Carbothermic reduction of oxides during nitrogen sitnering of manganese and chromium steels

Abstract

To interpret nitrogen sintering of chromium and manganese steels without the formation of deleterious oxides, but with manganese and carbon modifying the local microclimate, the role of the volatile Mn and carbothermic reactions were considered. Reduction of Cr2O3 by Mn vapour is always favourable. CO is an effective reducing agent, however, whereas at atmospheric pressure it will reduce FeO at ~730°C, temperatures some 500 and 700°C higher, i.e. above those for conventional sintering, are necessary for reducing Cr2O3 and MnO, respectively. Accordingly partial pressures must be considered and the sintering process is modelled at a conglomerate of several surface oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. The relevant partial pressures were calculated and reduction reactions become thermodynamically favourable from ~200°C. Kinetics, however, dictates availability of CO and the relevant reactions are the water-gas, C + H2O = CO + H2 from ~500°C and the Boudouard, C + CO2 = 2CO, from ~700°C. Discussion of sintering mechanisms is extended to processing in semi-closed containers, also possessing specific microclimates.