In the present work, N 2, N 2+H 2, Ar and Ar+H 2, were used as the sintering atmosphere of Metal Injection Molded 316L stainless steel respectively. The influences of the sintering atmospheres on C, O, N contents of t...In the present work, N 2, N 2+H 2, Ar and Ar+H 2, were used as the sintering atmosphere of Metal Injection Molded 316L stainless steel respectively. The influences of the sintering atmospheres on C, O, N contents of the sintered specimens, sintered density, grain morphology and mechanical properties were investigated. The results show that C, O, N contents of the sintered specimens can be controlled in permitted low values. The ultimate tensile strength and elongation of the specimen sintered in N 2+H 2 atmosphere are 765 MPa and 32% respectively. Using Ar and Ar+H 2 as the sintering atmosphere, the density of the sintered specimens is 98% of the theoretical density; the pores are uniformly distributed as small spherical shape and the grain size is about 50 μm. The mechanical properties of the specimen, i.e. ultimate tensile strength 630 MPa, yield strength 280 MPa, elongation 52%, HRB 71, are much better than those of the American Metal Powder Industries Federation(MPIF) 35 Standard after being sintered in Ar+H 2.展开更多
The mechanical behavior and microstructural evolution of an Fe-30Mn-3Al-3Si twinninginduced plasticity(TWIP)steel processed using warm forging was investigated.It is found that steel processed via warm forging improve...The mechanical behavior and microstructural evolution of an Fe-30Mn-3Al-3Si twinninginduced plasticity(TWIP)steel processed using warm forging was investigated.It is found that steel processed via warm forging improves comprehensive mechanical properties compared to the TWIP steel processed via cold rolling,with a high tensile strength(R_(m))of 793 MPa,a yield strength(R_(P))of 682 MPa,an extremely large R_(P)/R_(m)ratio as high as 0.86 as well as an excellent elongation rate of 46.8%.The microstructure observation demonstrates that steel processed by warm forging consists of large and elongated grains together with fine,equiaxed grains.Complicated micro-defect configurations were also observed within the steel,including dense dislocation networks and a few coarse deformation twins.As the plastic deformation proceeds,the densities of dislocations and deformation twins significantly increase.Moreover,a great number of slip lines could be observed in the elongated grains.These findings reveal that a much more dramatic interaction between microstructural defect and dislocations glide takes place in the forging sample,wherein the fine and equiaxed grains propagated dislocations more rapidly,together with initial defect configurations,are responsible for enhanced strength properties.Meanwhile,larger,elongated grains with more prevalently activated deformation twins result in high plasticity.展开更多
Three types of steels were designed on the basis of GX40CrNiSi25-12 austenitic heat resistant steel by adding different Mn contents(2wt.%,6wt.%,and 12wt.%).Thermodynamic calculation,microstructure characterization and...Three types of steels were designed on the basis of GX40CrNiSi25-12 austenitic heat resistant steel by adding different Mn contents(2wt.%,6wt.%,and 12wt.%).Thermodynamic calculation,microstructure characterization and mechanical property tests were conducted to investigate the effect of Mn addition on the microstructure and mechanical properties of the austenitic heat resistant steel.Results show that the matrix structure in all the three types of steels at room temperature is completely austenite.Carbides NbC and M_(23)C_(6)precipitate at grain boundaries of austenite matrix.With the increase of Mn content,the number of carbides increases and their distribution becomes more uniform.With the Mn content increases from 1.99%to 12.06%,the ultimate tensile strength,yield strength and elongation increase by 14.6%,8.0%and 46.3%,respectively.The improvement of the mechanical properties of austenitic steels can be explained by utilizing classic theories of alloy strengthening,including solid solution strengthening,precipitation strengthening,and grain refinement.The increase in alloy strength can be attributed to solid solution strengthening and precipitation strengthening caused by the addition of Mn.The improvement of the plasticity of austenitic steels can be explained from two aspects:grain refinement and homogenization of precipitated phases.展开更多
The microstructure and mechanical properties of titanium(Ti)-bearing medium-carbon nonquenched and tempered steel with different nitrogen content before and after hot forging were investigated through smelting,forging...The microstructure and mechanical properties of titanium(Ti)-bearing medium-carbon nonquenched and tempered steel with different nitrogen content before and after hot forging were investigated through smelting,forging,and laboratory tests.The results show that the grain size of nonquenched and tempered steel was gradually refined,and the ferrite content gradually increased with an increase in nitrogen content.The grain size of the material with low nitrogen content increased abnormally,and its impact properties significantly decreased after hot forging.The grain size of nonquenched and tempered steel with higher nitrogen content was slightly larger than that before forging,and the tensile and yield strength increased,but the impact toughness was not significantly reduced.The Ti-bearing nonquenched and tempered steel showed better strength and toughness after hot forging with the addition of 0.010%0.015%nitrogen.展开更多
A new thermomechanical process consisting of heavy cold rolling(HCR)and short-time heat treatment(STH)is developed to fabricate fine-grained martensite microstructure in a low-cost plain low-carbon steel.To achieve th...A new thermomechanical process consisting of heavy cold rolling(HCR)and short-time heat treatment(STH)is developed to fabricate fine-grained martensite microstructure in a low-cost plain low-carbon steel.To achieve the optimal mechanical properties after STH,three different ferrite-pearlite(F-P)dual-phase microstructures are prepared via hot rolling(HR),HR and austenitizing,and HR and HCR.The microstructure evolution and the comprehensive mechanical properties of the alloy during STH are then investigated.We find that the volume fractions of transformed martensite after STH increase with decreasing grain sizes of the pre-STH F-P dual phases.The rapid heating and short-time holding of STH promote grain nucleation and inhibit grain growth,resulting in microstructure refinement.The formation of martensites with different morphologies and different carbon concentrations in the HR and HCR+STH alloy is identified,owing to the inhomogeneous carbon distribution by STH.Tensile experiments demonstrate that STH greatly improves the comprehension mechanical properties of the alloy.Excellent mechanical properties,with a yield strength of 1224 MPa,a tensile strength of 1583 MPa,a uniform elongation of 4.0%and a total elongation of 7.3%are achieved in the HR and HCR+STH alloy.These excellent mechanical properties are principally attributed to the microstructure refinement and martensite formation induced by STH,with a yield strength improvement of 134%and a tensile strength improvement of 150%relative to the HR alloy.展开更多
The welding of aluminum(Al)and steel has attracted more and more interest due to the weight reduction trend in vehicle and aerospace manufacturing industries.5182-O/HC260YD+Z lap joint was produced by friction stir we...The welding of aluminum(Al)and steel has attracted more and more interest due to the weight reduction trend in vehicle and aerospace manufacturing industries.5182-O/HC260YD+Z lap joint was produced by friction stir welding(FSW),and the microstructure and mechanical property of the joint were systemically characterized.The microstructure in horizontal direction of the Al and steel near interface was similar to their corresponding conventional friction stir welded joint.The joint was divided into stir zone of Al(ST-Al),stir zone of interface(ST-I),thermal-mechanically affected zone of steel(TMAZ-Fe)and base material of steel(BM-Fe)according to their distinct microstructure vertically.Three kinds of intermetallic compounds(IMCs)of FeAl_(3),FeAl and Fe_(3)Al were formed at the interface.The horizontal micro hardness distribution exhibited a hat shape and“M”shape in Al and steel,respectively.The hardest region of the joint was located at the ST-I,with a hardness of 175 HV−210 HV.The joint was fractured along the hook structure,with an average shear strength of 73.9 MPa.Fractural morphology of Al and steel indicted a cleavage fracture mode.展开更多
Heterogeneous microstructure-induced mechanical responses in EH420 shipbuilding steel welded joint by electro-gas welding processed have been systematically studied by scanning electron microscopy,electron backscatter...Heterogeneous microstructure-induced mechanical responses in EH420 shipbuilding steel welded joint by electro-gas welding processed have been systematically studied by scanning electron microscopy,electron backscatter diffraction and mechanical testing.Comparing with the coarse-grained heat-affected zone(CGHAZ),the weld metal presents higher toughness(129.3 J vs.37.3 J)as it contains a large number of acicular ferrites with high-angle grain boundaries(frequency 79.2%)and special grain boundary∑3(frequency 55.3%).Moreover,coarse austenite grains in CGHAZ and slender martensite–austenite constituents between bainite laths may likely facilitate crack propagation.Polygonal ferrites and tempered pearlites formed at the junction of the fine-grained heat-affected zone and the intercritical heat-affected zone induced a softened zone with an average hardness of 185 HV0.5,which is the main reason for the occurrence of tensile fracture.展开更多
The effects of the microalloying element niobium(Nb) on the microstructure and mechanical properties of the hot stamped steel 38 Mn B5 were investigated. The impact of Nb addition on the microstructure was studied thr...The effects of the microalloying element niobium(Nb) on the microstructure and mechanical properties of the hot stamped steel 38 Mn B5 were investigated. The impact of Nb addition on the microstructure was studied through scanning electron microscopy(SEM), transmission electron microscopy(TEM), and X-ray diffraction(XRD). The experimental results indicated that the microstructures of the steel containing Nb were finer than those of the steel without Nb. Moreover, Nb mainly presented as a second-phase particle in 38MnB5 steel, and the particles included Nb carbonitrides. In addition, the tensile strength and elongation of the hot rolled and hot stamped steels were also measured, and they demonstrated that the appropriate addition of Nb was beneficial to the mechanical properties of 38 MnB5. Under the same conditions, the tensile strength of 38 MnB5 Nb was higher than that of 38 MnB5, which increased from 2011 to 2179 MPa. The yield strength also increased from 1316 to 1476 MPa, and the elongation increased from 5.92% to 6.64%. Overall, Nb had a positive effect on the performance of the hot stamped steel.展开更多
The tempering microstructure and mechanical properties of X80 steel used for heating-bent pipe were analyzed. The results show that the microstructure of X80 steel tempered at 550 ℃ and 600 ℃ is bainitic ferrite (BF...The tempering microstructure and mechanical properties of X80 steel used for heating-bent pipe were analyzed. The results show that the microstructure of X80 steel tempered at 550 ℃ and 600 ℃ is bainitic ferrite (BF)+granular bainite (GB), and partial ferrite laths in BF merge and broaden. The interior sub-lath boundary of some GB begins to disappear due to merging, the M/A constituent (a mixture of martensite plus retained austenite) in GB is orbicular. At the two tempering temperatures the tested X80 steel shows a certain degree of tempering stability. After being tempered at 650 ℃, the microstructure of X80 steel is GB+quasi-polygonal ferrite(QF), and the original BF laths have merged to form smaller GB crystal grains. The reason is that the steel shows better match of strength and toughness. After being tempered at 700 ℃ , the microstructure of X80 steel is composed mainly of QF, which can improve the plasticity but decline severely the yield strength of X80, and the M/A constituent assembles and grows up at the grain boundary of QF, resulting in excellent lower low-temperature toughness of X80.展开更多
Metastable 304 austenitic stainless steel was subjected to rolling at cryogenic and room temperatures, followed by annealing at different temperatures from 500 to 950°C. Phase transition during annealing was stud...Metastable 304 austenitic stainless steel was subjected to rolling at cryogenic and room temperatures, followed by annealing at different temperatures from 500 to 950°C. Phase transition during annealing was studied using X-ray diffractometry. Transmission electron microscopy and electron backscattered diffraction were used to characterize the martensite transformation and the distribution of austenite grain size after annealing. The recrystallization mechanism during cryogenic rolling was a reversal of martensite into austenite and austenite growth. Cryogenic rolling followed by annealing refined grains to 4.7 μm compared with 8.7 μm achieved under room-temperature rolling, as shown by the electron backscattered diffraction images. Tensile tests showed significantly improved mechanical properties after cryogenic rolling as the yield strength was enhanced by 47% compared with room-temperature rolling.展开更多
The mechanical properties and microstructure of the 3D-printed high Co–Ni secondary hardening steel fabricated by the laser melting deposition technique was investigated using a material testing machine and electron ...The mechanical properties and microstructure of the 3D-printed high Co–Ni secondary hardening steel fabricated by the laser melting deposition technique was investigated using a material testing machine and electron microscopy. A microstructure investigation revealed that the samples consist of martensite laths, fine dispersed precipitates, and reverted austenite films at the martensite lath boundaries. The precipitates are enriched with Co and Mo. Because the sample tempered at 486°C has smaller precipitates and a higher number of precipitates per unit area, it exhibits better mechanical properties than the sample tempered at 498°C. Although the 3D-printed samples have the same phase constituents as Aer Met 100 steel, the mechanical properties are slightly worse than those of the commercial wrought Aer Met 100 steel because of the presence of voids.展开更多
A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simula...A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel(DP600) was investigated using optical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength(YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient(n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength(UTS) and elongation(A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties(YS = 362 MPa, UTS = 638 MPa, A_(80) = 24.3%, n = 0.17) was obtained via process A.展开更多
The microstructure and mechanical properties of high nitrogen steel(HNS) weld metals prepared under air-and water-cooling conditions are investigated, and the effect of the cooling rate on these properties is discusse...The microstructure and mechanical properties of high nitrogen steel(HNS) weld metals prepared under air-and water-cooling conditions are investigated, and the effect of the cooling rate on these properties is discussed. The results indicate that an increase in the cooling rate could significantly increase the nitrogen content in HNS weld metals, especially for weld metals with a nitrogen content of 0.85%.Moreover, increasing the cooling rate could result in an increase in the tensile strength of HNS weld metals, which is found to be strongly dependent on the nitrogen content of the HNS sample. For high nitrogen austenitic stainless steel welding wire with lower nitrogen content, increasing the cooling rate could significantly improve its tensile strength, but a higher cooling rate has no influence on weld metals with nitrogen content less than 0.58%. The tensile strength of the joint reached 850 MPa.展开更多
The characteristics of microstructure, mechanical property and corrosion behavior of Cr26Mo3.5 super stainless steel joints by pulse tungsten inert gas(P-TIG)welding and laser welding were investigated. The results in...The characteristics of microstructure, mechanical property and corrosion behavior of Cr26Mo3.5 super stainless steel joints by pulse tungsten inert gas(P-TIG)welding and laser welding were investigated. The results indicate that the widths of the center equiaxed grain zone(EGZ)and the columnar grain zone(CGZ)increase with the increase of heat input in both welding processes. The precipitates of Nb and Ti carbides and nitrides are formed in the weld metal(WM)and the heat affected zone(HAZ). The joints by laser welding show better tensile and corrosion resistance properties than those by P-TIG welding due to the heat concentration and lower heat input. The tensile strength and elongation increase with the decrease of heat input, and the fracture mode of the joints turns into ductile-brittle mixed fracture from ductile fracture when the welding method turns into P-TIG welding from laser welding. Moreover, the corrosion resistance of all joints declines slightly with the increase of heat input. Hence, laser welding is more suitable for welding Cr26Mo3.5 super stainless steel in engineering applications.展开更多
The microstructure and mechanical properties of cast inserted dies for automobile covering components were studied. The results show that the as-cast microstructures of cast inserted dies are composed of pearlite, mar...The microstructure and mechanical properties of cast inserted dies for automobile covering components were studied. The results show that the as-cast microstructures of cast inserted dies are composed of pearlite, martensite, bainite, and austenite; and that the annealed microstructure is granular pearlite. The mechanical properties of cast inserted dies approach that of forged inserted dies. The tensile strength is 855 MPa, the elongation is 16%, the impact toughness is 177 J/cm2, and the hardness after annealing and quenching are HRC 19 and HRC 60-62. In addition, the cast inserted dies have good hardenability. The depth of the hardening zone and the hardness after flame quenching satisfy the operating requirements. The cast inserted dies could completely replace the forged inserted dies for making the dies of automobile covering components.展开更多
The low carbon Nb-Ti microalloyed tested steel was prepared by the process of vacuum induction furnace smelting,forging and hot rolling.The new steel aims to meet the demand of high strength,high toughness and high pl...The low carbon Nb-Ti microalloyed tested steel was prepared by the process of vacuum induction furnace smelting,forging and hot rolling.The new steel aims to meet the demand of high strength,high toughness and high plasticity for building facilities.The effects of quenching process on microstructure and mechanical properties of tested steel were investigated.The results showed that prior austenite grain size,phase type and precipitation behavior of(Nb,Ti)(C,N)play important roles in mechanical properties of the steel.Through modified appropriately,the model of austenite grain growth during heating and holding is d^(5.7778)=5.6478^(5.7778)+7.04×10^(22)t^(1.6136)exp(-427.15×10~3/(RT)).The grain growth activation energy is Q_g=427.15 kJ.During quenching,the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations.The content of phases,fine and coarsening(Nb,Ti)(C,N)precipitated changes during different quenching temperatures and holding time.Finally compared with the hardness value,the best quenching process can be obtained that heating temperature and holding time are900℃and 50 mins,respectively.展开更多
The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone(HAZ). In this work, the boron steel...The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone(HAZ). In this work, the boron steel and Q235 steel were selected for laser tailor welding,which obtained boron/Q235 steel tailor-welded blanks(TWBs). The method of welding with synchronous thermal field(WSTF) was utilized to eliminate the mismatch effects in TWBs. The WSTF was employed to adjust cooling rates of welded joints, thereby intervening in the solidification behaviors and phase transition of the molten pool. Boron/Q235 steel was welded by laser under conventional and WSTF(300-600 ℃) conditions, respectively. The results show that the microstructure of weld and HAZ(boron) was adequately transitioned to ferrites and pearlites instead of abundant martensite by WSTF. Meanwhile, the discrepancy of microhardness and yield strength between various regions of welded joints was greatly reduced, and the overall plasticity of welded joints was enhanced by WSTF. It is indicated that WSTF can effectively contribute to reducing plastic gradient and achieving mechanical congruity in welded joints by restraining the generation of hardbrittle phase, which could significantly improve the formability of TWBs in subsequent hot stamping.展开更多
Nitrogen can increase the strength of steels without weakening the toughness and improve the corrosion resistance at the same time. Compared with conventional nitrogen-free die steels, a new type of nitrogen-containin...Nitrogen can increase the strength of steels without weakening the toughness and improve the corrosion resistance at the same time. Compared with conventional nitrogen-free die steels, a new type of nitrogen-containing die steel was developed with many superior properties, such as high strength, high hardness, and good toughness. This paper focused on the effects of heat treatment on the microstruc-tures and mechanical properties of the new type of nitrogen-containing die steel, which were investigated by the optimized deformation process and heat treatment. Isothermal spheroidal annealing and high-temperature quenching as well as high-temperature tempering were ap-plied in the experiment by means of an orthogonal method after the steel was multiply forged. The mechanical properties of nitro-gen-containing die steel forgings are better than the standard of NADCA #207-2003.展开更多
Thermomechanical experiments were carried out to reproduce the hot stamping process and to investigate the effects of process parameters on the microstructure and mechanical properties of stamped parts. The process pa...Thermomechanical experiments were carried out to reproduce the hot stamping process and to investigate the effects of process parameters on the microstructure and mechanical properties of stamped parts. The process parameters, such as austenitizing temperature,soaking time, initial deformation temperature and cooling rate, are studied. The resulting microstructures of specimens were observed and analyzed. To evaluate the mechanical properties of specimens, tensile and hardness tests were also performed at room temperature. The optimum parameters to achieve the highest tensile strength and the desired microstructure were acquired by comparing and analyzing the results. It is indicated that hot deformation changes the transformation characteristics of 22MnB5 steel. Austenite deformation promotes the austenite-to-ferrite transformation and elevates the critical cooling rate to induce a fully martensitic transformation.展开更多
文摘In the present work, N 2, N 2+H 2, Ar and Ar+H 2, were used as the sintering atmosphere of Metal Injection Molded 316L stainless steel respectively. The influences of the sintering atmospheres on C, O, N contents of the sintered specimens, sintered density, grain morphology and mechanical properties were investigated. The results show that C, O, N contents of the sintered specimens can be controlled in permitted low values. The ultimate tensile strength and elongation of the specimen sintered in N 2+H 2 atmosphere are 765 MPa and 32% respectively. Using Ar and Ar+H 2 as the sintering atmosphere, the density of the sintered specimens is 98% of the theoretical density; the pores are uniformly distributed as small spherical shape and the grain size is about 50 μm. The mechanical properties of the specimen, i.e. ultimate tensile strength 630 MPa, yield strength 280 MPa, elongation 52%, HRB 71, are much better than those of the American Metal Powder Industries Federation(MPIF) 35 Standard after being sintered in Ar+H 2.
基金Funded by the National Natural Science Foundation of China(Nos.51701206 and 51671187)the Shanxi Natural Science Foundation(No.2019JQ-833)+2 种基金the Anhui Natural Science Foundation(1808085QE166)the Special Scientific Research Project of Shanxi Education Committee(No.19JQ0974)the Doctoral Research Initiation Project of Yan’an University(No.YDBD2018-21)。
文摘The mechanical behavior and microstructural evolution of an Fe-30Mn-3Al-3Si twinninginduced plasticity(TWIP)steel processed using warm forging was investigated.It is found that steel processed via warm forging improves comprehensive mechanical properties compared to the TWIP steel processed via cold rolling,with a high tensile strength(R_(m))of 793 MPa,a yield strength(R_(P))of 682 MPa,an extremely large R_(P)/R_(m)ratio as high as 0.86 as well as an excellent elongation rate of 46.8%.The microstructure observation demonstrates that steel processed by warm forging consists of large and elongated grains together with fine,equiaxed grains.Complicated micro-defect configurations were also observed within the steel,including dense dislocation networks and a few coarse deformation twins.As the plastic deformation proceeds,the densities of dislocations and deformation twins significantly increase.Moreover,a great number of slip lines could be observed in the elongated grains.These findings reveal that a much more dramatic interaction between microstructural defect and dislocations glide takes place in the forging sample,wherein the fine and equiaxed grains propagated dislocations more rapidly,together with initial defect configurations,are responsible for enhanced strength properties.Meanwhile,larger,elongated grains with more prevalently activated deformation twins result in high plasticity.
基金supported by the National Natural Science Foundation of China(Grant No.52275370)the Key R&D Program of Hubei Province,China(Grant Nos.2022BAD100,2021BAA048)the Open Fund of Hubei Longzhong Laboratory(Grant No.2022ZZ-04).
文摘Three types of steels were designed on the basis of GX40CrNiSi25-12 austenitic heat resistant steel by adding different Mn contents(2wt.%,6wt.%,and 12wt.%).Thermodynamic calculation,microstructure characterization and mechanical property tests were conducted to investigate the effect of Mn addition on the microstructure and mechanical properties of the austenitic heat resistant steel.Results show that the matrix structure in all the three types of steels at room temperature is completely austenite.Carbides NbC and M_(23)C_(6)precipitate at grain boundaries of austenite matrix.With the increase of Mn content,the number of carbides increases and their distribution becomes more uniform.With the Mn content increases from 1.99%to 12.06%,the ultimate tensile strength,yield strength and elongation increase by 14.6%,8.0%and 46.3%,respectively.The improvement of the mechanical properties of austenitic steels can be explained by utilizing classic theories of alloy strengthening,including solid solution strengthening,precipitation strengthening,and grain refinement.The increase in alloy strength can be attributed to solid solution strengthening and precipitation strengthening caused by the addition of Mn.The improvement of the plasticity of austenitic steels can be explained from two aspects:grain refinement and homogenization of precipitated phases.
文摘The microstructure and mechanical properties of titanium(Ti)-bearing medium-carbon nonquenched and tempered steel with different nitrogen content before and after hot forging were investigated through smelting,forging,and laboratory tests.The results show that the grain size of nonquenched and tempered steel was gradually refined,and the ferrite content gradually increased with an increase in nitrogen content.The grain size of the material with low nitrogen content increased abnormally,and its impact properties significantly decreased after hot forging.The grain size of nonquenched and tempered steel with higher nitrogen content was slightly larger than that before forging,and the tensile and yield strength increased,but the impact toughness was not significantly reduced.The Ti-bearing nonquenched and tempered steel showed better strength and toughness after hot forging with the addition of 0.010%0.015%nitrogen.
基金funded by the National Natural Science Foundation of China(No.52071212).
文摘A new thermomechanical process consisting of heavy cold rolling(HCR)and short-time heat treatment(STH)is developed to fabricate fine-grained martensite microstructure in a low-cost plain low-carbon steel.To achieve the optimal mechanical properties after STH,three different ferrite-pearlite(F-P)dual-phase microstructures are prepared via hot rolling(HR),HR and austenitizing,and HR and HCR.The microstructure evolution and the comprehensive mechanical properties of the alloy during STH are then investigated.We find that the volume fractions of transformed martensite after STH increase with decreasing grain sizes of the pre-STH F-P dual phases.The rapid heating and short-time holding of STH promote grain nucleation and inhibit grain growth,resulting in microstructure refinement.The formation of martensites with different morphologies and different carbon concentrations in the HR and HCR+STH alloy is identified,owing to the inhomogeneous carbon distribution by STH.Tensile experiments demonstrate that STH greatly improves the comprehension mechanical properties of the alloy.Excellent mechanical properties,with a yield strength of 1224 MPa,a tensile strength of 1583 MPa,a uniform elongation of 4.0%and a total elongation of 7.3%are achieved in the HR and HCR+STH alloy.These excellent mechanical properties are principally attributed to the microstructure refinement and martensite formation induced by STH,with a yield strength improvement of 134%and a tensile strength improvement of 150%relative to the HR alloy.
文摘The welding of aluminum(Al)and steel has attracted more and more interest due to the weight reduction trend in vehicle and aerospace manufacturing industries.5182-O/HC260YD+Z lap joint was produced by friction stir welding(FSW),and the microstructure and mechanical property of the joint were systemically characterized.The microstructure in horizontal direction of the Al and steel near interface was similar to their corresponding conventional friction stir welded joint.The joint was divided into stir zone of Al(ST-Al),stir zone of interface(ST-I),thermal-mechanically affected zone of steel(TMAZ-Fe)and base material of steel(BM-Fe)according to their distinct microstructure vertically.Three kinds of intermetallic compounds(IMCs)of FeAl_(3),FeAl and Fe_(3)Al were formed at the interface.The horizontal micro hardness distribution exhibited a hat shape and“M”shape in Al and steel,respectively.The hardest region of the joint was located at the ST-I,with a hardness of 175 HV−210 HV.The joint was fractured along the hook structure,with an average shear strength of 73.9 MPa.Fractural morphology of Al and steel indicted a cleavage fracture mode.
基金the National Natural Science Foundation of China(Grant Nos.U20A20277,51861130361,51861145312,51850410522,5201101443,and 52011530180)the Newton Advanced Fellowship by Royal Society(Grant No.RP12G0414)+5 种基金the Royal Academy of Engineering(No.TSPC1070)the Special Fund for Key Program of Science and Technology of Liaoning Province(Grant No.2019JH1/101000014)the Research Fund for Central Universities(Grant Nos.N172502004 and N2025025)the Xingliao Talents Program(Nos.XLYC1807024 and XLYC1802024)the Regional Innovation Joint Fund of Liaoning Province(No.2020-YKLH-39)funded in part by the National Research Foundation of South Africa(No.BRICS171211293679)。
文摘Heterogeneous microstructure-induced mechanical responses in EH420 shipbuilding steel welded joint by electro-gas welding processed have been systematically studied by scanning electron microscopy,electron backscatter diffraction and mechanical testing.Comparing with the coarse-grained heat-affected zone(CGHAZ),the weld metal presents higher toughness(129.3 J vs.37.3 J)as it contains a large number of acicular ferrites with high-angle grain boundaries(frequency 79.2%)and special grain boundary∑3(frequency 55.3%).Moreover,coarse austenite grains in CGHAZ and slender martensite–austenite constituents between bainite laths may likely facilitate crack propagation.Polygonal ferrites and tempered pearlites formed at the junction of the fine-grained heat-affected zone and the intercritical heat-affected zone induced a softened zone with an average hardness of 185 HV0.5,which is the main reason for the occurrence of tensile fracture.
文摘The effects of the microalloying element niobium(Nb) on the microstructure and mechanical properties of the hot stamped steel 38 Mn B5 were investigated. The impact of Nb addition on the microstructure was studied through scanning electron microscopy(SEM), transmission electron microscopy(TEM), and X-ray diffraction(XRD). The experimental results indicated that the microstructures of the steel containing Nb were finer than those of the steel without Nb. Moreover, Nb mainly presented as a second-phase particle in 38MnB5 steel, and the particles included Nb carbonitrides. In addition, the tensile strength and elongation of the hot rolled and hot stamped steels were also measured, and they demonstrated that the appropriate addition of Nb was beneficial to the mechanical properties of 38 MnB5. Under the same conditions, the tensile strength of 38 MnB5 Nb was higher than that of 38 MnB5, which increased from 2011 to 2179 MPa. The yield strength also increased from 1316 to 1476 MPa, and the elongation increased from 5.92% to 6.64%. Overall, Nb had a positive effect on the performance of the hot stamped steel.
基金Project(6990-HT-XEX-03-(2008)-0137) supported by the Major Special Project of Science and Technology of China National Petroleum Corporation
文摘The tempering microstructure and mechanical properties of X80 steel used for heating-bent pipe were analyzed. The results show that the microstructure of X80 steel tempered at 550 ℃ and 600 ℃ is bainitic ferrite (BF)+granular bainite (GB), and partial ferrite laths in BF merge and broaden. The interior sub-lath boundary of some GB begins to disappear due to merging, the M/A constituent (a mixture of martensite plus retained austenite) in GB is orbicular. At the two tempering temperatures the tested X80 steel shows a certain degree of tempering stability. After being tempered at 650 ℃, the microstructure of X80 steel is GB+quasi-polygonal ferrite(QF), and the original BF laths have merged to form smaller GB crystal grains. The reason is that the steel shows better match of strength and toughness. After being tempered at 700 ℃ , the microstructure of X80 steel is composed mainly of QF, which can improve the plasticity but decline severely the yield strength of X80, and the M/A constituent assembles and grows up at the grain boundary of QF, resulting in excellent lower low-temperature toughness of X80.
基金financially supported by the National Key Project of Research and Development Program of China (No. 2016YFB0300801)the National Natural Science Foundation of China (No. 51401016)State Key Laboratory for Advanced Metals and Materials of China
文摘Metastable 304 austenitic stainless steel was subjected to rolling at cryogenic and room temperatures, followed by annealing at different temperatures from 500 to 950°C. Phase transition during annealing was studied using X-ray diffractometry. Transmission electron microscopy and electron backscattered diffraction were used to characterize the martensite transformation and the distribution of austenite grain size after annealing. The recrystallization mechanism during cryogenic rolling was a reversal of martensite into austenite and austenite growth. Cryogenic rolling followed by annealing refined grains to 4.7 μm compared with 8.7 μm achieved under room-temperature rolling, as shown by the electron backscattered diffraction images. Tensile tests showed significantly improved mechanical properties after cryogenic rolling as the yield strength was enhanced by 47% compared with room-temperature rolling.
文摘The mechanical properties and microstructure of the 3D-printed high Co–Ni secondary hardening steel fabricated by the laser melting deposition technique was investigated using a material testing machine and electron microscopy. A microstructure investigation revealed that the samples consist of martensite laths, fine dispersed precipitates, and reverted austenite films at the martensite lath boundaries. The precipitates are enriched with Co and Mo. Because the sample tempered at 486°C has smaller precipitates and a higher number of precipitates per unit area, it exhibits better mechanical properties than the sample tempered at 498°C. Although the 3D-printed samples have the same phase constituents as Aer Met 100 steel, the mechanical properties are slightly worse than those of the commercial wrought Aer Met 100 steel because of the presence of voids.
基金financially supported by the National Natural Science Foundation of China (Nos.U1360202,51472030,and 51502014)
文摘A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel(DP600) was investigated using optical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength(YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient(n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength(UTS) and elongation(A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties(YS = 362 MPa, UTS = 638 MPa, A_(80) = 24.3%, n = 0.17) was obtained via process A.
文摘The microstructure and mechanical properties of high nitrogen steel(HNS) weld metals prepared under air-and water-cooling conditions are investigated, and the effect of the cooling rate on these properties is discussed. The results indicate that an increase in the cooling rate could significantly increase the nitrogen content in HNS weld metals, especially for weld metals with a nitrogen content of 0.85%.Moreover, increasing the cooling rate could result in an increase in the tensile strength of HNS weld metals, which is found to be strongly dependent on the nitrogen content of the HNS sample. For high nitrogen austenitic stainless steel welding wire with lower nitrogen content, increasing the cooling rate could significantly improve its tensile strength, but a higher cooling rate has no influence on weld metals with nitrogen content less than 0.58%. The tensile strength of the joint reached 850 MPa.
基金Supported by the Research Institute,Baoshan Iron&Steel Co.,Ltd
文摘The characteristics of microstructure, mechanical property and corrosion behavior of Cr26Mo3.5 super stainless steel joints by pulse tungsten inert gas(P-TIG)welding and laser welding were investigated. The results indicate that the widths of the center equiaxed grain zone(EGZ)and the columnar grain zone(CGZ)increase with the increase of heat input in both welding processes. The precipitates of Nb and Ti carbides and nitrides are formed in the weld metal(WM)and the heat affected zone(HAZ). The joints by laser welding show better tensile and corrosion resistance properties than those by P-TIG welding due to the heat concentration and lower heat input. The tensile strength and elongation increase with the decrease of heat input, and the fracture mode of the joints turns into ductile-brittle mixed fracture from ductile fracture when the welding method turns into P-TIG welding from laser welding. Moreover, the corrosion resistance of all joints declines slightly with the increase of heat input. Hence, laser welding is more suitable for welding Cr26Mo3.5 super stainless steel in engineering applications.
文摘The microstructure and mechanical properties of cast inserted dies for automobile covering components were studied. The results show that the as-cast microstructures of cast inserted dies are composed of pearlite, martensite, bainite, and austenite; and that the annealed microstructure is granular pearlite. The mechanical properties of cast inserted dies approach that of forged inserted dies. The tensile strength is 855 MPa, the elongation is 16%, the impact toughness is 177 J/cm2, and the hardness after annealing and quenching are HRC 19 and HRC 60-62. In addition, the cast inserted dies have good hardenability. The depth of the hardening zone and the hardness after flame quenching satisfy the operating requirements. The cast inserted dies could completely replace the forged inserted dies for making the dies of automobile covering components.
基金Sponsored by the Major State Basic Research Development Program of China(Grant No.2010CB630801)
文摘The low carbon Nb-Ti microalloyed tested steel was prepared by the process of vacuum induction furnace smelting,forging and hot rolling.The new steel aims to meet the demand of high strength,high toughness and high plasticity for building facilities.The effects of quenching process on microstructure and mechanical properties of tested steel were investigated.The results showed that prior austenite grain size,phase type and precipitation behavior of(Nb,Ti)(C,N)play important roles in mechanical properties of the steel.Through modified appropriately,the model of austenite grain growth during heating and holding is d^(5.7778)=5.6478^(5.7778)+7.04×10^(22)t^(1.6136)exp(-427.15×10~3/(RT)).The grain growth activation energy is Q_g=427.15 kJ.During quenching,the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations.The content of phases,fine and coarsening(Nb,Ti)(C,N)precipitated changes during different quenching temperatures and holding time.Finally compared with the hardness value,the best quenching process can be obtained that heating temperature and holding time are900℃and 50 mins,respectively.
基金the Natural Science Foundation of Fujian Province(2021J01299)school-enterprise cooperation project supported by Shandong Hongao Automotive Lightweight Technology Co.,Ltd.
文摘The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone(HAZ). In this work, the boron steel and Q235 steel were selected for laser tailor welding,which obtained boron/Q235 steel tailor-welded blanks(TWBs). The method of welding with synchronous thermal field(WSTF) was utilized to eliminate the mismatch effects in TWBs. The WSTF was employed to adjust cooling rates of welded joints, thereby intervening in the solidification behaviors and phase transition of the molten pool. Boron/Q235 steel was welded by laser under conventional and WSTF(300-600 ℃) conditions, respectively. The results show that the microstructure of weld and HAZ(boron) was adequately transitioned to ferrites and pearlites instead of abundant martensite by WSTF. Meanwhile, the discrepancy of microhardness and yield strength between various regions of welded joints was greatly reduced, and the overall plasticity of welded joints was enhanced by WSTF. It is indicated that WSTF can effectively contribute to reducing plastic gradient and achieving mechanical congruity in welded joints by restraining the generation of hardbrittle phase, which could significantly improve the formability of TWBs in subsequent hot stamping.
基金supported by the National Natural Science Foundation of China (Nos.50974014 and 51174026)
文摘Nitrogen can increase the strength of steels without weakening the toughness and improve the corrosion resistance at the same time. Compared with conventional nitrogen-free die steels, a new type of nitrogen-containing die steel was developed with many superior properties, such as high strength, high hardness, and good toughness. This paper focused on the effects of heat treatment on the microstruc-tures and mechanical properties of the new type of nitrogen-containing die steel, which were investigated by the optimized deformation process and heat treatment. Isothermal spheroidal annealing and high-temperature quenching as well as high-temperature tempering were ap-plied in the experiment by means of an orthogonal method after the steel was multiply forged. The mechanical properties of nitro-gen-containing die steel forgings are better than the standard of NADCA #207-2003.
基金financially supported by the Research Fund for the Doctoral Program of Higher Education,China(No.20120006110017)
文摘Thermomechanical experiments were carried out to reproduce the hot stamping process and to investigate the effects of process parameters on the microstructure and mechanical properties of stamped parts. The process parameters, such as austenitizing temperature,soaking time, initial deformation temperature and cooling rate, are studied. The resulting microstructures of specimens were observed and analyzed. To evaluate the mechanical properties of specimens, tensile and hardness tests were also performed at room temperature. The optimum parameters to achieve the highest tensile strength and the desired microstructure were acquired by comparing and analyzing the results. It is indicated that hot deformation changes the transformation characteristics of 22MnB5 steel. Austenite deformation promotes the austenite-to-ferrite transformation and elevates the critical cooling rate to induce a fully martensitic transformation.