The microstructures in several soft-nitrided steels have been examined by using the electron probe microanalysis on cross-section samples and the X-ray diffraction method combined with the polishing technique from sample surface. The volume fraction changes of ε-, γ'-, and/or α-Fe phases in the direction of the depth could be estimated from the integral intensities of X-ray diffraction peaks. The compound layers which included the porous layers, were characterized the following cases: (1) In the case of fused bath soft-nitriding, the compound layer was mainly constructed with ε-phase. At the upper of it, there was the porous layer which included M3O4 and γ'-phase. Then at the inner of it, there was evidently γ'-phase. The ε-phase lattice constants were clearly decreased with depth. (2) In the case of gas soft-nitriding, the compound layer was mainly constructed with ε-phase. At the upper of it, there was the porous layer which included M3O4. The ε-phase lattice constants were decreased with depth, though the changes were slight compared to the case (1). (3) In the case of ion nitriding, the compound layer was mainly γ'-phase. α-Fe phase and ε-phase existed at the upper of it, and the volume fraction of ε-phase increased with depth. The grain boundaries in the diffusion layers of all samples were nitrides. Therefore it was thought that the grain boundaries in the matrices were preferentially nitrided in the nitriding processes. As the nitriding processes for the samples were discussed by using the analytical results and the Fe-N-C phase diagram near the temperature of 600°C, it was suggested that the above cases of (1) and (2) were treated in the carbon-poor and rich atmospheres respectively.
