炭素鋼のバウシンガー効果に関する研究

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We made two kinds of experiments about this subject: for the first experiment, a number of structures of thin-walled cylindrical specimens of various carbon steels were made by heat treatment. From the stress-strain curves of these specimens drawn by the pendulum type torsion testing machine, the Bauschinger effect was estimated and the following results were gained.　1) In general, the less carbon is contained in the specimens, the less remarkably the Bauschinger effect is observed. Even in the annealed Flodin iron, the transient softening and the permanent softening are clearly seen when the order of 3 per cent shear prestrain is added.　The Bauschinger effect of carbon steels depends not only on the carbon contained but greatly on other impurities. The Bauschinger effect decreases when the ferrite crystals of the specimen show an axial fibre structure. It can be said that the Bauschinger effect of carbon steels is made remarkable when the structure of the steels is complicated macroscopically or microscopically or both.　2) The permanent softening curves are higher than the stless-strain curves of the virgin specimens when the Bauschinger effect is less appreciable, but this is revealed just contrary when the Bauschinger effect is remarkable. This shows that the work-hardening occurs both in the pre-strained direction and in the reverse direction simultaniously, containing the causes of the Bauschinger effect in the reverse direction.　3) When the comparatively small prestrain, under 5 per cent, being applied to the specimens, the Bauschinger effect of carbon steels is linearlly proporitonal to the amount of prestrains. The rate of increase of the Bauschinger effect with the prestrain of specimens is proportional to the ”r” power (r > 1) of the coefficient of work-hardening of the last part of prestrain curves.　4) The Bauschinger effect of carbon steels can be partly removed by low temperature ageing but cannot be completely removed before recrystallization, and in the course of its removal, though the Cottrell atmosphere is formed, sharp yielding is not always observed if the Bauschinger effect is pronounced. In this case the preyielding often appears in the same deformation as that of prestrain.　5) The Bauschinger effect of specimens with spheroidized pearlite structure is revealed smaller than that of fully annealed same specimens. The difference between these two Bauschinger effects becomes more remarkable as the spheroidization of cementite is promoted. This is due to the fact that the coefficient of work-hardening of prestrain of the specimens with spheroidized pearlite is smaller than that of the fully annealed same specimens.　6) With the carbon steel with spheroidized pearlite, the Bauschinger effect is more difficult to remove by low temperature ageing under 200℃ than the same carbon steel of the annealed structure, in spite of the Bauschinger effect of the former being smaller than the latter as mentioned in (5). A sharp yielding in the reverse direction does not return to the pre-strained spheroidized carbon steel unless it is aged above 200℃.　7) For the annealed specimen, the activation energy obtained by the process of raising the lower yielding point till the first saturation, with the removal of the Bauschinger effect by ageing at around l00℃, was 19,000 cal/mole, and for the specimens with spheroidized pearlite, the activation energy of the same process with ageing at around 220℃ was 24,000 cal/mole.　8) When the constant amplitude of stresses ±t_0 is repeated, the permanent strain with the opposite sign to the prestrain is not observed in the specimens which show a little Bauschinger effect, but in the specimens in which the Bauschinger effect is remarkably shown, the permanent strain with the opposite sign presents itself.　9) When ±t_0 is repeated, the width of the stress-strain hysteresis loop and the Bauschinger effect diminish remarkably at the second repition and then they diminish gradually to attain certain values after several repitions. But the effect of the prior deformation appears on the work-hardening and the Bauschinger effect of the subsequent stress-strain curves at least in the first few cycles of stressing.　10) Applying the prestrain, which is greater than the yield strain of the specimen, we repeated 10 cycles of constant amplitude of stress ±t_0 which were equal to the last stress of prestrain, and aged it. A sharp yielding returned in the positive direction. Though in the reverse direction the proportional limit rose somewhat, sharp yielding did not appear at all. If we begin with the same order of prestrain and ageing as above, but fail to apply the stress repetition, sharp yielding should appear in the reverse direction, and if the stress is repeated, the Bauschinger strain should be smaller than that of the pre-strained specimen as mentioned in (9).　For the second experiments, the fully annealed cylindrical specimens of low carbon steel were given 4.3% and 28.8% compressive prestrains. From these cylindrical specimens, test pieces for compression tests and tension tests with the axes of various directions to the pre-compression were machined. From the stress-strain curves of these test pieces resulted from the experiments, the following results were gained concerning the anisotropy of the Bauschinger effect of pre-compressed cylinders.　11) The Bauschinger effects are closely related to the kind and magnitude of the macroscopic plastic strain of each direction given in the prior deformation.　12) The more prominent the Bauschinger effect is, the more slowly the transient softening curve rises, and the larger Bauschinger strain is attained at the last stage of the transient softening.　13) As to the anisotropy of yield stress obtained from the permanent softening curves, the experimental results of the author showed good agreement with the results reduced from Yoshimura-Takenaka's Strain History Theory of Plasticity.　14) Once born anisotropy of the structure resulted from prestrain of only a small order of 4.3 per cent can hardly be removed even if quite a large plastic deformation in a different direction to the prior deformation is given to the structure, and as the deformation proceeds, an anisotropic effect can be observed in the sectional form of the specimen, too.　15) From the experimental results presented above, it can be concluded that the Bauschinger effect is largely attributed to the microscopic ununiformity of the internal stress and strain produced by the prestrain rather than to the macroscopic residual stress produced in the prior deformation.
山形大学の論文
1963-02-28

炭素鋼のバウシンガー効果に関する研究

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