Mechanism of austenite formation in heating steel

Study of Structural Inheritance of Austenite in Nb‐Microalloyed 18CrNiMo7–6 Steel

“…The higher the alloying content, the broader is this range . Lipchin has reported the effect of alloying elements on the spontaneous RX temperature of austenite, which is also supported by others (see section 3.4.3 for further details). Sadovskii suggests that with a high enough heating rate, it is possible to separate the two critical temperatures, A c3 and b point.…”

“…While the diffusive formation of globular austenite is conclusive, there is so far no unanimity on the mechanism of acicular austenite formation: diffusional or diffusionless. Sokolov and Sadovskii conclude that structural inheritance (restoration of PAGS) occurs by a diffusional mechanism on slow heating (≈2 °C min −1 ) and by a diffusionless mechanism on rapid heating (≈100 °C s −1 ). The diffusionless mechanism of austenite formation has also been reported by others, not only in steels, but also in Fe–Ni alloys, non‐ferrous systems like Cu, Cu–Sn, Al alloys .…”

“…Irrespective of the formation mechanism, recrystallization (RX) of acicular austenite occurs in both cases on further increase in austenitization temperature . In order for acicular austenite to nucleate by diffusion, a certain C enrichment is necessary.…”

“…This cannot be generalized to all steels. While Kimmins and Gooch, Hara et al, and Tao et al have all proved that the presence of retained austenite clearly promotes SI by assisting acicular austenite formation, there is also literature which shows that structural inheritance can occur in extremely lean alloyed steels, where retention of stable austenite to room temperature (RT) is unlikely. Thus, it can be argued that the choice (between lath boundary and retained austenite) of nucleation site for acicular austenite depends on the steel composition.…”

“…A consequence of these given conditions is the phenomenon called “structural heredity”, “structural inheritance (SI)”, “austenite memory effect” or “abnormal” α to γ transformation . It is an effect by which the newly formed austenite retains the austenite grain size, shape, and orientation of the previous microstructure. PAGS strongly influences the creep ductility of steels, thus a fine uniform grain size is intended in the final product .…”