The nitride in 30CrNi steel and its composition along boundaries have been studied by means of scanning Auger microprobe.The nitride identified metallographically is Zr(C,N)which contains a small amount of Ti and O.Th...The nitride in 30CrNi steel and its composition along boundaries have been studied by means of scanning Auger microprobe.The nitride identified metallographically is Zr(C,N)which contains a small amount of Ti and O.The interface width between the Zr(C,N)and the matrix is about 1.6 μm.An Fe-rich shell is.found on the surface of Zr(C,N)which appeared on the fracture surface.The thickness of shell is 0.1-0.8μm.The Fe concentration of its outmost layer is 3-18 at.-% and decreases exponentially along the depth direction.展开更多
A needle valve is a key component of a diesel injector.The needle valve body of the diesel engine, made of R18CrNi8 steel, cracked and failed during the working process.The cracking failure reasons for the carburized ...A needle valve is a key component of a diesel injector.The needle valve body of the diesel engine, made of R18CrNi8 steel, cracked and failed during the working process.The cracking failure reasons for the carburized injector valve body through chemical composition analysis, metallographic examination, scanning electron microscope(SEM) analysis, and energy spectrum analysis, were investigated.The results reveal that the original material of the needle valve is in conformity with the manufacturing requirement.Due to the high carburizing quenching temperature, the best carburized layer structure was not obtained, and the machining defect from which the crack emanated was not identified.The cracks expanded and eventually led to fracture under the action of altered stress and the high-temperature combustion environment during the operation of the engine.展开更多
The effect of martensite–austenite(M–A)constituents formed in thermo-mechanical controlled process on impact toughness of 20CrNi2MoV steel was investigated.The variation in fraction,size and morphology of M–A const...The effect of martensite–austenite(M–A)constituents formed in thermo-mechanical controlled process on impact toughness of 20CrNi2MoV steel was investigated.The variation in fraction,size and morphology of M–A constituent and its effect on toughness under different cooling rates were carried out.The result shows that there was no significant change in the fraction of M–A constituent under different cooling rates,but the distribution and size of M–A constituent were greatly influenced by cooling rate,which consequently influenced toughness.The amount of large blocky M–A constituents decreased from 4.7%to 1.7%,while that of elongated M–A constituents increased from 3.8%to 8.6%with the cooling rate increasing from 7 to 26°C/s,and the corresponding impact energy decreased from 132 to 84 J.The deterioration of impact toughness could be related to the increase in the elongated M–A constituents.The elongated M–A constituents existing along the prior austenite grain boundaries in samples of 26°C/s could easily lead to the formation of cleavage crack,which then results in the lower crack initiation energy than that of low cooling rate samples.展开更多
The effect of La on inclusions and fracture toughness of 40CrNi2Si2MoVA steel was investigated via the optical microscope,scanning electron microscope,image software and electronic universal testing machine.The result...The effect of La on inclusions and fracture toughness of 40CrNi2Si2MoVA steel was investigated via the optical microscope,scanning electron microscope,image software and electronic universal testing machine.The results reveal that the inclusions in steel without La are mainly MnS and Al_(2)O_(3)–MnS,while the inclusions in steels with La primarily contain La–O–S,La–S and other rare earth complex inclusions contain P and As.La–O–S and La–S are formed under the steelmaking temperature and act as the nucleation core of rare earth complex inclusions containing P and As.According to the segregation model,La–O–S–P–As and La–S–P–As are formed through chemical reactions during the solidification stage.As La content in steels increases from 0 to 0.032 mass%,the average spacing of inclusions is gradually increased from 5.28 to 15.91μm.The volume fraction of inclusions in steels containing less than 0.018 mass%La approaches 0.006%;however,it is significantly improved to 0.058%when La content is increased to 0.032 mass%.With the increase in La content,the fracture toughness is firstly improved from 63.1 to 80.0 MPa m^(1/2)due to the increase in average spacing of inclusions and then decreases to 69.6 MPa m^(1/2)owing to the excessive increase in volume fraction of inclusions.The optimal fracture toughness is found in 40CrNi2Si2MoVA steel with 0.018 mass%La.展开更多
Thermo-mechanical process and continuous cooling process were carried out on 20CrNi2MoV steel. Three cooling rates were implemented to optimize the mechanical properties. The microstructure evolution, precipitation be...Thermo-mechanical process and continuous cooling process were carried out on 20CrNi2MoV steel. Three cooling rates were implemented to optimize the mechanical properties. The microstructure evolution, precipitation behavior, and strengthening mechanisms were systematically investigated, and the fracture mechanisms were analyzed via combination of impact fracture morphologies and deflection-load curves. The experimental results indicate that the transformed microstructure of experimental steel is all complex consisting of granular bainitic ferrite and bainitic ferrite with dispersed martensite/austenite （M/A） constituents in the matrix at cooling rates of 13, 21, and 29 ℃/s. When the cooling rate increases, the grain of the steel is obviously refined. The sizes of the bainitic ferrite are 5.8, 4.7, and 3.1 μm under cooling rates of 13, 21, and 29 ℃/s, respectively. The refinement of the bainitic ferrite plays a dominant role in strength increasing and also contributes to high crack propagation energy. However, the morphologies of M/A constituents obtained under different cooling rates contribute to different crack initiation energies and then affect the impact property.展开更多
Laser Shock Peening(LSP)is a well-established surface treatment commonly used to improve mechanical properties of material’s surfaces.To further understand the relationship between tensile property and fatigue life i...Laser Shock Peening(LSP)is a well-established surface treatment commonly used to improve mechanical properties of material’s surfaces.To further understand the relationship between tensile property and fatigue life improvement of high strength low alloy steel in the LSP process,LSP treatment of 32 CrNi high strength low alloy steel was carried out by YAG laser with pulse energy of 15 J,and tensile property was tested by electronic universal material testing machine.Surface morphology,residual stress and tensile fracture of the specimens before and after LSP were observed by white light interferometer(WLI),X-ray measuring apparatus and scanning electron microscope(SEM).Result shows that LSP did not change tensile strength of 32 CrNi steel but cause yield characteristic transform from obvious yield point to no yield phenomenon which is the only factor benefiting fatigue life,indicating that the increment of fatigue life was probably related to the disappearance of yield phenomenon.Formation mechanisms of tensile fractures and yield phenomenon induced by LSP at room temperature were also discussed and completely revealed.Deeper compressive residual stress and flat grains contributed to the transition of yield characteristic and lower elongation rate of 32 CrNi steel subjected to LSP.展开更多
文摘The nitride in 30CrNi steel and its composition along boundaries have been studied by means of scanning Auger microprobe.The nitride identified metallographically is Zr(C,N)which contains a small amount of Ti and O.The interface width between the Zr(C,N)and the matrix is about 1.6 μm.An Fe-rich shell is.found on the surface of Zr(C,N)which appeared on the fracture surface.The thickness of shell is 0.1-0.8μm.The Fe concentration of its outmost layer is 3-18 at.-% and decreases exponentially along the depth direction.
文摘A needle valve is a key component of a diesel injector.The needle valve body of the diesel engine, made of R18CrNi8 steel, cracked and failed during the working process.The cracking failure reasons for the carburized injector valve body through chemical composition analysis, metallographic examination, scanning electron microscope(SEM) analysis, and energy spectrum analysis, were investigated.The results reveal that the original material of the needle valve is in conformity with the manufacturing requirement.Due to the high carburizing quenching temperature, the best carburized layer structure was not obtained, and the machining defect from which the crack emanated was not identified.The cracks expanded and eventually led to fracture under the action of altered stress and the high-temperature combustion environment during the operation of the engine.
文摘The effect of martensite–austenite(M–A)constituents formed in thermo-mechanical controlled process on impact toughness of 20CrNi2MoV steel was investigated.The variation in fraction,size and morphology of M–A constituent and its effect on toughness under different cooling rates were carried out.The result shows that there was no significant change in the fraction of M–A constituent under different cooling rates,but the distribution and size of M–A constituent were greatly influenced by cooling rate,which consequently influenced toughness.The amount of large blocky M–A constituents decreased from 4.7%to 1.7%,while that of elongated M–A constituents increased from 3.8%to 8.6%with the cooling rate increasing from 7 to 26°C/s,and the corresponding impact energy decreased from 132 to 84 J.The deterioration of impact toughness could be related to the increase in the elongated M–A constituents.The elongated M–A constituents existing along the prior austenite grain boundaries in samples of 26°C/s could easily lead to the formation of cleavage crack,which then results in the lower crack initiation energy than that of low cooling rate samples.
基金the National Natural Science Foundation of China(Nos.U1760114 and U1760206).
文摘The effect of La on inclusions and fracture toughness of 40CrNi2Si2MoVA steel was investigated via the optical microscope,scanning electron microscope,image software and electronic universal testing machine.The results reveal that the inclusions in steel without La are mainly MnS and Al_(2)O_(3)–MnS,while the inclusions in steels with La primarily contain La–O–S,La–S and other rare earth complex inclusions contain P and As.La–O–S and La–S are formed under the steelmaking temperature and act as the nucleation core of rare earth complex inclusions containing P and As.According to the segregation model,La–O–S–P–As and La–S–P–As are formed through chemical reactions during the solidification stage.As La content in steels increases from 0 to 0.032 mass%,the average spacing of inclusions is gradually increased from 5.28 to 15.91μm.The volume fraction of inclusions in steels containing less than 0.018 mass%La approaches 0.006%;however,it is significantly improved to 0.058%when La content is increased to 0.032 mass%.With the increase in La content,the fracture toughness is firstly improved from 63.1 to 80.0 MPa m^(1/2)due to the increase in average spacing of inclusions and then decreases to 69.6 MPa m^(1/2)owing to the excessive increase in volume fraction of inclusions.The optimal fracture toughness is found in 40CrNi2Si2MoVA steel with 0.018 mass%La.
基金financially supported by the National High Technology Research and Development Program of China(No.2012AA03A503)the National Natural Science Foundation of China(Nos.51174057 and 51274062)Research Fund for the Doctoral Program of Higher Education of China(No.20130042110040)
文摘Thermo-mechanical process and continuous cooling process were carried out on 20CrNi2MoV steel. Three cooling rates were implemented to optimize the mechanical properties. The microstructure evolution, precipitation behavior, and strengthening mechanisms were systematically investigated, and the fracture mechanisms were analyzed via combination of impact fracture morphologies and deflection-load curves. The experimental results indicate that the transformed microstructure of experimental steel is all complex consisting of granular bainitic ferrite and bainitic ferrite with dispersed martensite/austenite （M/A） constituents in the matrix at cooling rates of 13, 21, and 29 ℃/s. When the cooling rate increases, the grain of the steel is obviously refined. The sizes of the bainitic ferrite are 5.8, 4.7, and 3.1 μm under cooling rates of 13, 21, and 29 ℃/s, respectively. The refinement of the bainitic ferrite plays a dominant role in strength increasing and also contributes to high crack propagation energy. However, the morphologies of M/A constituents obtained under different cooling rates contribute to different crack initiation energies and then affect the impact property.
基金the National Natural Science Foundation of China(No.51375055)。
文摘Laser Shock Peening(LSP)is a well-established surface treatment commonly used to improve mechanical properties of material’s surfaces.To further understand the relationship between tensile property and fatigue life improvement of high strength low alloy steel in the LSP process,LSP treatment of 32 CrNi high strength low alloy steel was carried out by YAG laser with pulse energy of 15 J,and tensile property was tested by electronic universal material testing machine.Surface morphology,residual stress and tensile fracture of the specimens before and after LSP were observed by white light interferometer(WLI),X-ray measuring apparatus and scanning electron microscope(SEM).Result shows that LSP did not change tensile strength of 32 CrNi steel but cause yield characteristic transform from obvious yield point to no yield phenomenon which is the only factor benefiting fatigue life,indicating that the increment of fatigue life was probably related to the disappearance of yield phenomenon.Formation mechanisms of tensile fractures and yield phenomenon induced by LSP at room temperature were also discussed and completely revealed.Deeper compressive residual stress and flat grains contributed to the transition of yield characteristic and lower elongation rate of 32 CrNi steel subjected to LSP.
