The Structure of Cast Irons

John Campbell

doi:10.4028/www.scientific.net/msf.925.86

Abstract

There appear to be two main growth mechanisms for graphite in cast iron i) Coupled eutectic growth forms of gray irons which are classical growth modes of simultaneous parallel growth of graphite and austenite, not reliant on a bifilm mechanism. These are necessarily fine structures as a result of their control by the rate of diffusion of carbon in the liquid. These structures are well understood. (ii) Uncoupled eutectic mechanisms which appear to be much less well understood, including (a) growth of graphite on silica bifilm substrates floating freely in the melt, forming such structures as A-type graphite flakes. This prediction appears to have now been confirmed by direct observation. The transition to (b) nodular morphology occurs by Mg eliminating the silica bifilms by an exchange reaction. In this way the substrates for flake growth are instantly removed, and graphite can now wrap completely around nuclei, thereby growing as a nodule. Graphite structures in heavy sections such as chunky graphite may now be understandable in terms of the reorganisation by flotation of bifilms and/or nuclei.

Highlights

Bifilms are those defects in liquid metals which result from the turbulence of stirring and pouring, in which the surface oxide film becomes entrained in the melt
The entrainment mechanism ensures that the submergence of the surface film necessarily occurs with the upper dry surface of the film contacting an opposed upper dry surface, thereby forming a double film with no bonding between the two films
Both surface films and internal bifilms are common in cast irons

Summary

Introduction

Bifilms are those defects in liquid metals which result from the turbulence of stirring and pouring, in which the surface oxide film becomes entrained in the melt. The entrainment mechanism ensures that the submergence of the surface film necessarily occurs with the upper dry surface of the film contacting an opposed upper dry surface, thereby forming a double film with no bonding between the two films. For this reason the bifilm acts as a crack in the liquid. The structure and the properties of irons appear to be greatly influenced by the number, size and composition of the bifilms they contain. This short paper is a brief overview

Films in Cast Irons

Alloy Irons

Ductile Irons

Classical Coupled Eutectic Growth

Other Film Defects in Irons

Conclusions