Effect of aluminium and boron on the kinetics and morphology of graphite precipitation during malleablization

Abstract

The malleablization characteristics of white irons depend on the presence of certain elements. The effect of aluminium and boron on the kinetics of graphitization and morphology of the graphite nodules were studied in twenty iron-carbon-silicon alloys. In a particular group of four compositions, the aluminium was held constant, while boron was varied from 0.001 to 0.004% in expansion of the specimens during the graphitization reaction was determined by a dial gauge dilatometer immersed in a salt-bath held at a temperature of 950°C. The dilatation curves comprising initial thermal expansion, incubation period, followed by further expansion due to the prising initial thermal expansion, incubation period followed by further expansion due to the precipitation of graphite nodules, and finally no change in length, indicated completion of graphitization reaction. Simultaneously, test pieces of identical composition were withdrawn at predetermined time intervals, microstructures and hardness values of which disclosed the kinetics, the graphitization reaction and the rate of growth of graphite nodules. The morphology of the graphite nodules were studied by determining the rate of growth with time. It was found that the nodules grew at decreasing rate. It was observed that variation of boron at a definite level of aluminium did not materially alter the kinetics of malleablization, while decrease in aluminium promoted the graphitizing reaction. (Shri S.K. Palit, Senior Scientific Assistant, and Dr. A.B. Chatterjea, Scientist, National Metallurgical Laboratory. Dr. Chatterjea is now with M/s M.N. Dastur & Co.)