not be produced in a controlled and reproducible way, it re- mained a research curiosity.
Later, it was found that the addition of titanium could en- large the production window to some extent.3
Unfortunate-
ly, titanium combines to form carbo-nitrides which are ex- tremely hard. As engines require a lot of high speed machin- ing, the titanium route was excluded for series production.4 However, legislation and the need for better fuel efficiency pushed the manufacturers to start producing engines in com- pacted graphite cast iron. Examples are the Audi 3,3lV8, BMW 3,9lV8, DaimlerChrysler OM 501 LAR5 PSA 2,7lV6.
and Ford-
Today, possible production control is based on thermal analysis, acoustic resonance analysis and oxygen activity measurement. Thermal analysis requires about three min- utes, which is long in production circumstances. Moreover, sometimes a calibration is needed for each specific casting which makes production control difficult in case of small series.6
Mampaey7 examined the possibility of acoustic reso-
nance analysis. While this method is very suited to control castings after production, the research showed that exami- nation during production is difficult and time consuming too. Acoustic resonance analysis poses two major problems. When results are needed within three minutes, quenching of the specimens after solidification is required. The quenching reduces the difference between ductile iron and compacted graphite cast iron, making the method less accurate. Second- ly, undertreated iron solidifies gray without an indication of how much additional magnesium is still required to move the iron into the desired compacted graphite iron range. As a result, acoustic resonance analysis can only deliver quan- titative information for over-treated iron which is in fact not really the goal.
In previous research, the authors examined oxygen activity measurements in the case of ductile iron8 graphite cast iron.9
and for compacted With the sensor, the optimal condition
giving best mechanical properties could be determined ‘in real time’ during production since a result is obtained very quickly (12 seconds). The first results for compacted graph- ite cast iron, encouraged further research which is presented in this paper.
Review of Oxygen Activity Measurements in Cast Iron
The transition of graphite lamellae to spheroids occurs by adding magnesium to a melt. The magnesium combines with oxygen and sulfur. As long as sufficient magnesium is avail- able (about 0.040%), ductile iron is produced. Because the vapor pressure of magnesium in carbon-saturated liquid iron is relatively high, magnesium continuously evaporates from the melt.10
The basic idea is that the chemi-
cal equilibrium between oxygen and magnesium can be fol- lowed by determining the oxygen activity only.
and the base melt, which is much larger in ductile iron than in steel melts. On the other hand, sensors which measure ac- tivities based on the Nernst equation, have the advantage to be very sensitive at low concentrations, as the potential is related logarithmically to the concentration.11 version of a sensor became available.12
O3
Oxygen activity measurements are quite common in steel. Here, oxygen activities are in the order of 5 ppm to 500 ppm. How- ever, in ductile iron melts, oxygen activities are much lower, about 100 ppb, and pose more technological problems to sensor producers. In fact, a major problem arises from the difference of the oxygen activity of the reference material (Cr / Cr2
)
Recently, an improved In line with chemical
principles, oxygen activity changes rapidly with temperature. In order to compare measured activities, a recalculation to a con- stant reference temperature is a prerequisite. Accordingly, all oxygen activities listed in this paper are valid for the reference temperature of 1420°C (2588F). The temperature coefficients needed for this recalculation have been published.8
Finally, it is worth mentioning the present method used to determine nodularity. Mampaey13
developed software for
image analysis which determines the length to thickness ratio of graphite particles. Historically, this technique was introduced to discern compacted graphite particles (length to thickness ratio 2 - 10)14,15
to thickness ratio > 20). Using this classification, particles with length to thickness ratio
are both
accepted as nodular graphite, then the equivalent length to thickness ratio should be set to higher values (condition 2). Consequently, nodularity values listed here are smaller than in case of condition 2. The primary goal of the previous re- search was to find out if oxygen activity could be related to graphite form. Unfortunately, no international standard for image analysis is available to solve this matter. The length to thickness ratio has several advantages. First, it is easy for human interpretation. Second, a good correlation exists between mechanical properties of compacted – ductile iron and length to thickness related features.13
Finally, lamellar
graphite can be discerned from vermicular particles. The problem of nodularity will be discussed in more detail later when examining the graphite structure as a function of the oxygen activity for the experiments presented in this paper.
Putting too much magnesium in the melt has ad- verse effects. In previous papers, the chemical principles and governing equations of oxygen activity measurement have been discussed in detail.8
26
Figure 1 illustrates results obtained in the case of a ferritic ductile iron. The experimental procedure is identical to the one for compacted graphite iron which will be detailed be- low. As magnesium decreases, oxygen activity raises. Both nodularity and elongation present a maximum for the same oxygen activity value. Too much magnesium, giving oxygen activities below 72 ppb, decreases nodularity. On the other hand, when magnesium becomes too low, graphite spheroids gradually change to vermicular graphite particles. Similar results were noted for pearlitic and mixed matrix structures.
International Journal of Metalcasting/Spring 10 from lamellar graphite (length
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