From the sixties on, it is generally accepted that during the solidification of cast iron, graphite is formed directly from the melt and does not result from a decomposition of previous formed carbides or super-saturated austenite.
Nevertheless, it should be noted that under certain circumstances, graphite can form by these mechanisms, as is proved by the production of malleable cast iron and the occurence of graphitization in high carbon steels.
Critical literature reviews as published on the various pages of A New Approach to the Solidification of Cast Iron, showed that many additional assumptions have to be made to justify the presence of a graphite austenite eutectic and even then it remains difficult to explain the numerous graphite shapes that can occur in cast iron.
In The Iron carbon double diagram, a fundamentally wrong concept! it was shown that present day's views of the iron carbon diagram are not correct. A revised diagram is shown in Proposal for a new iron carbon diagram, in which the solidification of cast iron is governed by the formation of under-cooled and thus carbon super-saturated austenite. The actual mechanism of graphite formation in cast iron has thus to be revised too.
A graphite formation mechanism, which is normally not encountered in cast iron metallurgy, concerns the effects of metallic substrates on the decomposition of hydrocarbon gases [1,2]. In this process, iron, nickel or cobalt metallic foils are heated to 700-11000 C in a hydrocarbon atmosphere. As a result, platelet graphite is formed at the metallic surface. From these researches it was concluded that the metal substrate acted as a catalyst to transform carbon from the source into graphite. It seemed likely that the metal surface provided a medium for the diffusion of mobile carbon atoms, thus facilitating crystallite growth, the nature of the carbon source being unimportant to the process.
Catalytic reaction as the driving force behind graphite formation in cast iron has, to the author's best knowledge, never been proposed, but offers a unique opportunity to unify all existing graphitization mechanisms.
Process
Carbon_source
Condition
Second_phase
C_content
Temperature
Cast iron
Melt
Liquid
Austenite
Saturated
1150
Malleable
Iron carbide
Solid
Austenite
Saturated
>900
Steel
Iron carbide
Solid
Austenite
Saturated
>700
Pyrolysis
CH gas
Gaseous
Ni/Co/Austenite
Saturated
>800
This table shows the common factors related to all graphite formation mechanisms that occur in Iron Carbon alloys. These common factors are: -a high carbon source, -elevated temperature and -the presence of an austenite substrate. In all cases this austenite substrate is saturated with carbon before graphite will be deposited.
Does any indication exist that in the case of cast iron solidification, austenite acts as a catalytic substrate for graphite formation? Only in a few cases, austenite has been mentioned as possessing a "nucleating effect" on graphite formation in cast iron [3,4], showing that graphite forms directly on austenite.
A perfect explanation on the role of austenite in graphite formation was given in the case of hypereutectoid steel [5,6]. A summery of [6] is given below:
Heating experiments in vacuum of a hypereutoid steel, showed that surface graphite was formed on cooling, when the specimens were heated to temperatures below the austenite solubility limit for carbon.
Microscopic examination of the specimens at room temperature indicated that the rate of surface graphitization was sensitive to the orientation of the austenite.
The orientation dependence was determined from the traces of two different annealing twins in two surfaces mutually perpendicular to each other. The amount of graphite on these surfaces was the visually estimated from photomicrographs. The relationship between the austenite orientation and the amount of surface graphite is shown in figure 1. Surfaces whose orientation were near (111) were the most heavily graphitized. Surfaces which contain little or no graphite have an orientation near (100), the furthest from (111).
Figure 1, Effect of surface orientation on the amount of surface graphite formed in a 0.9% C iron-carbon alloy at 7060 C (3 min.).(according to [6]).
An electron diffraction pattern from extracted graphite is shown in figure 2. The orientation of the graphite layer is such that the basal plane of the graphite layer is parallel to the free surface. This orientation of the graphite layer was found for all the specimens studied.
The explanation of the marked orientation sensitivity of the surface graphitization rate evidently lies in the close matching that exists between the graphite basal plane and the austenite (111) plane as suggested by [5].
Figure 2. Electron diffraction pattern of surface graphite formed in a 0.88 pct C iron-carbon alloy at 7060C (according to [6]). The lattice parameter of austenite containing 0.88 pct C at 7000C was estimated to be 3.64 A from the work of Esser and Muller.
The lattice parameter of graphite was assumed to be the same as at room temperature.
The best fit between the lattices occurs when (111)A II(001)G and [101]AII[100]G. With this orientation relationship, the carbon atoms of the basal plane layer of graphite fit into the positions which iron atoms would normally fill in the ABC stacking sequence except that both the B and C positions are filled by the first graphite layer. The separation of these positions is 1.48 A on the austenite (111) plane while the interatomic distance between carbon atoms in the basal plane of graphite is 1.42 A, only a 4 pct misfit.
Fig.3 shows a 111plane of a unit cell.
A (111) projection of the austenite with the basal plane of the graphite lattice superimposed on it in such a way as to obtain the best possible fit is shown in figure 4.
Figure 4 Lattice matching of austenite (111) plane and graphite basal plane for a 0.88 pct C iron-carbon alloy at 7060C. (according to [6]).
If all new findings published on this Web site are combined with the above mentioned mechanism as described by G.R.Speich, then graphite formation in all cast irons can be explained by one simple mechanism. As under conditions known so far, austenite formation in cast iron compositions, always occurs with under-cooling and thus super-saturated with carbon, diffusion of carbon immediately after deposition of austenite starts within these dendrites. Only at sites within these dendrites where (111) planes are exposed, excess carbon will be deposited, using the austenite plane as template to form graphite. It remains a challenge to find the positions of these specific sites favourable for graphite deposition.
References:
[1] T.Baird, J.R.Fryer, B.Grant,
Carbon formation on iron and nickel foils by hydrocarbon pyrolysis-reactions at 7000C, Carbon, Vol.12 (1972), pp.591-602.
[2] F.J.Derbyshire, A.E.B.Presland, D.L.Trimm,
Graphite formation by the dissolution-precipitation of carbon in cobald,nickel and iron. Carbon,Vol.13 (1975),pp.111-113.
[3]K.D.Lakeland,
Directional solidification of Fe-C eutectic alloys containing various percentages of sulfur,
AFS Cast Metals Research journal, Vol.2 (1966), pp.78-86.
[4]K.R.Olen, R.W.Heine,
A revision of the Fe-C-Si system,
AFS Cast Metals Research journal, Vol.2 (1968), pp.28-43.
[5]M.J.Olney, G.C.Smith,
Surface effects occurring during the heating and cooling of plain carbon steels.
Journal of The iron and Steel institute, October 1959, pp.107-116.
[6}G.R.Speich,
Surface Graphitization of a Hypereutectoid Iron-carbon alloy,
Transactions of the metallurgical society of AIME, Vol.221 (1961), pp.417-419.
Figure 1, Effect of surface orientation on the amount of surface graphite formed in a 0.9% C iron-carbon alloy at 7060 C (3 min.).(according to [6]).
Figure 2. Electron diffraction pattern of surface graphite formed in a 0.88 pct C iron-carbon alloy at 7060C (according to [6]).
Fig.3 shows a 111plane of a unit cell.
Figure 4 Lattice matching of austenite (111) plane and graphite basal plane for a 0.88 pct C iron-carbon alloy at 7060C. (according to [6]).