Estudo dos parâmetros da nitretação a plasma no Inconel 718

Abstract

Nickel superalloys, such as Inconel 718, are known for their excellent mechanical properties and corrosion resistance at high temperatures and aggressive environments. However, when submitted to thermochemical treatments in order to enhance tribological properties, like wear resistance, these materials can display a reduction in their characteristic properties. With that in mind, this research’s goal is to evaluate the influence of plasma nitriding time and temperature in Inconel 718’s microstructure and hardness. To do so, several specimens were nitrided for 4 h, 8 h and 27 h, in temperatures of 400 ºC, 450 ºC and 500 ºC. Afterwards, the superficial hardness of these samples was measured, microhardness profiles were determined and x-ray diffractometry (XRD) and energy-dispersive spectroscopy (EDS) analysis were conducted in order to determine the phases and precipitates formed in the nitrided layer, as well as scanning electron microscopy (SEM) to determine the case depth and to evaluate the layer morphology. From the analysis performed, it was concluded that the plasma nitriding treatment of Inconel 718 resulted in the formation of nitrided layers for all of the cycles, resulting in case depths of 8.5 μm in average and a four-fold increase in the surface hardness. Moreover, the increase in treatment time and temperature resulted in an increase of superficial microhardness and layer thickness, although this trend is negligible when results are associated to measurement uncertainty. It was also concluded that the layer observed in all nitriding conditions is formed by an upper region composed by S phase, austenite and CrN, and a lower region most likely containing S phase, austenite and nitrides with lower quantities of nitrogen such as Cr2N. The quantity of each component, mainly in the upper layer, is temperature dependent. Finally, the formation of the nitrided layer was found to be linked to the crystallographic orientation of the grains on the material surface.