The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment.During the isothermal aging stage,the amount of a-phase was significant...The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment.During the isothermal aging stage,the amount of a-phase was significantly increased at beginning and gradually became constant.The aging time only slightly affected the size and morphology of the σ-phase.Conversely,during the isochronal aging stage,the amount ofσ-phase grew rapidly with the increase in the aging temperature.The σ-phase with a large amount of stacking faults was prone to nucleate around the（Ti,Mo）C particles or at the grain boundaries.The（Ti,Mo）C particles can act as effective nucleation sites,where the σ-phase directly precipitates from the austenitic matrix.From this work,the growth of σ-phase is found to be influenced by the aging temperature,and a new mechanism of σ-phase precipitation from austenite has been proposed.展开更多
The precipitation and evolution of secondary nitrides （S-nitrides） in the Alloy 690 electro-slag remelted （ESR） structure were investigated. Experimental results indicate that S-nitrides precipitated in the interd...The precipitation and evolution of secondary nitrides （S-nitrides） in the Alloy 690 electro-slag remelted （ESR） structure were investigated. Experimental results indicate that S-nitrides precipitated in the interdendritic region of the ESR structure at temperatures higher than 1100 ℃. S-nitrides could spread throughout the entire interdendritic area after sufficient exposure, and they were more numerous and much finer than primary nitrides. Furthermore, after studying the evolution of S-nitride particles at 1100, 1200 and 1300℃, it was determined that the precipitation of S-nitrides was controlled by the diffusion of nitrogen. In addition, by investigating the elemental segregation of ESR structure and calculating critical Ti concentrations, S-nitride precipitation was found to be thermodynamically inevitable in the Alloy 690 ESR structure.展开更多
The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied.The microstructure of the alloy without Co(K4750 alloy)and the alloy containing Co(K4750-Co alloy)w...The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied.The microstructure of the alloy without Co(K4750 alloy)and the alloy containing Co(K4750-Co alloy)were analyzed.Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation ofηphase during long-term aging treatment.In K4750-Co alloy,the morphology of MC carbide at the grain boundary(GB)remained dispersed blocky shape and noηphase was observed after aging at 750℃ for 3000 h.However,in K4750 alloy,almost all the MC carbides at GBs broke down into granular M23C6 carbide and needle-likeηphase.The addition of cobalt could delay the decomposition of MC carbides,which accordingly restricted the elemental supply for the formation ofηphase.The stress rupture tests were conducted on two alloys at 750℃/430 MPa.When Co is substituted for Fe in K4750 alloy,the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment.This was mainly attributed to increased concentration of Al,Ti and Nb inγ’phase in K4750-Co alloy,which further enhanced the strengthening effect ofγ’phase.After aging at 750℃ for 3000 h,substitution of Co for Fe can also cause the stress rupture life at 750℃/430 MPa to increase from 48.72 to 208.18 h.The reason was mainly because MC carbide degradation andηphase precipitation in K4750 alloy,which promoted the initiation and propagation of micro-crack during stress rupture testing.展开更多
This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750°C-1000°C in air for 100 h-1000 h by isothermal oxidation tests.The oxidation-kinetic curves were plotted by th...This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750°C-1000°C in air for 100 h-1000 h by isothermal oxidation tests.The oxidation-kinetic curves were plotted by the static discontinuous weight gain method.Observation and identification of oxidation products were conducted using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),electron probe micro-analysis(EPMA)and X-ray diffraction(XRD).X-ray photoelectron spectrometer(XPS)was also used to analyze the chemical state of elements and the distribution in depth.The results showed that the oxidation-kinetic curves of K4750 alloy basically obeyed the parabolic law.The average oxidation rate below 900°C was less than 0.1 g/m 2·h which meant the alloy was at a complete anti-oxidation grade,and the alloy was at an anti-oxidation grade at 1000°C.The predominant surface oxide was Cr_(2)O_(3),and a double layer structure of the oxide scale was observed at all tested temperatures as time increased.The outer oxide layer contained continuous Cr_(2)O_(3)and a small amount of oxides mixed TiO2 and NiCr2O4,while the inner oxide layer was composed with Al_(2)O_(3).The oxidation process could be interpreted by the competitive oxidation of different elements.The diffusion rate of Ti through Cr_(2)O_(3)layer increased with increasing temperature,and thus the generation of TiO2 was advantageous.The dispersed TiO2 reaching a certain amount destroyed the continuity of the surface oxide layer,which accounted for the reduction of oxidation resistance of K4750 alloy at high temperatures.展开更多
To verify the microalloying function and segregation behavior of trace Mg at grain boundary in steel,the 2.25Cr–1Mo steel doped with 0.056%P containing different Mg contents was refined with a vacuum-induction furnac...To verify the microalloying function and segregation behavior of trace Mg at grain boundary in steel,the 2.25Cr–1Mo steel doped with 0.056%P containing different Mg contents was refined with a vacuum-induction furnace.The effects of trace Mg addition on the temper embrittlement susceptibility of 2.25Cr–1Mo steel were studied by step-cooling test and the segregation behavior of Mg at grain boundary was explored by Auger electron spectroscopy.It is shown that P-induced temper embrittlement susceptibility can be reduced after subjecting to step-cooling treatment with trace Mg addition,mainly benefited from the segregation of Mg at grain boundary.This segregation can decrease the segregation amounts of P and S,especially for P,and increase the grain boundary cohesion,reducing the adverse effect on temper embrittlement caused by P and S.展开更多
Effects of silicon on the microstructure and mechanical properties of 20% cold-worked 15-15 Ti austenitic stainless steels are systemically investigated by uniaxial tensile tests,scanning and transmission electron mic...Effects of silicon on the microstructure and mechanical properties of 20% cold-worked 15-15 Ti austenitic stainless steels are systemically investigated by uniaxial tensile tests,scanning and transmission electron microscope observations,and strength calculations.The results reveal that a large number of deformation twins are formed in the 20% cold-worked steels with various silicon additions.The volume fraction of deformation twins and the density of dislocations increase with silicon content,while the twin thickness slightly decreases.A better strength-ductility combination is achieved by silicon addition,since the yield strength of the steel with 2% silicon is 61 MPa higher than that of the steel with 1% silicon,while their uniform elongations are almost both equal to 16%.The yield strength of the 15-15 Ti stainless steels is predominantly contributed by the solid solution,dislocation and deformation twin strengthening effects,which can be enhanced by silicon addition.展开更多
文摘The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment.During the isothermal aging stage,the amount of a-phase was significantly increased at beginning and gradually became constant.The aging time only slightly affected the size and morphology of the σ-phase.Conversely,during the isochronal aging stage,the amount ofσ-phase grew rapidly with the increase in the aging temperature.The σ-phase with a large amount of stacking faults was prone to nucleate around the（Ti,Mo）C particles or at the grain boundaries.The（Ti,Mo）C particles can act as effective nucleation sites,where the σ-phase directly precipitates from the austenitic matrix.From this work,the growth of σ-phase is found to be influenced by the aging temperature,and a new mechanism of σ-phase precipitation from austenite has been proposed.
基金supported by the Chinese Academy of Sciences Innovation Foundation under Grant No. CXJJ14-M44
文摘The precipitation and evolution of secondary nitrides （S-nitrides） in the Alloy 690 electro-slag remelted （ESR） structure were investigated. Experimental results indicate that S-nitrides precipitated in the interdendritic region of the ESR structure at temperatures higher than 1100 ℃. S-nitrides could spread throughout the entire interdendritic area after sufficient exposure, and they were more numerous and much finer than primary nitrides. Furthermore, after studying the evolution of S-nitride particles at 1100, 1200 and 1300℃, it was determined that the precipitation of S-nitrides was controlled by the diffusion of nitrogen. In addition, by investigating the elemental segregation of ESR structure and calculating critical Ti concentrations, S-nitride precipitation was found to be thermodynamically inevitable in the Alloy 690 ESR structure.
文摘The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied.The microstructure of the alloy without Co(K4750 alloy)and the alloy containing Co(K4750-Co alloy)were analyzed.Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation ofηphase during long-term aging treatment.In K4750-Co alloy,the morphology of MC carbide at the grain boundary(GB)remained dispersed blocky shape and noηphase was observed after aging at 750℃ for 3000 h.However,in K4750 alloy,almost all the MC carbides at GBs broke down into granular M23C6 carbide and needle-likeηphase.The addition of cobalt could delay the decomposition of MC carbides,which accordingly restricted the elemental supply for the formation ofηphase.The stress rupture tests were conducted on two alloys at 750℃/430 MPa.When Co is substituted for Fe in K4750 alloy,the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment.This was mainly attributed to increased concentration of Al,Ti and Nb inγ’phase in K4750-Co alloy,which further enhanced the strengthening effect ofγ’phase.After aging at 750℃ for 3000 h,substitution of Co for Fe can also cause the stress rupture life at 750℃/430 MPa to increase from 48.72 to 208.18 h.The reason was mainly because MC carbide degradation andηphase precipitation in K4750 alloy,which promoted the initiation and propagation of micro-crack during stress rupture testing.
文摘This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750°C-1000°C in air for 100 h-1000 h by isothermal oxidation tests.The oxidation-kinetic curves were plotted by the static discontinuous weight gain method.Observation and identification of oxidation products were conducted using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),electron probe micro-analysis(EPMA)and X-ray diffraction(XRD).X-ray photoelectron spectrometer(XPS)was also used to analyze the chemical state of elements and the distribution in depth.The results showed that the oxidation-kinetic curves of K4750 alloy basically obeyed the parabolic law.The average oxidation rate below 900°C was less than 0.1 g/m 2·h which meant the alloy was at a complete anti-oxidation grade,and the alloy was at an anti-oxidation grade at 1000°C.The predominant surface oxide was Cr_(2)O_(3),and a double layer structure of the oxide scale was observed at all tested temperatures as time increased.The outer oxide layer contained continuous Cr_(2)O_(3)and a small amount of oxides mixed TiO2 and NiCr2O4,while the inner oxide layer was composed with Al_(2)O_(3).The oxidation process could be interpreted by the competitive oxidation of different elements.The diffusion rate of Ti through Cr_(2)O_(3)layer increased with increasing temperature,and thus the generation of TiO2 was advantageous.The dispersed TiO2 reaching a certain amount destroyed the continuity of the surface oxide layer,which accounted for the reduction of oxidation resistance of K4750 alloy at high temperatures.
基金The authors thank the National Natural Science Foundation of China for their financial support under contract No.51801210.
文摘To verify the microalloying function and segregation behavior of trace Mg at grain boundary in steel,the 2.25Cr–1Mo steel doped with 0.056%P containing different Mg contents was refined with a vacuum-induction furnace.The effects of trace Mg addition on the temper embrittlement susceptibility of 2.25Cr–1Mo steel were studied by step-cooling test and the segregation behavior of Mg at grain boundary was explored by Auger electron spectroscopy.It is shown that P-induced temper embrittlement susceptibility can be reduced after subjecting to step-cooling treatment with trace Mg addition,mainly benefited from the segregation of Mg at grain boundary.This segregation can decrease the segregation amounts of P and S,especially for P,and increase the grain boundary cohesion,reducing the adverse effect on temper embrittlement caused by P and S.
基金financially supported by the Natural Science Foundation of Liaoning Province, China (Grant No. 2019-BS-248)。
文摘Effects of silicon on the microstructure and mechanical properties of 20% cold-worked 15-15 Ti austenitic stainless steels are systemically investigated by uniaxial tensile tests,scanning and transmission electron microscope observations,and strength calculations.The results reveal that a large number of deformation twins are formed in the 20% cold-worked steels with various silicon additions.The volume fraction of deformation twins and the density of dislocations increase with silicon content,while the twin thickness slightly decreases.A better strength-ductility combination is achieved by silicon addition,since the yield strength of the steel with 2% silicon is 61 MPa higher than that of the steel with 1% silicon,while their uniform elongations are almost both equal to 16%.The yield strength of the 15-15 Ti stainless steels is predominantly contributed by the solid solution,dislocation and deformation twin strengthening effects,which can be enhanced by silicon addition.
