繰り返し温度サイクルによる鋼の変態超塑性現象

抄録

The superplasticity in S15CK, SCM415 and SK5 steels due to temperature cycling under loading was studied in terms of elongation to fracture, m-value and microstructure of specimen fractured. Specimens subjected to constant tensile applied stress were thermal cycled through the austenite (A)\ ightleftarrowsferrite(F)+pearlite(P) or A\ ightleftarrowsP transformation. Main results obtained are as follows.
(1) As the applied stress σ decreases, the strain rate sensitivity m increases. The increase of elongation to fracture due to temperature cycling is in a good agreement with that of m-value.
(2) Elongations over 500% are observed on specimens thermal cycled to failure in the condition of m-value larger than 0.7.
(3) The specimen elongated largely under the small applied stress, where high m-value prevents necking, is fractured with the formation and the interlinkage of many voids. The occurrence of large necking causes the specimen to fracture with smaller elongation because of low m-value under the large applied stress. The true strain per cycle increases corresponding to the occurrence of necking or probably to the formation of many voids.