Solidification structure
5.5 Cast irons
5.5.3 Nucleation and growth of graphite The properties of most graphitic cast irons are
Solidification structure 163 distribution of the cracks control the properties.
(The analogy with light alloys containing a high density of bifilms is compelling! In the case of spheroidal graphite iron the spheroids are analogous to the convoluted form of the bifilms, whereas grey irons are analogous to the aluminium alloys with unfurled bifilm cracks.)
In the past, little attention has been paid to the structure of the iron dendrites, nor the as-cast grain size of the iron matrix. Despite the scientific interest of such questions, the approach seems actually sound and pragmatic and, in general, is adopted here.
This is a case where the as-cast matrix structure is accepted as relatively unimportant. The important features are (i) the high density of defects (the graphite particles acting as cracks) that dominate properties like elongation and ductility, and (ii) the room temperature structure of the metallic matrix, whether ferritic or pearlitic, etc., that dominates strength and hardness.
In view of the massive research effort devoted to cast iron, and the many books written on the subject, it may seem unnecessary to add to this impressive literature. Certainly, a review of cast iron properties is not intended. Nevertheless, recent thinking is assisting to clarify some of the traditional mysteries such as inoculation. Thus it is worthwhile to outline some of these new concepts.
The nucleation of graphite in cast irons by the deliberate addition of foreign nuclei is called inoculation. Inoculation of cast irons is beneficial to achieve a reproducible type and distribution of graphite, so important for the achievement of reproducible mechanical properties and good machinability.
Successful inoculants include ferrosilicon (an alloy of Fe and Si, usually denoted FeSi, and usually containing approximately 7.5 weight per cent silicon), calcium silicide and graphite. These are added to the melt as late additions, just prior to casting. Additions designed to work over a period of 1.5 to 20 minutes are used in a granular form, of size around 5 mm diameter, whereas very late additions (made to the pouring stream) are generally close to 1 mm. Late inoculation is carried out because the inoculation effect gradually disappears;
a process called ‘fade.’
Ferrosilicon is the normally preferred addition, and is known as a ‘clean’ inoculant. Calcium silicide is known to be a rather ‘dirty’ addition, almost certainly because the calcium will react with air to give solid CaO surface films (in contrast to FeSi that will cause liquid silicate films). The CaSi addition would probably be much more acceptable with better-designed filling systems that reduce surface turbulence, as is the case of ductile iron spherodized with magnesium.
It is immediately clear that the common inoculant FeSi does not perform any nucleating role itself.
difference may be the result of a higher Si content in this region. Finally, in this region, there is a high incidence of small inclusions that appear to be mainly magnesium silicates.
All these features are consistent with the defect being an oxide bifilm, probably a magnesium silicate, explaining the high Si content and the higher inclusion content, and possibly malformed spheroids as a result of local loss of Mg. The planar form arises from the bifilm being pushed by the raft of austenite dendrites a n d organized into an interdendritic sheet, similar to that commonly seen in other alloy systems (Figures 2.41-2.44). The vertical orientation is explained by the greater rate of heat transfer from the base of the casting where gravity retains the contact with the mould, so that these grains grow fastest and furthest. In addition, the buoyancy of the magnesium silicate bifilm will encourage its vertical orientation, and so assist the advancing dendrite to straighten the film. Spheroids in interdendritic regions would then be revealed at the regular spacing dictated by the dendrite arm size (normally, a section at a random angle to the dendrite growth directions would obscure this natural regularity that is almost certainly present in all ductile iron structures. Thus it should not be looked upon as a defective structure in itself, as has occasionally been assumed.) The bifilm probably disintegrates to some extent because of its surface energy tending to spherodize it; the high temperature also assisting this effect. What remains are the changes in chemistry and numerous silicate fragments as inclusions to encourage the direction of growth of the crack that finally causes failure.
Other features of plate fracture are its occurrence in slowly cooled regions, such as in a feeder neck.
This may be the result of the lower rate of growth allowing the dendrites to straighten films more successfully (at high growth velocity, the drag resistance of films would resist dendrite growth, and resist film straightening).
The less common appearance of plate fracture in irons of higher carbon equivalent value (above 2.9 per cent CEV), and its reduction in resin-bonded sand moulds reported by Barton (198.5) is probably not so much the result of a more rigid mould but an indication that the entrainment of the oxide film is less damaging in this m o r e carbonaceous environment.
5.5.3 Nucleation and growth of graphite
This is because liquid iron at its casting temperature is above the melting point of the FeSi intermetallic compound, so that the whole FeSi particle melts.
The evidence now suggests that the molten inoculant continues to exist as a high Si region in the liquid iron. Although the Si-rich region is liquid, and the iron is liquid, and the two liquids are completely miscible, the two nevertheless take time to inter-diffuse. This time is probably the fade time.
The Si-rich region slowly dissipates in the melt, eventually disappearing completely. However, in the meantime it provides a local environment with a highly effective carbon equivalent value (CEV).
To get some idea of the scale and importance of this effect it is instructive (although admittedly not really justified as we shall see) to calculate the carbon equivalent in one of these regions. For an iron of carbon content about 3 per cent, assuming CEV = (per cent C)
+
(per cent Si/3) we have CEV= 3
+
75/3 = 28 per cent C. Extrapolating the carbon liquidus line on the equilibrium diagram to an iron alloy with 28 per cent C predicts a liquidus temperature in the region of several thousand degrees Celsius. (This is actually not surprising because graphite itself has an effective melting point of over 10 000°C.) Clearly therefore there seems good reason for believing that the carbon in solution in the Si-rich regions is, in effect, enormously undercooled. It is a form of artificial constitutional undercooling (because the graphite is effectively undercooled as a result of a change in the constitution of the alloy).Now, in reality, it is not appropriate to extrapolate the CEV beyond the eutectic value of 4.3 per cent
I7O0 ~
C. In fact, when this part of the equilibrium phase diagram is calculated, the liquidus surface is nothing like linear, as seen in Figure 5.48 (Harding et al.
1997). Even so, this figure shows the liquidus in the hypereutectic region to be very high, so that the essential concept is not far wrong. The path of the dissolving particle is marked on the figure, confirming its progress though high constitutional undercoolings through high Si regions, where it will experience large driving forces for the precipitation of graphite.
The size of the driving force is almost certainly the reason why, over the years, so many different nuclei have been identified for the initiation of graphite. It seems that even nuclei that would hardly be expected to work at all are still coaxed into effectiveness by the extraordinary undercooling conditions that it experiences. Studies have shown that many particles that are found in the centres of graphite spherules, and thus appear to have acted as nuclei, are also seen to be floating freely in the melt of the same casting, having nucleated nothing (Harding et al. 1997). This is understandable if the nuclei are not particularly effective. They will only be forced to act as nuclei if they happen to float through a region that is highly constitutionally undercooled.
Studies by quenching irons just after inoculation have revealed a complex series of shells around the dissolving FeSi particle. Although FeSi itself contains almost no carbon, the carbon in the cast iron diffuses into the liquid FeSi region quickly.
Data from Figure 1.8 and Equation 5.21 indicate a time of 1 s for an average diffusion distance d =
40 60 80 100 Figure 5.48 The Fe-FeSi phase diagram showing Wt. % inoculant possible melting and mixing routes for a dissolving
FeSi inoculant particle (Harding et al. 1997).
01 + SIC
Solidification strticture 16.5
where they are needed, in the heart of the highly undercooled region.
This action of the inoculating material in providing a combination of good growth conditions and copious heterogeneous nuclei explains the action of graphitizers such as ferrosilicon, and the importance of the traces of impurities such as aluminium and rare earths that raise the efficiency of inoculation.
Ferrosilicon and calcium silicide are not, of course, the only materials that can act as inoculants.
Silicon carbide (Sic) is also effective, as is graphite itself. Both of these materials can be seen to provide in a similar way the transient conditions of high constitutional undercooling that are needed for the nucleation of graphite in cast irons.
Jacobs et al. (1974) were probably the first to carry out some elegant electron microscopy to demonstrate that within graphite nodules there is a central seed of a mixed (Ca,Mg) sulphide, surrounded by a mixed (Mg, Al, Si, Ti) oxide spinel.
There are matching crystal planes between the central sulphide, the spinel shell, and the graphite nodule, indicating a succession of nucleating reactions. This exemplary work has been confirmed a number of times, most recently by Solberg and Onsoien (2001).
However, because the undercooling is high at this initial time, once nucleated, graphite will be expected to grow dendritically as thin flakes (analogous to metal growth at high undercooling 0.1 mm, and 100 s ford = 1 mm. The flow resulting
from the buoyancy of the high Si melt, and the internal flows of metal in the mould cavity, will smear the liquid Si-rich region into streamers, reducing the diffusion distance to give the shorter estimated times of homogenization of carbon. Thus the shell of S i c particles around a dissolving FeSi particle (Figure 5.49) appears logical as a result of the high undercooling in the part of the phase diagram where S i c should be stable (Figure 5.48).
It seems likely that the S i c nucleates homogeneously because of the high constitutional undercooling. In a shell further out from the centre of the dissolving inoculant particle, graphite starts to form. It seems that graphite may not simply nucleate homogeneously by the generous undercooling but can also form in this region by the decomposition of some of the S i c particles.
If all this were not already complicated enough, there is even more complexity. In addition to the local solute enrichment from the dissolving particle there will also be a release of sundry complex inclusions including oxides and sulphides.
Commercially available inoculants contain various impurities, and various deliberate additions that supplement the natural nucleating action in this way. At least some of these may be good heterogeneous nuclei for the formation of new graphite crystals (or perhaps new S i c crystals that will subsequently transform to graphite particles).
Also, of course, these particles are provided exactly
Graphite SIC
100 pm
Fe-Si phases
Figure 5.49 Microsection of a dissolving FeSi particle in a ductile iron, quenched ,from the liquid state
(Bachelot 1997)
as thin dendrites). It seems unlikely therefore that the initial form of graphite is spheroidal as has often been supposed. Later, at the edges of the supercooled region, the thin dendritic form will start to coarsen, its form becoming more bulbous (Figure 5.50). As the embryonic particles of graphite move further out into the open liquid, growth conditions will reverse; the particles will become unstable and start to dissolve. Even so, of course, many will be expected to survive to approach the solidification front of the austenite, where their instability will be reduced. They will become fully stable when the eutectic is reached, and finally grow once again as further cooling takes the metal below the equilibrium eutectic temperature.
This complex chain of nucleating effects has the outcome that graphite particles exist in the melt at temperatures well above the eutectic. The prior existence of graphite particles in the liquid at high temperature, well above the temperature at which austenite starts to form, is quite contrary to normal expectations based on the equilibrium phase diagram, but explains many features of cast iron solidification. The expansion of graphitic irons prior to freezing (the so-called ‘pre-shrinkage expansion’) has in the past always been difficult to explain (Girshovich et nl. 1963). The existence of graphite spheroids growing freely in the melt above the eutectic temperature has been a similar problem, seemingly widely known, and seemingly widely ignored, but now provided with an explanation, despite the desirability of much confirmatory effort over future years.
Whether the subsequent growth of graphite occurs in the form of flakes or spheroids is a completely separate issue, unrelated to the nucleationhnoculation treatment. This is a growth problem. The separate nature of the problem can
I
Figure 5.50 Coarsening of gruphire particles on enierging froni the undercooled FeSi region (Benail!
19%)
be appreciated from a close look at the graphite structure around some central nucleating particles.
The structure in graphite spheroids close to the nucleating particle is usually seen to be highly irregular (Figure 5.51). The graphite form in this region appears almost turbulent. Clearly, after a very short growth distance, the crystallographic orientation is not under any influence of the nucleating particle. However, after a small further distance, the graphite organizes itself, and develops its nicely ordered radial grains typical of a good spheroid. Thus the organization of the growth takes time to develop, and is a macroscopic phenomenon.
The analogy with the planar growth condition of a metal under conditions of low constitutional undercooling is striking. The spheroidal growth has been widely proposed to be the result of a detailed atomic mechanism. For an elegant exposition the reader is recommended to the classic paper by Double and Hellawell(l974). However, in addition, if not actually dominant, the growth form almost certainly has at least some contribution from macroscopic influences. To influence the roundness of the growth form, a mechanism must act on the scale of the spheroid itself. Such mechanisms might include (i) a low constitutional undercooling condition in the surrounding liquid when in the free-floating state, or (ii) a mechanical constraint imposed on the expanding sphere when surrounded by solid, but plastically deforming, austenite. It is just possible that (iii) some adsorption on the surfaces of the growing crystal may be important.
There are no shortages of theories on this issue, and facts are hard to establish.
5.5.4 Nucleation and growth of the matrix