Nitride hardening of pure iron and iron-base alloys

Abstract

1. The solubility of vanadium and niobium nitrides in high-purity iron at 750° is 0.045 and 0.016%, respectively. These values can be taken as the maximum solubility of vanadium and niobium nitrides in α iron. The solubility of VN and NbN in γ iron increases by jumps and then increases sharply with rising temperatures, reaching ∼0.2 and 0.1%, respectively, at 1200°. 2. In the quenched condition the hardness of iron alloys with vanadium and niobium nitrides is higher than that of pure iron. The maximum hardening of alloys based on pure iron is reached with aging at 600–650°, when particles with a size <100 A are precipitated, which are evidently coherently bound to the ferrite lattice. 3. Undissolved particles of vanadium and niobium nitrides inhibit austenite grain growth, raising the initial temperature of grain growth, the effect being greater in alloys with NbN. In alloys containing VN grain growth occurs mainly by the "fusion" mechanism, but by the "migration" mechanism in alloys with NbN. 4. The basic laws established for alloys of pure iron with VN and NbN are also characteristic of more complex alloys that also contain C, Si, and Mn. 5. The presence of nitride-forming elements (V, N, Nb) in the solid solution increases the strength of iron (∼10–12 kgf/mm2) but hardly changes or leads to a slight increase of the ductile-brittle temperature. Precipitation of nitrides during aging induces somewhat greater hardening (by ∼13–14 kgf/mm2) but leads to a sharp rise of the ductile-brittle temperature (by ∼150–200°). In alloys with a ferritic-pearlitic structure the hardening effect is larger (as much as 30–40 kgf/mm2) due to precipitation of nitrides in the process of decomposition of austenite and grain refining; the ductile-brittle temperature of alloys hardened with nitrides is somewhat higher (by 20–30° for unalloyed) or lower (by 20–30° for alloyed) for alloys without nitrides. The hardening effect due to precipitation of nitrides during aging is negligible (5–7 kgf/mm2) for alloys with a structure of temper martensite, but the ductile—brittle temperature is lower (by 20–30°) despite the hardening.