The crystallographic structure and the chemical composition of precipitated δ-phase, δ′-phase and ε-phase were extensively investigated on those specimens which had been prepared from vacuum melted high-purity Fe-Nb-C, Fe-Nb-N and Fe-Nb-C-N alloys containing niobium up to 0.95%. The specimens were solution treated at 1300 or 1350°C for 2hr, and then these were again quenched into water after long time holding at respective temperatures of 1000 to 1300°C. The quantitative determination of soluble-niobium, -carbon and -nitrogen was also investigated in order to calculate the solubility products of niobium-carbide and -nitride in austenite. Results obtained are as follows;
1) The chemical composition of pure δ-niobium-carbide precipitates was determined to be NbC0.87 though the precipitated δ-phase was not pure carbide but carbonitride even in the Fe-Nb-C alloys containing little nitrogen.
2) The precipitated δ′-nitride in Fe-Nb-N alloys containing little carbon was substantially carbonitride containing a little amount of carbon, while the precipitated ε-nitride was estimated to be a pure niobium-nitride NbN.
3) The solubility products of niobium-carbide and -nitride in austenite were given respectively by the following equations;
log [%Nb][%C]0.87=-7, 700/T+3.18
log [%Nb][%N]=-10.150/T+3.79
4) It has been proved that oxygen has the stronger affinity to niobium than carbon and nitrogen in austenite. The preferential precipitation of a little amount of tapiolite [(Fe, Nb) O2 or FeO. Nb2O5] was recognized in all specimens.
