Gas phase deep nitriding of 33CrMoV12-9 steel: a microstructural characterization of the precipitates

With the increasing severity of operational conditions in terms of speed, load, temperature and environment applied to aerospace work pieces, suitable solutions must be developed to prevent parts failure. This is the case for gears and shafts used in power transmission applications. Thermochemical processes such as gas phase deep nitriding allows the improvement of mechanical properties (hardness and fatigue strength) of such structural parts. It is based on the incorporation of atomic nitrogen at the surface of the metal, obtained by heterogeneous reaction of ammonia at the interface of the gaseous atmosphere and the metal in the range of temperature of 380°C to 580°C. Nitrogen diffuses and reacts with high affinity elements, such as chromium, to precipitate semi-coherent nitrides of nanometer scale. These precipitates are responsible for the superficial hardness improvement as well as the creation of the residual compressive stresses within the nitrided layer. The aim of this study is to determine the morphology of the precipitation that occurs during the process and correlate it to the properties of the nitrided layer. Thus gaseous deep nitriding of low alloyed steel 33CrMoV12-9 was performed at 520°C for more than 100 h with a nitrogen potential (K_N) of 5 atm^1/2. Results will be discussed on the basis of microhardness profiles, nitrogen and carbon depth profiles obtained by electron probe microanalysis (EPMA), crystallographic considerations with high resolution transmission electron microscopy (HRTEM) observations and chemical composition of the precipitates measured by electron diffraction spectroscopy (EDS) analysis. It will be shown that a partial substitution of chromium with iron atoms occurs in the semi-coherent finely dispersed chromium nitrides (CrN) precipitates. This can be related to the excess nitrogen phenomenon, which manifests itself by a higher experimental nitrogen mass fraction than what can be calculated by thermodynamics.