The Ginzbury-Landau theory for bainitic transformation was devised, which contains two first-order phase transformations, one being reconstructive represented by the diffusional proeutectoidal precipitation of ferrite...The Ginzbury-Landau theory for bainitic transformation was devised, which contains two first-order phase transformations, one being reconstructive represented by the diffusional proeutectoidal precipitation of ferrite, and the other the displacive transformation. It provides a coupled mechanism for the formation of bainite. With the numerical simulation results, a diffusion-induced nucleation and a diffusion-accompanied growth of displacive transformation were suggested. This theory can be helpful to over- throw the thermodynamic difficulty of displacive transformation above the Ms temperature, and also helpful to understand the Bs temperature, the partial supersaturation, the single variation of bainitic carbides, and the incomplete-reaction phenomenon of bainitic transformation, etc..展开更多
The microstructure evolution and bainitic transformation of an Fe-0.19C-4.03Mn-1.48Si steel subjected to near-M_(s)austempering treatment were systematically investigated by combining dilatometer,X-ray diffraction,and...The microstructure evolution and bainitic transformation of an Fe-0.19C-4.03Mn-1.48Si steel subjected to near-M_(s)austempering treatment were systematically investigated by combining dilatometer,X-ray diffraction,and electron microscopy.Three additional austempering treatments with isothermal temperatures above M_(s)were used as benchmarks.Results show that the incubation period for the bainitic transformation occurs when the medium Mn steel is treated with the austempering temperature above M_(s).However,when subjected to near-M_(s)isothermal treatment,the medium Mn steel does not show an incubation period and has the fastest bainitic transformation rate.Moreover,the largest volume fraction of bainite with a value of 74.7%is obtained on the condition of near-M_(s)austempering treatment after cooling to room temperature.Dilatometer and microstructure evolution analysis indicates that the elimination of the incubation period and the fastest rate of bainitic transformation are related to the preformed martensite.The advent of preformed martensite allows the specimen to generate more bainite in a limited time.Considering bainitic ferrite nucleation at austenite grain boundaries and through autocatalysis at ferrite/austenite interfaces,a model is established to understand the kinetics of bainite formation and it can describe the nucleation rate of bainitic transformation well when compared to the experimental results.展开更多
In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at ...In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.展开更多
The phase transformation behaviors during continuous cooling of low-carbon boron steels with different vanadium contents were studied by means of dilatometric measurement and microstructure observation. The bainite tr...The phase transformation behaviors during continuous cooling of low-carbon boron steels with different vanadium contents were studied by means of dilatometric measurement and microstructure observation. The bainite transformation behavior is not noticeably altered when the vanadium content is 0.042 and 0.086 wt%, and these steels exhibit full bainitic microstructure even at a cooling rate of 5 ℃/s. When vanadium content is increased to 0.18 wt%, ferrite is still present in the microstructure even at a cooling rate of 40 ℃/s. Vickers hardness of the steels with 0.042 and 0.086 wt% V is remarkably higher than that of the steel with 0.18 wt% V at a cooling rate higher than 10 ℃/s, and the difference is increased with the increase in cooling rate. Moreover, the amount of coarse vanadium precipitates formed in austenite is increased with the increase in vanadium content. The optimum content of vanadium to obtain bainitic microstructure is 0.086 wt% in this experimental low-carbon boron steels.展开更多
ULCB steels have a great potential for structural as well as automotive applications due to the lean alloying concept and the favorable combination of strength and ductility.These steels are being considered as the ma...ULCB steels have a great potential for structural as well as automotive applications due to the lean alloying concept and the favorable combination of strength and ductility.These steels are being considered as the material of choice for many advanced applications where yield strength above 500 MPa is required to reduce component weight.Traditionally Mo has been a key alloying element in producing such steels.In order to push the property envelope of ULCB steels it is interesting to combine Mo alloying with the microalloying elements Nb and B.Cross effects between these elements bear synergies that cannot be achieved by single alloying.The paper demonstrates how these synergies can be beneficially used in combination with appropriate processing.展开更多
The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity（TRIP） steels with polygonal ferrit...The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity（TRIP） steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3 rd generation advanced high strength steels（AHSS）. It is shown from the dilatometry study,microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than450℃ whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450℃.Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450℃ while Si-added steel has a better strength-elongation combination at temperatures lower than 450℃. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite（MA） blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.展开更多
The addition of boron（B） is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affe...The addition of boron（B） is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affects the bainitic transformation kinetics after ausforming. By systematic high-resolution dilatometry tests, the present work reveals that the bainitic transformation kinetics is accelerated in a low C steel with B addition after ausforming from all aspects including incubation time, transformation velocity and transformed volume fraction. In contrast, for the same steel without B addition, both transformation velocity and transformed volume fraction are retarded after ausforming. It is proposed that ausforming can reduce B segregation at prior austenite grain boundaries as some boron can interact with dislocations and therefore enhance bainite nucleation rate. Furthermore, auforming can refine the average volume of bainitic sheaf. Based on the competing mechanisms between increase of nucleation rate and refinement of bainitic sheaf, the effects of B and ausforming on the bainitic transformation kinetics are discussed.展开更多
Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the result...Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the resulting mechanical properties is still unclear.LT-AF was applied to ultrahigh-strength bainitic steel before austempering.The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy(TEM).The planar dislocation structures produced during deformation at 350℃ accelerate the bainitic transformation kinetics during isothermal holding.The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement,TEM,scanning electron microscopy,and X-ray diffraction.It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability.This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF,the decrease in bainitic sheaves size,and the increase in filmy RA content compared to the sample without ausforming.A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.展开更多
Two driving forces △G^((β_1)+α')and △G^(β_1→β_2+α)for the bainitic transformation have been calcu- lated for Cu-24 at.-% Al alloy.Results reveal △G^(β_1→α')>0 within the transformation tem- pera...Two driving forces △G^((β_1)+α')and △G^(β_1→β_2+α)for the bainitic transformation have been calcu- lated for Cu-24 at.-% Al alloy.Results reveal △G^(β_1→α')>0 within the transformation tem- perature range 600—750 K.Therefore,the bainitic transformation cannot be initiated by the β_1→α' shearing mechanism.On the other hand,the driving force △G^(β_1→β_2+α)to produce 5% of the α becomes negative only when the composition of the α satisfies x_(Al)~α≤0.204 at 700 K or x_(Al)~α=0.209 at 750 K.So the bainitic transformation can only proceed by β_1→β_2+α diffusional reaction with different diffusion amount at different temperatures.展开更多
Based on the assumptions of parabolic variation of nucleation rate versus time and transformation kinetics depending mainly on nucleation rate, a different model for bainitic transformation kinetics in hypoeutectoid s...Based on the assumptions of parabolic variation of nucleation rate versus time and transformation kinetics depending mainly on nucleation rate, a different model for bainitic transformation kinetics in hypoeutectoid steels was established. And this model was proved to be effective in the description of bainitic transformation by comparison with the result of conventional Avrami equation.展开更多
The behaviour of the pre bainitic transformation in Fe Ni alloy was investigated by using SAM, X ray diffraction and TEM. The results show that there is segregation of Ni atoms in austenite and that bainite forms i...The behaviour of the pre bainitic transformation in Fe Ni alloy was investigated by using SAM, X ray diffraction and TEM. The results show that there is segregation of Ni atoms in austenite and that bainite forms in depleted regions of Ni atoms.展开更多
The integrated processing of chromizing and austempering(termed chro-austempering)treatments was proposed.The microstructure and properties of a medium-carbon high-strength bainitic steel treated by chro-austempering ...The integrated processing of chromizing and austempering(termed chro-austempering)treatments was proposed.The microstructure and properties of a medium-carbon high-strength bainitic steel treated by chro-austempering treatments were investigated by metallography,scanning vibrating electrode technique,electrochemical workstation,and microhardness test.The results show that the high-strength bainitic steel with carbide-free bainite as matrix and the chromized layers on surfaces was successfully fabricated by chro-austempering treatment.The hardness of surface layers was about 3.5 times that of the bainite matrix.Meanwhile,the corrosion started from exposed bainitic matrix and proceeded along the depth direction,testifying that the surface corrosion resistance was significantly improved by chro-austempering treatment due to the formation of Cr_(7)C_(3)and(Cr,Fe)7_(7)C_(3)on the surface.展开更多
The present investigation showed that the re- laxation peak at about 200℃ (f≈1Hz) was related to bainitic transformation in a CuZnAl alloy.The peak decreased when bainite precipitated.The activation energy of the pe...The present investigation showed that the re- laxation peak at about 200℃ (f≈1Hz) was related to bainitic transformation in a CuZnAl alloy.The peak decreased when bainite precipitated.The activation energy of the peak was the same as the apparent activation energy of bainitic transforma- tion.According to the relations between the peak and the bainitic transformation,the mechanism of the peak has been discussed.展开更多
The lower bainitic carbides precipitation in 40CrMnSiMoV steel previously austempered at 310℃ was observed in situ by employing a high temperature stage of an ultra high voltage TEM(JEM-1000).The typical lower bamiti...The lower bainitic carbides precipitation in 40CrMnSiMoV steel previously austempered at 310℃ was observed in situ by employing a high temperature stage of an ultra high voltage TEM(JEM-1000).The typical lower bamitic carbides were found to have precipitated from within the carbide-free bainitic ferrite after tempering for certain period at a temperature higher than that of the isothermal transformation.This revealed that the lower bainitic ferrite is supersaturated with carbon to some extent.The carbide may also precipitate from austenite, but they have no typical morphological features of lower bainitic carbide.展开更多
TEM observation and analysis have been conducted on bainitie transformation with and without the influence of externally applied tensile sress for alloyed steel 35MV7. Recrystallizafion was found to occur within the b...TEM observation and analysis have been conducted on bainitie transformation with and without the influence of externally applied tensile sress for alloyed steel 35MV7. Recrystallizafion was found to occur within the bainitic structures transformed at 450 ℃ in cases of both with and without the application of external stress, and coupling between the reconstructive and displacive mechanisms is expected, due to the relatively high holding temperature and high dislocation density yielded with the displacive mechanism. RecrystaUization was not observed within the bainitic structures transformed at lower temperature of 350 ℃, both with and without the application of stress; However, for the stressed specimen, the structure with very fine subgrains was found to be preserved and not reconstructed thermically, due to the low temperature and short holding time.展开更多
The crystallographic nature of initial isothermal phase transformation in Cu-26Zn-4Al (wt-％) was investigated. The kinetic transforma- tion curve, morphology, crystal structure, substructure, orientation relationship...The crystallographic nature of initial isothermal phase transformation in Cu-26Zn-4Al (wt-％) was investigated. The kinetic transforma- tion curve, morphology, crystal structure, substructure, orientation relationship and twin relationship of bainite plates have been studied by means of optical and transmission electron microscopy. The experimental results showed that the characteristics of initial 8→bainite transformation are not exactly consistent with that found in martensite transformation, for example, orientation relationship between matrix and bainite does not exist in martensite transformation.展开更多
The microstructure of bainite ferrite in NANOBAIN steel transformed at different temperatures was investigated by scanning electron microscopy, transmission electron microscopy, electron back-scattered diffraction, an...The microstructure of bainite ferrite in NANOBAIN steel transformed at different temperatures was investigated by scanning electron microscopy, transmission electron microscopy, electron back-scattered diffraction, and vickers hardness tester in detail. It is found that the average width of bainitic ferrite （BF） plates can be refined to be thinner with the reduction of temperature （473-573 K）, and the bainitic ferrite plates can reach up to 20-74 nm at 473 K. Crystallographic analysis reveals that the bainitic ferrite laths are close to the Nishiyama-Wasserman orientation relationship with their parent austenite. Temperature shows a significant effect on the variant selection, and a decrease in temperature generally weakens the variant selection. Thermodynamic analyses indicates that the Lacher, Fowler and Guggenheim （LFG） model is more suitable than the Kaufman, Radcliffe and Cohen （KRC） model dealing with NANOBAIN steel at a low temperature range. The free energy change △G^γ→BF is about -1500 J.mol^-1 at 473 K, which indicates that nucleation in NANOBAIN steel is the shear mechanism. Finally, the formation of carbon poor regions is thermodynamically possible, and the existence of carbon poor regions can greatly increase the possibility of the shear mechanism.展开更多
The influence of prior austenite deformed at different temperature on the subsequent continuous cooling bainitic transformation has been investigated in an C-Ma-Cr-Ni-Mo plastic die steel. The results show that the pr...The influence of prior austenite deformed at different temperature on the subsequent continuous cooling bainitic transformation has been investigated in an C-Ma-Cr-Ni-Mo plastic die steel. The results show that the prior deformation in low temperature region of austenite retards significantly the bainitic transformation. For the same continuous cooling schedule, as austenite deformed at lower temperature, the quantity of the classical sheaf-like bainite becomes less. The present results show that severe deformation leads to mechanical stabilization of austenite and causes the difficulty of bainitic ferrite propagation into the austenite.展开更多
Based on the experimental results that solute-depleted zone was observed in Cu-28Zn-4AI (mass fraction) at 523 K, AG is calculated as a positive according to the thermodynamic criteria for the spinodal decomposition o...Based on the experimental results that solute-depleted zone was observed in Cu-28Zn-4AI (mass fraction) at 523 K, AG is calculated as a positive according to the thermodynamic criteria for the spinodal decomposition of a ternary systems. So, the solute-depleted zone cannot be formed by spinodal decomposition. Dislocation density required by the formation of solute-depleted zone is estimated greater than 7.89×109 cm-2, according to the segregation of solute atoms around dislocations, which is not consistent with the practical situation for the alloy at 523 K. Associated with the internal friction experimental fact that internal friction peaks appear within the incubation for bainitic transformation in Cu-Zn-AI alloy, the equilibrium temperature, TO, is evaluated as 433 K for solute-depleted Cu-25Zn-3.4AI, which is lower than the experimental temperature 523 K. Thus, nucleation by shear mechanism is impossible in this circumstance. Therefore, it is concluded that, like bainite in steels and Ag-Cd, bainite in Cu-Zn-AI alloys nucleates by diffusional mechanism, just implied by the experimental existence of solute-depleted zone.展开更多
First,strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique.Next,the isothermal growth of austenite grain during the re...First,strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique.Next,the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope(CLSM).The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000℃.And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region(above 1000℃)and 693.2 kJ/mol in the low temperature region(below 1000℃),respectively.Then,the kinetics model of austenite isothermal growth was established,which can predict the austenite grain size during isothermal hold very well.Besides,high density of second phase particles with small size was found during the isothermal hold at the low temperature region,leading to the refinement of austenite grain.After isothermal hold at different temperature for 1800 s,the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process.And growth rates of bainite plates with different nucleation positions and different prior austenite grain size(PAGS)were calculated.It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.展开更多
文摘The Ginzbury-Landau theory for bainitic transformation was devised, which contains two first-order phase transformations, one being reconstructive represented by the diffusional proeutectoidal precipitation of ferrite, and the other the displacive transformation. It provides a coupled mechanism for the formation of bainite. With the numerical simulation results, a diffusion-induced nucleation and a diffusion-accompanied growth of displacive transformation were suggested. This theory can be helpful to over- throw the thermodynamic difficulty of displacive transformation above the Ms temperature, and also helpful to understand the Bs temperature, the partial supersaturation, the single variation of bainitic carbides, and the incomplete-reaction phenomenon of bainitic transformation, etc..
基金support from the National Natural Science Foundation of China(Grant Nos.52201101 and 52274372)the National Key R&D Program of China(2021YFB3702404)the Fundamental Research Funds for the Central Universities(FRF-TP-22-013A1)。
文摘The microstructure evolution and bainitic transformation of an Fe-0.19C-4.03Mn-1.48Si steel subjected to near-M_(s)austempering treatment were systematically investigated by combining dilatometer,X-ray diffraction,and electron microscopy.Three additional austempering treatments with isothermal temperatures above M_(s)were used as benchmarks.Results show that the incubation period for the bainitic transformation occurs when the medium Mn steel is treated with the austempering temperature above M_(s).However,when subjected to near-M_(s)isothermal treatment,the medium Mn steel does not show an incubation period and has the fastest bainitic transformation rate.Moreover,the largest volume fraction of bainite with a value of 74.7%is obtained on the condition of near-M_(s)austempering treatment after cooling to room temperature.Dilatometer and microstructure evolution analysis indicates that the elimination of the incubation period and the fastest rate of bainitic transformation are related to the preformed martensite.The advent of preformed martensite allows the specimen to generate more bainite in a limited time.Considering bainitic ferrite nucleation at austenite grain boundaries and through autocatalysis at ferrite/austenite interfaces,a model is established to understand the kinetics of bainite formation and it can describe the nucleation rate of bainitic transformation well when compared to the experimental results.
基金the National Natural Science Foundation of China(No.51274154)the National High-Tech Research and Development Program of China(No.2012AA03A504)+1 种基金the State Key Laboratory of Development and Application Technology of Automo-tive Steels(Baosteel Group)the Key Project of Hubei Education Committee(No.D20121101)
文摘In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.
基金financially supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2011BAE13B03)the Natural Science Foundation of Liaoning Province of China (No. 201202062)
文摘The phase transformation behaviors during continuous cooling of low-carbon boron steels with different vanadium contents were studied by means of dilatometric measurement and microstructure observation. The bainite transformation behavior is not noticeably altered when the vanadium content is 0.042 and 0.086 wt%, and these steels exhibit full bainitic microstructure even at a cooling rate of 5 ℃/s. When vanadium content is increased to 0.18 wt%, ferrite is still present in the microstructure even at a cooling rate of 40 ℃/s. Vickers hardness of the steels with 0.042 and 0.086 wt% V is remarkably higher than that of the steel with 0.18 wt% V at a cooling rate higher than 10 ℃/s, and the difference is increased with the increase in cooling rate. Moreover, the amount of coarse vanadium precipitates formed in austenite is increased with the increase in vanadium content. The optimum content of vanadium to obtain bainitic microstructure is 0.086 wt% in this experimental low-carbon boron steels.
文摘ULCB steels have a great potential for structural as well as automotive applications due to the lean alloying concept and the favorable combination of strength and ductility.These steels are being considered as the material of choice for many advanced applications where yield strength above 500 MPa is required to reduce component weight.Traditionally Mo has been a key alloying element in producing such steels.In order to push the property envelope of ULCB steels it is interesting to combine Mo alloying with the microalloying elements Nb and B.Cross effects between these elements bear synergies that cannot be achieved by single alloying.The paper demonstrates how these synergies can be beneficially used in combination with appropriate processing.
文摘The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity（TRIP） steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3 rd generation advanced high strength steels（AHSS）. It is shown from the dilatometry study,microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than450℃ whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450℃.Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450℃ while Si-added steel has a better strength-elongation combination at temperatures lower than 450℃. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite（MA） blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.
基金the financial support by the Steel Joint Funds of the National Natural Science Foundation of China(Grant No.U1560204)National Natural Science Foundation of China(Grant No.51301148)+2 种基金the Research Grants Council of Hong Kong(Grant Nos.HKU719712E,HKU712713E,17203014)Shenzhen Science,the Technology and Innovation Commission(Grant No.JCYJ20150629151046886)financial support from National Natural Science Foundation of China(Grant No.51574080)
文摘The addition of boron（B） is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affects the bainitic transformation kinetics after ausforming. By systematic high-resolution dilatometry tests, the present work reveals that the bainitic transformation kinetics is accelerated in a low C steel with B addition after ausforming from all aspects including incubation time, transformation velocity and transformed volume fraction. In contrast, for the same steel without B addition, both transformation velocity and transformed volume fraction are retarded after ausforming. It is proposed that ausforming can reduce B segregation at prior austenite grain boundaries as some boron can interact with dislocations and therefore enhance bainite nucleation rate. Furthermore, auforming can refine the average volume of bainitic sheaf. Based on the competing mechanisms between increase of nucleation rate and refinement of bainitic sheaf, the effects of B and ausforming on the bainitic transformation kinetics are discussed.
基金The financial support provided by the National Natural Science Foundation of China(No.U1808208)the Fundamental Research Funds for the Central Universities(N2107005)is gratefully acknowledged.
文摘Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the resulting mechanical properties is still unclear.LT-AF was applied to ultrahigh-strength bainitic steel before austempering.The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy(TEM).The planar dislocation structures produced during deformation at 350℃ accelerate the bainitic transformation kinetics during isothermal holding.The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement,TEM,scanning electron microscopy,and X-ray diffraction.It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability.This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF,the decrease in bainitic sheaves size,and the increase in filmy RA content compared to the sample without ausforming.A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.
文摘Two driving forces △G^((β_1)+α')and △G^(β_1→β_2+α)for the bainitic transformation have been calcu- lated for Cu-24 at.-% Al alloy.Results reveal △G^(β_1→α')>0 within the transformation tem- perature range 600—750 K.Therefore,the bainitic transformation cannot be initiated by the β_1→α' shearing mechanism.On the other hand,the driving force △G^(β_1→β_2+α)to produce 5% of the α becomes negative only when the composition of the α satisfies x_(Al)~α≤0.204 at 700 K or x_(Al)~α=0.209 at 750 K.So the bainitic transformation can only proceed by β_1→β_2+α diffusional reaction with different diffusion amount at different temperatures.
文摘Based on the assumptions of parabolic variation of nucleation rate versus time and transformation kinetics depending mainly on nucleation rate, a different model for bainitic transformation kinetics in hypoeutectoid steels was established. And this model was proved to be effective in the description of bainitic transformation by comparison with the result of conventional Avrami equation.
文摘The behaviour of the pre bainitic transformation in Fe Ni alloy was investigated by using SAM, X ray diffraction and TEM. The results show that there is segregation of Ni atoms in austenite and that bainite forms in depleted regions of Ni atoms.
基金The National Natural Science Foundation of China(NSFC)(51874216 and 52104381)the Key Project of Hebei Iron and Steel Group(HG219313)+2 种基金the Open Fund of Hubei Provincial Key Laboratory for New Processes of Ironmaking and Steelmaking(KF-20-4)Natural Science Foundation of Hubei(2021CFB127)China Postdoctoral Science Foundation(2021M702539).
文摘The integrated processing of chromizing and austempering(termed chro-austempering)treatments was proposed.The microstructure and properties of a medium-carbon high-strength bainitic steel treated by chro-austempering treatments were investigated by metallography,scanning vibrating electrode technique,electrochemical workstation,and microhardness test.The results show that the high-strength bainitic steel with carbide-free bainite as matrix and the chromized layers on surfaces was successfully fabricated by chro-austempering treatment.The hardness of surface layers was about 3.5 times that of the bainite matrix.Meanwhile,the corrosion started from exposed bainitic matrix and proceeded along the depth direction,testifying that the surface corrosion resistance was significantly improved by chro-austempering treatment due to the formation of Cr_(7)C_(3)and(Cr,Fe)7_(7)C_(3)on the surface.
文摘The present investigation showed that the re- laxation peak at about 200℃ (f≈1Hz) was related to bainitic transformation in a CuZnAl alloy.The peak decreased when bainite precipitated.The activation energy of the peak was the same as the apparent activation energy of bainitic transforma- tion.According to the relations between the peak and the bainitic transformation,the mechanism of the peak has been discussed.
文摘The lower bainitic carbides precipitation in 40CrMnSiMoV steel previously austempered at 310℃ was observed in situ by employing a high temperature stage of an ultra high voltage TEM(JEM-1000).The typical lower bamitic carbides were found to have precipitated from within the carbide-free bainitic ferrite after tempering for certain period at a temperature higher than that of the isothermal transformation.This revealed that the lower bainitic ferrite is supersaturated with carbon to some extent.The carbide may also precipitate from austenite, but they have no typical morphological features of lower bainitic carbide.
文摘TEM observation and analysis have been conducted on bainitie transformation with and without the influence of externally applied tensile sress for alloyed steel 35MV7. Recrystallizafion was found to occur within the bainitic structures transformed at 450 ℃ in cases of both with and without the application of external stress, and coupling between the reconstructive and displacive mechanisms is expected, due to the relatively high holding temperature and high dislocation density yielded with the displacive mechanism. RecrystaUization was not observed within the bainitic structures transformed at lower temperature of 350 ℃, both with and without the application of stress; However, for the stressed specimen, the structure with very fine subgrains was found to be preserved and not reconstructed thermically, due to the low temperature and short holding time.
文摘The crystallographic nature of initial isothermal phase transformation in Cu-26Zn-4Al (wt-％) was investigated. The kinetic transforma- tion curve, morphology, crystal structure, substructure, orientation relationship and twin relationship of bainite plates have been studied by means of optical and transmission electron microscopy. The experimental results showed that the characteristics of initial 8→bainite transformation are not exactly consistent with that found in martensite transformation, for example, orientation relationship between matrix and bainite does not exist in martensite transformation.
基金financially supported by the National Natural Science Foundation of China (No.51271035)
文摘The microstructure of bainite ferrite in NANOBAIN steel transformed at different temperatures was investigated by scanning electron microscopy, transmission electron microscopy, electron back-scattered diffraction, and vickers hardness tester in detail. It is found that the average width of bainitic ferrite （BF） plates can be refined to be thinner with the reduction of temperature （473-573 K）, and the bainitic ferrite plates can reach up to 20-74 nm at 473 K. Crystallographic analysis reveals that the bainitic ferrite laths are close to the Nishiyama-Wasserman orientation relationship with their parent austenite. Temperature shows a significant effect on the variant selection, and a decrease in temperature generally weakens the variant selection. Thermodynamic analyses indicates that the Lacher, Fowler and Guggenheim （LFG） model is more suitable than the Kaufman, Radcliffe and Cohen （KRC） model dealing with NANOBAIN steel at a low temperature range. The free energy change △G^γ→BF is about -1500 J.mol^-1 at 473 K, which indicates that nucleation in NANOBAIN steel is the shear mechanism. Finally, the formation of carbon poor regions is thermodynamically possible, and the existence of carbon poor regions can greatly increase the possibility of the shear mechanism.
文摘The influence of prior austenite deformed at different temperature on the subsequent continuous cooling bainitic transformation has been investigated in an C-Ma-Cr-Ni-Mo plastic die steel. The results show that the prior deformation in low temperature region of austenite retards significantly the bainitic transformation. For the same continuous cooling schedule, as austenite deformed at lower temperature, the quantity of the classical sheaf-like bainite becomes less. The present results show that severe deformation leads to mechanical stabilization of austenite and causes the difficulty of bainitic ferrite propagation into the austenite.
文摘Based on the experimental results that solute-depleted zone was observed in Cu-28Zn-4AI (mass fraction) at 523 K, AG is calculated as a positive according to the thermodynamic criteria for the spinodal decomposition of a ternary systems. So, the solute-depleted zone cannot be formed by spinodal decomposition. Dislocation density required by the formation of solute-depleted zone is estimated greater than 7.89×109 cm-2, according to the segregation of solute atoms around dislocations, which is not consistent with the practical situation for the alloy at 523 K. Associated with the internal friction experimental fact that internal friction peaks appear within the incubation for bainitic transformation in Cu-Zn-AI alloy, the equilibrium temperature, TO, is evaluated as 433 K for solute-depleted Cu-25Zn-3.4AI, which is lower than the experimental temperature 523 K. Thus, nucleation by shear mechanism is impossible in this circumstance. Therefore, it is concluded that, like bainite in steels and Ag-Cd, bainite in Cu-Zn-AI alloys nucleates by diffusional mechanism, just implied by the experimental existence of solute-depleted zone.
基金supported from the National Natural Science Foundation of China(No.52130408)the Hunan Scientific Technology Project,China(Nos.2019RS3007,2020WK2003)the Fundamental Research Funds for the Central Universities of Central South University,China。
文摘First,strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique.Next,the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope(CLSM).The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000℃.And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region(above 1000℃)and 693.2 kJ/mol in the low temperature region(below 1000℃),respectively.Then,the kinetics model of austenite isothermal growth was established,which can predict the austenite grain size during isothermal hold very well.Besides,high density of second phase particles with small size was found during the isothermal hold at the low temperature region,leading to the refinement of austenite grain.After isothermal hold at different temperature for 1800 s,the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process.And growth rates of bainite plates with different nucleation positions and different prior austenite grain size(PAGS)were calculated.It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.
