Thermodynamic model-assisted evaluation of phase transformations in subcritical austenitic nitriding

摘要

Conventional nitriding of low-alloy steels is a multi-hour surface hardening process carried out at ferritic temperatures and concluding a complete heat treatment cycle: normalizing, austenitizing, martensitic quenching and tempering. An alternative, subcritical-temperature austenitic nitriding process is evaluated with the purpose of accelerating the treatment and optimizing the hardness and toughness of nitrided layers while minimizing the distortion of steel parts treated. The alternative process involves cryogenic nitrogen quenching as well as aging. This paper presents results of experimental work on AISI 4140 steels, examining the interplay between the nitriding and cooling conditions and phase transformations in both ferritic (525°C) and subcritical, nitrogen-austenitic (610°C) processes. Thermodynamic models, used to design process conditions, are applied also in the microstructural interpretation of nitrided layers, examined via SEM, EPMA and EDS. Kinetics of interstitial diffusion and phase transformation-based dimensional control of nitrided parts are also presented.