The eutectic morphology of three grades of gray cast iron samples is shown in the figure.
Eutectic clusters in gray cast iron are a mixture of austenite and graphite. At room temperature, the eutectic contains many small essential grains, such as pearlite + cementite or ferrite + graphite. On the outside of the matrix (eutectic grain boundary), there are often more impurities such as sulfur and phosphorus and other mesophase, which presents a network in the middle, so it is also called white phase. It is not only different from ordinary carbides, but also different from the matrix. It contains a variety of alloy elements that are easy to form carbides. The strength and hardness of the white phase are higher than that of the matrix, so the boundary of the eutectic is also a strengthening phase. At low magnification, it forms a white network, in which there is often a thick flake pearlite tissue.
Formation of eutectic group: eutectic group is formed in the liquid metal between primary dendrites in the second stage of solidification of cast iron. When single or multiple eutectic grow, sulfur, phosphorus and other impurities and some alloy elements are enriched in the eutectic grain boundary. Cast iron eutectic is a combination (austenite, graphite or austenite, cementite). At room temperature, austenite can be decomposed into eutectic mixture of pearlite or ferrite and graphite. After nucleation, graphite and austenite grow cross at the same time. Austenite nucleates with the base surface of graphite as the substrate, and finally forms a eutectic group of graphite austenite two-phase symbiotic growth centered on each graphite core.
The effect of eutectic on the mechanical properties of gray cast iron. Generally, the smaller the size of eutectic and the more the number of eutectic per unit area, the better the mechanical properties of gray cast iron. It can be seen from Fig. 3-3 that the grain size of ordinary HT250 eutectic is large, and the eutectic distribution presents an uneven grid shape, which is easy to produce stress concentration and reduce its mechanical properties. The eutectic grains of low alloy HT250 and HT300 added with trace alloying elements are obviously refined, the number of eutectic clusters per unit area is increased, and the eutectic grain size is more uniform, which improves the tensile and fatigue properties of gray cast iron.