9Cr ferritic/martensitic(9Cr F/M)steels are considered ideal structural materials for various nuclear energy systems.However,δ-ferrite(δ),as a controlled phase,may occur in its welds.Three deposited metals with diff...9Cr ferritic/martensitic(9Cr F/M)steels are considered ideal structural materials for various nuclear energy systems.However,δ-ferrite(δ),as a controlled phase,may occur in its welds.Three deposited metals with different carbon contents(0.04,0.07,and 0.10 wt%)were investigated using experimental and finite element simulation methods.The results showed that the incomplete peritectic reaction,the incompleteδto austenite phase transition,and the segregation of ferrite-stabilized elements led to the residualδ.The amount and morphology ofδsignificantly influence the mechanical properties.After increasing the carbon content,the increase in strength comes mainly from precipitation strengthening and dislocation strengthening,the presence ofδwill reduce the strength.During the impact process,δaffects the absorbed energy for the stable crack growth through its morphology,and M_(23)C_(6)affects the crack formation energy through its quantity.By decreasing the carbon content to a certain extent,the reduction of M_(23)C_(6)content and the generation of large polygonalδcan effectively improve the toughness of 9Cr-steel deposited metals.展开更多
FF sample(nearly free ofδ-ferrite)and CF sample(containing∼4%δ-ferrite)were prepared from the AISI 316 austenitic stainless steel plate to elucidate the role ofδ-ferrite in the fatigue crack growth under the solut...FF sample(nearly free ofδ-ferrite)and CF sample(containing∼4%δ-ferrite)were prepared from the AISI 316 austenitic stainless steel plate to elucidate the role ofδ-ferrite in the fatigue crack growth under the solution treated and accelerated aged conditions.It is found that the fatigue crack growth resistance of the CF sample is higher than the FF sample under the solution treated condition.However,a significant deterioration of the fatigue crack growth resistance is observed in the CF sample while little variation is found in the FF sample after accelerated aging treatment at 750°C for 10 h.In the solution treated condition,deflected crack growth path is present when the main crack encounters theδ-ferrite in the CF sample due to the differences in the fatigue responses between austenite andδ-ferrite.The measured growth rate of the deflected crack is significantly slower than that of the flat crack of the same length.After the accelerated aging treatment,microcracks are produced at the M_(23)C_(6)/δinterface due to the strain incompatibility between M_(23)C_(6) and retainedδ-ferrite when the decomposedδ-ferrite is subject to plastic deformation in the crack tip plastic zone.The preexisting microcracks in the front of crack tip provide a viable path for crack propagation,resulting in the relatively flat crack path.展开更多
The microstructure evolution and mechanical properties of the 316 H stainless steel weld metals with different C contents were studied at the aging temperature of 550℃ for different aging holding time.The transformat...The microstructure evolution and mechanical properties of the 316 H stainless steel weld metals with different C contents were studied at the aging temperature of 550℃ for different aging holding time.The transformation behavior of δ-ferrite and precipitation mechanisms of M(23)C6 and σ phase in the as-aged weld metal were investigated.The results indicated that for the as-welded weld metal,with increasing C content,the yield and tensile strengths increased,while the elongation decreased owing to the increase of C solid solution strengthening effect.Moreover,both the high δ-ferrite content in low C weld metal and the precipitated M(23)C6 carbide in high C weld metal deteriorated the impact energy obviously.During the aging process,the rapid precipitation of M(23)C6 carbide occurred in σ-ferrite firstly owing to the high diffusion rate of C.Once the carbon is depleted by precipitation of M(23)C6,the slow formation of σ phase occurred through eutectoid transformation(δ→σ+γ) depending on the diffusion of Cr and Mo.Moreover,increasing C content promoted the formation of M(23)C6 carbides and inhibited the formation of σ phase.Therefore,increasing C content accelerated the transformation of σ ferrite in weld metal during aging process.Furthermore,after a long enough aging time,a transformation from M(23)C6 to σ occurred.The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions.For the low C weld metal aged at 550℃,with the increase of the aging time,fine M(23)C6 first precipitated,then coarsened,after that σ phase formed,which caused that the yield and tensile strengths first increased,then decreased,and finally increased slightly again.For the medium C weld metal,as the aging time increased,first the depletion of the solid solution C as a result the M(23)C6 precipitation deteriorated the strength,and then the formation of σ phase improved the strength.For the high C weld metal,with the increase of the aging time,the depletion of the solid solution C and the coarsening of the M(23)C6 precipitates deteriorated the strength.Furthermore,with increasing aging time,both the precipitation and coarsening of M(23)C6 and increasing σ phase content deteriorated the elongation and impact ene rgy.展开更多
The morphology and the evolution of δ-ferrite existing in B410D slabs, hot-rolled plates, annealed plates and quenched plates were studied through metallographic observation. Results show that δ-ferrite forms during...The morphology and the evolution of δ-ferrite existing in B410D slabs, hot-rolled plates, annealed plates and quenched plates were studied through metallographic observation. Results show that δ-ferrite forms during the solidification process and that it easily grows and increases in quantity during high temperature annealing. Band-shaped δ-ferrite in hotrolled plates is difficult to be eliminated by conventional heat treatment and hard to recrystallize.展开更多
It was presented the in situ observation of growth behavior and morphology of delta-ferrite as a function of solidification rate in an AISI304 stainless steel. The specimens have been solidified and observed using con...It was presented the in situ observation of growth behavior and morphology of delta-ferrite as a function of solidification rate in an AISI304 stainless steel. The specimens have been solidified and observed using confocal scanning laser microscopy (CSLM). The δ-phase always appears like cells on the sample surface when critical supercooling occurs, during which the L→δ transformation starts. The solid-liquid (S-L) interface is found to be finger shaped and has no faceted shape. γ phase appears among δ grains due to partitioning of Ni into the melt during solidification, when solidification rate is higher. The mergence of observed δ cells is possible for the steel sample cooled at 7.5℃/min. The formation of dendrites can be observed on the free surface of the steel sample cooled at 150℃/min. The size of solidified delta grains decreases from 120 to 20-80μm, and the volume fraction of solidified austenite increases with increase in solidification rate from 7.5 to 150℃/min. The relation between the tip radius of δ cell and its growth rate is deduced, and the results agree with the experimental values.展开更多
The strength-ductility trade-off in low-Mn lightweight steels is a significant challenge due to the low thermal stability of austenite and the presence ofδ-ferrite.Two types of low-Mn lightweight steels containing V ...The strength-ductility trade-off in low-Mn lightweight steels is a significant challenge due to the low thermal stability of austenite and the presence ofδ-ferrite.Two types of low-Mn lightweight steels containing V and NbVMo microalloying elements were developed by warm rolling.Among these,NbVMo steel demonstrated superior properties,achieving a tensile strength of~1.2 GPa and a product of strength and elongation exceeding 45 GPa%.In-depth mechanism analysis by atom probe tomography and quasi-in-situ electron backscatter diffraction revealed that different microalloying compositions influence the mechanical properties by strengtheningδ-ferrite,refining retained austenite and homogenizing matrix strain.In NbVMo steel,δ-ferrite strengthening is attributed to the synergistic effects of(V,Mo,Cr,Nb)C composite precipitation,fine NbC and MoC precipitates,and the solid solution strengthening of Mo.These mechanisms collectively contribute to a higher yield strength andδ-ferrite microhardness compared to V steel.Consequently,δ-ferrite and the surrounding matrix in NbVMo steel exhibit coordinated elongation during deformation,enhancing the ductility.The improved microstructural and strain uniformity in NbVMo steel mitigates stress concentration effects onδ-ferrite deformation and serves as a barrier that delays the transformation of retained austenite.In contrast,the retained austenite in V steel exhibits a blocky morphology with larger grain sizes,resulting in lower stability.Combined with localized stress concentrations due to non-uniform strain distribution,this leads to premature transformation of retained austenite to alleviate stress,ultimately impairing elongation and the continuity of strain hardening.Furthermore,the precipitation mechanisms of(V,Mo,Cr,Nb)C composite precipitates are elucidated.展开更多
Two types of 316 butt welds with carbon contents of 0.016%and 0.062%have been produced using the gas tungsten arc welding process.Theδ-ferrite content decreased from 7.2 to 2.8%in volume as the carbon content increas...Two types of 316 butt welds with carbon contents of 0.016%and 0.062%have been produced using the gas tungsten arc welding process.Theδ-ferrite content decreased from 7.2 to 2.8%in volume as the carbon content increased.The creeprupture strength and creep ductility of the two types of weld metals have been measured at 550℃over the stress range of 290-316 MPa and at 600℃over 230-265 MPa.The microstructure change and precipitation behavior of the weld metals were observed and related to the creep rupture properties.The creep rupture strength of the C2(0.062%C)weld metal was higher than that of the Cl(0.016%C)weld metal at both 550℃and 600℃.At 550℃,as the decrease in the applied stress,the difference of the creep-rupture life between the two weld metals diminished due to the higher depletion rate of carbon by precipitation of M_(23)C_(6)in the C2 weld metal,while at 600℃,the difference enlarged due to the massive precipitation ofσphase and extensive crack formation and propagation alongσ/austenite boundaries in the C1 weld metal.For both the C1 and C2 weld metal,the decrease in ductility was adverse with the transformation percentage and related to products of theδ-ferrite transformation.展开更多
In order to clarify the recrystallization mechanism of low-densityδ-ferrite steel Fe-4AI-2Ni,interrupted and single-pass compression tests were carried out.In this regard,five deformation temperatures(750-950 at an i...In order to clarify the recrystallization mechanism of low-densityδ-ferrite steel Fe-4AI-2Ni,interrupted and single-pass compression tests were carried out.In this regard,five deformation temperatures(750-950 at an interval of 50°C)and different hold time were selected.It was observed that the softening and recrystallization fraction was enhaneed with increased deformation temperature and hold time.The original grain bounclaries were the preferred nucleation sites for recrystallized grains,and recrystallization had an impact on obtaining homogeneous and fine-grained structure.Recrystallization in the ferritic alloy commenced after a significant degree of softening,and the softening associated with recovery was appreciably less.The optimum rolling deformation temperature was identified to be greater than 900°C.展开更多
The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ agin...The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ aging on the α'-martensitic transformation of the as-welded and the SHT weld metals were investigated.The results showed that the weld metal had poor thermal stability of austenite.The precipitation of NbC during the 850℃ SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α'-martensite.The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α'-martensite at the early stage of 550℃ aging.The homogenized diffusion of C and Cr in the matrix during 550℃ aging led to the restoration of austenitic thermal stability and the decrease of α'-martensite content.The C-depleted α'-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix,while the Cr-depleted α'-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.展开更多
Microstructures of casting samples of Fe-9%Cr steel and samples subjected to different heat treatments were investigated to determine their formation and evolution mechanism. The results show that there is no macrosco...Microstructures of casting samples of Fe-9%Cr steel and samples subjected to different heat treatments were investigated to determine their formation and evolution mechanism. The results show that there is no macroscopic segregation in the casting Fe-9%Cr steel. During cooling from solidification temperature to room temperature, 6-ferrite → austenite transformation is obviously influenced by cooling rate, while subsequent transformation of austenite does not obviously depend on the cooling rate. In the casting; samples, a great number of precipitates distribute inside martensitic laths while there are almost no precipitates inside δ-ferrite. When the casting samples were reheated to and isothermally held at 800 ℃, the original precipitates and the lath boundaries disappeared gradually. Meanwhile, new precipitates nucleate and grow at the prior lath boundaries.展开更多
The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings.The 9Cr-1.5Mo-1Co cast steel,referred to as CB2,is one of the most promising alloys for various cast compone...The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings.The 9Cr-1.5Mo-1Co cast steel,referred to as CB2,is one of the most promising alloys for various cast components to be used under ultrasupercritical conditions.In this study,HRTEM,SEM,and XRD methods were used for qualitative and quantitative analyses of contents,phases,and sizes of the inclusions and precipitates,as well as microstructure observation of the tempered martensitic steel at different cooling rates.Traditional tensile tests were conducted to characterize the material mechanical properties.Results show that the size of the boron nitride and precipitate,the width of the martensite lath and the content of theδ-ferrite are reduced as the cooling rate increases from 5-8℃·min^-1(CB2-S steel)to 50-60℃·min^-1(CB2-F steel).The precipitates are mainly composed of M23C6 and a small amount of M3C.The average diameters of the M23C6 precipitates in CB2-F and CB2-S are 202 nm and 209 nm,respectively.The inclusions are mainly composed of BN,Al2O3 and MnO2,and the inclusion ratios are 0.1969%for the CB2-F and 0.06556%for CB2-S steel.The average martensite lath widths of CB2-F and CB2-S steels are 289 nm and 301 nm,respectively.Furthermore,the M3C having a diameter of about 150 nm and a thickness of 20 nm is observed in theδ-ferrite of the tempered CB2-S steel.The presence of theδ-ferrite reduces the precipitation strengthening and dislocation density in CB2-S steel.In addition,the lower cooling rate stabilizes theδ-ferrite structure at room temperature.展开更多
The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-p...The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-phase structure and crevice corrosion.The three-dimensional structure of the leakage point was demonstrated using X-ray diffraction topography.The results of scanning electron microscope examination show the microstructure of the weld to be columnar and dendritic.It is found by electron probe microscope analysis and transmission electron microscopy that columnar dendrites consisted of γ-dendrite and an amount of δ-ferrite phases at the dendrite trunk.Simulated corrosion test results confirmed that the corrosion medium was the chloride ion.Crevice corrosion of chloride ions occurred at weld defects on the inner wall thus forming a concentration cell.Grains of columnar dendrites were then corroded by chloride ions and δ-ferrite phases on the grain boundaries were retained,which formed the particular skeleton corrosion structure.As a result,it led to leakage when the corrosion of weld occurred from the inner wall to the outer wall.展开更多
Press hardened steel(PHS)plays a key role in the manufacture of anti-collision structural components.The formation of δ-ferrite is a suffering issue for the laser welding of Al-Si coated PHS.Oscillating laser was use...Press hardened steel(PHS)plays a key role in the manufacture of anti-collision structural components.The formation of δ-ferrite is a suffering issue for the laser welding of Al-Si coated PHS.Oscillating laser was used to weld Al-Si coated 1.5 GPa PHS and novel 2 GPa PHS,and the effect of laser offset on the microstructure and properties of the dissimilar welded joints was studied.The results showed that a perfect weld profile was achieved by laser offset welding(LOW),without any welding defects.The δ-ferrite formed in as-received welds of laser alignment welding(LAW)with high Al content(up to 2.9 wt.%),but it disappeared with the laser offset to 2 GPa PHS,and the maximum Al content in the segregation zone reduced to 1.2 wt.%.After post-welding heat treatment,the δ-ferrite was coarsened and theα-ferrite formed in the secondary Al-rich area for the high Ac3 temperature,but the α-ferrite was few and fine in LOW welds.The hardness in the LAW welds was lower than that in the LOW welds,due to the presence of δ-ferrite,as well as less carbon content and Ti and V alloying elements.The tensile strength(1561 MPa)and elongation(5.4%)with LOW were higher than those with LAW(1490 MPa,3.1%),and the fracture occurred in the Al-Si coated PHS.It is indicated that adjusting the laser offset is effective to prevent the formation of δ-ferrite and is potential to avoid the formation of α-ferrite.It also provides a wide heat treatment temperature window for the dissimilar welds of 1.5 GPa PHS and novel 2 GPa PHS.展开更多
基金supported by the LingChuang Research Project of China National Nuclear Corporation,China Institute of Atomic Energy(E141L803J1)the Natural Science Foundation of Heilongjiang Province(No.TD2021E006).
文摘9Cr ferritic/martensitic(9Cr F/M)steels are considered ideal structural materials for various nuclear energy systems.However,δ-ferrite(δ),as a controlled phase,may occur in its welds.Three deposited metals with different carbon contents(0.04,0.07,and 0.10 wt%)were investigated using experimental and finite element simulation methods.The results showed that the incomplete peritectic reaction,the incompleteδto austenite phase transition,and the segregation of ferrite-stabilized elements led to the residualδ.The amount and morphology ofδsignificantly influence the mechanical properties.After increasing the carbon content,the increase in strength comes mainly from precipitation strengthening and dislocation strengthening,the presence ofδwill reduce the strength.During the impact process,δaffects the absorbed energy for the stable crack growth through its morphology,and M_(23)C_(6)affects the crack formation energy through its quantity.By decreasing the carbon content to a certain extent,the reduction of M_(23)C_(6)content and the generation of large polygonalδcan effectively improve the toughness of 9Cr-steel deposited metals.
基金financially supported by the National Natural Science Foundation of China (No. 51871218)the Youth Innovation Promotion Association, CAS (No. 2018227)+1 种基金the Ling Chuang Research Project of China National Nuclear CorporationCNNC Science Fund for Talented Young Scholars
文摘FF sample(nearly free ofδ-ferrite)and CF sample(containing∼4%δ-ferrite)were prepared from the AISI 316 austenitic stainless steel plate to elucidate the role ofδ-ferrite in the fatigue crack growth under the solution treated and accelerated aged conditions.It is found that the fatigue crack growth resistance of the CF sample is higher than the FF sample under the solution treated condition.However,a significant deterioration of the fatigue crack growth resistance is observed in the CF sample while little variation is found in the FF sample after accelerated aging treatment at 750°C for 10 h.In the solution treated condition,deflected crack growth path is present when the main crack encounters theδ-ferrite in the CF sample due to the differences in the fatigue responses between austenite andδ-ferrite.The measured growth rate of the deflected crack is significantly slower than that of the flat crack of the same length.After the accelerated aging treatment,microcracks are produced at the M_(23)C_(6)/δinterface due to the strain incompatibility between M_(23)C_(6) and retainedδ-ferrite when the decomposedδ-ferrite is subject to plastic deformation in the crack tip plastic zone.The preexisting microcracks in the front of crack tip provide a viable path for crack propagation,resulting in the relatively flat crack path.
基金financially supported by the Opening Foundation of Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences (No. 2019NMSAKF04)China Institute of Atomic Energy (No. 2016DGB-I-KYSC-0024)。
文摘The microstructure evolution and mechanical properties of the 316 H stainless steel weld metals with different C contents were studied at the aging temperature of 550℃ for different aging holding time.The transformation behavior of δ-ferrite and precipitation mechanisms of M(23)C6 and σ phase in the as-aged weld metal were investigated.The results indicated that for the as-welded weld metal,with increasing C content,the yield and tensile strengths increased,while the elongation decreased owing to the increase of C solid solution strengthening effect.Moreover,both the high δ-ferrite content in low C weld metal and the precipitated M(23)C6 carbide in high C weld metal deteriorated the impact energy obviously.During the aging process,the rapid precipitation of M(23)C6 carbide occurred in σ-ferrite firstly owing to the high diffusion rate of C.Once the carbon is depleted by precipitation of M(23)C6,the slow formation of σ phase occurred through eutectoid transformation(δ→σ+γ) depending on the diffusion of Cr and Mo.Moreover,increasing C content promoted the formation of M(23)C6 carbides and inhibited the formation of σ phase.Therefore,increasing C content accelerated the transformation of σ ferrite in weld metal during aging process.Furthermore,after a long enough aging time,a transformation from M(23)C6 to σ occurred.The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions.For the low C weld metal aged at 550℃,with the increase of the aging time,fine M(23)C6 first precipitated,then coarsened,after that σ phase formed,which caused that the yield and tensile strengths first increased,then decreased,and finally increased slightly again.For the medium C weld metal,as the aging time increased,first the depletion of the solid solution C as a result the M(23)C6 precipitation deteriorated the strength,and then the formation of σ phase improved the strength.For the high C weld metal,with the increase of the aging time,the depletion of the solid solution C and the coarsening of the M(23)C6 precipitates deteriorated the strength.Furthermore,with increasing aging time,both the precipitation and coarsening of M(23)C6 and increasing σ phase content deteriorated the elongation and impact ene rgy.
文摘The morphology and the evolution of δ-ferrite existing in B410D slabs, hot-rolled plates, annealed plates and quenched plates were studied through metallographic observation. Results show that δ-ferrite forms during the solidification process and that it easily grows and increases in quantity during high temperature annealing. Band-shaped δ-ferrite in hotrolled plates is difficult to be eliminated by conventional heat treatment and hard to recrystallize.
基金This work was supported by the China Postdoctoral Science Foundation (No. 20060390150).
文摘It was presented the in situ observation of growth behavior and morphology of delta-ferrite as a function of solidification rate in an AISI304 stainless steel. The specimens have been solidified and observed using confocal scanning laser microscopy (CSLM). The δ-phase always appears like cells on the sample surface when critical supercooling occurs, during which the L→δ transformation starts. The solid-liquid (S-L) interface is found to be finger shaped and has no faceted shape. γ phase appears among δ grains due to partitioning of Ni into the melt during solidification, when solidification rate is higher. The mergence of observed δ cells is possible for the steel sample cooled at 7.5℃/min. The formation of dendrites can be observed on the free surface of the steel sample cooled at 150℃/min. The size of solidified delta grains decreases from 120 to 20-80μm, and the volume fraction of solidified austenite increases with increase in solidification rate from 7.5 to 150℃/min. The relation between the tip radius of δ cell and its growth rate is deduced, and the results agree with the experimental values.
基金supported by the Fundamental Research Funds for the Central Universities(Grant No.FRF-BD-25-001)Development and Application of Ultra-High Strength Hot Stamping Steel Strip for Automobiles(Grant No.20232BCJ22030)Manufacturing and Application Innovation and Integration of High-Safety Automotive Steel(Grant No.24431002D).
文摘The strength-ductility trade-off in low-Mn lightweight steels is a significant challenge due to the low thermal stability of austenite and the presence ofδ-ferrite.Two types of low-Mn lightweight steels containing V and NbVMo microalloying elements were developed by warm rolling.Among these,NbVMo steel demonstrated superior properties,achieving a tensile strength of~1.2 GPa and a product of strength and elongation exceeding 45 GPa%.In-depth mechanism analysis by atom probe tomography and quasi-in-situ electron backscatter diffraction revealed that different microalloying compositions influence the mechanical properties by strengtheningδ-ferrite,refining retained austenite and homogenizing matrix strain.In NbVMo steel,δ-ferrite strengthening is attributed to the synergistic effects of(V,Mo,Cr,Nb)C composite precipitation,fine NbC and MoC precipitates,and the solid solution strengthening of Mo.These mechanisms collectively contribute to a higher yield strength andδ-ferrite microhardness compared to V steel.Consequently,δ-ferrite and the surrounding matrix in NbVMo steel exhibit coordinated elongation during deformation,enhancing the ductility.The improved microstructural and strain uniformity in NbVMo steel mitigates stress concentration effects onδ-ferrite deformation and serves as a barrier that delays the transformation of retained austenite.In contrast,the retained austenite in V steel exhibits a blocky morphology with larger grain sizes,resulting in lower stability.Combined with localized stress concentrations due to non-uniform strain distribution,this leads to premature transformation of retained austenite to alleviate stress,ultimately impairing elongation and the continuity of strain hardening.Furthermore,the precipitation mechanisms of(V,Mo,Cr,Nb)C composite precipitates are elucidated.
基金financially supported by the Opening Foundation of CAS Key Laboratory of Nuclear Material and Safety Assessment under Grant No.2019NMSAKF04the China Institute of Atomic Energy under Grant No.2016-DGB-I-KYSC-0024。
文摘Two types of 316 butt welds with carbon contents of 0.016%and 0.062%have been produced using the gas tungsten arc welding process.Theδ-ferrite content decreased from 7.2 to 2.8%in volume as the carbon content increased.The creeprupture strength and creep ductility of the two types of weld metals have been measured at 550℃over the stress range of 290-316 MPa and at 600℃over 230-265 MPa.The microstructure change and precipitation behavior of the weld metals were observed and related to the creep rupture properties.The creep rupture strength of the C2(0.062%C)weld metal was higher than that of the Cl(0.016%C)weld metal at both 550℃and 600℃.At 550℃,as the decrease in the applied stress,the difference of the creep-rupture life between the two weld metals diminished due to the higher depletion rate of carbon by precipitation of M_(23)C_(6)in the C2 weld metal,while at 600℃,the difference enlarged due to the massive precipitation ofσphase and extensive crack formation and propagation alongσ/austenite boundaries in the C1 weld metal.For both the C1 and C2 weld metal,the decrease in ductility was adverse with the transformation percentage and related to products of theδ-ferrite transformation.
文摘In order to clarify the recrystallization mechanism of low-densityδ-ferrite steel Fe-4AI-2Ni,interrupted and single-pass compression tests were carried out.In this regard,five deformation temperatures(750-950 at an interval of 50°C)and different hold time were selected.It was observed that the softening and recrystallization fraction was enhaneed with increased deformation temperature and hold time.The original grain bounclaries were the preferred nucleation sites for recrystallized grains,and recrystallization had an impact on obtaining homogeneous and fine-grained structure.Recrystallization in the ferritic alloy commenced after a significant degree of softening,and the softening associated with recovery was appreciably less.The optimum rolling deformation temperature was identified to be greater than 900°C.
基金supported by the National Key Research and Development Program of China(2018YFA0702902)the Innovation Project of Shenyang National Laboratory for Materials Science(SYNL-2022)the China Institute of Atomic Energy(E141L803J1).
文摘The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ aging on the α'-martensitic transformation of the as-welded and the SHT weld metals were investigated.The results showed that the weld metal had poor thermal stability of austenite.The precipitation of NbC during the 850℃ SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α'-martensite.The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α'-martensite at the early stage of 550℃ aging.The homogenized diffusion of C and Cr in the matrix during 550℃ aging led to the restoration of austenitic thermal stability and the decrease of α'-martensite content.The C-depleted α'-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix,while the Cr-depleted α'-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.
基金supported by the National Magnetic Confinement Fusion Program (No. 2011GB108002)the National Natural Science Foundation of China (No. 51071021)
文摘Microstructures of casting samples of Fe-9%Cr steel and samples subjected to different heat treatments were investigated to determine their formation and evolution mechanism. The results show that there is no macroscopic segregation in the casting Fe-9%Cr steel. During cooling from solidification temperature to room temperature, 6-ferrite → austenite transformation is obviously influenced by cooling rate, while subsequent transformation of austenite does not obviously depend on the cooling rate. In the casting; samples, a great number of precipitates distribute inside martensitic laths while there are almost no precipitates inside δ-ferrite. When the casting samples were reheated to and isothermally held at 800 ℃, the original precipitates and the lath boundaries disappeared gradually. Meanwhile, new precipitates nucleate and grow at the prior lath boundaries.
文摘The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings.The 9Cr-1.5Mo-1Co cast steel,referred to as CB2,is one of the most promising alloys for various cast components to be used under ultrasupercritical conditions.In this study,HRTEM,SEM,and XRD methods were used for qualitative and quantitative analyses of contents,phases,and sizes of the inclusions and precipitates,as well as microstructure observation of the tempered martensitic steel at different cooling rates.Traditional tensile tests were conducted to characterize the material mechanical properties.Results show that the size of the boron nitride and precipitate,the width of the martensite lath and the content of theδ-ferrite are reduced as the cooling rate increases from 5-8℃·min^-1(CB2-S steel)to 50-60℃·min^-1(CB2-F steel).The precipitates are mainly composed of M23C6 and a small amount of M3C.The average diameters of the M23C6 precipitates in CB2-F and CB2-S are 202 nm and 209 nm,respectively.The inclusions are mainly composed of BN,Al2O3 and MnO2,and the inclusion ratios are 0.1969%for the CB2-F and 0.06556%for CB2-S steel.The average martensite lath widths of CB2-F and CB2-S steels are 289 nm and 301 nm,respectively.Furthermore,the M3C having a diameter of about 150 nm and a thickness of 20 nm is observed in theδ-ferrite of the tempered CB2-S steel.The presence of theδ-ferrite reduces the precipitation strengthening and dislocation density in CB2-S steel.In addition,the lower cooling rate stabilizes theδ-ferrite structure at room temperature.
文摘The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-phase structure and crevice corrosion.The three-dimensional structure of the leakage point was demonstrated using X-ray diffraction topography.The results of scanning electron microscope examination show the microstructure of the weld to be columnar and dendritic.It is found by electron probe microscope analysis and transmission electron microscopy that columnar dendrites consisted of γ-dendrite and an amount of δ-ferrite phases at the dendrite trunk.Simulated corrosion test results confirmed that the corrosion medium was the chloride ion.Crevice corrosion of chloride ions occurred at weld defects on the inner wall thus forming a concentration cell.Grains of columnar dendrites were then corroded by chloride ions and δ-ferrite phases on the grain boundaries were retained,which formed the particular skeleton corrosion structure.As a result,it led to leakage when the corrosion of weld occurred from the inner wall to the outer wall.
基金financially supported by the funds of the National Natural Science Foundation of China(Nos.52005357 and U1908224)the support of Postdoctoral Research Foundation of China(2022M712308)Liaoning Revitalization Talents Program(No.XLYC2007066).
文摘Press hardened steel(PHS)plays a key role in the manufacture of anti-collision structural components.The formation of δ-ferrite is a suffering issue for the laser welding of Al-Si coated PHS.Oscillating laser was used to weld Al-Si coated 1.5 GPa PHS and novel 2 GPa PHS,and the effect of laser offset on the microstructure and properties of the dissimilar welded joints was studied.The results showed that a perfect weld profile was achieved by laser offset welding(LOW),without any welding defects.The δ-ferrite formed in as-received welds of laser alignment welding(LAW)with high Al content(up to 2.9 wt.%),but it disappeared with the laser offset to 2 GPa PHS,and the maximum Al content in the segregation zone reduced to 1.2 wt.%.After post-welding heat treatment,the δ-ferrite was coarsened and theα-ferrite formed in the secondary Al-rich area for the high Ac3 temperature,but the α-ferrite was few and fine in LOW welds.The hardness in the LAW welds was lower than that in the LOW welds,due to the presence of δ-ferrite,as well as less carbon content and Ti and V alloying elements.The tensile strength(1561 MPa)and elongation(5.4%)with LOW were higher than those with LAW(1490 MPa,3.1%),and the fracture occurred in the Al-Si coated PHS.It is indicated that adjusting the laser offset is effective to prevent the formation of δ-ferrite and is potential to avoid the formation of α-ferrite.It also provides a wide heat treatment temperature window for the dissimilar welds of 1.5 GPa PHS and novel 2 GPa PHS.
