Gaseous carbonitriding of high carbon chromium bearing steel: Correlation between composition, microstructure and stability

Gaseous carbonitriding of AISI 52100 steel was investigated under various processing conditions. The process parameters studied involve ammonia content (2.5–7.5 vol%), carbon potential (0.8–1.2%), and temperature (780–880 °C). The carbon/nitrogen concentration-depth profiles that developed during carbonitriding were determined by light element analysis.

The microstructural and residual stress evolution, and the thermal-mechanical/dimensional stability, associated with the dissolving of carbon and nitrogen, were studied by reflected light microscopy, X-ray diffraction, hardness indentation and dimensional analysis, respectively. The resulting carbon/nitrogen concentration depth-profiles and microstructure of the carbonitrided layer vary with the combination of parameters applied, associated with the carbonitriding thermodynamics.

The content of retained austenite and the state and distribution of residual stresses in the carbonitrided specimen depend strongly on the total content (distribution) of carbon and nitrogen dissolved in the carbonitrided surface layer. Carbonitriding leads to exceptional tempering resistance over conventional treatment (austenitization and quenching), but results in inferior dimensional stability due to the higher content of retained austenite.