Catalytic Effect of Carbonates in Iron–Graphite Compacts

Abstract

Additions of certain alkali metal carbonates to irongraphite compacts cause the generation of a carburising gas atmosphere within the pores during sintering, and carbon alloying takes place efficiently and homogeneously at temperatures as low as 850°C if sintering is conducted in an inert gas furnace atmosphere. The process is believed to invoke the same catalytic cycle of reactions as that which is involved in pack carburising. However, in the sintering case only those carbonates which react with carbon to produce alkali metal are effective, and then only if the vapour pressure of the alkali metal is high at the sintering temperature. Both sodium and potassium carbonate are efficient catalysts at 850–950°C. It is necessary to add more carbonate than will theoretically sustain the catalytic cycle within the pores. The excess is required to ensure that there is a net flow of reaction products from the pores, so that furnace gases cannot enter and dilute the carburising pore gases before all the graphite in the compact has reacted. PM/0399