軟鋼の時効とバウシンガ効果

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Thin walled cylindrical specimens were made from annealed S25C mild steel and were aged for 30 min at 200℃ after torsionally prestrained, and then the forward torsion or the reverse torsion with prestrains were applied respectively for the strain age hardening and the diminution of the Bauschinger effect to be studied. Also dislocation structures of the same material, to which the torsional prestrain, or the torsional prestrain and reverse torsion, or the torsional strain ageing and reverse torsion were applied, were observed with the transmission electron microscope. The main results obtained were as follows.　(1) The B.E. in the annealed mild steel is partially removed by 30 min ageing at 200℃ after prestrain, and the amount of removal becomes larger as the prestrain increases. This removal is caused by the precipitation of carbide particles and nitride particles, the formation of the Cottrell atmosphere, and the partial diminution of the residual internal stress which is caused by the precipitation of particles and the formation of the atmosphere and the recovery resulted from the ageing.　(2) The strengthening effect of the matrix by the precipitates accorded by the ageing becomes larger as the prestrain increases and gradually approaches to a certain value. This effect contributes to the removal of permanent softening.　(3) The increase of the yield stress with the formation of the Cottrell atmosphere contributes to the removal of transient softening but the removal of internal stress caused by the recovery and so on plays the larger part in it.　(4) A thin foi1 specimen was taken from the test specimen given 2% reverse torsional strain after 5% torsional forward prestrain. By the observation of the foil with TEM, it was found that the cell walls which had been formed during the prestrain were partially dissolved, and also long parallel dislocation lines which were considered to be newly formed during the reverse torsional strain were found. The dissolution of cell walls suggests that the back stress is one of the causes of the B.E., and the newly formed dislocations will cause the strain hardening in the reverse direction.　(5) The foil was taken from the specimen which was given 5% torsional prestrain and then aged for 30 min at 200℃ to lock the dislocation firmly and then given 2% reverse torsional strain. It was clarified from the observation of the foil with TEM that the dissolution of the cell walls was much smaller and the iong parallel dislocations were denser than those of the unaged specimen. The dislocation density of the aged specimen was higher than that of the unaged one. That smaller dissolution of the cell walls suggests that the B.E. will be partially removed by ageing, and it can be said that it is one of the causes of reappearance of the proportionallimit on the Bauschinger curve.
山形大学の論文
1981-01-20

軟鋼の時効とバウシンガ効果

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