Nitriding of Fe--V alloys

Fe-V alloys containing 1.06 wt percent V, 5.23 wt percent V and 15.55 wt percent V, have been nitrided in purified NH3 gas in temperatures ranging between 7730K and 11730K. The nitriding kinetics of all these alloys in this temperature range obey a parabolic rate law. The comparison of the nitriding rate constants evaluated from experimental results and from the calculations based on Wagner's internal oxidation model show a deviation which is explained in terms of the effect of the lattice strains on the solubility and diffusivity of nitrogen in the Fe matrix. The hardness of the nitrided zone increases with the vanadium content in solution and reaches a saturation value of about 1300 VHN (12.75 GNm-2) which corresponds to about 4 wt percent V. The hardening in the nitrided region is caused by the precipitation of VN which cannot be observed on specimens nitrided at the lower limits of the temperature range. Precipitates grown in size can be seen on specimens nitrided at 10730K and 11730K