Solidification structure
5.5 Cast irons
5.5.1 Films on liquid cast iron
been investigated. Perhaps these are indications that Sr aids the liquid metal to wet the dry inner sides of the bifilm, and thus creeps between the films, separating the two halves. The separation is only apparent in those cases where iron-rich particles are attached. If there is any truth in this suggestion, such twin intermetallic phases should not, of course, contain a central crack, and both sides on the intermetallic would be expected to be well bonded to the matrix. The question also arises, what happens to the entrained air between the films? This would be pushed ahead of the advancing liquid, and might be exuded as a bubble. Alternatively it may react and be consumed by reaction with the advancing fresh interface.
In summary, the most likely explanation of the action of Sr is that most operators using poorly designed gravity filling systems will benefit because the new bifilm defects introduced by pouring are partially healed. Some will enjoy a useful net benefit if the increase of hydrogen can be controlled. In contrast, those operators using a process such as the Cosworth process will have few new films and so cannot benefit from any healing process, and will only suffer from the extra porosity as a result of any increase in hydrogen. The refinement of the eutectic, much sought-after by metallurgists and assumed to be the main mechanism of property enhancement, appears to have little effect either way.
In passing, it is noted that even with the Cosworth process, where gas content of the continuously processed liquid metal is nicely controlled, the hydrogen level will rise during its passage into the mould because of the reaction with the organic sand binder. In heavy sections, there will be several minutes for the hydrogen to diffuse into the casting sections. From Figure 1.6 the coefficient of diffusion of hydrogen is seen to be close to 10" m's-'. From Equation 5.21, for a typical time around 100 s the average diffusion distance f o r hydrogen in aluminium is close to 10 mm. Thus the gas will easily penetrate the thickest sections of castings such as automotive cylinder blocks.
This interesting observation calls into question the self-imposed task that the foundry adopts to reduce hydrogen levels. Clearly, if sand casting, or even semi-permanent mould casting (i.e. a metal mould with sand cores), the degradation of the sand binder will always raise the hydrogen level at the worst moment, as the melt enters the mould.
Extremely low hydrogen content of the melt therefore is not feasible.
It seems inescapable therefore that the really important quality requirement should perhaps be the absence of nuclei for pores, i.e. absence of bifilms. This means really clean metal and excellent designs of melt handling. In such a case, the gas content probably need not be controlled.
Solidification structure IS7
a dry, solid film, rather grey in colour. This film cannot be removed by wiping the surface, since it constantly re-forms.
At a temperature of l30O0C, and in alloys that contain some manganese, it is clear from the Ellingham diagram that MnO is the least stable, S i 0 2 is intermediate and CO the most stable. Thus manganese is oxidized away preferentially, followed by silicon and finally by carbon. The contribution of MnO to the film at this stage may reduce the melting point of the film, causing it to become liquid.
At around 1200"C, iron oxide, FeO, contributes to the further lowering of the melting point at the ternary eutectic between FeO, MnO and S O 2 . If sulphur is also present in the iron then MnS will contribute to a complex eutectic of melting point 1066°C (Heine and Loper 1966a).
The author finds that, in general terms, the above considerations nicely explain his observations in an iron foundry where he once worked. For a common grade of grey iron, the surface of the iron was seen to be clear at 1420°C. As the temperature
-
I
C a, 0 W
L
c
-
2 2 -.-
._ I v) Q 0 E I + - 0
0
preferentially, and is therefore lost at a higher rate than silicon, as is seen in Figure 5.44. Here the blowing of air on to the surface of a small crucible of molten metal serves to accentuate the effect.
Silicon is observed to fall only after all the carbon has been used up. At this high temperature no film is present on the melt - any silicon oxide, S O 2 , would be immediately reduced to silicon metal, which would be dissolved in the melt, simultaneously forming CO, which would escape to atmosphere.
At around 1420°C the stability of the carbon and silicon oxides is reversed. The exact temperature of this inversion seems to be dependent on the composition of the iron; de Sy (1967) reports a range of 1410-1450°C for the irons that h e investigated, whereas the Ellingham diagram (Figure 1 S ) predicts an inversion temperature for pure Fe- C alloys of about 1500°C. The agreement is, perhaps, as good as can be expected because Merz and Marincek point out that the inversion temperature is sensitive to composition. Below approximately 1400"C, therefore, S i 0 2 appears on the surface as
C 31FOPOp
\
.A \\
1300°C
\.
'
si A \A
\
GFigure 5.44 Change in composition of 3.6 kg qf molten grey iron held in LI
.-=--.-.
Mn I- -.
I k - A k - silica crucible, while air was directed2 3
k
over its surface at the rate of 22 mUs:1550°C
fell, patches of solid grey film were first observed at about 1390°C. These grew to cover the surface completely at 1350°C. The grey film remained in place until about 128O"C, at which temperature it started to break up by melting, finally becoming completely liquid at 1 150°C.
When casting grey iron in a n oxidizing environment, the falling temperature during the pour and the filling of the mould will ensure that the surface film will be liquid at this critical late stage.
If it becomes entrained in the molten metal it will therefore quickly spherodize into compact droplets.
The droplets are of much lower density than the iron, and so will float out rapidly. On meeting the surface of the casting they will mutually assimilate, and be assimilated by, the existing liquid film, and so spread over the casting surface. The glassy sheen of some grey iron castings may be this solidified skin. The harmless dispersal of the oxide film in this way is the reason for the good natured behaviour of cast iron when cast into greensand moulds; it is one of the very few metal/mould combinations that is tolerant of surface turbulence.
Only on one occasion has the accumulation of liquid oxide at a casting surface given the author some problems. This was in a grey iron casting where a small amount of surface turbulence was known to be present just inside the ingate, because it was not easy to lower the velocity below 0.4 ms-' at this point, and was judged to be a negligible risk of any kind of internal defect. However, so much liquid surface was entrained, and so much floated out at a point just downstream, that the layer of surface slag accumulated at this location exceeded the machining allowance, scrapping the casting.
In ductile irons the entrainment of the surface is nearly always a serious matter. The small percentage of magnesium that is required to convert the iron f r o m flake t o the spheroidal graphite type dramatically alters the nature of the oxide film.
Above 1454°C Heine and Loper (1966b) find that the surface of liquid ductile iron remains clear of any film. Below this temperature a film starts to form, increasing in thickness to 1350"C, at which point the surface exhibits solidified crusty particles.
By the time the temperature has reached 1290°C the entire surface is covered with a dry dross.
Magnesium vapour distils off through dross since the molten iron is above the boiling point of magnesium. Presumably the oxidation of the vapour to powdery MgO at the upper surface of the dross is a major contributor, causing the dross to grow quickly and copiously. The dross makes life difficult for the ductile iron foundryman, forming films and agglomerating into dry, non-wetting heaps, which, if entrained, spoil otherwise excellent castings.
Ductile iron is renowned for being difficult to cast cleanly, without unsightly dross defects.
5.5.1.2 Graphite films (lustrous carbon)
The liquid film present o n cast iron at low temperature in an oxidizing environment has made iron easy to cast free from serious defects, This marvellous natural benefit of cast iron when cast into moulds made from sand bonded with clay and water must have played an important part in the success of the industrial revolution. In general, apart from a few infamous and tragic exceptions, the bridges did not fall into the river and the steam engines continued to power machinery. Later this benefit was to be extended to moulds made using one of the first widely used chemical binders: sodium silicate. This environmentally friendly chemical is still widely used today as a low-cost sand binder for the production of strong moulds (despite a number of significant disadvantages that some foundries are prepared to live with).
However, it is one of those ironies of history that the arrival of modern chemical binders was to change all this.
Binders based on various kinds of resins - furane, phenolic, acrylic, polyurethane, etc.
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were heralded as the breakthrough of the twentieth century. Indeed, the new binders had many desirable properties, making accurate and stable moulds, with excellent surface finish, good breakdown after casting and at good rates of production from simple low-cost equipment.However, when iron was poured into some of these early resin-bonded moulds, especially those based on polyurethane, a new defect was discovered.
It became known as lustrous carbon. Although it had been occasionally seen, especially if the volatile additions to greensand had been high, it was never so common nor so damaging. This shiny, black material resulted in casting skins wrinkled like elephant hide. Studies concluded that it was pyrolytic carbon (a microcrystalline form) that was deposited from the gas phase on to hot surfaces in the temperature range of at least 6.50-1000°C (Draper
1976).
The hot surface was always assumed to be the sand grains of the mould, and somehow the deposit was pushed ahead of the advancing liquid front, to become incorporated into the surface as folds (Naro and Tanaglio 1977). The explanation is clearly problematical on several fronts: in most instances where lustrous carbon causes problems the sand surface is rather cold, and thus incapable of chemically 'cracking' (i.e. breaking down) the polymeric gases to precipitate graphite. Also, it is difficult to imagine how a film deposited on the complex and rough sand grains could be detached from its grip on these three-dimensional shapes before the arrival of the liquid metal. After the arrival of the metal the film would be assisted
Solidification structure 159
The reader is recommended to Petrzela’s engaging chatty and candid account. He was clearly one of our great foundry characters. His writing contains other fascinating asides to some of his observations on the release of carbon from hydrocarbons. On one occasion he recorded a sooty deposit that had a fibrous, woolly appearance among (hot?) sand grains.
Other work has studied the generation of lustrous carbon in greensand moulds. It is clear that the mould atmosphere can provide a hydrocarbon environment for the liquid metal if sufficiently high concentrations of hydrocarbons are added to the sand mixture. Such additives help the mould to resist wetting by the metal, and so improve surface finish, as appreciated in the original work of Petrzela (1968), and later by Bindernagel et al. (1975). Excess additions have sometimes been claimed to give lustrous carbon defects. However, it is certain that the defects form only if the surface turbulence can cause the film to be entrained.
The mechanism for the improvement of surface finish by the addition of hydrocarbons to the mould repays examination in some detail. The carbon film forms on the front of the advancing liquid. It becomes trapped between the melt and the mould, and is held there by friction. Thus, as the meniscus advances, it is forced to tear, splitting apart. The film is therefore continuously formed and laid down between the melt and the mould by the advancing metal, as though the advancing metal were rolling out its own track like a track-laying vehicle. The film forms a mechanical barrier between the metal and the mould. It seems most likely that it is the mechanical rigidity of this barrier, helping to bridge in keeping its place by being held against the
surface of the sand grains by the pressure of the liquid.
The only explanation that fits all the facts is that the graphitic film forms on the surface of the molten metal itself. Photographs of lustrous carbon defects, particularly those seen on the fracture surfaces of parts that have suffered brittle failure, beautifully reveal their origin as the surface film on the liquid metal (Bindernagel et al. 1975; Naro and Tanaglio 1977). The caption to the photograph of the oxide skin on an aluminium alloy (Figure 5.45) could be changed to read that it was a graphitic skin on a grey iron; to the unaided eye the appearance of the two types of film is practically identical.
Part of the confusion that has surrounded the lustrous carbon film, and that has claimed that it deposited on sand grains, dates from a misreading of the brilliant original work by Petrzela (1 968).
This Czech foundry researcher devised a test in which he demonstrated that the vapours released from coal tar and other hydrocarbon additions to moulding sands would decompose, depositing carbon as a shiny, silvery film on a metal strip, resistance heated to at least 1300°C. In his test, it happened that the sand was also heated to this temperature. Thus he observed carbon to be deposited directly on to the sand grains in addition to that deposited on the heated metal strip. The mistake of subsequent generations of researchers has been to assume that lustrous carbon always deposits on to sand grains, even though, at the instant that the metal is filling the mould, the sand is usually nowhere near the temperature at which a hydrocarbon vapour could be decomposed.
Figure 5.45 ( a ) Oxide skin on liquid Al-9Si4Mg alloy wrinkled by repeated disturbance of the suflace: and ( b ) a cmss-section of the solidified metal. The appearance in both cases is extremely similar to graphite films on grey iron (courtesy of Agema and Fray 1990).
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.