Influence of Phosphorus, Tin, and Heat Treatments on Matrix Structures in Nodular Cast Irons

The matrix of cast iron is usually pearlite and ferrite. There are two types of pearlite; namely lamellar and granular. Recently, only the lamellar is called pearlite and granular is called spheroidite. This paper discusses the formation mechanism of both types using the granular types. The influence of P, Sn, and heat treatments on matrix structures in nodular cast irons (chemical composition is 3.65%C －2.5%Si －0.3%Mn －0.04%Mg) were studied. Group P added with 0mass% to 0.2mass% P and Group Sn added with 0mass% to 0.06mass% Sn were studied. The samples were heat-treated to obtain spherical pearlite in matrix structures using method A (heated to the intermediate temperature between Ac1 and Ar1) and method B (two step heat treatment). The results of these experiments showed that the matrix structure of cast sample to be lamellar pearlite. The matrix of samples treated by method A was spherical pearlite When the spherical pearlite observed in optical micrographs (2-dimentional morphology) were stereographically observed by SEM, the morphology was found to be stick-like cementite in samples heat-treated at 973K, 1023K, and coral-like cementite heat-treated at 1073K. The forming conditions of stick-like were pound to be related to the content of dissolved carbon in austenite. This investigation shows that spherical pearlite is obtained at heating temperatures between Ar1 and Ac1.