TEMPERING OF IRON-CARBON-NITROGEN MARTENSITES

The tempering behavior of ternary FeCN martensitic specimens, with a total amount of interstitials of about 5.5 at. pct and carbon and nitrogen contents between about 1.5 and 3.9 at. pct, was investigated in the temperature range 110 to 830 K. Analysis of the corresponding changes in crystalline structure (X-ray diffraction), volume (dilatometry), and hardness and enthalpy (calorimetry) revealed that the following processes occurred: (a) martensitic transformation of retained austenite between 110 and 200 K; (b) redistribution of interstitials in martensite up to 370 K; (c) formation of nitrogen containing alpha" precipitates and carbon containing epsilon/eta precipitates between 370 and 450 K; (d) conversion of alpha" nitride into gamma' nitride and coarsening of epsilon/eta-carbide between 450 and 560 K; (e) decomposition of retained austenite above 540 K; and (f) conversion of epsilon/eta-carbide into cementite above 570 K. Significant precipitation of carbon and nitrogen together, as nitrocarbides or carbonitrides, was not observed. From a comparison with the tempering behavior of binary FeC and FeN alloys of similar interstitial content, it was concluded for the ternary FeCN alloy that the transformation of the transition nitride (alpha") into the "equilibrium" nitride (gamma') was advanced and that the precipitation of the transition carbide (epsilon/eta) and its conversion into the equilibrium carbide (cementite) and the decomposition of retained austenite were retarded.