The Effect of Graphitization on the Decarburization of Cast Iron (Ist Report)

Four kinds of cast irons (2.5%C, 1% or 2%Si, 0.1%Mn) were cast into chill mould, quenched in oil from 900° after being kept for 10 minutes in charcoal powder, or annealed in charcoal powder after being plated with Cu previously.Using those specimens the decarburization phenomena at 900°, 950° or 1000° (2 hrs for each) in CaO powder were examined from microstructures and weight changes. Generally, cast iron in easier to decarburize when carbon exists in combined state (cementite) than when it exists as free graphite. Cementite is easy to be dissolved into austenite and be decarburized at high temperature because of its unstability, while graphite is difficult to be dissolved because of the strong stability. Hence, the more the special elements like Cu or Si which promote graphitization are added in white iron, the more difficult the becarburization becomes.In 1%Si, 1%Si-1%Cu and 2%Si irons, the specimens as cast are easier to be decarburized at first than the quench-annealed ones, for in the former the decarburization proceeds by dissolving graphite into austenite. But, afterwards the specimens as cast become more difficult to be decarburized than the quench-annealed ones. It can be explained as follows : after the completion of the graphitization, the decarburization proceeds by dissolving graphite into austenite.As the graphite particle size which is very large in the former, is very fine in the latter, the decarburization becomes easier in the latter than in the former.The quenched specimens are the easiest to be decarburized; the quench-annealed 1%Si-1%Cr iron was not graphitized but its cementite was globularized, and the iron was the easiest to be decarburized. The quenched specimens in which cementite is easy to be globularized are easier to be decarburized than the specimens as cast in which cementite is difficult to be globularized.