Formation and Growth Kinetics of Reverted Austenite During Tempering of a High Co-Ni Steel

Microstructural Effects on the Toughness of a High Co–Ni Steel

“…In prior investigations, the increase of austenite in this steel grade during tempering was attributed to the formation of reverted austenite . In maraging and high Co–Ni steels, the formation of reverted austenite in was found to be controlled predominantly by the diffusion of Ni atoms .…”

“…Since, this steel forms secondary hardening carbides upon tempering, tempering leads to an increase of hardness . Regardless, whether tempering is conducted at 482 °C for 1, 1 + 4, or 5 h or if a cryogenic treatment is applied, the resulting hardness increase shows similar values of 581–593 HV.…”

“…It was found that in high Co–Ni steels, both retained and reverted austenite are present after tempering at martensite lath boundaries mainly in form of thin films . After tempering, the retained austenite in these steels is neither enriched in C nor in Ni and, thus, it should exhibit a lower stability . Therefore, usually a cryogenic treatment is applied before tempering to reduce the phase fraction of retained austenite .…”

“…It is assumed that cryogenic treatment also reduces the thickness of retained austenite films, which has a stabilizing effect on them . On the contrary, reverted austenite, that forms in high Co–Ni steels during tempering above temperatures of 450–480 °C, is enriched in Ni and, therefore, it is mostly stable against martensitic transformation . Detailed analyses showed that the formation of reverted austenite is controlled by the diffusion of Ni to retained austenite‐martensite interfaces or preexistent austenite nuclei .…”

“…On the contrary, reverted austenite, that forms in high Co–Ni steels during tempering above temperatures of 450–480 °C, is enriched in Ni and, therefore, it is mostly stable against martensitic transformation . Detailed analyses showed that the formation of reverted austenite is controlled by the diffusion of Ni to retained austenite‐martensite interfaces or preexistent austenite nuclei . According to Ayer and Machmeier and Lippard, reverted austenite in high Co–Ni steels is the beneficial type of austenite for toughness enhancement.…”