To study the effect of tempering temperature on strain hardening exponent and flow stress curve,one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed,and the standard uniaxial te...To study the effect of tempering temperature on strain hardening exponent and flow stress curve,one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed,and the standard uniaxial tensile tests were conducted at room temperature.A new flow stress model,which could predict the flow behavior of the tested steels at different tempering temperatures more efficiently,was established.The relationship between mobile dislocation density and strain hardening exponent was discussed based on the dislocation-stress relation.Arrhenius equation and an inverse proportional function were adopted to describe the mobile dislocation,and two mathematical models were established to describe the relationship between tempering temperature and strain hardening exponent.Nonlinear regression analysis was applied to the Arrhenius type model,hence,the activation energy was determined to be 37.6kJ/mol.Moreover,the square of correlation coefficient was 0.985,which indicated a high reliability between the fitted curve and experimental data.By comparison with the Arrhenius type curve,the general trend of the inverse proportional fitting curve was coincided with the experimental data points except of some fitting errors.Thus,the Arrhenius type model can be adopted to predict the strain hardening exponent at different tempering temperatures.展开更多
The effect of the strain hardening exponent(n)of a material on the changes in shape,strain inhomogeneity,and contact pressure was investigated during wire flat rolling to understand its effect on the deformation behav...The effect of the strain hardening exponent(n)of a material on the changes in shape,strain inhomogeneity,and contact pressure was investigated during wire flat rolling to understand its effect on the deformation behavior of a flat-rolled wire and to determine the optimal working conditions with materials.The deformation behaviors of wires with various n values were systematically compared using finite element method.The shape of the deformed wire was found to depend on the n value of the material.Both the contact width and lateral spreading of the wire decrease with increasing n,resulting in a large reduction in area with the n value.The strain homogeneity of the wire increases with the n value of the wire.The improvement in the strain homogeneity with the n value is attributable to two factors:a lower strain concentration in the central region and a higher overall elongation as n increases.In addition,the average effective strain of the wire cross section decreases with the n value of a material during wire flat rolling.The contact pressure distribution on the surface of the wire differs significantly depending on the n value.In materials with a low n value,the contact pressure exhibits a higher value at the entry and edge zones of the contact surface.By contrast,materials with high n values exhibit a higher contact pressure at the exit zone.This behavior can be explained by the strain hardening behavior of the material during wire flat rolling.展开更多
Complete experimental investigation on the instantaneous strain hardening behaviour of powder metallurgy (P/M) preforms of pure iron, Fe 0.35%C, Fe-0.75%C and Fe-1.1%C was carried out. The strain hardening be haviou...Complete experimental investigation on the instantaneous strain hardening behaviour of powder metallurgy (P/M) preforms of pure iron, Fe 0.35%C, Fe-0.75%C and Fe-1.1%C was carried out. The strain hardening be haviour of the above-mentioned P/M sintered steel preforms with aspect ratio of 0.4 under triaxial stress state condition was determined by cold upsetting under nil/no and graphite lubricant conditions. The instantaneous strain hardening value (ni), strength coefficient (Ki), and the stress as a function of strain and densification were obtained and analyzed. Furthermore, a relation was obtained from a semi-log plot of stress against relative density and analyzed to study the hardening behaviour owing to densification as stress was a function of induced strain as well as densification in the P/M materials.展开更多
Surface yielding of metallic material was measured with strain gage and X-ray diffraction methods. The results show that.when the residual stress in the transverse direction is involved,the surface yield strength shou...Surface yielding of metallic material was measured with strain gage and X-ray diffraction methods. The results show that.when the residual stress in the transverse direction is involved,the surface yield strength should be evaluated with biaxial Mises criterion.For a medium carbon high strength steel, the yield strength of the bulk material is 581 MPa and the surface yield strengths for 0.05% and 0.1%plastic strain are about 436 MPa and 463 MPa respectively.The 0.05% yield strength will approximately increase to 788 MPa after shot peening.In the early stage of plastic deformation, strain hardening in the surface layer is quite different from that of the bulk sample.展开更多
The hot deformation behavior of GCr15SiMn steel was studied through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator. The initiation and evolution of dynamic recrystallization (DRX...The hot deformation behavior of GCr15SiMn steel was studied through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator. The initiation and evolution of dynamic recrystallization (DRX) were investigated with microstructural analysis and then the process variables were derived from flow curves. In the present deformation conditions, the curves of strain hardening exponent (n) and the true strain (e) at the deformation temperature of 1423 K and strain rates of 0.1, 1 and 10 s^-1 exhibit single peak and single valley. According to Zener-Hollomon and Ludwik equation, the experimental data have been regressed by using linear method. An expression of Z parameter and hot deformation equation of the tested steel were established. Moreover, the Q values of GCrlSSiMn and GCr15 steels were compared. In order to determine the recrystallization fraction under different con ditions, the volume fraction of DRX as a function of process variables, such as strain rate (ε), temperature (T), and strain (ε), was established. Itwas found that the calculated results agreed with the mierostructure of the steel at any deformation conditions.展开更多
Two kinds of steels (YP960 and YP690) with low carbon bainite structure were designed, and their flow stress and strain hardening exponents were studied. The results showed that, when Hollomon relation was applied t...Two kinds of steels (YP960 and YP690) with low carbon bainite structure were designed, and their flow stress and strain hardening exponents were studied. The results showed that, when Hollomon relation was applied to descrihe the flow stress, there were significanl errors between the experimental and calculated points in specimens tempered below 400 ℃, while a high precision was ohserved in samples tempered above 400℃. Whereas, the modijied Voce relation could effectively predici the flow stress as well as the strain hardening exponent at different tempe ring temperatures, which was verified by unbiased estimators such as maximum relative error (MRXE) and average ahsolute relative error (AARE). Besides, the modified Voee relation was also applied to estimate the maximum uniform strain, and the correlation coefficients (R) between the experimental data and calculated maximum uniform strain were more than 0.91. The high correlation coefficients indicated that the modified Vote relation could effec lively predict the uniform deformation ability of high strength steels with low carbon bainite structure at different tempering temperatures.展开更多
With mean yield(MY)criterion,an analytical solution of the collapse load for a defect-free pipe elbow under internal pressure is first obtained.It is a function of ratio of thickness to radius t0/r0,strain hardening...With mean yield(MY)criterion,an analytical solution of the collapse load for a defect-free pipe elbow under internal pressure is first obtained.It is a function of ratio of thickness to radius t0/r0,strain hardening exponent n,curvature influence factor mand ultimate tensile strength.The collapse load increases with the increase of m,and it is the same as the burst pressure of straight pipe if m=1is assumed.The MY-based solution is compared with those based on Tresca,Mises and twin shear stress(TSS)yield criteria,and the comparison indicates that Tresca and twin shear stress yield criteria predict a lower bound and an upper bound to the collapse load respectively.However,the MY-based solution lies just between the TSS and Tresca solutions,and almost has the same precision with the Mises solution.展开更多
文摘To study the effect of tempering temperature on strain hardening exponent and flow stress curve,one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed,and the standard uniaxial tensile tests were conducted at room temperature.A new flow stress model,which could predict the flow behavior of the tested steels at different tempering temperatures more efficiently,was established.The relationship between mobile dislocation density and strain hardening exponent was discussed based on the dislocation-stress relation.Arrhenius equation and an inverse proportional function were adopted to describe the mobile dislocation,and two mathematical models were established to describe the relationship between tempering temperature and strain hardening exponent.Nonlinear regression analysis was applied to the Arrhenius type model,hence,the activation energy was determined to be 37.6kJ/mol.Moreover,the square of correlation coefficient was 0.985,which indicated a high reliability between the fitted curve and experimental data.By comparison with the Arrhenius type curve,the general trend of the inverse proportional fitting curve was coincided with the experimental data points except of some fitting errors.Thus,the Arrhenius type model can be adopted to predict the strain hardening exponent at different tempering temperatures.
基金This work was partially supported by the National Research Foundation of Korea(NRF)grant funded by the Korea government(MSIT,South Korea)(No.2021R1A2C1011700).
文摘The effect of the strain hardening exponent(n)of a material on the changes in shape,strain inhomogeneity,and contact pressure was investigated during wire flat rolling to understand its effect on the deformation behavior of a flat-rolled wire and to determine the optimal working conditions with materials.The deformation behaviors of wires with various n values were systematically compared using finite element method.The shape of the deformed wire was found to depend on the n value of the material.Both the contact width and lateral spreading of the wire decrease with increasing n,resulting in a large reduction in area with the n value.The strain homogeneity of the wire increases with the n value of the wire.The improvement in the strain homogeneity with the n value is attributable to two factors:a lower strain concentration in the central region and a higher overall elongation as n increases.In addition,the average effective strain of the wire cross section decreases with the n value of a material during wire flat rolling.The contact pressure distribution on the surface of the wire differs significantly depending on the n value.In materials with a low n value,the contact pressure exhibits a higher value at the entry and edge zones of the contact surface.By contrast,materials with high n values exhibit a higher contact pressure at the exit zone.This behavior can be explained by the strain hardening behavior of the material during wire flat rolling.
文摘Complete experimental investigation on the instantaneous strain hardening behaviour of powder metallurgy (P/M) preforms of pure iron, Fe 0.35%C, Fe-0.75%C and Fe-1.1%C was carried out. The strain hardening be haviour of the above-mentioned P/M sintered steel preforms with aspect ratio of 0.4 under triaxial stress state condition was determined by cold upsetting under nil/no and graphite lubricant conditions. The instantaneous strain hardening value (ni), strength coefficient (Ki), and the stress as a function of strain and densification were obtained and analyzed. Furthermore, a relation was obtained from a semi-log plot of stress against relative density and analyzed to study the hardening behaviour owing to densification as stress was a function of induced strain as well as densification in the P/M materials.
文摘Surface yielding of metallic material was measured with strain gage and X-ray diffraction methods. The results show that.when the residual stress in the transverse direction is involved,the surface yield strength should be evaluated with biaxial Mises criterion.For a medium carbon high strength steel, the yield strength of the bulk material is 581 MPa and the surface yield strengths for 0.05% and 0.1%plastic strain are about 436 MPa and 463 MPa respectively.The 0.05% yield strength will approximately increase to 788 MPa after shot peening.In the early stage of plastic deformation, strain hardening in the surface layer is quite different from that of the bulk sample.
基金Item Sponsored by National High-tech Research and Development Program(863Program)of China(2012AA03A503)
文摘The hot deformation behavior of GCr15SiMn steel was studied through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator. The initiation and evolution of dynamic recrystallization (DRX) were investigated with microstructural analysis and then the process variables were derived from flow curves. In the present deformation conditions, the curves of strain hardening exponent (n) and the true strain (e) at the deformation temperature of 1423 K and strain rates of 0.1, 1 and 10 s^-1 exhibit single peak and single valley. According to Zener-Hollomon and Ludwik equation, the experimental data have been regressed by using linear method. An expression of Z parameter and hot deformation equation of the tested steel were established. Moreover, the Q values of GCrlSSiMn and GCr15 steels were compared. In order to determine the recrystallization fraction under different con ditions, the volume fraction of DRX as a function of process variables, such as strain rate (ε), temperature (T), and strain (ε), was established. Itwas found that the calculated results agreed with the mierostructure of the steel at any deformation conditions.
基金Item Sponsored by National Natural Science Foundation of China(51274036)
文摘Two kinds of steels (YP960 and YP690) with low carbon bainite structure were designed, and their flow stress and strain hardening exponents were studied. The results showed that, when Hollomon relation was applied to descrihe the flow stress, there were significanl errors between the experimental and calculated points in specimens tempered below 400 ℃, while a high precision was ohserved in samples tempered above 400℃. Whereas, the modijied Voce relation could effectively predici the flow stress as well as the strain hardening exponent at different tempe ring temperatures, which was verified by unbiased estimators such as maximum relative error (MRXE) and average ahsolute relative error (AARE). Besides, the modified Voee relation was also applied to estimate the maximum uniform strain, and the correlation coefficients (R) between the experimental data and calculated maximum uniform strain were more than 0.91. The high correlation coefficients indicated that the modified Vote relation could effec lively predict the uniform deformation ability of high strength steels with low carbon bainite structure at different tempering temperatures.
基金Sponsored by National Natural Science Foundation of China(51074052,50734002)
文摘With mean yield(MY)criterion,an analytical solution of the collapse load for a defect-free pipe elbow under internal pressure is first obtained.It is a function of ratio of thickness to radius t0/r0,strain hardening exponent n,curvature influence factor mand ultimate tensile strength.The collapse load increases with the increase of m,and it is the same as the burst pressure of straight pipe if m=1is assumed.The MY-based solution is compared with those based on Tresca,Mises and twin shear stress(TSS)yield criteria,and the comparison indicates that Tresca and twin shear stress yield criteria predict a lower bound and an upper bound to the collapse load respectively.However,the MY-based solution lies just between the TSS and Tresca solutions,and almost has the same precision with the Mises solution.