Recrystallization of the Austenite Transformed Reversely and Structure of Martensite in 18Ni Maraging Steel

Abstract

A recrystallization of austenite during heating at the temperatures above A_f was investigated in the solution-treated and the 50% rolled 18 Ni maraging steels. The characteristics of martensite structure formed from the unrecrystallized austenite was also studied. The main results obtained are as follows:
1) Just after the α′-γ transformation, the austenite structure (i. e., prior austenite boundary) of the starting materials is inherited into the reverted austenite in non-deformed and 50% rolled specimens. The reverted austenite is recrystallized to a finer grain size by heating it for longer time or at higher temperatures.
2) In the case of non-deformed materials, the reverted austenite grain boundary is the preferential nucleation site of recrystallization. On the other hand, in the 50% rolled materials, the recrystallization occurs finely and uniformly in the reverted austenite grain. Owing to such a difference of recrystallization behavior, a deformation prior to heating enhances the refinement of austenite grain.
3) During the cycle of α′→γ→α′ transformation, the structure memory effect of martensite (packet and block structures) is observed provided martensites are formed from the unrecrystallized austenite. However, the width of martensite laths formed from the unrecrystallized austenite is fine in comparison with that formed from the recrystallized austenite.