Ideas and theories about the solidification mechanism of gray cast iron have been developed 30 to 40 years ago and only slight modifications have been made to it since.
A detailed description of the solidification mechanism of gray cast iron can hardly be found in present literature and if so, it remains limited to an explanation with the help a traditional cooling-curve as is shown in fig.1.
Solidification begins with the separation of primary austenite crystals as soon as the liquidus temperature is reached. This separation continues until after a certain undercooling and recalescence, the eutectic temperature is reached and the graphite-austenite eutectic forms [1] Attention has always been focussed on the formation of this eutectic, whilst the earlier formed primary phase remains virtual un-discussed. This fact was already recognized in the thirties when the very first publications on the primary solidification of gray iron appeared.However, our attitude has little changed over the past sixty years as is shown by the principles applied in solidification simulation software which were rapidly developing from the eighties on and which are mainly concerned with the eutectic part of the solidification [2,3,4].
From the relatively few publications that, at first sight do exist on the field of primary solidification of gray cast iron, however, it appears that the influence of the primary solidification on for instance mechanical properties is large and can even exceed the influence of graphite formation (the eutectic part of the solidification). Despite this, interest for primary solidification has remained limited. The following reasons may have played an important role in that:
1) Primary austenite dendrites are hidden within the matrix structure and will hardly be noticed.
Under normal conditions, these austenite dendrites must be made visible with special techniques.
2)Magnifications of 100 X or more, which are normally used for microscopic examinations, do not allow the recognition of bigger dendrite shapes.
3)Dendrites in cast iron possess a rather negative image. Dendrites, visible with the naked eye often appear together with casting defects, as in gas-defects or porosity's.
Since the introduction of standards for graphite shapes in 1941 [5], a relationship was established between dendrites and certain graphite types, which are generally regarded as undesirable for good mechanical properties. Reason for such structures are usual found in melting and treating practices of the material.Fig.2 shows typical D and E graphite,. The graphite phase outlines the dendritic structure of the matrix.
4) Common cast iron compositions, expressed as Carbon Equivalent, are in the range 0.8 to 1. This means that, according to the existing iron-carbon diagram, only a very limited amount of primary dendrites can be expected. If, however, more dendrites are found than should theoretically be allowed, it is assumed that this is caused by a slow-down of the eutectic solidification, which in turn causes under-cooling.Under-cooling also depends on melting technique and melt treatment, which means that the occurrence of dendrites seems to point to a less well controlled melting process, and no foundry engineer or metallurgist likes to be linked with this!
5)Importance laid on eutectic cells.
As a result of research efforts, undertaken by BCIRA during the fifties and sixties, attention was fully focussed on the formation of eutectic cells. Relationships were established between the number of cells and mechanical properties, inoculation and feeding characteristics [6,7,8]. A relative unimportant role was left over for the primary austenite phase. To emphasize the importance of the eutectic solidification, Oldfield[9] even suggests that it is possible to leave out the formation of primary austenite dendrites.
6) The importance laid on graphite-shape.
The shape, dimensions and distribution of graphite particles in cast iron play a dominant role on the final mechanical properties of the material. So it seems understandable that these aspects are emphasized in judging this material.
Macro-structure of the primary solidification.
Roll [10] was one of the very first to study the primary structure of cast iron. Still little was known on the way the primary structure formed . The visual appearance of cast iron fractures seemed to indicate that cast iron crystallizes in course grains (globular); fractures that showed a directional (dendritic) crystallization were rare. To find out whether cast iron solidified dendritic or globular, Roll investigated many cast iron samples of various compositions and origin. When a cast iron sample is strongly etched with an acid etchant like nital, the segregated fosfide network becomes visible. The lighter fosfide network in fig.3 seems to indicate a globular solidification.
A Baumann print of the same specimen, however, is shown in fig.4 and shows clearly that the primary solidification is dendritic.
Such a print represents the distribution of manganese sulfides in the sample. This phase, which was the very first to form, will be enclosed by austenite dendrites and delineates these.
The first method ( comparable with etching for eutectic cells) points to a completely different solidification mechanism as compared to the last one.
The wording dendritic and globular can easily introduce misunderstandings. Wlodawer [11] gives a better and clearer description of a dendritic zone and a globular region:
-The outer-zone consists of columnar grains , that is, the external boundary of these grains is columnar. In the interior of these columnar grains are dendritic crystals.
-The core consists of roughly rounded grains which are locally flattened and resemble those of eutectic cells. In the interior of these rounded or globular grains are also the dendritic crystals as indicated in fig 5.
Patterson and Engler [12,13] interpret solidification morphology as exogenous (smooth walled, rough walled) and endogenous (pasty and shell-forming). Between the various exogenous and endogenous solidification types many transition types can be found. Figure 6.
Although a further sub-division is made in cellular and dendrite formation, this is no longer visible in the name and direct pointing to the presence of dendrites is thus omitted.
Evaluation of dendrites
Dendrites and the various parts of them can be measured accurately on photographs and evaluated, according. to Underwood [17] or very detailed according Wlodawer [11a]. Figure 7 shows such a calculation. For actual measurements as well as the theoretical background information, reference is made to the original work.
Such measurements are rather time consuming, reason why more simple methods have been developed to estimate dendrite shape and quantity.
Guidelines for the evaluation of dendritic structures in malleable cast iron, were constructed by Patterson en Döpp [18] , which can also be used for cast iron. See fig.8
Another simplified method was adopted by Ruff and Wallace [14,15,19].
By using a special heat-treatment, the original pearlite matrix is transformed into a mixed structure of ferrite and pearlite. Dendrites that are present become better visible after etching.
The amount of dendrites is indicated as "dendrite interaction area" the percentage of the surface in which dendrites are clearly interwoven. See fig.9.
Calculation of the theoretical quantity of dendrites (primary phase) can be done with the corrected carbon-equivalent according to [20]. From the chemical composition, the relationship between primary phase and eutectic can be calculated:
C-(1.3+.1Si)
Sr= -------------
2.93-.21Si
The fact that silicon present not only lowers the maximum amount of eutectic carbon, but also the fact that the maximum solubility of carbon in austenite decreases.
A Sr value of 0.9 means that the structure in equilibrium consists for 90% eutectic and 10 % primary austenite dendrites.
Influence of primary structure on the mechanical properties of gray cast iron.
It has been mentioned earlier, eutectic cells have always been emphasized in relationship with mechanical properties in cast iron. In all cases, however, where also the primary structure has been examined, it was shown that the effect of the primary structure exceeds that of eutectic cells [21,22,26].
Highest tensile strengths are obtained when primary dendrites are large , the lowest tensile strengths are obtained when dendrites are short and globular.[14,15,21,22,23,24].
This seems to indicate that Patterson's "beton-stahl hypothese", in which a strong network of primary dendrites contribute largely to the final mechanical strength of the material, is correct.
Relationships with other structural components.
The introduction of the eutectic cell theory made it necessary to emphasize the fact the formation of these eutectic cells took place fully independent of the primary phase. The only possible influence of the primary phase on the formation of eutectic cells remained limited to a spatial effect, i.e. formation of eutectic cells, graphite or carbides can only take place in the space that is left over by the primary dendrites.
From the literature [11b,25,26,28,29,30,31,32,33,34,35,36,37] as well as from own research work, however, it appears that the primary phase plays a far more active role in the formation of these phases.
Factors that influence the formation of the primary structure in gray iron.
Melt and pouring temperature.
Under normal casting and cooling conditions, there exists a clear relationship between melt- and pouring temperature and the shape of the primary austenite.
A high melting temperature increases a clear dendritic and oriented structure [25,38,39].
A low melting temperature on the other hand , causes smaller dendrites which are more randomly oriented.
This effect also occurs when a low pouring temperature is obtained by holding the liquid metal for a longer period of time.
Patterson et al [22] observed that when the time interval between tapping and pouring is short, respectively pouring with a high temperature, mainly elongated and long dendrites will be found. A low pouring temperature, obtained by holding, showed short and globular dendrites. When the material is held for a longer period after inoculation, the oriented dendritic structure disappears [25].
Patterson en Döpp [18] also find increasing endogenous solidification in malleable iron when this is kept at a low pouring temperature.
The use of quenching tests [13], however, shows different results, which means that interpretations of data obtained with quenching tests should be interpreted very carefully,.
Composition.
Patterson en Döpp [18] believe that the higher carbon content forces cast iron to solidify exogenous.
Dendrite length and quantity are maximal with a low carbon equivalent [14,15,25] or a low sulfur content [25].
Under these circumstances, dendrite arms are also courser [14,15]. With higher carbon equivalent or higher sulfur content, dendrites are finer and more randomly oriented.
At still higher carbon equivalent no differences in primary structure could be established.
Inoculation.
Inoculation of gray iron is mainly related to an improvement of graphite formation or to prevent carbide formation. It is surprising however, that inoculants also have a clear effect on the shape and dimension of primary austenite dendrites.
The majority of commercial available inoculants cause a decrease [14,15] or an increase [23] of dendrite dimensions.
The effect of special additions such as Sr, Al,Ti,Zr or Ce is also severe. Results, however are not consistent [14,15,23].
Research results do not show a uniform picture. Comparable differences can be found when inoculation is evaluated with respect to graphite formation.
Mechanical inoculation.
A primary structure consisting of long and oriented dendrites changes into compacted endogenous dendrites when the melt is vibrated by electro-mechanical means.[42].
After reaching the eutectic temperature, dendrites cannot be changed any more. Also the dendrite structure in cast steel can be influenced by vibration [43], which points to a common mechanism for the formation of austenite dendrites in steel and in cast iron.
The influence of a vacuum treatment on the primary structure of cast iron.
Thieme [44] noticed that when cast iron is melted under normal atmospheric conditions and given a vacuum treatment afterwards, the primary structure changes dramatically.
After vacuum treatment, the material crystallizes globular (endogenous), but solidified dendritic before that.
Other research work, although not dedicated to the primary structure, as in [45], obtained quite different and opposite results as after a vacuum treatment , dendrite length increased as well as its compactness.
Influence of a desulfurisation treatment on the primary structure of cast iron.
Specialized research work on this field is unknown. However, from research results of desulfurisation tests, it can be concluded that not only the external dimensions of austenite dendrites increase, but that also the internal construction of these dendrites is so strongly altered that at a later stage, ferrite formation is promoted. This ferrite formation takes also place in more massive parts of the dendrite, where the influence of finely divided graphite as a carbon collector can be excluded. Fig. 10 shows an example .
When gray cast iron is treated with a small amount of magnesium, the dimensions of the primary austenite dendrites changes dramatically, even before graphite nodules appear. Figure 12.
The influence of mold coatings on the primary structure.
Bryant en Moore [47] were able to surpress a severe dendritic and hot-tearing sensitive structure in malleable iron by using mold-coating which contained zinc powder, cokes or cobalt. The exogenous dendritic structure changed into fine endogenous crystals.
Concluding remarks.
When gray cast iron structures are examined and evaluated, attention is focussed on the eutectic part of the solidification. The primary structure is hardly ever mentioned. From this literature review, however, this appears not to be justified. On the contrary, in all cases where the formation of the primary phase has been studied, its influence on mechanical properties, overrules that of the eutectic cells.
Another surprising fact is that all actions undertaken to influence graphite formation such as super-heating, inoculation, casting temperature, vacuum treatment etc. also change the primary structure.
This could lead to the conclusion that these actions influence primary structure as well as the graphite phase. As, however, the primary phase has been highly neglected when theories on graphite formation were developed, the following conclusion might be preferred:
The aforementioned actions change the primary structure and as a consequence, this primary structure force the graphite phase to change its dimension, distribution and shape.
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