低炭素鋼の引張変形挙動と衝撃値におよぼす低応力振幅予加工の影響

抄録

Low carbon steel (SM 41) was cyclically pre-loaded under a low stress amplitude in tension-compression to 1, 3, 5, 10, 15 and 30% of fatigue life. Then the effect of cyclic pre-loading on the monotonic tensile properties and Charpy impact values was investigated. The microstructures were also observed by transmission electron microscopy. The results are as follows:
(1) Grains containing bundle and/or isolated dislocation structures were observed in the specimens cyclically pre-loaded to 5 to 15% of fatigue life. Dislocation network structures were also observed in some grains pre-loaded to 30% of fatigue life. In the case of cyclic loading under a low stress amplitude, the dislocation structure was quite different among individual grains in a specimen. These results suggest that the deformation was inhomogeneous among grains in contrast with monotonic tensile pre-deformation.
(2) All of the cyclically pre-loaded specimens showed apparent upper yield points and Lüders strain during a monotonic tensile test. These phenomena are probably caused by a dynamic strain aging of isolated dislocations during cyclic pre-loading.
(3) The increase in tensile strength of the specimens, cyclically pre-loaded up to 10% of fatigue life, is caused by the work hardening due to bundle dislocation structures, and conversly, the decrease in tensile strength of the specimens cyclically pre-loaded to 15 and 30% is caused by a certain dynamic recovery during cyclic pre-loading.
(4) The maximum impact value (I_max) decreased and the temperature at the onset of ductility (T_d) increased with increasing amount of cyclic pre-loading. These results suggest that cydic pre-loading always makes the specimens brittle.
(5) The transition temperature (T_c) decreased with increasing amount of cyclic pre-loading. However, the decrease in T_c does not relate to the increase in toughness.