Precipitation and reverted austenite formation in maraging 350 steel: Competition or cooperation?

Aged maraging steels offer unique strength and toughness via the presence of finely dispersed precipitates, allowing the material to reach values of up to 2400 MPa in yield strength in the case of 18Ni350. However, when aging heat treatments above 550 degrees C are conducted, simultaneous precipitation and austenite reversion can occur changing the mechanical behavior of the material. Although many studies related to the physical metallurgy of maraging steels have already been published, less attention has been given to a detailed understanding of the initial formation of austenite and its relationship with the precipitates. In this study, undeformed and cold rolled commercial 18Ni350 maraging steel samples were submitted to short aging heat treatments of 1800s at 600, 650, and 700 degrees C. The influence of the initial microstructure on subsequent phase evolution was studied using in-situ synchrotron X-ray diffraction and final microstructural products using transmission electron microscopy and atom probe tomography. Results show that cold rolled samples did not present faster kinetics of transformation of reverted austenite as expected, but this condition presented austenite in a different morphology than the undeformed condition. However, cold rolling changed the morphology of reverted austenite from elongated (undeformed case) to equiaxed; and induced a higher density of smaller Ni3Ti and Fe2Mo precipitates, especially after low-temperature aging. Besides, the deformation extinguished retained austenite, which influenced the reverted austenite formation, concluding that the simple increase in dislocation density is not a unique and direct factor to increase the reverted austenite kinetics.