Influence of Reverted Austenite on Tensile Deformation and Fracture in 18Ni 250 Maraging Steel

Overaging of C250 maraging steel leads to reversion of austenite. The percentage of reverted austenite in the microstructure increases with an increasing extent of overaging. The effect of volume fraction of reverted austenite on the deformation behavior during tensile testing was studied. Specimens in the solution-annealed condition of the steel were overaged using different temperature/time combinations to produce a range of reverted austenite volume fractions. Changes in the lattice parameter of the martensite phase as a function of overaging treatment were monitored. X-ray diffraction technique was used to measure the volume fraction of reverted austenite and monitor the changes in lattice parameter of martensite. True stress-true strain curves were calculated from the engineering stress-engineering strain diagrams obtained for different overaging treatments, and plots showing the rate of work hardening as a function of true strain were derived. The present investigation shows that the reverted austenite produced at 538 degrees C and 620 degrees C induces serrated flow during tensile testing, whereas reverted austenite produced at 510 degrees C does not. It is believed that the reverted austenite formed at the two higher temperatures is not stable because of the relatively lower amount of nickel and transforms to martensite on tensile deformation, leading to serrated flow. After aging at 510 degrees C and 538 degrees C, after the initial steep drop in the work hardening rate, the rate remains nearly constant over a certain range of true strain before it starts dropping again. After aging at 620 degrees C, in contrast, the work hardening rate continuously decreases with increasing true strain. The precipitates formed after aging at 510 degrees C/538 degrees C are believed to be important contributions to the occurrence of the plateau. The results show that not only the quantity of reverted austenite but also the temperature at which reversion took place influence the fracture mechanisms and fracture surface appearance.