treatment in the same base melt. Previous research has shown that such an extra treatment has no influence on oxygen activ- ity value corresponding to the transition from compacted to lamellar graphite.9
In Figure 6 the minimal value for the ten-
sile strength (300 MPa, Table 2) is plotted. Basically, Figure 6 shows 3 distinct areas which are schematically shown by the 2 vertical dashed lines V1 and V2. The data in the Figure show that up to about 300 ppb (left of V1), all Y-blocks sat- isfy the ISO requirement for tensile strength. Between V1 and V2, a transition area is present where the minimal required ISO tensile strength is not met anymore but still a completely compacted graphite structure is present. Above 350 ppb oxy- gen activity, lamellar graphite appears in the tensile bars and strength drops below 133 MPa. The low tensile strength of these lamellar graphite structures is due to the fact that the base iron composition contains very low sulfur content which does not correspond to a good gray iron base composition. In these circumstances, the graphite appears for a consider- able part as relatively thin graphite ‘films’ in the standard Y- blocks. It is important to mention that all experimental data appearing close to the transition zone V1-V2 in Figure 6, all present a sulfur content of 0.007 percent. The sulfur content corresponds to the one determined in the Y-blocks poured in the transition zone. Indeed, typically in our experiments, the initial sulfur content gradually decreases during holding the melt in the furnace.
Figure 7 plots proof strength versus oxygen activity. Tensile bars with lamellar graphite in most cases do not have a proof strength anymore. In order to show the transition in the Fig- ure, these points are attributed a zero value. The two points in Figure 6 which do not satisfy the minimal value for the tensile strength, do so for the proof strength. This phenomenon has been frequently noticed . In the vicinity of the transition zone V1-V2, shown in Figure 6, ISO minimal requirements may be met for one specification (tensile strength, proof strength or elongation) but not for all three simultaneously. Elongation as a function of oxygen activity for a ferritic matrix is shown in Figure 8. The Figure illustrates the advantage of the ferritic matrix for determination of the transition from a compacted to a (partly) lamellar graphite structure. It is accompanied by a sudden and pronounced decrease of the elongation. This was the major reason why the first series of experiments were carried out with a ferritic matrix. During these initial experi- ments, the goal was to follow the behavior of oxygen activity during the transition from spheroidal graphite to compacted and finally lamellar graphite. Variation of the sulfur content was not envisaged here.
The data of the elongation are relatively high. It is due to the use of Sorel iron for composing the entire charge. The ben- eficial effect of high purity pig iron for ferritic ductile iron is well known26,27
The previous experiments also show that mechanical prop- erties are needed to determine if a Y-block meets the ISO requirements. Examination of the graphite structure alone is not sufficient.
In some of the previous experiments, a smaller quantity of mag- nesium wire was added when some residual magnesium was still active in the melt. In these circumstances, the measured oxygen activity is well below the original oxygen activity in the melt prior to the first addition. These extra Y-blocks are labeled as E points. Figures 6-8 show that the extra Y-blocks poured after an extra magnesium addition to the melt, are in line with the ‘normal’ data (corresponding to Y-blocks poured after the initial magnesium wire addition). The extra E samples are ob- tained after a two step procedure in which extra magnesium is added to a melt containing insufficient magnesium.
Pearlitic Matrix
Compacted graphite cast iron is mentioned frequently in lit- erature as the material of choice for future engines. Because of legislation, exhaust gases need to contain less harmful com- ponents. This can only be achieved by increasing the com- bustion pressure in the cylinders above 200 bars. In these cir- cumstances, material properties of about 350 MPa are needed. This can be achieved with compacted graphite cast iron, pro- vided a pearlitic matrix is present. Indeed, the proof strength of the pearlitic composition is 350 MPa (Table 3).
A pearlitic composition has been prepared by adding man- ganese, copper and some tin to the charge (Table 1B). The basic charge composition remained the same. Tin addition was small in order to avoid the embrittling effect for tin con- tents above 0.1 percent.
Mechanical specifications for pearlitic compacted graphite cast iron, listed in Table 3, have been applied to define the minimal values for the mechanical properties.
and the same effect is noted here for com- pacted graphite cast iron. It was deliberately chosen in order to minimize the number of processing parameters. Nodular- ity versus oxygen activity will be discussed together with the pearlitic matrix data.
32
Figure 8. Elongation for a ferritic 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 2).
International Journal of Metalcasting/Spring 10
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