This contribution addresses the dissolution of primary M7C3 carbides and the simultaneous precipitation of nitrides during plasma nitriding of an AISI D2 tool steel. Samples of quenched and tempered D2 tool steel were plasma nitrided at 793 K (520°C) for times varying between 2.52 and 39.6 ks (between 42 min and 11 h). Microstructure and chemical composition of nitrided samples were then characterized by scanning electron microscopy and wavelength dispersive spectrometry. It was observed that nitrogen absorption induces M7C3 dissolution and simultaneous in situ precipitation of MN nitrides. Calphad computational thermodynamic simulations were carried out in order to evaluate phase stability and atomic partitioning in the microstructure. Atomic partitioning between MN and the martensitic matrix was associated with a thermodynamic metastable state. At 793 K, the negligible metallic-element diffusion inside both M7C3 and MN phases makes the metastable state kinetically favored instead of the thermodynamic stable state.