Nodularity
In the previous Figures mechanical properties were com- pared with the minimal values as defined in ISO 16112 Standard for compacted graphite cast irons. Additionally, nodularity should be maximum 20 percent. Nodularity is considered as all particles belonging to type V and VI as specified in ISO945-1. Unfortunately, a well defined inter- national standardized procedure to determine nodularity us- ing image analysis is not available today.
In order to bring some clarity, three methods for image analy- sis have been compared on the same series of structures. The structures have been obtained from two experiments. The first one covers graphite structures from spheroidal to compacted graphite within a ferritic matrix. The second one is close to a perfect compacted graphite and stems from a pearlitic matrix. Results of nodularity are plotted as a function of the elonga- tion as measured during tensile testing for a ferritic matrix elongation is high and is very susceptible to graphite form. The methods are the French method (NF A 04-197), the rec- ommendation from ISO 16112 and the length to thickness ra- tio used by the present authors before.13
Graphite structure has
been measured as the average of 10 pictures, taken at magni- fication 200. Figure 14 summarizes the results. An example of the sample with 22 percent elongation is shown in Figure 15, with 8 percent elongation in Figure 16 and with 1 percent elongation in Figure 17. The method defined in the French Standard gives high values for nodularity in case of spheroidal graphite cast iron. However, in the transition zone between spheroidal and compacted graphite iron, nodularity remains high. The method recommended in ISO 16112 as well as the present method give practically identical results, confirming the previous match with the SinterCast wall charts.13
However,
both methods give low values for the nodularity for spheroidal graphite cast iron. The explanation is simple. The informative method from ISO 16112 calculates nodularity as
Figure 12. Elongation for a pearlitic matrix as a function of the oxygen activity at 1420°C. The red line corresponds to the minimal elongation required by the ISO Standard (Table 3). Data points are colored according to sulfur content: red S ≥ 0.014%; black 0.007 ≤ 0.007%; blue S ≤ 0.002%.
Nodularity (ISO 16112) = type VI + 0.5 (type V + type IV)
Good spheroidal structures do not present much type IV which means that the contribution of type V is only considered as half. On the other hand, considering particles with length to thick- ness
Figure 18 presents nodularity of all experiments as a function of oxygen activity. Only structures without lamellar graphite are shown. For ease of examination, data points have been
Figure 13. Sulfur content and oxygen activity at 1420°C corresponding to the transition from compacted to lamellar graphite (blue line V2 and symbols). Left of the red line V1 (and red symbols), minimal mechanical properties required by ISO16112 are met.
International Journal of Metalcasting/Spring 10
Figure 14. Nodularity determined by three methods on 5 samples of 1 experiment, and plotted as function of the elongation measured in the test bar from a Y-block.
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