Direct Observation of Austenite Reversion During Aging of 18Ni (350 Grade) Maraging Steel Through In-Situ Synchrotron X-Ray Diffraction

Austenite reversion in Fe–Ni alloys plays a significant role in determining their mechanical properties. Increasing the austenite content in these alloys improves their toughness but reduces their mechanical strength. In this work, austenite-reversion heat treatments between 600 °C and 700 °C were studied in-situ using synchrotron X-ray diffraction and scanning electron microscopy (SEM) in an 18Ni (350) maraging steel. Aging heat treatments were simulated in three steps: slow heating (0.666 °C s−1), soaking for 1 hour at temperatures between 600 °C and 700 °C, and cooling with forced Ar. After the treatments, the samples were prepared for SEM and microhardness analyses. The reverted austenite fraction increases with the aging temperature, and this phase proved to be partially unstable after aging at a temperature above 675 °C, and it is transforming partially to martensite after cooling. In addition to the increase in the reverted austenite fraction, the coalescence and dissolution of precipitates occurred simultaneously and decreased the hardness.