On the α″ to γ′ nitride transformation after plasma nitriding and aging a low carbon steel

Abstract

Phase transformations during nitriding are dependent on the nitrogen content diffused into the ferritic matrix during the thermochemical treatment. According to the iron-nitrogen phase diagram [1] the nitrogen solubility limit in ferrite is 0.1% in mass. The increase in nitrogen content leads to iron nitride precipitation. The equilibrium iron nitrides in the Fe-N system are the Fe4N (γ ′) and Fe2-3N (e). According to Jack [2] for certain substrate compositions and processing parameters a metastable nitride is formed before γ ′ precipitation, the α′′-Fe16N2 face centred tetragonal nitride. This nitride is formed on {001} planes of the ferritic matrix [2–4]. The α′′ nitride precipitates show a plate like morphology, whereas the γ ′ phase precipitates as needles. Precipitation of α′′ occurs in the 100–220 ◦C temperature range, with an activation energy of 115 kJ/mol.