Abstract
From the viewpoint of protective atmosphere for heat treatment, the effects of annealingatmospheres on the primary and secondary graphitization in making black heart malleablecast iron were studied. Namely, the relations between the graphitizing process of cementitein ledeburite or pearlite structures and the annealing atmospheres were investigated by observingthe graphitizing behaviour of several single atmospheres such as Ar, N2, H2 CO, CO2, H20 and CH4, as well as some practical furnace atmospheres which contain hydrogen as acomponent such as the mixed gas of N2-H2 system, the charcoal gas generated by blowingwet air and the exothermic gas produced through partial burning of propane. Results obtainedare as follows.When heated in the elemental gas which contains hydrogen such as 112, H20 or CH4, atomichydrogen penetrated quickly into the cast iron. In these atmospheres, the number of graphiteparticles formed was decreased and simultaneously the diffusion of carbon in austeniteor ferrite matrix was lowered because the meta-stable cementite in Fe-C system was stabilizedremarkably with dissolved hydrogen. In consequence, primary graphitizing was retardedand secondary graphitizing almost stopped. In the other annealing atmospheres the penetrationinto the cast iron was so sluggish that the difference from that in the argon atmosphere wasnot recognized.In every annealing atmosphere except hydrogen, the “bull's eyes” structure was formed bythe furnace being cooled after primary graphitization was completed. However, it was foundthat free ferrite phase was not precipitated in the hydrogen atmosphere. It was observedthat the degree of super cooling and the dilatic value at Al transformation became lowerthan in the other atmospheres.When heated in various practical furnace atmospheres, both primary and secondary graphitizationwere disturbed rapidly with rising hydrogen and water vapor contents. The particularpreventive tendency of dissolved hydrogen was very remarkable at the secondary graphitizing stage.
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