Microstructure Change of Decarburized Spheroidal Graphite Cast Iron during Cooling

Microstructure of decarburized spheroidal graphite cast iron is affected by the temperature and time of decarburization, as well as by cooling rate thereafter. To make clear these decarburizing conditions, experiments and numerical simulation were carried out . The isothermal growth rate of decarburized layer obtained experimentally is in good coincidence with that calculated formerly. Precipitations of ferrite and carbon take place beside decarburization during cooling. On further cooling below eutectoid temperature, ferritization proceeds in hyper-eutectoid zone of decarburized layer as well as non-decarburized part, while ferrite-pearlite mixed structure is formed in hypo-eutectoid zone. The latter striking structure interposed by ferritic matrix zones is called “pearlite band'¥Numerical simulation shows that pearlite band is likely to be formed when the product of the initial decarburized depth by the square root of cooling rate is larger than about 0.01 cm' CK/s) 1/2 for cooling from 1273 K, and 0.006 cm' CK/s) 1/2 for cooling from 1173 K.