Materials

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Materials and Design 27 (2006) 1007–1015

& Design

Perspectives on research on the formation ofnodular graphite in cast iron

Gustaf Ostberg *

University of Lund, Department of Engineering Materials, P.O. Box 118, S-22100 Lund, Sweden

Received 26 May 2004; accepted 10 February 2005Available online 18 April 2005

Abstract

Current views on the mechanism of the formation of nodular graphite in cast iron are examined with respect to their explanatorypower. The aim of this review is to find, if possible, a rationale for further studies. Arguments are presented in favour of a compre-hensive model, encompassing the characteristics of both the nucleation and the growth of the graphite. A plea is made for focusingon the most salient features rather than on phenomenological aspects.� 2005 Published by Elsevier Ltd.

Keywords: Graphite spherulites; Nodules; Nucleation; Growth; Causal relationships; Epistemology; Comprehensiveness

1. Scope

1.1. A long-standing issue

The mechanism of the formation of the nodular orspherulitic structure of graphite in cast iron, as an alter-native to the flake variety, has been studied extensivelyand intensively ever since the first detailed observationswere published in the 1940s [1–3], Fig. 1, following ear-lier evidence [4], Fig. 2. Despite the considerable effortsfor many years of a large number of researchers benefit-ing from steadily improved experimental techniques,there is still disagreement about, on the one hand, theimportance and necessity of heterogeneous nucleationof the graphite and, on the other hand, the dependenceon preferred growth in the c-axis direction as comparedwith the a-axes directions of the graphite.

1.2. Differences of opinions

Observations of foreign particles in the centre of graph-ite nodules have prompted the idea that heterogeneous

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nucleation is crucial for their formation. It has also beensuggested that gas bubbles may serve the same purpose.

It has been claimed that heterogeneous nucleationfollowed by growth along the interphase in question inthe a-axis directions of the graphite accounts for the radialorientation of the c-axes of the crystallites in the nodule.

According to a different view, the reason why growthtakes place in the c-axis direction is that some mecha-nism enhances such growth in comparison with the nor-mal flake variety.

Analogies of spherulite formation in other systemshave led to the opinion that the radial arrangement ofcrystallites develops from an initial flake-like crystal bybranching and/or curvature during the subsequentgrowth.

Finally, it has been claimed that it is the spheruliticstructure that is the natural form of graphite in cast iron.This idea is based on observations of nodules in iron with-out any special treatment, provided the melt is clean, i.e.free of normal impurities such as oxygen, sulphur andphosphorus as well as so-called subversive elements.

These differences of opinion will be treated in thisreview in two stages. First, an account of the physical-metallurgical aspects of the different opinions will be given

Fig. 1. Schematic representation of the orientation of graphitecrystallites in spherulites (2).

Fig. 2. An early observation of a graphite nodule in cast iron,apparently not resulting in any detailed studies (4). Magnification600·.

1008 G. Ostberg / Materials and Design 27 (2006) 1007–1015

in a survey of conspicuous features and phenomena.Thereafter, a comparison will be made of the relevanceand merits of the arguments submitted in support ofthe respective standpoints.

Before that, in order to motivate, and possibly alsojustify, the mode of reasoning and of analysing the issueto be dealt with, a brief introduction to the cognitiveperspective will be presented.

2. Some principles for analysis

2.1. Searching for causality

The following analysis of the intriguing situation out-lined above is an attempt to assess the relative impor-tance of the two processes of nucleation and growth,

respectively, from the point of view of their possible cru-cial roles as the decisive causes of the spherulitic crystal-lization of graphite in cast iron. The complexity of theaggregate of disparate findings makes it necessary tobase these considerations on a philosophical ground.This means, among other things, that queries have tobe raised about the causal relevance of the two alterna-tive explanations, including an assessment of their com-plementary nature.

In order to prepare for such an examination of causalrelationships, a review will first be made of the knowl-edge that has been accumulated during many years ofstudying various cases. It would be instructive for theconcluding discussion if this account of microstructuralfeatures could be perceived and interpreted against thebackground of the aim and intention to assess theexplanatory value of different current views.

2.2. Concepts of knowledge acquisition

The aim of this paper is to call the attention ofresearchers in the field in question to the need for a com-prehensive understanding of the phenomenon in ques-tion, namely the development of a model which takesboth the radial orientation of the graphite crystals ingraphite nodules and their preferred c-axis growth intoaccount.

The point of departure of the present review is theobservation of a pervading tendency in the relevant lit-erature to restrict the focus to either the orientation as-pect or the characteristics of growth. This implies that inany particular study one of the issues to be considered isregarded as the crucial one, while the other ones are re-garded as only of secondary importance.

Apparently, this approach is an application of theprinciple of so-called causal selection. When one oftwo possible issues or aspects of a phenomenon to be ex-plained is selected, the scene is set for the one primarilyfocused to play the main and leading role in the presen-tation of relationships between the two possible causesto be studied.

In analyses of the progress of sciences, this situationhas been recognized and described in terms of differ-ences in thought style [5]. This concept would probablybe valid also for the present review. In particular, thisappears to be true when it comes to dealing with theimportance of the nucleation of nodules in comparisonwith their subsequent growth. Experience shows that acertain prevailing thought style tends to exclude the pos-sibility to accept the existence of alternative styles.

As a consequence, falsification is rarely attempted asa means of evaluating the conclusions arrived at. Ques-tions are seldom raised about either the necessity or thesufficiency of various conditions for the occurrence ofthe phenomenon studied. It is often assumed that a par-ticular effect of the change of a certain variable is the

G. Ostberg / Materials and Design 27 (2006) 1007–1015 1009

only factor among other possible alternatives that mat-ters. Furthermore, the general validity of a suggestedmechanism is often disregarded.

It is against this background of some general con-cepts of knowledge acquisition that current views andopinions about the formation of graphite nodules in castiron will be surveyed. This review is not intended topresent a complete record of all the various findings re-ported in the literature. The account will be limited tothose salient features that may provide a basis for somepragmatic conclusions about the state of knowledgewith respect to the understanding of the phenomenonin question.

3. Salient features and phenomena

3.1. Heterogeneous nucleation

Many independent observations of non-metallic par-ticles or inclusions in the centre of graphite nodules sup-port the view that the first stage in the formation of theparticular nodular structure in such cases has been het-erogeneous nucleation [6], Fig. 3. This process is some-times described and explained in terms of favourablecrystallographic relationships between substrate andgraphite, i.e. epitaxy. Another aspect of enhancementof heterogeneous nucleation is the role of surface tensionat the interface [7].

The number of nodules can be affected and controlledby inoculation [8]. This effect is usually attributed to theintroduction of particles for heterogeneous nucleation,but it has also been claimed that the phenomenon inquestion is due to changes in the conditions for growthby the action of dissolved elements from the addition ofinoculants on the surface tension mentioned above.

Furthermore, the opinion has been expressed that thesites for heterogeneous nucleation are the same for nod-ules and flakes. Accordingly, it is the mode of growththat determines the resulting outer shape of graphitespherulites.

Fig. 3. Formation of graphite spherulites by nucleation on the surfacesof sulphide particles and growth along their interfaces, eventuallydeveloping into nodules (6). Magnification 1500·.

Finally, in this very brief account of evidence of het-erogeneous nucleation of spherulitic graphite, a remarkon so-called ‘‘clean’’ or ‘‘pure’’ cast iron melts seems tobe warranted. As will be mentioned elsewhere in this re-view, it has been found that the formation of nodules isenhanced by decreasing the contents of certain minorelements that would otherwise have an adverse effectin this respect. On the basis of experience from researchon so-called ‘‘clean steel’’, however, it cannot be con-cluded that such ‘‘clean’’ cast iron should be completelyfree of inclusions that could act as substrates for heter-ogeneous nucleation.

3.2. Homogeneous nucleation

The simplest model for the initiation of spheruliticcrystallization of graphite is probably the one that as-sumes that the very first stage is the formation of a hex-agonal array of carbon atoms bounded by facetsadvancing by growth in the c-axis direction [9]. In thisway crystallites are formed in radial orientation of theirc-axis directions.

The discovery of the C-40 structure prompted thevisualization of this spherical arrangement of a largenumber of carbon atoms, so-called fullerenes, as theorigin of graphite spherulites. The negative result ofsearching for fullerenes in nodular cast iron falsifiesthis hypothesis, however. Instead, on the basis of cal-culations of the molecular attachment kinetics, it hasbeen suggested that the first stage of formation ofgraphite spheres may be the assembling of carbonatoms into two-dimensional hexagons, which are sub-sequently extended by growth in the a-axis directions[10,11].

Eventually, these monolayers are bent and wrappedup into loose balls or tangles to form ordered concentricshells, which evolve into spherulites. There is thus noclear distinction between nucleation and growth.

Obviously, this model cannot be valid for the forma-tion of graphite nodules during the annealing of whitecast iron, since it is based on the assumption that the ori-ginal monolayer sheets are bent and wrapped up. Theonly way of applying this model to the solid state forma-tion of graphite nodules would be the presupposition ofthe existence of carbon spheres already in the white ironas cast. Until evidence in support of that prerequisitehas been presented, such an assumption remains aspeculation.

The model�s mechanism for spherulitic surfacegrowth by extension of steps arising at emergent twistboundaries is presumably more generally applicable[10,11], Fig. 4. This could explain how the preferredgrowth in the c-axis direction of radiating crystals is pre-served even according to other alternative processes forthe original formation of a nodular structure, for in-stance the one dealt with in the next section.

Fig. 4. Growth by extension of steps arising at emergent twistboundaries (10) (11).

1010 G. Ostberg / Materials and Design 27 (2006) 1007–1015

One important feature of this mechanism is the factthat it requires the absence of impurities such as oxygen,sulphur and phosphorus. This is true also for the othermodels referred to, for instance the one requiring pertur-bations for the extension of facets of crystallites perpen-dicular to the basal plane. Hence, flakes are formedinstead of spherulites if no scavenging additions aremade in order to render such elements innocuous.

Accordingly, this is the sole role of nodularizingagents such as magnesium [13]. Their occurrence in thegraphite has been the object of much attention as a pos-sible crucial factor in the formation of nodular graphite.In view of the findings regarding the role of impuritiesreferred to, it seems that their presence in spheruliticgraphite is simply the result of incapsulation. Obviously,this standpoint is in opposition to the claim that theyplay the causal role of enhancing the preferred growthof graphite in the c-axis direction [13].

There is another model that appears morphologicallysimilar to the previous one by being based on observa-tions of curved crystal growth [12], Fig. 5. The crystallo-graphic mechanism leading to such curvature is theformation of twin-tilt boundaries. Obviously, this effectdoes not require any substrate as in the case of curvaturebeing governed by the shape of the interface ofinclusions.

Admittedly, a metallographic observation of curva-ture of a crystal in a two-dimensional section does not

Fig. 5. Curved flake (12). Magnification 500·.

in itself necessarily permit definite conclusions aboutthe geometry in three dimensions. Alternatively, thecrystal may be either tubular or spheroidal. However,in the former case it should be possible to observe alsoaxial sections of tubular shape, which has, in fact, notbeen reported. Furthermore, falsification of the conclu-sion of spheroidal rather than tubular growth of graph-ite surrounding a particle like a sulphide inclusion wouldrequire corresponding observations of tangential devia-tions of an initiated flake from the particle surface.Again, such a feature is not known from the literature.

3.3. Growth

There is a wealth of information about the influenceof various elements in the iron on the preference ofgrowth in the c-axis direction of graphite. Most of theknowledge acquired in this field relates to the subversiveeffects of certain impurities. Some elements have beenfound to prevent nodular growth completely, and othersmay change the shape of an initially nodular structureby forming protrusions in the a-axis directions. All theseeffects on the growth of graphite in cast iron are mostlyexplained by reference to adsorption of the elements inquestion on the graphite surface.

There are also reports of the beneficial role of minornon-metallic elements, the action of which may cause achange opposite to the one described above for detri-mental elements. Again, adsorption as well as incorpo-ration of such elements into the graphite is referred toin order to explain their promoting function [14]. How-ever, this conclusion contrasts with the opinion thattheir only role is as a scavenger, i.e. to free the melt fromoxygen, sulphur and phosphorus [12,13].

It is important to realize in this connection that pre-ferred growth in the c-axis direction does not in itselfgive rise to a nodular structure. If there is no mechanismfor radial orientation of crystallites in the nodule, the re-sult is merely flakes or rods with a higher ratio betweenthe thickness and length as viewed in microscopicsections.

At this point, attention is called to the above remarkabout the effect of additions of inoculants by altering thepropensity for growth in the c-axis direction.

One intriguing growth feature is the competition be-tween the two different modes illustrated in Fig. 6. Itcannot be assessed which of them had appeared beforethe other one on the basis of one single metallographicsection. Nevertheless, it is noticeable that the transitionbetween them during their growth is quite distinct andsharp. Again, such observations pose a challenge forthose who want to claim that nodularizing conditionshave an effect solely on growth.

A comment might also be warranted regarding thesometimes eclectic manner of arguing about the pre-ferred c-axis growth as the sufficient prerequisite for

Fig. 6. Growth of spherulites and flake nests together (6). Magnifi-cation 1200·.

G. Ostberg / Materials and Design 27 (2006) 1007–1015 1011

spherulitic crystallization of graphite in cast iron. In onestudy, support of this opinion was sought in two earlypublications on spherulite formation in other systems[14]. However, a close examination of the two papers re-ferred to shows that they are mutually contradictorywith respect to the claimed promoting role of impuritiesor, rather, foreign particles for the radial orientation ofthe crystallites.

Similarly, establishing a radiating structure has oncebeen ascribed to a mechanism that is, unfortunately,not sufficiently specified to make possible a comparisonwith the alternatives dealt with previously in this review[15].

3.4. Analogous systems

From time to time, researchers have drawn conclu-sions on the basis of knowledge and information aboutspherulitic crystallization in other systems in order tosupport their studies of the formation of graphitespherulites. Analogous cases may thus be found amongsilicon alloys, minerals and polymers.

Early examples from the latter field have been quotedfor the sake of complementing observations of preferredc-axis growth with a mechanism that accounts for radialorientation [14]. Similar references have been mademore recently, exploiting ideas about the branchingand curvature of growing elongated polymer crystalseventually producing a crystal of radiating sections witha spherical outer shape [9,16].

According to an elaborate study of this issue, impu-rity effects may cause defects in graphite responsiblefor branching [17]. Disregarding this particular aspectof the mechanism for the establishment of radial orien-

tation, it may nevertheless provide the complementneeded for a comprehensive view based on the conclu-sion mentioned previously that spherulites rather thanflakes should be considered the natural form of graphitein cast iron [12,13].

4. Assessment of causal relationships

4.1. Inventory of aspects

From the survey of current issues presented above, itappears that the phenomena and features that have tobe considered in an attempt to identify and characterizetheir possible causal relationships are the following,listed in an order that is not intended to indicate any rel-ative importance:

� radial orientation of crystallites,� preferential c-axis growth,� heterogeneous nucleation as such and its role for

radial orientation and c-axis growth, respectively,� homogeneous nucleation,� presence and absence of alloying elements and impu-

rities in the iron,� the same for the graphite,� effects of inoculation,� bending of crystals,� branching and curvature of growing crystals,� initiation of monolayer spheroids.

In the graph below, Fig. 7, a tentative network of prob-able relationships is presented. The lines indicate causalconnections of varying significance, which will be dealtwith in the next section in the form of a comparativerecapitulation of the previous accounts of different viewson particular aspects of the formation of nodular graph-ite in cast iron.

4.2. Comparison of alternative explanations

The point of departure for this analysis, admittedlyeclectic, is the view that the most salient feature of thestructure of nodules besides their spheroidal shape, isthe radial orientation of the crystallites in the spheruliticaggregates. This assumption may later be rejected or re-vised if the final conclusions prove to be unreasonable orincompatible with other facts or observations.

The models referred to above for the development ofthe spherulitic texture all share the perception of a pri-mary stage of formation of a graphite crystal growingin the basal plane or a-axes directions and eventuallyattaining a curved shape. Different mechanisms havebeen proposed for the curvature. On the basis of micro-scopic observations it has been claimed that the initiallyformed flake-like crystal is curved by either bending or

EXTERNAL OPERATIONS

CHEMICAL CHANGES AND EFFECTS

STRUCTURAL PROCESSES

GRAPHITE MORPHOLOGY

Treatment with nodularizing agent

Inoculation

Additions against subversive elements

Introduction of additional elements

Scavenging

Annihilation

Heterogeneous nucleation

Non-hetoregeneous

nucleation

Adsorptionon graphite

Preferred c-axis growth

Branching and curvature of crystallites

Bending of flakes

Radial c-axis orientaion

Spherical symmetry

Fig. 7. Possible causal relationships in the formation of nodular graphite in cast iron.

1012 G. Ostberg / Materials and Design 27 (2006) 1007–1015

following the interface between a substrate for heteroge-neous nucleation and the surrounding matrix.

In addition to these two observed mechanisms of cur-vature, two alternatives have been suggested. One model,apparently based on the perception of C-40 fullerenes,assumes that the initial graphite structure consists of amonolayer of hexagons, which is subsequently trans-formed by growing into a spherially shaped body form-ing the core of a spherulite.

However, this model poses a problem in as much as itis difficult to apply to the formation of nodules in the so-lid state, where curvature by bending should obviouslydiffer from the same process in a cast iron melt. Forthe sake of fairness it should be mentioned that this defi-ciency of the model in question has been admitted by itsproponents, which suggests that there may be a differentmechanism for the formation of nodules by decomposi-tion of cementite [10].

Based on an analogy with spherulitic crystallizationin other systems, it has been proposed as another alter-native that the initially formed crystal is divided intobranches, forming an aggregate of crystallites with theirc-axes eventually oriented radially. However, in the caseof the previous model, the experimental evidence forsuch a process is lacking.

Returning to the first two mechanisms for curvaturerecapitulated above, the next step in comparing differentviews on the formation of graphite nodules is to assesstheir relative merits with respect to the ultimate aim of

providing a comprehensive explanation of the develop-ment of the spherulitic structure. Obviously, the obser-vation of nucleating particles in the centre of nodules,as well as of flakes growing along the interface of suchsubstrates for nucleation, indicates that this mechanismof curvature is indeed acceptable as a comprehensivemodel for nodule formation.

However, this fact should not rule out the alternativeof curvature of flakes in a single phase matrix withoutany supporting substrate. This is true even if it couldbe argued that the available evidence of that mechanismis relatively limited in comparison with that of nucle-ation and growth initiated on foreign particles.

For the sake of completeness it may be added herethat the promoting effect of inoculation could possiblysupport the opinion that heterogeneous nucleation is anecessary requirement for nodule formation. Clearly,this argument is based on the assumption that the mainrole of inoculation is to provide foreign particles fornucleation rather than acting as a scavenger.

This is probably the proper point for changing theperspective over to considerations of the importance ofpreferred growth in the c-axis direction. First it shouldbe recognized that the previous reasoning neitherpresupposes nor excludes the possibility that this phe-nomenon might contribute to the formation of a spher-ulitic graphite structure. Among other things, it may beargued that the domination of c-axis growth over a-axisgrowth may be the reason why the surface of the graphite

G. Ostberg / Materials and Design 27 (2006) 1007–1015 1013

nodules is relatively smooth with no protrusions in thea-axes directions.

However, this argument may be countered by refer-ring to the suggestion of a twist-twin mechanism forthe development of the observed surface structure ofgraphite nodules. Furthermore, spherulitic crystalliza-tion has been found even in the absence of those addi-tions that are known to cause preferred c-axis growthof graphite in cast iron. Nevertheless, in view of theattention that preferred c-axis growth has received, thisphenomenon needs to be considered further, but forother reasons than as a prerequisite for the formationof a spherulitic graphite structure.

4.3. Complementary phenomenological aspects

In principle, nodules and flakes are alternative struc-tures of graphite in cast iron. Which one of the two willpredominate can be the consequence of either theenhancement of the occurrence of one of them or thefact that the formation of the other one is rendered dif-ficult. The latter implies a retardation that makes it pos-sible for the alternative to develop.

If retardation of the occurrence of flakes is the crucialcondition for predominance of nodules, the nodularalternative would require a particular mechanism forthe formation of this peculiar structure. If, on the otherhand, the absence of flakes is not due to retardation butto the enhancement of spherulitic crystallization, thatwould also indicate the existence of favourable condi-tions for such a process.

According to both of the two different accounts of thepossibility for nodules to form instead of flakes, the nod-ular or spherulitic crystallization appears to involvemore complicated prerequisites than those for flakes.Hence it would seem that flakes should be consideredas the more natural variety of graphite structure.

This view is supported by historical evidence, as itwere. With one exception [4], Fig. 2, there has been a con-spicuous absence of observations of other than flakegraphite in cast iron for more than half a century. Inevi-tably, this has had an impact on the classification of flakesas compared with nodules with respect to normality.

This view thus assumes that the prerequisites for theformation of nodules are not normally present in castiron. On the basis of different kinds of observations ithas also been claimed, however, that this is a prejudicethat has emerged from the historical experience men-tioned above. In other words, it should be quite possiblefor nodules to form if this is not prevented by the inter-ference of subversive elements in the cast iron. Conse-quently, it would then not be considered farfetched toadopt the view that graphite nodules are, in fact, morenatural or normal than flakes in clean cast iron.

The attention of the majority of researchers in thisfield has been focussed on separate particular features

of spherulitic crystallization of graphite in cast iron, notablyheterogeneous nucleation and preferred c-axis growth.While those aspects are evidently of interest as such, theyseem to have obscured the fundamental issue of the occur-rence of nodular graphite in cast iron. What has mostlybeen dealt with in the past has been secondary phenomenathat are redundant as causes of the formation of nodules.

Actually, this comment on the eclecticism in questionis not unique and new. A similar remark was made al-ready in the 1960s by one of the earliest researchers inthis field. With some two decades of experience, he com-plained that ‘‘people are prepared to take isolated littlefacts and develop general theories from them’’ [18].

4.4. Concluding remarks

Of the different models for nodule formation in castiron dealt with in this discussion, only those involvingmechanisms for curvature, i.e. by initial growth on asubstrate, spontaneous bending and/or branching, seemto be comprehensive in the sense that they do not needany complement relating, for instance, to preferred c-axis growth. Inversely, the alternatives that refer onlyto the latter phenomenon cannot be called comprehen-sive in the same sense as the previous model, since theylack consideration or evidence of a mechanism for estab-lishing radial orientation of the crystallites in the nodule.

One standard objection against a certain model for acomplicated phenomenon is the claim that it relates onlyto a special condition and is not relevant to ordinarycases. With respect to nodular graphite in normal varie-ties of cast iron, the requirement for general validitymay rule out virtually all of the present models andmechanisms. Therefore, some questions that remain tobe answered in this discussion are to what extent thesimilarities between the model systems and normal castiron justify an analogy, and whether there are differencesthat warrant doubts about their validity.

Obviously, these are issues of judgement based onknowledge about and experience of the complexity ofthe respective systems in the analogy. For instance, thereis no reason to discredit the model based on observa-tions of nucleation and growth of graphite, initiatedon sulphide inclusions, by simply stating that such par-ticles do not exist in normal cast iron. Rather, it seemsjustified to claim that the inclusions that are presenteven in so-called ‘‘clean’’ cast iron, may, in principle,function in the same way as sulphide particles.

The analogy between graphite in cast iron andspherulites in the other systems referred to in the litera-ture is a more sensitive issue. Apparently, the experi-mental evidence of a mechanism similar to that inthese systems is, at best, meagre. This fact cannot beused as an argument against the analogy in question,since branching and bending graphite crystals may takeplace on such a small scale that it escapes observation.

1014 G. Ostberg / Materials and Design 27 (2006) 1007–1015

The same is true for the process of establishing aspheroidal carbon monolayer that serves as a nucleusfor the development of a graphite spherulite. Actually,this may also account for the fact that the correspondingfullerenes of carbon have not been found, if they haveever existed in cast iron.

4.5. Consequences for research

In the perspective outlined above of the relative rolesof heterogeneous nucleation and preferred growth in thec-axis direction, respectively, it appears that a combina-tion of these two prerequisites would provide a moregenerally valid basis for developing an understandingof the formation of graphite nodules in cast iron undermost practical circumstances. Accordingly, suggestionsof mechanism dealing with only one or the other ofthe two aspects in question should be considered lessinteresting from the point of view of rationalizing theinformation and knowledge available.

Consequently, research on other conditions forgraphite nodule formation in cast iron should be closelyrelated to a comprehensive model of nucleation andgrowth. For instance, this is true for studies of the effectsof various elements on surface tension at the interface ofinterest, their adsorption on graphite and incorporationin the crystals.

In an attempt to assess the relative importance ofsuch varying conditions and factors, one decisive phaseis that of identifying the primary cause of the effect un-der consideration. The result of this effort should serveas the point of departure for a subsequent examinationof the potential causal relationships between the differ-ent factors or conditions that influence the process inquestion.

For the purpose of understanding the formation ofnodular graphite in cast iron, it seems natural to basesuch a search for a primary event on the assumptionthat this should be the structural behaviour of carbonatoms at the surface of the heterogeneous nuclei and/or the growing nodule.

The other contributing factors and conditions men-tioned should then be considered phenomenological.By this is meant that accounts of their appearance aremerely descriptive and not dependent on any presuppo-sitions about their causal importance for the mechanismunder consideration.

If the assumption made about the primary cause ofnodule formation proves not to be valid, other partsof the system have to be investigated with respect to fac-tors or conditions which might possibly play the role ofprimary causes replacing the one originally assumed.For the sake of completeness, this next attempt shouldalso include such phenomena in this context as the ac-tion of nodularizing agents as well as that of subversiveelements and remedial additions.

It is true, of course, that work related to the latterphenomena may possibly yield technical and economicrewards. However, a sharper focus on the underlyingmechanisms would shorten the time required to arriveat a clearer insight into the detailed mechanism ofgraphite nodule formation in cast iron according tothe comprehensive model advocated above.

4.6. A concluding plea

In analyses of research policies in different disciplines,a distinction can sometimes be made between an orien-tation towards general descriptive theories and effortsaiming at prescriptive conclusions. The former are fo-cused on comprehensive understanding, whereas the lat-ter are eclectically limited to selected phenomena orfeatures that can be acted upon or used for a particularpurpose including support of a predetermined principleor rationality.

It appears that a considerable portion of the researchon the formation of nodular graphite in cast iron be-longs to the intentionally rationalizing category withnarrow perspectives. This seems to be true, for instance,for investigations of the roles, effects and importance ofcertain alloying elements and additions, aimed at opti-mizing various industrial conditions.

The instrumental emphasis may exclude immutablebut still theoretically and practically important aspectsfrom the perception of the phenomenon in question asa whole. The resulting picture is a fragmented one withmore or less arbitrary connections between the parts.

Obviously, this difference in approaches creates a di-lemma for research as long as the discrepancy betweenthem is not fully recognized. Those who try to interpretinformation and knowledge retrieved from the literaturecannot always identify their character. Hence there is arisk that the apprehension of the issue and problem tobe considered, and its possible solution, will becomebiased or even prejudiced.

Acknowledgements

This review is based on literature retrieved by Mr.Anders Martenson, librarian of the Swedish Institutefor Metal Research, in a dedicated effort for which theauthor is very grateful. His thanks are due also to Prof.Hasse Fredriksson of the Royal Institute of Technologyin Stockholm for stimulating discussions and provisionof literature.

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