Several Solidification Characteristics of As-Cast Heavy Section Ferritic Ductile Iron

摘要

Cooling curves of center of heavy section ductile iron cuboid castings.with wall thickness from 350millimeters to 500 millimeters.have been recorded.Several solidification characteristics have been analysed.Because of very slow cooling rate in heavy section ductile iron,especially in its thermal center.crystallization advances in the equilibrium pattern .Primary nodular graphites that precipitate in hypereutectic ductile iron may degenerate or break up into pieces due to slow cooling speed and no austenite shell outside those nodular graphites to protect them.These degenerated or broken graphites act as nucleation sites for eutectic graphites and lead to a special divorced eutectic crystallization that is different from the normal ductile's divorce growth and result in the formation of chunky graphites. As hypereutectic iron take those pieces of primary graphites as its nucleation base, it's eutectic crystallization speed is fast and releases much latent heat and so makes it's eutectic temperature higher than that of hypoeutectic ductile iron. By contrast, hypoeutectic iron does not produce chunky graphites. To eliminate chunky graphites and get a high elongation and tensile strength of cast iron,it is important to control the chemical composition of heavy section ductile iron not to exceed eutectic composition ,that is.carbon equivalent should be less than 4.3 percent.The role of the chiller and the effect of antimony.are also analysed in this article.