The wear resistances of austempered ductile iron （ADI） were improved through intxoduction of a new phase （carbide） into the ma- txix by addition of chromium. In the present investigation, low-caxbon-equivalent duc...The wear resistances of austempered ductile iron （ADI） were improved through intxoduction of a new phase （carbide） into the ma- txix by addition of chromium. In the present investigation, low-caxbon-equivalent ductile iron （LCEDI） （CE = 3.06%, and CE represents cax- bon-equivalent） with 2.42% chromium was selected. LCEDI was austeintized at two difl＇erent temperatures （900 and 975~C） a^ld soaked for 1 h and then quenched in a salt bath at 325~C for 0 to 10 h. Samples were analyzed using optical microscopy and X-ray diffraction. Wear tests were carded out on a pin-on-disk-type machine. The efl＇ect of austenization temperature on the wear resistance, impact strength, and the mi- crostructure was evaluated. A stxucture-property correlation based on the observations is established.展开更多
The influences of nitrogen content and austenitization temperature on Nb（C, N） precipitation in niobium micro-alloyed steels were studied by different methods： optical microscopy, tensile tests, scanning electron m...The influences of nitrogen content and austenitization temperature on Nb（C, N） precipitation in niobium micro-alloyed steels were studied by different methods： optical microscopy, tensile tests, scanning electron mi- croscopy, transmission electron microscopy, physicochemical phase analysis, and small-angle X-ray scattering. The results show that the strength of the steel with high nitrogen content is slightly higher than that of the steel with low nitrogen content. The increase in the nitrogen content does not result in the increase in the amount of Nb（C,N） precipitates, which mainly depends on the niobium content in the steel. The mass fraction of small-sized Nb（C, N） precipitates （1--10 nm） in the steel with high nitrogen content is less than that in the steel with low nitrogen con- tent. After austenitized at 1 150 ℃, a number of large cuboidal and needle-shaped particles are detected in the steel with high nitrogen content, whereas they dissolve after austenitized at 1 200 ℃ and the Nb（C,N） precipitates become finer in both steels. Furthermore, the results also show that part of the nitrogen in steel involves the formation of al- loyed cementite.展开更多
In this paper, the austenitization and homogenization process of Q235 plain carbon steel during reheating is predicted using a two-dimensional model which has been developed for the prediction of diffusive phase trans...In this paper, the austenitization and homogenization process of Q235 plain carbon steel during reheating is predicted using a two-dimensional model which has been developed for the prediction of diffusive phase transformation (e.g. α to γ). The diffusion equations are solved within each phase (α and γ) and an explicit finite volume technique formulated for a regular hexagonal grid are used. The discrete interface is represented by special volume elements α/γ, an volume element a undergoes a transition to an interface state before it becomes γ. The procedure allows us to handle the displacement of the interface while respecting the flux condition at the interface. The simulated microstructure shows the dissolution of ferrite particles in the austenite matrix is presented at different stages of the phase transformation. Specifically, the influence of the microstructure scale and the heating rate on the phase transformation kinetics has been investigated. The experimental results agree well with the simulated ones.展开更多
In order to investigate the carbide dissolution mechanism of high carbon-chromium bearing steel during the intercritical austenitization, the database of TCFE7 of Thermo-calc and MOBFE of DICTRA software were used to ...In order to investigate the carbide dissolution mechanism of high carbon-chromium bearing steel during the intercritical austenitization, the database of TCFE7 of Thermo-calc and MOBFE of DICTRA software were used to calculate the elements diffusion kinetic and the evolution law of volume fraction of carbide. DIL805 A dilatometer was used to simulate the intercritical heat treatment. The microstructure was observed by scanning electron microscopy（SEM）, and the micro-hardness was tested. The experimental results indicate that the dissolution of carbide is composed of two stages： initial austenite growth governed by carbon diffusion which sharply moves up the micro-hardness of quenched martensite, and subsequent growth controlled by diffusion of Cr elements in M3 C. The volume fraction of M3 C decreases with the increasing holding time, and the metallographic analysis shows a great agreement with values calculated by software.展开更多
The austenitization behaviors of two high niohium-containing X80 pipeline steels with different titanium contents, including the dissolution of microalloying precipitates and the austenite grain growth, were investiga...The austenitization behaviors of two high niohium-containing X80 pipeline steels with different titanium contents, including the dissolution of microalloying precipitates and the austenite grain growth, were investigated by using physical-chemical phase analysis method and microstructural observation. The results illustrated that most niobium could be dissolved into austenite during soaking at 1 180 ℃, whereas little amount of titanium could be dissolved. It was found that during soaking, the austenite grain growth rate was initially high, and then decreased after soaking for 1 h; moreover, the austenite grains grew up more rapidly at temperatures above 1 180 ℃ than below 1 180 ℃. The results show that the steel with titanium content of 0. 016% has a larger austenite grain size than that with titanium content of 0. 012% under the same soaking conditions, which is explained by considering the particle size distribution.展开更多
A modified cellular automaton model is developed to depict the interface evolution inside the cementite plus ferrite lamellar microstructures during the reaustenitization of a pearlite steel. In this model, migrations...A modified cellular automaton model is developed to depict the interface evolution inside the cementite plus ferrite lamellar microstructures during the reaustenitization of a pearlite steel. In this model, migrations of both the austenite- ferrite and austenite-cementite interfaces coupled with the carbon diffusion and redistribution are integrated. The capil- laxity effect derived from local interface curvatures is also carefully considered by involving the concentration given by the phase diagram modified by the Gibbs-Thomson effect. This allows the interface evolution from a transient state to a steady state under different annealing conditions and various interlamellar spacings to be simulated. The proposed cellular automaton approach could be readily used to describe the kinetics of austenite formation from the lamellar pearlites and virtually reveal the kinematics of the moving interfaces from the microstructural aspect.展开更多
The effect of austenitization heat treatment on magnetic properties was examined by means of M6ssbauer spectroscopy on an Fe-40wt%Ni-2wt%Mn alloy. The morphology of the alloy was obtained by using scanning electron mi...The effect of austenitization heat treatment on magnetic properties was examined by means of M6ssbauer spectroscopy on an Fe-40wt%Ni-2wt%Mn alloy. The morphology of the alloy was obtained by using scanning electron microscopy （SEM） under different heat treatment conditions. The magnetic behavior of the non heat-treated alloy is ferromagnetic. A mixed magnetic structure including both paramagnetic and ferromagnetic states was obtained at 800℃ after 6 and 12 h heat treatments. In addition, the magnetic structure of the heat-treated alloy at 1150~C for 12 h was ferromagnetic. With the volume fraction changing, the effective hyperfine field of the ferromagnetic austenite phase and isomery shift values were also determined by Mtssbauer spectroscopy.展开更多
The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel(Fe-0.10C-5Mn)was investigated during uniaxial tensile testing.In-situ observations under scanning electron microscopy,transmiss...The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel(Fe-0.10C-5Mn)was investigated during uniaxial tensile testing.In-situ observations under scanning electron microscopy,transmission electron microscopy,and X-ray diffraction analysis were conducted to characterize the progressive transformation-induced plasticity process and associated fracture initiation mechanisms.These findings were discussed with the local strain measurements via digital image correlation.The results indicated that Lüders band formation in the steel was limited to 1.5%strain,which was mainly due to the early-stage martensitic phase transformation of a very small amount of the less stable large-sized retained austenite(RA),which led to localized stress concentrations and strain hardening and further retardation of yielding.The small-sized RA exhibited high stability and progressively transformed into martensite and contributed to a stably extended Portevin-Le Chatelier effect.The volume fraction of RA gradually decreased from 26.8%to 8.2%prior to fracture.In the late deformation stage,fracture initiation primarily occurred at the austenite/martensite and ferrite/martensite interfaces and the ferrite phase.展开更多
In 316L austenitic stainless steel,the presence of ferrite phase severely affects the non-magnetic properties.316L austenitic stainless steel with low-alloy type(L-316L)and high-alloy type(H-316L)has been studied.The ...In 316L austenitic stainless steel,the presence of ferrite phase severely affects the non-magnetic properties.316L austenitic stainless steel with low-alloy type(L-316L)and high-alloy type(H-316L)has been studied.The microstructure and solidification kinetics of the two as-cast grades were in situ observed by high temperature confocal laser scanning microscopy(HT-CLSM).There are significant differences in the as-cast microstructures of the two 316L stainless steel compositions.In L-316L steel,ferrite morphology appears as the short rods with a ferrite content of 6.98%,forming a dual-phase microstructure consisting of austenite and ferrite.Conversely,in H-316L steel,the ferrite appears as discontinuous network structures with a content of 4.41%,forming a microstructure composed of austenite and sigma(σ)phase.The alloying elements in H-316L steel exhibit a complex distribution,with Ni and Mo enriching at the austenite grain boundaries.HT-CLSM experiments provide the real-time observation of the solidification processes of both 316L specimens and reveal distinct solidification modes:L-316L steel solidifies in an FA mode,whereas H-316L steel solidifies in an AF mode.These differences result in ferrite and austenite predominantly serving as the nucleation and growth phases,respectively.The solidification mode observed by experiments is similar to the thermodynamic calculation results.The L-316L steel solidified in the FA mode and showed minimal element segregation,which lead to a direct transformation of ferrite to austenite phase(δ→γ)during phase transformation after solidification.Besides,the H-316L steel solidified in the AF mode and showed severe element segregation,which lead to Mo enrichment at grain boundaries and transformation of ferrite into sigma and austenite phases through the eutectoid reaction(δ→σ+γ).展开更多
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 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
Austenitic stainless steel(ASS)is a common material used in high-pressure hydrogen systems.Prolonged exposure to high-pressure hydrogen can cause hydrogen embrittlement(HE),raising significant safety concerns.Selectiv...Austenitic stainless steel(ASS)is a common material used in high-pressure hydrogen systems.Prolonged exposure to high-pressure hydrogen can cause hydrogen embrittlement(HE),raising significant safety concerns.Selective Laser Melting(SLM),known for its high precision,is a promising additive manufacturing technology that has been widely adopted across various industries.Studies have reported that under certain SLM manufacturing conditions and process parameters,the HE resistance of SLM ASS is significantly better than that of conventionally manufactured(CM)ASS,showing great potential for application in high-pressure hydrogen systems.Thus,studying the HE of SLM ASS is crucial for further improving the safety of high-pressure hydrogen systems.This paper provides an overview of the SLM process,reviews the mechanisms of HE and their synergistic effects,and analyzes the HE characteristics of SLM ASS.Additionally,it examines the influence of unique microstructures and SLM process variables on HE of SLM ASS and offers recommendations for future research to enhance the safety of high-pressure hydrogen systems.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
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
This work investigated the original microstructure of cold-worked alumina-forming austenitic steel,along with its precipitation and dissolution corrosion behaviors in lead-bismuth eutectic with 10-8 wt.%oxygen at 600...This work investigated the original microstructure of cold-worked alumina-forming austenitic steel,along with its precipitation and dissolution corrosion behaviors in lead-bismuth eutectic with 10-8 wt.%oxygen at 600℃,using solution-annealed steel for comparison.Anomalously,cold-worked steel presented milder corrosion compared to solution-annealed steel,with average corrosion depths of 314.3 and 401.0μm after 1700 h exposure.Cold working-induced de-twinning transformed the annealing twin boundaries into normal high-angle grain boundaries(NGBs),increasing NGBs proportion from 36%to 89%.The increased NGBs provided more nucleation sites for intergranular barriers composed of alternate NiAl and M23C6 precipitates,thus better obstructing the dissolution attack.展开更多
In this study, novel reconstruction methods, including grain graph and variant graph, were established to reconstruct parent austenite on the basis of electron backscatter diffraction (EBSD) data. The evaluation indic...In this study, novel reconstruction methods, including grain graph and variant graph, were established to reconstruct parent austenite on the basis of electron backscatter diffraction (EBSD) data. The evaluation indicators included boundary identification and variant distribution. Moreover, an innovative variant pair analysis method was proposed. The results indicated that the Kurdjumov-Sachs orientation relationship was the most appropriate because it had the smallest refinement error and deviation. In addition, the variant graph reconstruction was more effective in reducing mis-indexing areas than the grain graph, exhibiting a robust capacity to accurately identify austenite grain boundaries. Additionally, the variant graph reconstruction induced the transformation of variants, variant pairs, close-packed plane (CP) groups, and Bain groups. Moreover, various reconstructed datasets (calc-grain data and EBSD data) affected the distribution of variants. The austenite grains reconstructed from the calc-grain data featured two or more variants clustered within the same region due to the preprocessing (calculating, filtering, and smoothing) of the EBSD data. These variations did not impede the microstructural analysis when consistent original data and reconstruction methods were used. The reconstruction of parent austenite grains holds promise for providing a fresh perspective and a deeper understanding of strengthening and toughening mechanisms in the future.展开更多
The hardenability of steel is crucial for its durability and performance in engineering applications,significantly influencing mechanical properties such as hardness,strength,and wear resistance.As the engineering fie...The hardenability of steel is crucial for its durability and performance in engineering applications,significantly influencing mechanical properties such as hardness,strength,and wear resistance.As the engineering field continuously demands higher-performance steel materials,a deep understanding of the key influencing factors on hardenability is crucial for developing quality steel that meets stringent application requirements.The effects of some specific elements,including carbon(C),vanadium(V),molybdenum(Mo),and boron(B),as well as heat treatment process parameters such as austenitizing temperature,austenitizing holding time,and cooling rate,were examined.It aims to elucidate the interactions among these factors and their influence on steel hardenability.For each influencing factor,the heat treatment procedure,characteristic microstructure resulting from it,and corresponding Jominy end quench curves were discussed.Furthermore,based on the continuous development of big data technology in the field of materials,the use of machine learning to predict the hardenability of steel and guide the design of steel material was also introduced.展开更多
High-temperature long-term microstructural instability is an urgent problem to be solved for high-silicon Fe-Cr-Ni austenitic stainless steel.In this study,we propose a novel strategy to improve the microstructural th...High-temperature long-term microstructural instability is an urgent problem to be solved for high-silicon Fe-Cr-Ni austenitic stainless steel.In this study,we propose a novel strategy to improve the microstructural thermal stability of Si-modified Fe-Cr-Ni austenitic steels via N doping.The microstructural evolution behaviors of N-free and N-doping steels were systematically investigated during aging at 783-923 K.The findings indicate that N doping results in substantial grain refinement and improves the strength of the steel.Importantly,it is found that N doping inhibits the premature segregation of Ni,Cr,Si,and Mo at grain boundaries by reducing their diffusion coefficients,thereby suppressing the generation of intergranular M_(6) C carbides during aging at 783 K,achieving superior thermal stability.In contrast,N-free steel exhibits microstructural instability due to theγ→M_(6) C+ferrite transformation during aging at 783 K.At 823 and 873 K,it is concluded that the diffusion of alloying elements accelerates,resulting in the formation of M_(6) C and ferrite in N-doping steel and subsequent microstructural instability.It contributes to a decrease in impact toughness,as microcracks tend to form at the ferrite domain and M_(6) C/ferrite interface with high strain concentration.Notably,when aged at 923 K,N-doping steel exhibits a cellular structure composed of M_(23) C_(6) and M_(6) C carbonitrides,with Nb(C,N)serving as the nucleation site within the grains.This differs from the intragranularχ-phase observed in N-free steel,as the nucleation driving force of theχ-phase decreases with an increasing N content.The study offers valuable insights for the development of fastener materials intended for utilization in lead-cooled fast reactors.展开更多
Medium-manganese steel exhibits excellent strength and toughness,which are essential features in wear resistance applications.This study examines the impact of annealing temperature on impact abrasive wear.The results...Medium-manganese steel exhibits excellent strength and toughness,which are essential features in wear resistance applications.This study examines the impact of annealing temperature on impact abrasive wear.The results have indicated that samples annealed at different temperatures display plowing and fatigue wear effects.In the initial wear stage,the hightemperature annealed steel outperforms samples annealed at a lower temperature in terms of anti-plowing wear performance.This phenomenon is mainly due to the lower initial hardness of the samples subjected to low-temperature annealing.However,with prolonged wear time,the low-temperature annealed samples exhibit improved plowing wear performance,which is ascribed to a refinement of the lamellar microstructure and an increased residual austenite(RA),which enhances the work hardening effect,improving the hardness of the worn surface.The low-temperature annealed samples consistently delivered superior fatigue wear performance when compared with samples annealed at the higher temperature.The latter effect may be attributed to two factors.Firstly,the finer lamellar microstructure in the low-temperature annealed samples,coupled with greater RA,results in transformation-induced plasticity or twin-induced plasticity effect that hinders crack formation and propagation.Secondly,the low-temperature annealed samples form nanoscale equiaxed grains near the worn surface during the wear process.These grains can withstand crack driving forces in fine-grained regions,suppressing the formation and propagation of cracks.展开更多
Liquefied natural gas storage and transportation as well as space propulsion systems have sparked inter-est in the martensitic transformation and behaviours of 316 L stainless steels(SS)under ultra-cryogenic deformati...Liquefied natural gas storage and transportation as well as space propulsion systems have sparked inter-est in the martensitic transformation and behaviours of 316 L stainless steels(SS)under ultra-cryogenic deformation.In this study,high-resolution transmission electron microscopy(HRTEM)and molecular dy-namics(MD)simulations were used to investigate the atomic arrangements and crystalline defects of deformation-induced γ-austenite→ε-martensite→α'-martensite and γ→α'martensitic transforma-tions in 316 L SS at 15 and 173 K.Theγ→εtransformation involves the glide of Shockley partial dislocations on(111)γplanes without a change in atomic spacing.The formation of anα'inclusion in a singleε-band is achieved by a continuous lattice distortion,accompanied by the formation of a tran-sition zone ofα'and the expansion of the average atomic spacings due to dislocation shuffling.Asα'grows further intoγ,the orientation relationship(OR)of theα'changes by lattice bending.This pro-cess follows the Bogers-Burgers-Olson-Cohen model despite it not occurring on intersecting shear bands.Stacking faults and twins can also serve as nucleation sites forα'at 173 K.We also found that direct transformation of γ→α'occurs by the glide of √6aγ[11(2)]/12 dislocations on every(111)γplane with misfit dislocations.Overall,this study provides,for the first time,insights into the atomic-scale mech-anisms of various two-step and one-step martensitic transformations induced by cryogenic deformation and corresponding local strain,enhancing our understanding of the role of martensitic transformation under ultra-cryogenic-temperature deformation in controlling the properties.展开更多
文摘The wear resistances of austempered ductile iron （ADI） were improved through intxoduction of a new phase （carbide） into the ma- txix by addition of chromium. In the present investigation, low-caxbon-equivalent ductile iron （LCEDI） （CE = 3.06%, and CE represents cax- bon-equivalent） with 2.42% chromium was selected. LCEDI was austeintized at two difl＇erent temperatures （900 and 975~C） a^ld soaked for 1 h and then quenched in a salt bath at 325~C for 0 to 10 h. Samples were analyzed using optical microscopy and X-ray diffraction. Wear tests were carded out on a pin-on-disk-type machine. The efl＇ect of austenization temperature on the wear resistance, impact strength, and the mi- crostructure was evaluated. A stxucture-property correlation based on the observations is established.
基金Item Sponsored by the CITIC-CBMM Niobium Steel Research and Development Program of China(2013-D065)
文摘The influences of nitrogen content and austenitization temperature on Nb（C, N） precipitation in niobium micro-alloyed steels were studied by different methods： optical microscopy, tensile tests, scanning electron mi- croscopy, transmission electron microscopy, physicochemical phase analysis, and small-angle X-ray scattering. The results show that the strength of the steel with high nitrogen content is slightly higher than that of the steel with low nitrogen content. The increase in the nitrogen content does not result in the increase in the amount of Nb（C,N） precipitates, which mainly depends on the niobium content in the steel. The mass fraction of small-sized Nb（C, N） precipitates （1--10 nm） in the steel with high nitrogen content is less than that in the steel with low nitrogen con- tent. After austenitized at 1 150 ℃, a number of large cuboidal and needle-shaped particles are detected in the steel with high nitrogen content, whereas they dissolve after austenitized at 1 200 ℃ and the Nb（C,N） precipitates become finer in both steels. Furthermore, the results also show that part of the nitrogen in steel involves the formation of al- loyed cementite.
基金The finiancial support of the National 973 Super-Steel Iterne1998061512 is greatly acknowledged.
文摘In this paper, the austenitization and homogenization process of Q235 plain carbon steel during reheating is predicted using a two-dimensional model which has been developed for the prediction of diffusive phase transformation (e.g. α to γ). The diffusion equations are solved within each phase (α and γ) and an explicit finite volume technique formulated for a regular hexagonal grid are used. The discrete interface is represented by special volume elements α/γ, an volume element a undergoes a transition to an interface state before it becomes γ. The procedure allows us to handle the displacement of the interface while respecting the flux condition at the interface. The simulated microstructure shows the dissolution of ferrite particles in the austenite matrix is presented at different stages of the phase transformation. Specifically, the influence of the microstructure scale and the heating rate on the phase transformation kinetics has been investigated. The experimental results agree well with the simulated ones.
基金Funded by the National High-tech Research and Development Program of China(863 Program)
文摘In order to investigate the carbide dissolution mechanism of high carbon-chromium bearing steel during the intercritical austenitization, the database of TCFE7 of Thermo-calc and MOBFE of DICTRA software were used to calculate the elements diffusion kinetic and the evolution law of volume fraction of carbide. DIL805 A dilatometer was used to simulate the intercritical heat treatment. The microstructure was observed by scanning electron microscopy（SEM）, and the micro-hardness was tested. The experimental results indicate that the dissolution of carbide is composed of two stages： initial austenite growth governed by carbon diffusion which sharply moves up the micro-hardness of quenched martensite, and subsequent growth controlled by diffusion of Cr elements in M3 C. The volume fraction of M3 C decreases with the increasing holding time, and the metallographic analysis shows a great agreement with values calculated by software.
基金Item Sponsored by National Key Technologies Research and Development Program of China(2006BAE03A15)
文摘The austenitization behaviors of two high niohium-containing X80 pipeline steels with different titanium contents, including the dissolution of microalloying precipitates and the austenite grain growth, were investigated by using physical-chemical phase analysis method and microstructural observation. The results illustrated that most niobium could be dissolved into austenite during soaking at 1 180 ℃, whereas little amount of titanium could be dissolved. It was found that during soaking, the austenite grain growth rate was initially high, and then decreased after soaking for 1 h; moreover, the austenite grains grew up more rapidly at temperatures above 1 180 ℃ than below 1 180 ℃. The results show that the steel with titanium content of 0. 016% has a larger austenite grain size than that with titanium content of 0. 012% under the same soaking conditions, which is explained by considering the particle size distribution.
基金financially supported by the National Natural Science Foundation of China (Nos. 51371169 and 51401214)
文摘A modified cellular automaton model is developed to depict the interface evolution inside the cementite plus ferrite lamellar microstructures during the reaustenitization of a pearlite steel. In this model, migrations of both the austenite- ferrite and austenite-cementite interfaces coupled with the carbon diffusion and redistribution are integrated. The capil- laxity effect derived from local interface curvatures is also carefully considered by involving the concentration given by the phase diagram modified by the Gibbs-Thomson effect. This allows the interface evolution from a transient state to a steady state under different annealing conditions and various interlamellar spacings to be simulated. The proposed cellular automaton approach could be readily used to describe the kinetics of austenite formation from the lamellar pearlites and virtually reveal the kinematics of the moving interfaces from the microstructural aspect.
文摘The effect of austenitization heat treatment on magnetic properties was examined by means of M6ssbauer spectroscopy on an Fe-40wt%Ni-2wt%Mn alloy. The morphology of the alloy was obtained by using scanning electron microscopy （SEM） under different heat treatment conditions. The magnetic behavior of the non heat-treated alloy is ferromagnetic. A mixed magnetic structure including both paramagnetic and ferromagnetic states was obtained at 800℃ after 6 and 12 h heat treatments. In addition, the magnetic structure of the heat-treated alloy at 1150~C for 12 h was ferromagnetic. With the volume fraction changing, the effective hyperfine field of the ferromagnetic austenite phase and isomery shift values were also determined by Mtssbauer spectroscopy.
基金supported by the National Key R&D Program of China(No.2017YFB0304402)。
文摘The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel(Fe-0.10C-5Mn)was investigated during uniaxial tensile testing.In-situ observations under scanning electron microscopy,transmission electron microscopy,and X-ray diffraction analysis were conducted to characterize the progressive transformation-induced plasticity process and associated fracture initiation mechanisms.These findings were discussed with the local strain measurements via digital image correlation.The results indicated that Lüders band formation in the steel was limited to 1.5%strain,which was mainly due to the early-stage martensitic phase transformation of a very small amount of the less stable large-sized retained austenite(RA),which led to localized stress concentrations and strain hardening and further retardation of yielding.The small-sized RA exhibited high stability and progressively transformed into martensite and contributed to a stably extended Portevin-Le Chatelier effect.The volume fraction of RA gradually decreased from 26.8%to 8.2%prior to fracture.In the late deformation stage,fracture initiation primarily occurred at the austenite/martensite and ferrite/martensite interfaces and the ferrite phase.
基金support of the Research Project Supported by Shanxi Scholarship Council of China(2022-040)"Chunhui Plan"Collaborative Research Project by the Ministry of Education of China(HZKY20220507)+2 种基金National Natural Science Foundation of China(52104338)Applied Fundamental Research Programs of Shanxi Province(202303021221036)Shandong Postdoctoral Science Foundation(SDCX-ZG-202303027,SDBX2023054).
文摘In 316L austenitic stainless steel,the presence of ferrite phase severely affects the non-magnetic properties.316L austenitic stainless steel with low-alloy type(L-316L)and high-alloy type(H-316L)has been studied.The microstructure and solidification kinetics of the two as-cast grades were in situ observed by high temperature confocal laser scanning microscopy(HT-CLSM).There are significant differences in the as-cast microstructures of the two 316L stainless steel compositions.In L-316L steel,ferrite morphology appears as the short rods with a ferrite content of 6.98%,forming a dual-phase microstructure consisting of austenite and ferrite.Conversely,in H-316L steel,the ferrite appears as discontinuous network structures with a content of 4.41%,forming a microstructure composed of austenite and sigma(σ)phase.The alloying elements in H-316L steel exhibit a complex distribution,with Ni and Mo enriching at the austenite grain boundaries.HT-CLSM experiments provide the real-time observation of the solidification processes of both 316L specimens and reveal distinct solidification modes:L-316L steel solidifies in an FA mode,whereas H-316L steel solidifies in an AF mode.These differences result in ferrite and austenite predominantly serving as the nucleation and growth phases,respectively.The solidification mode observed by experiments is similar to the thermodynamic calculation results.The L-316L steel solidified in the FA mode and showed minimal element segregation,which lead to a direct transformation of ferrite to austenite phase(δ→γ)during phase transformation after solidification.Besides,the H-316L steel solidified in the AF mode and showed severe element segregation,which lead to Mo enrichment at grain boundaries and transformation of ferrite into sigma and austenite phases through the eutectoid reaction(δ→σ+γ).
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finncially supported by the National Natural Science Foundation of China(No.52075183)the Guangdong Basic and Applied Research Fundamental(No.2023A1515010692)the Key-Area Research and Development Program of Guangdong Province(Nos.2024B1111080002 and 2020B0404020004).
文摘Austenitic stainless steel(ASS)is a common material used in high-pressure hydrogen systems.Prolonged exposure to high-pressure hydrogen can cause hydrogen embrittlement(HE),raising significant safety concerns.Selective Laser Melting(SLM),known for its high precision,is a promising additive manufacturing technology that has been widely adopted across various industries.Studies have reported that under certain SLM manufacturing conditions and process parameters,the HE resistance of SLM ASS is significantly better than that of conventionally manufactured(CM)ASS,showing great potential for application in high-pressure hydrogen systems.Thus,studying the HE of SLM ASS is crucial for further improving the safety of high-pressure hydrogen systems.This paper provides an overview of the SLM process,reviews the mechanisms of HE and their synergistic effects,and analyzes the HE characteristics of SLM ASS.Additionally,it examines the influence of unique microstructures and SLM process variables on HE of SLM ASS and offers recommendations for future research to enhance the safety of high-pressure hydrogen systems.
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 Nuclear Technology R&D Program.
文摘This work investigated the original microstructure of cold-worked alumina-forming austenitic steel,along with its precipitation and dissolution corrosion behaviors in lead-bismuth eutectic with 10-8 wt.%oxygen at 600℃,using solution-annealed steel for comparison.Anomalously,cold-worked steel presented milder corrosion compared to solution-annealed steel,with average corrosion depths of 314.3 and 401.0μm after 1700 h exposure.Cold working-induced de-twinning transformed the annealing twin boundaries into normal high-angle grain boundaries(NGBs),increasing NGBs proportion from 36%to 89%.The increased NGBs provided more nucleation sites for intergranular barriers composed of alternate NiAl and M23C6 precipitates,thus better obstructing the dissolution attack.
基金the National Natural Science Foundation of China(Grant Nos.52325406,52374331,and U1960203)the Program of Introducing Talents of Discipline to Universities(Grant No.B21001).
文摘In this study, novel reconstruction methods, including grain graph and variant graph, were established to reconstruct parent austenite on the basis of electron backscatter diffraction (EBSD) data. The evaluation indicators included boundary identification and variant distribution. Moreover, an innovative variant pair analysis method was proposed. The results indicated that the Kurdjumov-Sachs orientation relationship was the most appropriate because it had the smallest refinement error and deviation. In addition, the variant graph reconstruction was more effective in reducing mis-indexing areas than the grain graph, exhibiting a robust capacity to accurately identify austenite grain boundaries. Additionally, the variant graph reconstruction induced the transformation of variants, variant pairs, close-packed plane (CP) groups, and Bain groups. Moreover, various reconstructed datasets (calc-grain data and EBSD data) affected the distribution of variants. The austenite grains reconstructed from the calc-grain data featured two or more variants clustered within the same region due to the preprocessing (calculating, filtering, and smoothing) of the EBSD data. These variations did not impede the microstructural analysis when consistent original data and reconstruction methods were used. The reconstruction of parent austenite grains holds promise for providing a fresh perspective and a deeper understanding of strengthening and toughening mechanisms in the future.
基金supported by the National Natural Science Foundation of China(Nos.52122408 and 52071023)Hong-hui Wu also thanks the financial support from the Fundamental Research Funds for the Central Universities(University of Science and Technology Beijing,Nos.FRF-TP-2021-04C1 and 06500135)supported by USTB MatCom of Beijing Advanced Innovation Center for Materials Genome Engineering.
文摘The hardenability of steel is crucial for its durability and performance in engineering applications,significantly influencing mechanical properties such as hardness,strength,and wear resistance.As the engineering field continuously demands higher-performance steel materials,a deep understanding of the key influencing factors on hardenability is crucial for developing quality steel that meets stringent application requirements.The effects of some specific elements,including carbon(C),vanadium(V),molybdenum(Mo),and boron(B),as well as heat treatment process parameters such as austenitizing temperature,austenitizing holding time,and cooling rate,were examined.It aims to elucidate the interactions among these factors and their influence on steel hardenability.For each influencing factor,the heat treatment procedure,characteristic microstructure resulting from it,and corresponding Jominy end quench curves were discussed.Furthermore,based on the continuous development of big data technology in the field of materials,the use of machine learning to predict the hardenability of steel and guide the design of steel material was also introduced.
基金funded by the LingChuang Research Project of China National Nuclear Corporation and the Natural Science Foun-dation of Liaoning Province(No.2023-MS-019).
文摘High-temperature long-term microstructural instability is an urgent problem to be solved for high-silicon Fe-Cr-Ni austenitic stainless steel.In this study,we propose a novel strategy to improve the microstructural thermal stability of Si-modified Fe-Cr-Ni austenitic steels via N doping.The microstructural evolution behaviors of N-free and N-doping steels were systematically investigated during aging at 783-923 K.The findings indicate that N doping results in substantial grain refinement and improves the strength of the steel.Importantly,it is found that N doping inhibits the premature segregation of Ni,Cr,Si,and Mo at grain boundaries by reducing their diffusion coefficients,thereby suppressing the generation of intergranular M_(6) C carbides during aging at 783 K,achieving superior thermal stability.In contrast,N-free steel exhibits microstructural instability due to theγ→M_(6) C+ferrite transformation during aging at 783 K.At 823 and 873 K,it is concluded that the diffusion of alloying elements accelerates,resulting in the formation of M_(6) C and ferrite in N-doping steel and subsequent microstructural instability.It contributes to a decrease in impact toughness,as microcracks tend to form at the ferrite domain and M_(6) C/ferrite interface with high strain concentration.Notably,when aged at 923 K,N-doping steel exhibits a cellular structure composed of M_(23) C_(6) and M_(6) C carbonitrides,with Nb(C,N)serving as the nucleation site within the grains.This differs from the intragranularχ-phase observed in N-free steel,as the nucleation driving force of theχ-phase decreases with an increasing N content.The study offers valuable insights for the development of fastener materials intended for utilization in lead-cooled fast reactors.
基金financial support to the Application Foundation Frontier Project of the Major Program(JD)of Hubei Province(2023BAA019-4)the National Natural Science Foundation of China(U20A20279,12072245,52071238)+2 种基金the Science and Technology Program of Guangxi Province(AA22068080)the Key Research and Development Program of Hubei Province(2021BAA057)the Taishan Industry-Leading Talent Project Special Funding and Subject Innovation and Talent Introduction Program in Colleges and Universities(111 programs No.D18018)。
文摘Medium-manganese steel exhibits excellent strength and toughness,which are essential features in wear resistance applications.This study examines the impact of annealing temperature on impact abrasive wear.The results have indicated that samples annealed at different temperatures display plowing and fatigue wear effects.In the initial wear stage,the hightemperature annealed steel outperforms samples annealed at a lower temperature in terms of anti-plowing wear performance.This phenomenon is mainly due to the lower initial hardness of the samples subjected to low-temperature annealing.However,with prolonged wear time,the low-temperature annealed samples exhibit improved plowing wear performance,which is ascribed to a refinement of the lamellar microstructure and an increased residual austenite(RA),which enhances the work hardening effect,improving the hardness of the worn surface.The low-temperature annealed samples consistently delivered superior fatigue wear performance when compared with samples annealed at the higher temperature.The latter effect may be attributed to two factors.Firstly,the finer lamellar microstructure in the low-temperature annealed samples,coupled with greater RA,results in transformation-induced plasticity or twin-induced plasticity effect that hinders crack formation and propagation.Secondly,the low-temperature annealed samples form nanoscale equiaxed grains near the worn surface during the wear process.These grains can withstand crack driving forces in fine-grained regions,suppressing the formation and propagation of cracks.
基金supported by the Henry Royce Institute for Advanced Materials,funded through Engineering and Physical Sciences Research Council(EPSRC)grants EP/R00661X/1,EP/S019367/1,EP/P025021/1,and EP/P025498/1.
文摘Liquefied natural gas storage and transportation as well as space propulsion systems have sparked inter-est in the martensitic transformation and behaviours of 316 L stainless steels(SS)under ultra-cryogenic deformation.In this study,high-resolution transmission electron microscopy(HRTEM)and molecular dy-namics(MD)simulations were used to investigate the atomic arrangements and crystalline defects of deformation-induced γ-austenite→ε-martensite→α'-martensite and γ→α'martensitic transforma-tions in 316 L SS at 15 and 173 K.Theγ→εtransformation involves the glide of Shockley partial dislocations on(111)γplanes without a change in atomic spacing.The formation of anα'inclusion in a singleε-band is achieved by a continuous lattice distortion,accompanied by the formation of a tran-sition zone ofα'and the expansion of the average atomic spacings due to dislocation shuffling.Asα'grows further intoγ,the orientation relationship(OR)of theα'changes by lattice bending.This pro-cess follows the Bogers-Burgers-Olson-Cohen model despite it not occurring on intersecting shear bands.Stacking faults and twins can also serve as nucleation sites forα'at 173 K.We also found that direct transformation of γ→α'occurs by the glide of √6aγ[11(2)]/12 dislocations on every(111)γplane with misfit dislocations.Overall,this study provides,for the first time,insights into the atomic-scale mech-anisms of various two-step and one-step martensitic transformations induced by cryogenic deformation and corresponding local strain,enhancing our understanding of the role of martensitic transformation under ultra-cryogenic-temperature deformation in controlling the properties.
