Anomalies in graphite crystallization taking place in iron-carbon alloys, particularly in cast iron, that always entail drastic deterioration of material properties, above all the mechanical and dynamic capacities of a material, are referred to as graphite degeneration.
Since this defect always occurs in connection with the chemical composition of iron, with melt control, inoculation treatment (s. Inoculation), and in any case with the presence of interfering and/or accompanying elements, it can be found with any molding and casting processes. It has also been observed that the defect is dependent on wall thickness (see Influence of wall thickness).
Graphite degeneration in flake graphite cast iron
Two typical and frequently occurring types of graphite degeneration in gray cast iron (s. Flake graphite cast iron) are:
1. Supercooled graphite (e.g. D graphite according to EN 20945, Figures 1 and 2), that becomes visible in metallographic specimen in the form of a cohesive net. Strictly speaking, this is not graphite degeneration yet; but at least unfavorable graphite distribution which will have a massive influence on the dynamic and mechanical properties of the material.
2. Widmannstätten graphite (Figure 3), characterized by particularly pointed ends and projections of the graphite lamellae whose surfaces may be covered in what looks like a fringe. This defect can also be made visible in metallographic specimen.
Graphite degeneration in nodular graphite cast iron
Significant deviations from the spheroid shape to such an extent that burst spheres (the graphite appears as if complete spheres had been crashed through subsequent explosion), great proportions of flake and vermicular graphite (partly with rounded, partly with pointed lamellae), “crab" or “spikey” graphite (star-shaped graphite type with graphite points with notch effect) are visible in the metallographic specimen (Figures 4 to 10).
In addition, nodular graphite degeneration is present underneath the casting skin as a result of magnesium loss along the surface during solidification. Rim zone degeneration up to type III (s. Classification charts for graphite form acc. to EN 20945, and Rim zone graphite degeneration) in depths of some 1/10 of mm are common and can be hardly prevented. However, massive rim zone degeneration up to type I according to EN 20945 may occur in thick-walled castings and mold material with high sulfur contents, reaching down to depths of several millimeters below the surface and having a greatly negative effect on dynamic properties (Figure 11).
Slight deviations from the spheroid shape that always occur in nodular graphite cast iron are tolerable, whereas greater proportions of degenerated graphite (quasi flakes) from around 10 % of the overall graphite quantity may imply considerable losses with regard to mechanical properties. With increasing proportions of degenerated graphite, it is impact resistance that is reduced first, followed by elongation and tensile strength. In contrast to that, the 0.2% yield strength remains nearly unaffected by degenerated graphite. This is important insofar as the most critical value for engineers and designers, i.e. the yield strength is the most impervious to quality losses.
The following modes of formation are viable:
Flake graphite cast iron:
1. Supercooled graphite
In cast iron with eutectic composition the graphite eutectic is diffused during gray solidification (austenite graphite). The eutectic grain increases spheroidally (Bild 12) and the diffused graphite in the eutectic grain branches off in the shape of a dendritic structure. Shape and size of the eutectic grain and the graphite skeleton depend on the growth of the grain. This growth is mainly influenced by the composition of the cast iron, the number of nuclei in the melt, overheating and the cooling rate. Therefore, the graphite form changes accordingly (Figure 13).
In the micrograph the graphite skeleton appears in the form of graphite flakes after cutting. Consequently, it becomes clear that these elements are by no means independent and unconnected bodies but branching of the skeleton. Since every graphite skeleton develops from a crystal nucleus or crystallizer, many eutectic grains and graphite skeletons are generated when a large amount of nuclei is available.
Under technical conditions, the eutectic gray solidification starts not immediately when reaching the equilibrium temperature, however, with more or less delay (= supercooling). Rapid solidification and/or a low number of effective nuclei for graphite crystallization may increase supercooling of the eutecticsolidification in such a way that it does not start before the temperature falls below the equilibrium temperature of white solidification leading to the feared edge hardness and poor processing properties of the entire casting (Fig. 14).
The beginning of eutectic graphite diffusion depends on the presence of effective crystallization nuclei. A great number of nuclei results in less supercooling and fine eutectic grains. The cell borders are punctured by graphite lamellae to a far lesser extent and therefore function as the load-carrying regions in cases of tensile loads. Consequently, fine types of eutectic grain have strength-increasing functions.
In melts that have not undergone supercooling, coarse primary graphite (C graphite – hypereutectic graphite) is predominantly diffused in hypereutectic cast iron.
Rosette graphite (B graphite) predominantly originates from melts with slight or medium supercooling while high supercooling leads to interdendritic graphite (D graphite and E graphite).
Increasing superheating of the iron prior to casting and/or insufficient or missing inoculation, i.e. a very low nucleation of the iron, leads to a decrease in eutectic grain and increase in grain size. Depending on the cooling rate (wall thickness, molding process), this results in dentritic solidification (s. Solidification type) with very fine intermediate graphite eutectic.
2. Widmannstätten graphite
Lead impurities (s. Lead) cause pointed, deltoid graphite diffusion and deposits at the lamellae that are also referred to as Widmannstätten graphite. For this scenario the lead content may even be below the minimum detection limit applicable for wet-chemical analysis (<0.004 %). This type of graphite can only be identified in very finely polished microtome cuts and with considerable magnification. The pointed, needle-like projections increase notch sensitivity and reduce endurance strength of the material.
Nodular graphite cast iron
In cast iron, trace elements influence the graphite precipitation shape, the ferrite/pearlite ratio and the tendency towards metastable solidification.
In particular with nodular graphite cast iron, a number of trace elements lead to degeneration of the spheroidal graphite formation which is why these elements are referred to as “interfering elements”. Such interferences were shown in the presence of lead, bismuth, antimony, tin, arsenic, aluminum, cadmium, silver, uranium, gallium, zinc, tellurium, thallium, selenium, boron, indium, phosphorus, sulfur, oxygen, titanium, vanadium, zircon, magnesium, cerium, lanthanum, yttrium and thorium. When present at higher concentrations, copper (especially in combination with any of the above elements), manganese and nickel (!) also result in graphite degeneration.
The trace elements regarded as technically significant are classified as elements with direct (lead, bismuth, tin, antimony, arsenic, magnesium, copper) and indirect (titanium, aluminum) effect. However, an additional classification of the direct-acting elements into traces practically insoluble in iron and those slightly soluble in iron is also necessary.
The group of trace elements nearly insoluble in iron mainly includes lead and bismuth. A noticeable interference effect on graphite formation can already be detected at a concentration of these elements in the range of 1·10-3 %. The interference effect starts with the occurrence of vermicular graphite inclusions in the immediate vicinity of ideally structured spherulites. Once a certain concentration limit (interference threshold) has been exceeded, a sudden drop in the degree of nodule formation occurs. The graphite then precipitates either in a lamellarpattern with split processes or as compact inclusions structured like temper carbon. The secondary graphite can preferably be seen in a Widmanstätten pattern.
The interference threshold concentration is largely defined by the cooling rate, with an additive effect originating from lead and bismuth elements. With low additions of Bi an increase in the nodule count can be identified; this fact is made use of in Bi-containing inoculants. The considerably pearlitizing effect caused by slightest bismuth and lead concentrations must be taken into account.
Other direct-acting interfering elements having low solubility in austenite, which mainly includes antimony, zinc, arsenic, magnesium (copper), progressively affect graphite formation as the level of interfering elements increases. The interference effects start with necking at the spherulite surface, proceeding to “crab” graphite, “exploded” graphite, and “spiky” graphite at higher concentration .
The extent of the interference effect is not so much dependent on the cooling rate, but rather on the fraction of indirect-acting interfering elements. A cumulative effect of the elements in this group is probable.
Only titanium can be definitely counted among the indirect-acting interfering elements. However, aluminum obviously also belongs to this group, even though this element also showed direct interferences.
The degree of interference by titanium and aluminum is exceptionally dependent on the material’s trace element concentration and to a large extent on the cooling rate. The interference with graphite formation starts with increasing titanium and aluminum levels, being hardly detectable at first (partly, an improvement of graphite formation may even be seen). When certain concentrations are exceeded, titanium and aluminum result in a rapid drop in the degree of nodule formation, with the appearing graphite shapes (in particular flake graphite, partly in the vicinity of well-formed spherulites) being similar to interferences caused by lead and bismuth (Fig. 7).
The effect of trace/interfering elements, in particular titanium, lead and bismuth, is generally enhanced as the wall thickness increases. Moreover, their effect is cumulative, i.e. low concentrations of several trace elements which would be without effect if only one element were present have a detrimental effect on nodule formation if co-existent.
As has been previously known, the detrimental effect fo interfering elements in gray-solidified Fe-C alloys is the higher the thicker the casting walls and/or the lower the cooling rate. The interfering elements that are initially present in the melt in dissolved condition can only reach the graphite/melt phase boundary by diffusion. Consequently, a clearly time-dependent process, i.e. diffusion of interfering elements, takes place. If a melt containing interfering elements is only given little time for diffusion of those elements due to high cooling rates, the elements are only capable of traveling short distances; and if, in certain scenarios with low interfering element contents, the distances are too large, the elements will not be able to reach the phase boundary. Consequently graphite crystallization takes place without the presence of interfering elements although they are analytically measurable.
If interfering element levels increase and/or the cooling rate decreases, the interfering elements will have enough time to diffuse to the graphite melt boundary surface, and the diffusion paths will be reduced at higher interfering element levels, disrupting graphite crystallization.
The difficulties encountered in formation of nodular graphite in hypoeutectic, gray solidifying Fe-C alloys in comparison with eutectic alloys are presumably caused (among others) by the delayed growth of the austenite shell and above all by the effect of interfering elements accumulating in the eutectic residual melt which is increased with decreasing degrees of saturation. Consequently, the regions in which graphite crystallization takes place display greater concentrations of interfering elements than measured by chemical analyses.
Influences resulting from mold material parameters may also cause nodule degeneration in the rim zones of castings during casting of nodular graphite cast iron. Graphite degeneration is found not only in mold materials which contain sulfur but also in those which do not. The oxygen content in the mold material (MgO formation) is seen as the cause for this.
However, the most common cause can be found in the sulfurization of the rim zone through phenol or furan resin-bound mold materials which are hardened with PTS. In this process, the sulfur in the mold material uses up the dissolved magnesium which forms nodular graphite on the surfaces where the mold material touches the iron through the formation of magnesium sulfide, meaning that there is no longer any available to form nodular graphite. The extent of this sulfurization depends on the wall thickness, the sulfur content in the mold material and the casting temperature. The spilt-off of the sulfur is a temperature-dependent process.
Measures for prevention (acc. to S. Hasse, FT&E):
Flake graphite cast iron
1. Supercooled graphite
2. Widmannstätten graphite
Thorough inspection of all charge materials for contaminations, lead above all. If copper is used as alloying element, the presence of lead must be particularly taken into account.
Nodular graphite cast iron
1. Use of pure charge material, with as low trace element contents as possible.
The following sources can be specified for the major interfering elements:
2. Utilization of cerium or rare earths for neutralization of interfering elements. Cerium or cerium mixed metal may be present in the master alloy or be added separately. The concentration of cerium or cerium mixed metal required for compensation of interfering elements amounts up to 0.005% of cerium mixed metal and is obviously very low. Attention: With pure charge materials cerium functions as interfering element (s. Chunky graphite).
3. Reduction of the residual magnesium content to the required minimum content (0.05 %).
4. Optimum late inoculation for achieving maximum nodule counts, good nodularity, reduction of nodule diameter, and supercooling.
5. Adjustment of a carbon equivalent at 4.2 to 4.3 % for thick-walled parts and use of chills or chill plates, as necessary.
6. Keeping hardener contents in the form of synthetic resin-bound elements, particularly PTS, as low as possible.
7. Control of sulfur accumulation in reclaimed material (S < 0.15%).
8. Use of facing coats and particularly thick application.
9. Casting at possibly low temperatures if feasible.