Diffusion of nitrogen and carbon in tempered martensite

During nitriding and FNC heat treating nitrogen and carbon diffuse into a tempered martensite matrix to form a hardened case beneath the compound layer. The initial tempered martensite is comprised of a two phase microstructure with low carbon martensite and carbides (cementite). During the heat-treating nascent nitrogen diffuses into the tempered martensite to form iron nitrides and /or carbonitrides. The diffusion process can be interpreted using a diffusion with a chemical reaction process or a diffusion with trapping model (McNabb-Foster). Experimental results for nitrogen and carbon concentration profiles along with microhardness and microstructure profiles are presented and analyzed using the diffusion with trapping models.