Ion nitriding of stainless steels. Real time surface characterization by synchrotron X-ray diffraction

Three types of steels with different Cr and Ni contents were ion nitrided by means of 100 Hz glow discharge in a N2-H2 gas mixture using a special reactor installed in the SAXS beamline of the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. The studied materials were austenitic (AISI 304 and DIN WNr 1.4882) and ferritic (SAE HNV3) steels. The experimental setup allowed the study of the structural evolution in situ and in real time of steel surfaces during the ion nitriding process itself, using the X-ray diffraction technique. The use of a well-collimated and intense X-ray beam of the synchrotron radiation source allowed successive short exposure diffraction patterns to be taken. A very fast ion nitriding process was verified for the three studied steels. The measurements concerning the nitriding kinetics of both austenitic steels indicated the formation of an S phase. Nevertheless, while in the AISI 304 steel, the S phase was later replaced by a compound layer of γ′-Fe4N, in the DIN WNr 1.4882 steel, the S phase remained until the end of the nitriding process. In both cases, CrN was formed during the process. Instead, in the ferritic SAE HNV3 steel, the S and CrN phases were not observed.