Phase Transformations during Low Temperature Nitrided Inconel 718 Superalloy

Abstract

The effects of the salt bath nitriding parameters on the microstructure, microhardness and corrosion behavior of Inconel 718 superalloy at temperature ranging from 425–500°C were investigated by X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and corrosion test. Experimental results indicated that the microstructure and phase constituents of the surface nitridization are highly process dependent. When Inconel 718 superalloy was subjected to salt bath nitriding, the predominant phases of the nitrided layer were identified as expanded austenite (S phase), austenite and CrN. The thickness of the nitrided layer increased with the time of nitridization. Meanwhile, salt bath nitriding improved the surface hardness dramastically. The maximum value of hardness measured from the treated surface was 2100 HV_0.1 after 16 h at 500°C, which is about 5 times as hard as the untreated material (420 HV_0.05). Proper low temperature nitriding can improve the erosion corrosion resistance. The sample that was nitrided for 4 h at 475°C had the best corrosion resistance.