The isothermal transformation of high-nitrogen austenite occurring at 225 °C was found to be a γ → α-Fe + γ′-Fe 4 N 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′-Fe 4 N) bainitic packets and discrete retained austenite patches. The (α-Fe + γ′-Fe 4 N) bainitic packets were morphologically strikingly different from the tempered martensite produced in the same material.