Solidification structure
5.5 Cast irons
5.5.2 Entrained films
Castings
the sand grains that confers the improved smoothness to the cast surface. The action is that shown in Figure 2.2 for all film-forming alloys.
History now appears to have turned full circle because some resin binders for sands have recently been developed to yield iron castings with reduced incidence of lustrous carbon defects. It is not clear whether the surface finish of the castings has suffered as a result.
Lustrous carbon films cause troublesome defects in lost-foam castings where the foam consists of polystyrene. In this situation the vaporization of the polystyrene to styrene, and the subsequent decomposition of the styrene to lower hydrocarbons, deposits thick carbonaceous films on the advancing surface of the iron (Figure 4.9 shows the decomposition products). Gallois et a1 (1987) found the film to consist of three main layers: (1) an upper lustrous multilayered structure of amorphous carbon; (2) an intermediate layer of sooty fibres consisting of strings of crystallites; and (3) a layer adhering strongly to the surface of the iron consisting of polycrystalline graphite enriched in manganese, silicon and sulphur. Clearly there has been some exchange of solutes from the iron into the film.
It is probably worth bearing in mind that the visible evidence of the thermal decomposition of hydrocarbons C,H, on the surface of the melt is the surface film of remaining carbon. But there is also an invisible legacy: the hydrogen that will have dissolved in the liquid.
Graphite films that have grown on molten iron have been studied in the form of crystals formed on the surface of Fe-C alloys held in graphite crucibles, and so saturated with carbon before being allowed to cool. These shiny black sheets that float on the surface are analogous to the kish graphite that separates from hypereutectic cast irons during cooling. (For the scientifically minded, the graphite films in this research had interesting features. They were single crystals with numerous cracks along certain crystal directions, and hexagonal growth steps on the underside that showed how the film grew by gradual deposition of carbon atoms, probably on to ledges from emergent screw dislocations.)
The growth of graphite on melts saturated with carbon, as above, is easy to understand; but how does graphite grow in the case of lustrous carbon films where the composition of the iron is far from saturated? Such films should go into solution in the iron! Every student of metallurgy knows that at the eutectic temperature the carbon in solution in iron has to exceed 4.3 weight per cent before free (kish) graphite will precipitate. At higher temperatures the carbon concentration for saturation increases, following the liquidus line for the solidification of kish graphite in hypereutectic irons, as is clear from the Fe-C phase diagram.
In an atmosphere containing hydrocarbons, if the rate of arrival of reactants at the free liquid surface is low, then both carbon and hydrogen can diffuse away from the surface into the bulk liquid.
However, in a highly concentrated environment of hydrocarbon gases the rate of arrival of reactants may exceed the rate of diffusion away into the bulk. Thus carbon will become concentrated on the surface (hydrogen less so, because its rate of diffusion is much higher) and may exceed saturation, allowing carbon to build up at the surface as a solid in equilibrium with the local high levels of carbon. Once formed, it would then take time to go into solution again, even if the conditions for growth and stability were removed. Thus it would appear to have a pseudo stability, with a life just long enough so that in some conditions the film could be frozen into the casting if a chance event of surface turbulence were to enfold the surface into the melt.
Recent research has indicated that the conditions for the growth of the graphite film on liquid metals are similar to the conditions required for the growth of diamond films. Reviews by Bachmann and Messier (1984) andyarborough and Messier (1990) list conditions for the growth of diamond as the breakdown of hydrocarbons and the presence of hydrogen. In the case of iron, the temperature is a little too high, and would tend to stabilize the growth of graphite films. But for metals such as aluminium in a hydrocarbon environment, the conditions seem optimum for the creation of diamond on the metal surface. Prospectors and investors will be disappointed to note, however, that the rate of growth is slow, only one micrometre per hour. Thus in the time that most liquid metal fronts exist, the diamond layers, if any, will be so thin as to be a disappointing investment.
Solidification structure 16 1
course, lead to a variety of additional problems.
One potential problem is discussed below.
distinct possibility that many of the graphite flakes seen on a polished section of grey iron will not be formed as a result of a metallurgical precipitation reaction, but may be the remnants of entrained graphitic bifilms. The occasional appearance on microsections of what seem to be isolated large flakes amid uniform smaller flakes is suggestive of the bi-modal distribution to be expected if such a mixed source of graphite were present.
5.5.2.1 Nitrogen fissures in grey iron
Nitrogen fissures in grey iron castings are large cracks, often measured in centimetres, that appear to have been associated with the use of sand binders that contain high levels of nitrogen. They are an enigma that has never been satisfactorily explained.
The high-nitrogen binders that are blamed for these features usually contain amines, whose breakdown probably contributes both nitrogen and hydrogen to the liquid iron. However, although such binders have been associated with fissure defects, their use does not always result in fissures. Perhaps entrained bifilms, perhaps consisting of nitride films, are also required, so that the filling system may also be highly influential. Any involvement of the filling system has not previously been suspected, but would explain the confusion in results. Once entrained, the high hydrogen and nitrogen pressure in the iron might be sufficient to open any bifilms to some extent, revealing their presence as crack-like features.
5.5.2.2 Bifilms in ductile iron
The ductile casting industry has referred to entrained surface films as ‘dross stringers’. This name, based on their one-dimensional appearance on a polished section, has led to a comforting self-deception, concealing their obvious real nature as extensive two-dimensional defects in the form of films. The occasional appearance of clusters of graphite nodules that have floated up and been trapped under such
‘stringers’ corroborates their real nature as films.
Also, as we are now aware, if the film is solid, the entrainment process will fold them in dry side to dry side, thus forming a crack.
The films appear only at low temperature as we have seen, and seem to be mainly magnesium silicate, Mg0.Si02 (alternatively written as MgSi03) probably with a thick upper layer of solid MgO as discussed earlier. If the ductile iron is cast at a low temperature, and if the surface is entrained, the creation of seriously damaging bifilms is guaranteed.
Naturally, as the iron cools during its passage through the running system it is likely to cool to the temperature at which the solid film can form, so that defects will be expected in most filling systems in which surface turbulence is not controlled. Once entrained, the defects can, of
5.5.2.3 Plate fracture defect in ductile iron Like nitrogen fissures in grey irons, plate fracture in ductile irons has also never been satisfactorily explained. Ductile irons, should, of course, always exhibit a ductile mode of failure. Sometimes, however, a casting will exhibit poor strength and poor elongation to failure, with the fracture surface appearing to consist of large embrittled grains. These unpredictable events give rise to serious concern that the material is not under the proper control that either the foundry or the customer would like to see. Everyone’s faith is shaken. The question naturally arises, ‘Is ductile iron a reliable engineering material?’ This is a question that should never arise, and that no one wishes to hear.
Following the description given by Karsay (1980) and Gagne and Goller (1983), the features of the plate fracture are large, flat, apparently brittle fracture planes, in ductile irons that should exhibit only ductile failure (Figures 5.46 and 5.47). When viewed closely, the planes are seen to be studded with small, irregularly shaped graphite spheroids, that are arranged with an accuracy almost resembling a crystal lattice (Figure 5.47b). The planes are nearly always close to a right angle with the cast surface, and grow mainly vertically. The plane is in a matrix that is somewhat lighter than the rest of the casting after etching. Karsay suggests that the colour
Figure 5.46 Plate fracture in the feeder neck of a ductile iron casting (Karsay 1980).
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