Fe-Ni-(C)合金における加工オーステナイトおよび逆変態オーステナイトの再結晶挙動

抄録

Recrystallization behavior of austenite cold rolled or formed by the α′→γ reverse martensitic transformation was investigated using Fe-33Ni, Fe-35Ni and Fe-25Ni-0.4C austenitic alloys. The main results obtained are as follows.
(1) In the case of Fe-35Ni alloy in which slip deformation occurs by cold rolling of 20∼80%, recrystallized grains are preferentially formed along the austenite grain boundaries. The number of recrystallized grains formed along austenite boundaries increases with increase in the amount of prior deformation. When specimens were slightly deformed by 20%, the recrystallization also occurs by the bulging mechanism of existing austenite grain boundaries.
(2) The Fe-25Ni-0.4C alloy is deformed not only by slip but also by twinning when cold rolled by 50%. In such a case, many recrystallized grains are preferentially formed at the intersections of deformation twins within austenite grains. Because of such a preferential nucleation at deformation twins, austenite grain size after recrystallization in Fe-25Ni-0.4C becomes much smaller than that in Fe-35Ni.
(3) The Fe-33Ni martensite which are formed by subzero cooling to 77 K transforms to austenite by shear mechanism during heating. Reversed austenite boundaries correspond to prior austenite boundaries before heating. Recrystallized grains are preferentially formed along grain boundaries of reversed austenite. The number of recrystallized grains in the reversed austenite are very few. Therefore, the austenite grain refinement by recrystallization is not observed in the case of reversed austenite in Fe-33Ni alloy.
(4) When the Fe-25Ni-0.4C martensites which are formed by subzero cooling to 77 K are heated up above the A_f temperature, the reversed transformation occurs mainly by the shear mechanism, but partly by the diffusional process because of the rapid carbide formation in martensites on heating. In this case, fine austenite grains which are formed by the diffusional transformation are observed within reversed austenite grains, in addition to the few recrystallized grains at the reversed austenite grain boundaries.