The formation of reverted austenite in 18% Ni 350 grade maraging steel

Abstract

Austenite reversion was studied in 18% Ni 350 grade maraging steel. The samples were heat treated from room temperature to the austenite phase field and, without holding, they were cooled again to ambient temperature. The reverted austenite which was retained after this heat treatment was examined using a scanning transmission electron microscope equipped with an energy dispersive spectroscopic system. Two morphologies of the austenite were observed. The first forms at the martensite lath boundaries and the other nucleates inside the martensite laths in the form of Widmanstatten plates. These Widmanstatten plates mostly appear as coupled twins. The coupled twins have a distinct midrib which was found parallel to (1 1 1)γ and (1 1 0)α planes. The latter morphology of austenite appeared only after the formation of Ni3Ti precipitates. Growth of Fe2Mo precipitates was not observed in this heat-treatment cycle. Both Nishyama–Wassermann and Kurdjumo–Sachs orientation relationships were found between the austenite and martensite phases. On the basis of these results, it can be suggested that intra-lath-reverted austenite is formed on or by the local dissolution of Ni3Ti precipitates. © 1998 Kluwer Academic Publishers