Introduction.
1998 is generally recognized as the 50th. Anniversary of the invention of ductile (or nodular) cast iron, which meant a dramatic change in the application of a material that had been known for centuries. In the course of a detailed literature survey on one century of theories treating graphite formation in cast iron, specific articles were found that makes one wonder why ductile cast iron was not invented at an earlier date. In the following text, some freely translated quotations are given to demonstrate that all the ingredients that could have lead to the invention of ductile iron early this century, were actual present at that time.
The use of magnesium, nowadays indispensable for the production of ductile cast iron, was already known in the beginning of this century: F.Wüst (1900) [1] "The application of magnesium in cast iron remained only a single case."
Even the favourable influence of magnesium on tensile strength was established and the need for further research desired: Geilenkirchen (1908) [2]: "Recently, magnesium, being an important deoxidation product for copper and nickel alloys, was re-introduced in the cast iron foundry industry. The use of magnesium for the deoxidation of cast iron was recommended by Griesheim-Electron, which has made some practical tests. Magnesium is said to show the same effect as aluminium, only much stronger. A cast iron of very low viscosity can be obtained, giving rise to homogeneous casting of higher compression and tensile strengths. For practical use, a magnesium containing ferro alloy is recommended that can be prepared by the foundry itself, using nickel-magnesium or any other magnesium alloy. Test results that were obtained:
">Mg% Addition
Tensile strength kg/mm2, test1
Tensile strength kg/mm2, test2
0
12.2
13.8
0.05
18.9
17.4
0.10
18.0
18.5
As can be seen, the addition of only 0.05% of magnesium is sufficient to increase the tensile strength enormously. The use of still higher amounts of magnesium, however, seems no longer to be effective. In order to make a good evaluation possible, more test results of systematically conducted test are required."
The metallurgical microscope, in use for more than a decade, provided the first signs of spherulitic graphite inclusions in a high silicon gray cast iron. As nucleation and undercooling were the main items used for the explanation of the iron-carbon diagram at that time, the favourable ball-shape was overlooked and dedicated to a nucleus. E.Heyn (1906) [3] "In both specimens, tiny ball-shaped graphite-nuclei can be found in the matrix."
And this, despite the fact that one claimed to be looking for a ball-shaped graphite form: (1906) [4]
"Maybe it is even possible to obtain graphite in special shapes, as compact balls instead of long flakes..." And even kept on looking for this typical graphite shape during the next decades: P.Goerens (1925) [5] "We have been looking for and are in fact still looking for a method, that will transform graphite during the solidification and not after a heat treatment as in temper cast iron, in this shape."
But even although the typical nodular shape is noticed as such, it is easily theorized away by introducing other well-known mechanisms: Wüst, Meißner (1914) [6] "Graphite occurs in two shapes, in the form of flakes and in the shape of small, more or less spheroidal inclusions. These small inclusions consist of graphite that did not crystallize during the solidification interval, but during the cooling period between the end of solidification and the pearlite point. It resembles in its appearance temper carbon, although it is surrounded with pearlite instead of ferrite."
Or by using a vague hypothetical explanation, using general mechanisms like nucleation and undercooling: E.Piwowarski (1925) [7] "As can be seen in fig.1, the difference in color in the concentric rings only depends on the shape of the graphite. The lighter border and core region show strange, uniform rounded graphite inclusions. The solidification is assumed to be the following:
Because of the low superheating temperature, the tendency for gray solidification was already present. The mold, which was coated with graphite containing coating, had an additional nucleating effect, because it prevented undercooling. The nucleation effect of the coating, however, stopped after a circular part had solidified. The neighboring region solidified therefore with an increasing undercooling. The crystallization velocity increased more than the number of nuclei, so causing a coarsening of the flakes. It is not simple, however, to explain, why the core zone again solidified without noticeable undercooling. Graphite in the region that had already solidified, might have prevented undercooling of the still liquid part, causing fining and at the end ball-shaped graphite formation in the core-zone. The expansion of the outer regions, causing tensile forces on the core zone, may have had an identical influence. Anyway, remarkable remains the fact that when graphite that is formed at the lowest undercooling, it will be transformed into a spherical shape, a fact that up till now was unknown. It is desirable that in literature, far more accidental structures like these would be shown."
It was shown that the high tensile strengths that could be obtained with fine undercooled graphite cast iron, was even beaten by "point" shaped graphite separations. The occurrence of plasticity, one of the main advantages of ductile cast iron, was recognized: P.Bardenheuer (1927) [8] "The very high tensile strength that can be obtained in these test bars, show that the point shaped separation of carbon has a still more favourable influence on tensile properties as the fine eutectiform graphite. Remarkable is the high bending tendency and elongation of these testbars."
So it was repeatedly established that rounded graphite inclusions should be aimed at: Bardenheuer (1928) [9] "The best results are obtained when we succeed to force the graphite to crystallize in a shape like that in temper carbon."
Although some spherulitic graphite could be seen in later publications [10,11],it was not until 1938 that a method was found to produce nodular graphite, using high temperatures and a strong basic slag. The outside world, however, would not know of it but 10 years later [12].
Finally, the first public announcement on the production of nodular cast iron appeared in 1946:
Dr.Harold Hartley's presidential address to BCIRA (1946) [13]: "We have succeeded in casting iron into the mould with a graphite structure of nodular or spherical form instead of being in the stringy, longated flake structure usually found in grey cast iron." It would take only a couple of years, befre a practical method was developed, that would revolutionize cast iron metallurgy and application. The main ingredient being magnesium, the same element that was used fifty years earlier....
Concluding remarks.
Reading through these historical remarks, with today's knowledge on ductile cast iron, it is hard to understand why nodular cast iron was not invented decades earlier. Technical equipment and pure materials were available, at least on a laboratory scale, the need to obtain spherical graphite shapes was recognized, the better mechanical properties more or less established and actual graphite nodules were found. And yet, no one was able to collect all facts and transform these into what one was looking for. Even the remark that was made by Piwowarsky [14] at a later date is not good enough, especially (his) figure 1 shows the best nodules that were found during that period:
"It is possible, that during the trials on superheating by E.Piwowarski, the graphite shapes that were obtained already showed a more or less spherulitic structure. The low magnifications that were generally used in microscopical examination at that time (1920-1930) was probably the reason why this graphite shape remained unnoticed by researchers."
It will hardly be possible to establish the reason why ductile cast iron was not invented at an earlier date. Compared with the relatively huge technical achievements that were obtained during that period, a method for the production of nodular graphite can certainly not be regarded as the most difficult one.
The author's personal opinion, however, is that the main reason for the "missed chance" lies is the fact that the majority of people concerned with the metallurgy of cast iron, is ultimately fixed at existing solidification theories and pre-conceived ideas and that this attitude prevents a broader view on what is really taking place. The main issue that remains, however, is the question whether we are making the same mistake today! Future developments of this unique material might very well strongly depend on the way this question will be answered.
References :
[1] F.Wüst, Ueber die Ursachen des Entstehens von Fehlgüssen. Stahl und Eisen, Vol.20 (1900), p.1042.
[2] Th. Geilenkirchen, Herstellung dichter Güsse durch desoxydierende Zuschläge. Stahl und Eisen, Vol.28 (1908), pp. 592-96.
[3] E.Heyn, Metallographische Untersuchungen für das Gießereiwesen, Stahl und Eisen, Vol.26 (1906), pp.1295-1301+1386-93.
[4] M.Ballay, R.Chavy, L'évolution des fontes depuis 25 ans, International Foundry Congress 1949, Amsterdam Preprint no 2. Contains quotation from: Revue de Métallurgie (1906)
[5] P.Goerens, Wege und Ziele zur Veredelung von Gußeisen. Stahl und Eisen, Vol.45 (1925), pp. 137-40.
[6] F.Wüst, Meissner Über den Einfluß von Mangan auf die mechanischen Eigenschaften des grauen Gußeisens, Ferrum, Vol.11 (1914), pp.111
[7] E.Piwowarsky, Bemerkenswerte Erscheinungen über die Graphitbildung in grau erstarrten Roheisensorten. Stahl und Eisen, Vol.45 (1925), pp. 457-58.
[8] P.Bardenheuer, Der Graphit im grauen Gusseisen. Mitt. Kaiser-Wilhelm-Institut, 1927, pp. 215-25.
[9] P.Bardenheuer, K.L.Zeyen, Beiträge zur Kenntnis des Graphits im grauen Gußeisen und seines Einflusses auf die Festigkeit. Stahl und Eisen, Vol.48 (1928), p.517.
[10] H.Nipper, Graphitbildung im Grauguß, Die Giesserei, Vol.22 (1935), pp. 280-87.
[11] H.Gröber, H.Hanemann, Aufbau des Graphits und Zementits in übereutektischen Eisen-Kohlenstoff-Legierungen. Archiv für das Eisenhüttenwesen, Vol.11 (1937), pp. 199-202.
[12] C.F.Adey, Das veredelte Graphiteutektikum mit kugeligem, sphärolithischem Graphit.
Die neue Giesserei, Vol.35 (1948), pp. 67-74.
[13] H. Hartley, Developments in cast iron research, Foundry trade journal, Vol.80 (1946) 408
[14] E.Piwowarsky, Hochwertiges Gusseisen. Zweiter Neudruck. Springer-Verlag, Berlin, 1961.Quotation page 209.
