Mechanism of α′→γ reverse transformation has been examined by means of dilatometric measurements, X-ray analysis and structural observations in an 18% Ni maraging steel.
The mechanism is changed in the range of heating rate between 100 and 500°C/min. At the lower heating rates, the stabilization (decrcase in Ms and increase in the content of retained γ) and the unstabilization (increase in Ms) of reverted γ are observed, and this phenomenon is concerned with the partitioning of the solute atoms in α′ during heating. Under these conditions, surface relieves are observed during the reverse transformation and γ grain boundaries inherit the former features completely when heated to the temperature range from Af to γ recrystallization temperature. From the above results, the mechanism of α′→γ reverse transformation at lower heating rates is inferred as follows; first, α′ is partitioned to solute rich and solute poor regions by short range diffusion, and in the subsequent α′→γ transformation, martensitic character participates partially.
On the other hand, at the higher heating rates, either stabilization or unstabilization of the reverted γ is not observed, and accordingly it is considered that α′ transforms to γ martensitically without the diffusion process.
