Simulation , optimization and development of thermo - chemical diffusion processes by Yingying Wei

An alternative, subcritical-temperature austenitic nitriding process is evaluated in this paper with the purpose of accelerating the treatment cycle, optimizing the hardness gradient in nitrided layers and seeking improved process control. Comparing to the conventional nitriding processed at ferritic temperature and followed by slow cooling, the alternative process includes nitriding at elevated austenitic temperature in nitrogen-dilute ammonia atmospheres and cryogenic quenching in liquefied nitrogen. This paper presents results of experimental work on low alloy steel I I en the nitriding and cooling conditions as well as phase transformations. Thermodynamic models were used to design the process conditions and assist in the microstructural interpretation of the nitrided layers produced. Results were verified using the SEM, EPMA, and microhardness measurements. It is observed that in subcritical temperature austenitic nitriding, nitrogen absorption rate is accelerated, a transitional nitrogen-austenite sublayer is formed between compound layer and diffusion zone, meanwhile, the hardness in diffusion zone increases with the cryogenic soaking time ranging from 1 hour to 48 hours, suggesting that more mechanisms are operational than the classic nitrogen martensitic 36 transformation. The kinetics of interstitial diffusion and layer growth in non-equilibrium gas nitriding atmosphere are also discussed.