CARBIDE REACTIONS AND PHASE-EQUILIBRIA IN LOW-ALLOY CR-MO-V STEELS TEMPERED AT 773-993 K - PART II - THEORETICAL CALCULATIONS

The coexistence of carbide phases in thermodynamic equilibrium was stu died by means of theoretical calculations and compared with experiment al results. The study was carried out for systems corresponding to exp erimentally used low alloy Cr steel (approx. 2.5 wt%) with different M o (0.70-1.0 wt%) and V (0.02-0.32 wt%) contents. The carbon content in experimental materials was about 0.1 wt%. Theoretical calculations we re realized by the PD-pp software and the modelling of thermodynamic e quilibria in the Fe-Cr-Mo-V-C system for the concentration correspondi ng to the experimental materials based on the Hillert-Staffansson subl attice model was carried out. The equilibrium phase coexistence of car bidic phases with b.c.c. ferrite was calculated in dependence on tempe rature. It was found that M23C6 carbide is thermodynamically stable fo r low temperatures (up to 800-850 K) and then gradually replaced by M7 C3 for higher temperatures. M6C is stable up to 890-950 K and dissolve s completely. MC carbide was found to be thermodynamically stable for all temperatures for systems with vanadium content above 0.1 wt%. Good qualitative agreement between the theoretical and experimental result s was reached. (C) 1997 Acta Metallurgica Inc.