On the formation of austenite during ion nitriding of armco iron

The ion-nitriding behaviour of Armco-iron in the temperature range of 475–550 °C in a 25N 2:75H 2 gas mixture has been studied using optical metallography and X-ray diffraction techniques. The austenite phase was detected at short treatment times in the temperature range 500–550 °C but it disappeared with increasing duration of treatment. It has been shown that nitrogen supersaturation in ferrite adjacent to the nitride layer causes its transformation to austenite. This austenite disappears by reacting with nitrogen to form the γ′ nitride.