Microstructure and graded hardness control in Fe-Cr alloy by plasma nitriding technique

Fe-Cr alloys have been nitrided by plasma process in the temperature range of 773 to 893 K to develop a controlled nitrided microstructure. Typical microstructures of nitriding layers were observed on the cross section of specimens plasma nitrided at temperature below 823 K. A novel nitriding layer of stripe pattern was observed on the cross section of Fe-13Cr alloy, which was plasma nitrided at 873 K for 176.4 ks. This microstructure consisted of sub-layers, which seems to have different dissolved chromium content each other. Also it was observed that the hardness gradually increased from the specimen surface toward the nitriding front before changed sharply to the same level as matrix hardness. Such phenomena can not be explained only by nitrogen diffusion process during the nitriding, but also diffusion of alloying elements i.e. Cr should be considered to contribute significantly to the stripe-pattern microstructure. A series of frictional wear examination result showed that such nitrided layer properties should be a good candidate for improving wear resistance.