Isothermal transformation of high-nitrogen austenite

The isothermal transformation of high-nitrogen austenite occurring at 225 °C was found to be a γ → α-Fe + γ′-Fe4N upper bainitic one, which inhomogeneously and preferentially occurred at the grain boundaries and at the dislocations in the interior of the prior austenite grains, producing a microstructure composed of (α-Fe + γ′-Fe4N) bainitic packets and discrete retained austenite patches. The (α-Fe + γ′-Fe4N) bainitic packets were morphologically strikingly different from the tempered martensite produced in the same material.