Technical Review & Discussion
On Line Oxygen Activity Measurements to Determine Optimal Graphite Form During Compacted Graphite Iron Production F. Mampaey; Sirris, Gent, Belgium D. Habets, J. Plessers, F. Seutens; Heraeus Electro- Nite Intl., Houthalen, Belgium
Reviewer: The author used his own image analysis method (ref 13) to determine nodularity, therefore it is difficult to relate this work to industry practice. This really hurts the usefulness of the data.
Authors: In discussing ‘nodularity’, it is appropriate to make a distinction between compacted and spheroidal graphite cast iron. Nodularity is defined in ISO 16112, in Annex B, only as informative (reference 25) and represents the areal sum of type VI (nodular) and half of the intermedi- ates (consisting of type V and IV). Graphite types are speci- fied in ISO/DIS 945-1 (reference 16). As shown in the pa- per, the present method based on length to thickness ratio of graphite particles gives practically the same results as ISO 16112 for nodularity for a series of photographs (Figure 14) as well as for the SinterCast wall charts (reference 13). As a conclusion, the present method is a good method for nodu- larity calculation in case of compacted graphite cast iron, which is the topic of the paper.
For spheroidal graphite cast iron, nodularity is not defined in CEN 1563:1997 (“Founding – Spheroidal graphite cast irons”). In this document is only specified that the graphite structure should be mainly in type V and VI. Also ASTM A 536 (“Standard Specifications for Ductile Iron Castings”) does not specify nodularity, but nodularity is defined in ASTM A 644 (“Standard Definitions of Terms Relating to Iron Castings”) as type I and II from ASTM A 247 (“Standard Method for Evalu- ating the Microstructure of Graphite in Iron Castings”), which corresponds to ISO 945 type VI and V, respectively. Hence, nodularity is defined differently in ISO 16112 and ASTM A 644, and not defined in CEN 1563. Consequently, in order to avoid ambiguities, it seems better to use the sum of type V + VI rather than nodularity to specify the graphite structure for spheroidal graphite cast iron as specified in CEN 1563.
In our first paper (reference 8), we demonstrated that opti- mal spheroidal graphite properties occur as a function of oxygen activity, also for the average nodule quality of the graphite structure. In the present paper as well as the pre- vious one we used exactly the same method to determine ‘nodularity’. We did not use type V + VI in the first paper because type IV and type V cannot be distinguished perfectly from each other as shown by next references:
• Reference 25 (ISO 16112) • Fargues, J., Stucky, M., “Caractérisation de la forme du graphite à l’aide d’un analysateur d’images,” Fonderie, no. 129, pp 13-21 (1993).
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• Velichko, A., Mücklich, F., “Shape Analysis and Classification of Irregular Graphite Morphology in Cast Iron,” Praktische Metallographie, vol. 43, no. 4, pp 192-207 (2006).
Finally, the magnification of the pictures also influences nodularity. At lower magnification, an irregular outline of spheroidal graphite particles becomes less clear which in- creases nodularity. In the present research, all pictures are taken at magnification 200x in order to detect changes in nodule quality clearly. In ductile iron, nodularity as the sum of type V and VI is not accurate enough for our purposes. Indeed, any graphite structure consisting entirely of type V and VI only, all invariably gives 100 percent nodularity. The method based on length to thickness ratio gives differences in this case. As a conclusion, nodularity as used in our pa- pers gives a correct value in case of compacted graphite cast iron but does not correspond to the sum of type V + VI in case of spheroidal graphite cast iron.
Reviewer: The authors did chemical analysis on castings, not chilled spectrometer samples. There are few calibration standards for castings so the results may not be accurate. Please comment.
Authors: We used GDOES (Glow Discharge Optical Emis- sion Spectrometry, see reference below). The advantages of this type of spectrometer are its ability to analyze samples in depth (max. 0,15 mm) and the structure independence: it analyzes samples solidified gray and / or white iron with the same accuracy. The disadvantage is a longer analysis time (about 75 seconds) and the need for plane and pol- ished surfaces. The calibration standard samples used are white. Carbon and sulfur are determined using combustion analysis (LECO equipment). The advantage of using a gray sample from the Y-block lies in the fact that sulfur and mag- nesium correspond exactly to the test piece. Indeed during holding, the content of both elements changes continuously. (Reference: van der Mey, M., “Modernes Spektrometer ar- beitet nach dem GDOES-Prinzip,” Giesserei, vol. 91, no. 2, pp 86-87,2004).
Reviewer: Inoculation and inoculants are mentioned but their effect on the results is not discussed.
Authors: In general, the amount of inoculation as well as the type of the inoculant have an influence on nodularity. A more efficient inoculation will increase nodularity, an ef- fect which is well-known. For compacted graphite cast iron, inoculation treatment should be low to decrease nodular- ity however high enough to avoid carbides. This behavior makes production of thin wall compacted iron casting so difficult since the efficient inoculation treatment required to avoid carbides inherently increases nodularity too. In the present work, 0.3 percent inoculant was manually added in the stream during filling the ladle. In these circumstances, inoculant type did not have an influence on the position of
International Journal of Metalcasting/Spring 10
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