resistant bearing steel,especially in terms of strength and ductility synergy.A modified CSS-42L steel was obtained by multicomponent modulation,and an exceptional strength and ductility synergy of the alloy was attained through the adjusted tempering treatment.The tensile strength of 1986 MPa,the yield strength of 1622 MPa,and the uniform elongation of more than 17% were attained in the specimen tempered at 540℃,being superior to the mechanical properties of many previously reported alloys.Such superior performance originates from the generation of the high-density intermetallic compound Laves-Fe_(2)Mo,the reduction of the larger M_(6)C phase,and the formation of film-like reverse austenite.As a result,the strong interaction between the high-density dislocations of the matrix and the nanoprecipitates dominates the deformation microstructures of the specimen,strengthening the alloy.Meanwhile,the reverse austenite effectively inhibits premature crack initiation and localized stresses,increasing the uniform elongation.展开更多
The microstructures and mechanical properties of Cr13 super martensitic stainless steel after different heat treatments were studied. The results show that the structures of the steel after quenching are of lath marte...The microstructures and mechanical properties of Cr13 super martensitic stainless steel after different heat treatments were studied. The results show that the structures of the steel after quenching are of lath martensite mixed with a small amount of retained austenite. With the raising quenching temperature, the original austenite grain size increases and the lath martensite gradually becomes thicker. The structures of the tempered steel are mixtures of tempered martensite and reversed austenite dispersed in the martensite matrix. The amount of reversed austenite is from 7.54% to 22. 49%. After different heat treatments, the tensile strength, the elongation and the HRC hardness of the steel are in the range of 813 1 070 MPa, 10.1%--21.2% and 21.33--32.37, respectively. The steel displays the best comprehensive mechanical properties after the sample is quenched at 1 050 ℃ followed by tempering at 650 ℃.展开更多
Mechanical properties of quenching,intercritical quenching and tempering （QLT） treated steel containing Ni of 9% were evaluated from specimens subject to various tempering temperatures. The detailed microstructures ...Mechanical properties of quenching,intercritical quenching and tempering （QLT） treated steel containing Ni of 9% were evaluated from specimens subject to various tempering temperatures. The detailed microstructures of steel containing Ni of 9% at different tempering temperatures were observed by optical microscope （OM） and transmission electron microscope （TEM）. The volume fraction of austenite was estimated by XRD. The results show that high strength and cryogenic toughness of steel containing Ni of 9% are obtained when the tempering temperature are between 540 and 580 ℃. The microstructure keeps the dual phase lamellar structure after the intercritical quenching and there is cementite created in the Ni-rich constituents when tempering temperature is 540 ℃. When tempering temperatures are between 560 and 580 ℃,the reversed austenites （γ′） grow up and the dual phase lamellar structure is not clear. The γ′ becomes instable at 600 ℃. When tempered at temperature ranging from 500 to 520 ℃,the increase of dislocation density in the lamellar matrix makes both tensile strength and yield strength decrease. When tempered at 540 ℃ and higher temperature,the yield strength decreases continuously because the C and alloying elements in the matrix are absorbed by the cementite and the γ′,so the yield ratio is decreased by the γ′. There are two toughness mechanisms at different tempering temperatures. One is that the precipitation of cementite absorbs the carbon in the steel which plays a major role in improving cryogenic toughness at lower temperature. Another is that the γ′ and the purified matrix become major role at higher tempering temperature. When the tempering temperature is 600 ℃,the stability of γ′ is decreased quickly,even the transformation takes place at room temperature,which results in a sharp decrease of Charpy-V impact energy at 77 K. The tempering temperature range is enlarged by the special distribution of cementite and the lamellar structure.展开更多
The microstructural evolution and mechanical property of 00Crl3NiSMo2 supermartensitic stainless steel （SMSS） subjected to different heat treatments were investigated. Room tensile tests, hardness tests, scanning el...The microstructural evolution and mechanical property of 00Crl3NiSMo2 supermartensitic stainless steel （SMSS） subjected to different heat treatments were investigated. Room tensile tests, hardness tests, scanning elec- tron microscopy, transmission electron microscopy and X ray diffraction were conducted on the heat-treated steels. It is found that the microstructure of the heat-treated steel is composed of tempered lath martensite, retained austenite and ^-ferrite. The austenitizing temperature and tempering temperature have a significant effect on the microstrtlctur al changes, which leads to the complex variations of mechanical properties. The fine tempered lath martensite and more dispersed reversed austenite in the microstructure facilitate improving the comprehensive mechanical properties of the studied steel. The optimal heat treatment process of 00Crl3Ni5Mo2 SMSS is obtained by austenitizing at 1000 ℃ for 0.5 h＋air cooling followed by tempering at 630 ℃ for 2 h＋air cooling, where the excellent combination of ten- sile strength, elongation and hardness can be achieved.展开更多
文摘A new method was used to analyze the factors affecting the precipitation of reversed austenite during tempering. The samples were kept at various tempering temperatures for 10 min and their length changes were recorded. Then, the precipitation of reversed austenite which led to the length reduction was shown by thermal expansion curves. The results show that the effects of process parameters on the precipitation of reversed austenite can be determined more accurately by this method than by X-ray diffraction. When the quenching and tempering process is adopted, both the lower quenching temperature and higher tempering temperature can promote the precipitation of reversed austenite during tempering; and when the quenching, lamellarizing, and tempering process is used, intercritical quenching is considered beneficial to the precipitation of reversed austenite in the subsequent tempering because of Ni segregation during holding at the intercritical temperature.
基金supported by the Fundamental Research Funds for the Central Universities(No.N120807001)the National High-tech Research and Development Program of China (863 Program)(No.2007 AA03Z504)
文摘It has been widely demonstrated that addition of Ni in low-carbon steels can effectively improve the cryogenic toughness, but the mechanism behind it has yet to be clarified. In the present work, the evolutions of microstructure and mechanical properties after quenching and tempering for Ni-containing cryogenic steels with different Ni contents （3.5-9 wt%） were investigated. The results showed that after quenching and tempering, the Ni-containing cryogenic steels were composed of tempered martensite and reversed austenite. The volume fraction of reversed austenite has increased from 0 up to 6.3% when the Ni content increases from 3.5% to 9%. The Charpy impact tests indicated that the low- temperature toughness was markedly improved with the increase in Ni content, which can be correlated with the increase in reversed austenite amount. The main contribution of reversed austenite to the toughness lies in： （1） the elimination of cementite precipitates improved the plastic deformation capacity of matrix, and （2） the crack propagation is hindered through plastic deformation.
文摘The effect of different heat treatments on the reversed austenite in Cr15 super martensitic stainless steel was investigated. The experimental results indicate that the microstructure of the steel is composed of tempered martensite and diffused reversed austenite after quenching at 1 050 ℃ and tempering from 550 to 750 ℃. The volume fraction and size of reversed austenite increase with increasing tempering temperature and both of them reach the maximum value at 700 ℃. The volume fraction and size of reversed austenite decrease when the temperature is above 700 ℃. The transmission electron microscope （TEM） results indicate that the orientation relationship between tempered martensite and reversed austenite belongs to Kurdjmov-Sach （K-S） relationship.
基金supported by the National Natural Science Foundation of China under Grant No.50871110
文摘The effect of compressive stress on the stability of reversed austenite in gNi steel was investigated by uni- axial and hydrostatic compression. It was found that the uniaxial compressive pressure promoted the Υ-α transformation, while the hydrostatic pressure suppressed the -Υ-α transformation. The pressure dependent transformation behavior can be explained according to thermodynamic analysis.
基金financially supported by the National High Technology Research and Development Program of China (No. 2007AA03Z506)the National Basic Research Program of China (No. 2015CB654803)
文摘High-resolution transmission electron microscopy(HRTEM) and X-ray diffraction(XRD) were used to investigate Ni segregation and thermal stability of reversed austenite(RA) in a Fe–Ni alloy processed by quench–lamellarize–temper(QLT) heat treatment. The results show that the 77 K impact energy of the alloy increases with RA content increasing. As an austenite-stabilizing element, Ni is found to segregate in RA, though Ni is not evenly distributed within RA. The amount of segregations increases near the boundary(twice as high as the balanced content)and decreases to some extent in the center of the RA regions. Ni concentration in matrix near the boundary is lower than that in matrix far from the boundary because of Ni atom transportation from a to c near the boundary. RA in this alloy has high heat and mechanical stability but is likely to lose its stability and transform to martensite when a mechanical load is applied at ultralow temperatures(77 K), which induces plasticity.
文摘The effect of deep cryogenic treatment on the formation of reversed austenite （RA） in super martensitic stainless steel was investigated. RA was found to form in steels without （A） and with （B） deep cryogenic treatment. The volume fraction of RA initially increased and then decreased with increasing tempering temperature over 550-- 750 ℃ for the two steels, which were quenched at 1050 ℃. In addition, for both with and without deep cryogenic treatment, the RA content reached a maximum value at 650 ℃ although the RA content in steel B was greater than that in steel A over the entire range of tempering temperatures. Furthermore, the hardness （HRC） of steel B was greater than that of steel A at tempering temperatures of 550--750 ℃. From these results, the basic mechanism for the formation of RA in steels A and B was determined to be Ni diffusion. However, there were more Ni enriched points, a lower degree of enrichment, and a shorter diffusion path in steel B. It needed to be noted that the shapes of the RA consisted of blocks and stripes in both steels. These shapes resulted because the RA redissolved and trans- formed to martensite along the martensitic lath boundaries when the tempering temperature was 650--750 ℃, and a portion of RA in the martensitie lath divided the originally wider martensitic laths into a number of thinner ones. In- terestingly, the RA redissolved more rapidly in steel B and consequently resulted in a stronger refining effect.
文摘The alloy was reheated to 580℃for tempering at rates of 2,5,10,20,and 40℃/s,respectively,after quenching.The amount,distribution,and stability of reversed austenite were investigated by X-ray diffraction(XRD)and electron back scatter diffraction(EBSD).The microstructure and cryogenic impact energy were studied by scanning electron microscope(SEM),transmission electron microscope(TEM)and Charpy V-notch(CVN)tests.The results showed that when the sample was heated at 10℃/s,the volume fraction of reversed austenite exhibited maximum of 8%;the reversed austenite was uniform along all kinds of boundaries;the reversed austenite contained higher concentration of carbon which enabled it to be more stable.The cryogenic toughness of the alloy was greatly improved when heated at 10℃/s,as the fracture surface observation showed that it mainly fractured in ductile rupture mode,which was consistent with the results of cryogenic impact energy.
基金financially supported by the National Natural Science Foundation of China(Nos.52271089 and 52001023)the Basic Research and Application Basic Research Foundation of Guangdong Province(Nos.2022A1515240016 and 2023B1515250006)the Fundamental Research Funds for the Central Universities(No.FRF-BD-23-01).
文摘The austenite(γ)reversely transformed from lath martensite(LM),lath bainite(LB),granular bainite(GB)and pearlite+ferrite(P+F)in a high-strength steel was studied at high temperatures using in-situ electron backscatter diffraction(EBSD).The memory effect of initial γ significantly affects the nucleation of the reverted γ in LM and GB structures,while a weak influence on that of LB and P+F structures.This results in a significant difference in γ grain size after complete austenitization,with the first two obtaining larger γ grains while the latter two are relatively small.Crystallographic analysis revealed that the reverted γ with acicular morphology(γA),most of which maintained the same orientation with the prior γ,dominated the reaustenitization behavior of LM and GB structures through preferential nucleation within γ grains and coalesced growth modes.Although globular reverted γ(γ_(G))with random orientation or large deviation from the prior γ can nucleate at the grain boundaries or within the grains,it is difficult for it to grow and play a role in segmenting and refining the prior γ due to the inhibition of γ_(A) coalescing.For LB and P+F structures,the nucleation rate of intragranular γ_(G) increases with increasing temperature,and always shows a random orientation.These γ_(G) grains can coarsen simultaneously with the intergranular γ_(G),ultimately playing a role in jointly dividing and refining the finalγgrains.Research also found that the differences in the effects of four different microstructures on revertedγnucleation are closely related to the variant selection of the matrix structure,as well as the content and size of cementite(θ).High density of block boundaries induced by weakening of variant selection and many fineθformed in the lath are the key to promoting LB structure to obtain more intragranular γ_(G) formation,as well as the important role of the large-sized θ in P+F structure.
基金support from the National Natural Science Foundation of China(Grant Nos.52321001 and 52071322).
文摘In the present study,a simple but effective two-step annealing processing strategy via manipulating the austenite reversion path is proposed to obtain a large fraction of retained austenite in low-Mn medium-Mn steels.Initially,the Fe-3Mn-0.2C-1.5Si(wt%)steel is intercritically annealed to form Mn-enriched lamellar martensite precursors.Subsequently,the austenite reversion transformation is manipulated to occur within the martensite lamellae during the second annealing process,resulting in an ultra-fine duplex microstructure of laminated austenite and ferrite.This process can not only allow a large fraction of austenite to be retained in low-Mn medium-Mn steels,but also increase the elongation by up to 41%without sacrificing the strength level compared to the conventional annealing.
基金supported by the National Natural Science Foundation of China(Nos.52130110,52271116,52431002)the Fundamental Research Funds for the Central Universities(No.D5000220052)the Aeronautical Science Foundation of China(2023Z053053003).
文摘How to describe the austenite reverse transformation(ART)has always been considered as a key problem of controlling microstructures and mechanical properties in high-strength steels.So far,numerous studies have been conducted,unfortunately,without fully considering diffusion of elements,interface migration,and interaction between trans-interface diffusion and interface migration,as well as synergy of thermodynamic and kinetic for interfacial migration.A more flexible modeling for the ART is herein developed using thermodynamic extremal principle,where the concept of trans-interface diffusion in two steps,i.e.,from the parent phase to the interface and from the interface to the product phase,as well as the Gibbs energy balance approach,was introduced to predict the behavior of interface migration and element trans-interface diffusion within the migrating interface.Subsequently,the thermodynamic driving force ΔG and the effective kinetic energy barrier Q_(eff) for the ART were also analytically performed,as well as a unified expression for so-called generalized stability(GS).It is demonstrated that the higher driving force in the ART generally results in the increased yield strength,while the larger GS tends to yield improved uniform elongation,thus forming a correspondence between the thermo-kinetics trade-off and the strength-ductility trade-off.Applying a proposed criterion of high ΔG-high GS,the present model can be adopted to design the ART,which will produce the austenite microstructure with high strength and high plasticity,as evidenced by the current experiments.
基金supported by the Project of Scientific and Technical Supporting Program of China during the 11th Five-Year Plan(No.2006BAE03A06)
文摘Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.
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.
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The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft(DFG)within the Collaborative Research Center(SFB)761‘Steel-ab initio:Quantum mechanics guided design of new Fe-based materials’and the project BL402/49-1.H.W.Luo is thankful for the financial supports from the National Natural Science Foundation of China(Nos.51861135302 and 51831002).Dr.Bernd Böttger at ACCESS e.V.is acknowledged for the helpful discussions.The synchrotron high-energy X-ray diffraction measurements were carried out at the Powder Diffraction and Total Scattering Beamline P02.1 of PETRA III at DESY(No.I-20181007),a member of the Helmholtz Association(HGF),which is gratefully acknowledged.Dr.Martin Etter at DESY is acknowledged for his support of acquiring HEXRD data.
文摘Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite.Such a unique microstructure is processed by intercritical annealing,where austenite reversion occurs in a fine martensitic matrix.In the present study,austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS®coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2.In particular,a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite.The simulated microstructural morphology and phase transformation kinetics(indicated by the amount of phase)concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction,respectively.
基金support received from the National Natural Science Foundation of China(Grant No.U1964204).
文摘In this work,we proposed a novel Cu/θdual nanoparticles strategy to tailor the austenite characteris-tics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties.We explored the effects of Cu-rich particles and cementite precipitated in the tempering process on the austenite reversion during the subsequent annealing process.Both experiments and numerical simula-tions verified that the austenite inherited from cementite had a finer size and a higher Mn enrichment compared with the austenite originating from the tempered martensite matrix.In addition,quantitative evaluations revealed that the pinning effect exerted by the Cu-rich particles could significantly hinder theα/γinterface migration and the recrystallized grain growth,thereby further refining the final mi-crostructure.With contributions from the effects of dual nanoprecipitates on the austenite reversion,the heterogeneous austenite grains inherited from varying nucleation sites ensured the sustained and gradual deformation-induced martensite and twinning formation.Therefore,the Cu-added steels subjected to a tempering-annealing process achieved synergetic enhancement of the tensile strength from 1055 MPa to 1250 MPa and elongation from 33%to 45%.This strategy may provide new guidance for the development and alloy design of high-performance medium Mn steels.
基金financially supported by the National Key K&D Program of China(No.2023YFE0200300)the National Natural Science Foundation of China(Nos.52174303and 51874084)the Program of Introducing Talents of Discipline to Universities(No.B21001)。
文摘Multistage heat treatment involving quenching(Q),lamellarizing(L),and tempering(T)is applied to marine 10Ni5CrMoV steel.The microstructure and mechanical properties were studied by multiscale characterizations,and the kinetics of reverse austenite transformation,strain hardening behavior,and toughening mechanism were further investigated.The lamellarized specimens possess low yield strength but high toughness,especially cryogenic toughness.Lamellarization leads to the development of film-like reversed austenite at the martensite block and lath boundaries,refining the martensite structure and lowering the equivalent grain size.Kinetic analysis of austenite reversion based on the JMAK model shows that the isothermal transformation is dominated by the growth of reversed austenite,and the maximum transformation of reversed austenite is reached at the peak temperature(750℃).The strain hardening behavior based on the modified Crussard-Jaoul analysis indicates that the reversed austenite obtained from lamellarization reduces the proportion of martensite,significantly hindering crack propagation via martensitic transformation during the deformation.As a consequence,the QLT specimens exhibit high machinability and low yield strength.Compared with the QT specimen,the ductile-brittle transition temperature of the QLT specimens decreases from-116 to-130℃due to the low equivalent grain size and reversed austenite,which increases the cleavage force required for crack propagation and absorbs the energy of external load,respectively.This work provides an idea to improve the cryogenic toughness of marine 10Ni5CrMoV steel and lays a theoretical foundation for its industrial application and comprehensive performance improvement.
基金financial support from the funding of National Science and Technology Major Project,China(No.J2019-VI-0019-0134).
文摘The rapid development in the aerospace industry requires the service performance of high-strength and heat-resistant bearing steel,especially in terms of strength and ductility synergy.A modified CSS-42L steel was obtained by multicomponent modulation,and an exceptional strength and ductility synergy of the alloy was attained through the adjusted tempering treatment.The tensile strength of 1986 MPa,the yield strength of 1622 MPa,and the uniform elongation of more than 17% were attained in the specimen tempered at 540℃,being superior to the mechanical properties of many previously reported alloys.Such superior performance originates from the generation of the high-density intermetallic compound Laves-Fe_(2)Mo,the reduction of the larger M_(6)C phase,and the formation of film-like reverse austenite.As a result,the strong interaction between the high-density dislocations of the matrix and the nanoprecipitates dominates the deformation microstructures of the specimen,strengthening the alloy.Meanwhile,the reverse austenite effectively inhibits premature crack initiation and localized stresses,increasing the uniform elongation.
文摘The microstructures and mechanical properties of Cr13 super martensitic stainless steel after different heat treatments were studied. The results show that the structures of the steel after quenching are of lath martensite mixed with a small amount of retained austenite. With the raising quenching temperature, the original austenite grain size increases and the lath martensite gradually becomes thicker. The structures of the tempered steel are mixtures of tempered martensite and reversed austenite dispersed in the martensite matrix. The amount of reversed austenite is from 7.54% to 22. 49%. After different heat treatments, the tensile strength, the elongation and the HRC hardness of the steel are in the range of 813 1 070 MPa, 10.1%--21.2% and 21.33--32.37, respectively. The steel displays the best comprehensive mechanical properties after the sample is quenched at 1 050 ℃ followed by tempering at 650 ℃.
基金Item Sponsored by National High Technology Research and Development Program of China (2007AA03Z506)
文摘Mechanical properties of quenching,intercritical quenching and tempering （QLT） treated steel containing Ni of 9% were evaluated from specimens subject to various tempering temperatures. The detailed microstructures of steel containing Ni of 9% at different tempering temperatures were observed by optical microscope （OM） and transmission electron microscope （TEM）. The volume fraction of austenite was estimated by XRD. The results show that high strength and cryogenic toughness of steel containing Ni of 9% are obtained when the tempering temperature are between 540 and 580 ℃. The microstructure keeps the dual phase lamellar structure after the intercritical quenching and there is cementite created in the Ni-rich constituents when tempering temperature is 540 ℃. When tempering temperatures are between 560 and 580 ℃,the reversed austenites （γ′） grow up and the dual phase lamellar structure is not clear. The γ′ becomes instable at 600 ℃. When tempered at temperature ranging from 500 to 520 ℃,the increase of dislocation density in the lamellar matrix makes both tensile strength and yield strength decrease. When tempered at 540 ℃ and higher temperature,the yield strength decreases continuously because the C and alloying elements in the matrix are absorbed by the cementite and the γ′,so the yield ratio is decreased by the γ′. There are two toughness mechanisms at different tempering temperatures. One is that the precipitation of cementite absorbs the carbon in the steel which plays a major role in improving cryogenic toughness at lower temperature. Another is that the γ′ and the purified matrix become major role at higher tempering temperature. When the tempering temperature is 600 ℃,the stability of γ′ is decreased quickly,even the transformation takes place at room temperature,which results in a sharp decrease of Charpy-V impact energy at 77 K. The tempering temperature range is enlarged by the special distribution of cementite and the lamellar structure.
基金sponsored by Science and Technology Projects from Xi'an City(CX1261⑤)the State Key Laboratory of Solidification Processing in NWPU(SKLSP201322)
文摘The microstructural evolution and mechanical property of 00Crl3NiSMo2 supermartensitic stainless steel （SMSS） subjected to different heat treatments were investigated. Room tensile tests, hardness tests, scanning elec- tron microscopy, transmission electron microscopy and X ray diffraction were conducted on the heat-treated steels. It is found that the microstructure of the heat-treated steel is composed of tempered lath martensite, retained austenite and ^-ferrite. The austenitizing temperature and tempering temperature have a significant effect on the microstrtlctur al changes, which leads to the complex variations of mechanical properties. The fine tempered lath martensite and more dispersed reversed austenite in the microstructure facilitate improving the comprehensive mechanical properties of the studied steel. The optimal heat treatment process of 00Crl3Ni5Mo2 SMSS is obtained by austenitizing at 1000 ℃ for 0.5 h＋air cooling followed by tempering at 630 ℃ for 2 h＋air cooling, where the excellent combination of ten- sile strength, elongation and hardness can be achieved.
