Internal carbonitriding behavior of Ni-V, Ni-Cr, and Ni-3Nb alloys

Ni-2V, Ni-5V, Ni-12V, Ni-10Cr, Ni-20Cr, and Ni-3Nb alloys were carbonitride d in C3H6 and NH3 gas mixtures (bal. H-2) over the range 700-1000 degrees C . Carbonitridation of Ni-12V and Ni-20Cr in C3H6/NH3/H-2 (1.5/1.5/97 v/o) a nd (1.5/10/88.5 v/o) produced duplex subscales consisting of near-surface n itrides with underlying carbides. Growth of each zone obeyed the parabolic rate law under most conditions. The presence of carbon generally did not ef fect the depth of the nitride zones compared to nitriding the alloys in NH3 /H-2 (10/90 v/o). However, at 700 degrees C, the nitride zones were deeper in the carbonitrided Ni-V alloys and Ni-20Cr. The presence of nitrogen gene rally increased the depth of the carbide zones in Ni-12V and Ni-20Cr compar ed to carburizing these alloys in C3H6/H-2 (1.5/98.5 v/o). VN, CrN, and NbN formed in Ni-V, Ni-Cr, and Ni-Nb alloys, respectively, whereas the underly ing carbide layers contained V4C3 in Ni-12V, Cr3C2 above a zone of Cr7C3 in Ni-20Cr, and NbC in Ni-3Nb. The solubilities and diffusivities of nitrogen and carbon in nickel were determined Nitrogen and carbon each exhibited re trograde solubility with temperature in pure Ni in both carbonitriding envi ronments. Nitrogen diffusion in nickel was generally lower in each carbonit riding mixture compared to nitrogen diffusion in a nitriding environment, e xcept at 700 degrees C when nitrogen diffusion was higher. Carbon diffusion in nickel was generally higher in the carbonitriding environments compared to carbon diffusion in a carburizing environment.