Image analysis of graphite particles in cast iron uses a new method which determines the length to thickness ratio of each graphite particle. This feature is easy for human inter- pretation and should be the preferred method for all non- spherical objects like compacted graphite particles and graphite lamellae. The length to thickness calculation differs basically from current techniques, which rely on a compari- son of particle features with those of a circular object. A de- tailed description of the method as well as its application to compacted graphite cast iron has been published before.7,13 All graphite particles are divided into 4 classes, based on the length to thickness ratio. Class SG is considered group- ing spheroidal graphite particles. Compacted graphite cast iron is generally defined to contain particles with length to thickness ratio between 2 and 10. This class is denoted by CG. During the research, it became clear that two additional classes instead of just one class were needed for particles with a length to thickness ratio larger than 10. These are de- noted G10-20 for particles with a length to thickness ratio between 10 and 20, and LG>20 for particles with a length to thickness ratio larger than 20. Although this last class (LG>20) is generally considered as lamellar graphite, a small fraction of compacted graphite particles has length to thickness ratio higher than 20. For example 0.92% on a to- tal of 1731 particles, with maximal length to thickness ratio of 31.75. These samples were taken from test bars exempt of lamellar graphite. Indeed, traces of lamellae are accom- panied by a drop in tensile strength (and elongation for a ferritic matrix). In the present research, the fraction (or per- centage) of SG particles corresponds to the nodularity and is calculated as an area fraction. Nodularity determined in this way, gives a perfect match with the SinterCast wall chart as previously shown.13
The great advantage of the length to
thickness method is the automatic recognition of graphite lamellae within compacted – spheroidal graphite. The infor- mative procedure for nodularity evaluation published in the ISO standard for compacted graphite irons25 lar graphite in the microstructure.
excludes lamel-
In the present research, oxygen activity has been measured using commercially available equipment and sensors.12
In
order to examine experimental results, the measured oxy- gen activities need to be compared. Because oxygen activity
Table 1A . Chemical Analysis for All Heats with a Ferritic Matrix (Mass Percent)
strongly varies with temperature, a constant reference tem- perature of 1420ºC (2588F) has been chosen for compari- son. This holding temperature has been chosen because it represents a suitable pouring temperature. The recalculation to the reference temperature is done automatically by the equipment. However, the procedure for recalculation as well as the temperature coefficients have been published in de- tail.8
Previous research has shown that the temperature cor- rection for the oxygen activity is not constant but varies with oxygen activity, especially within the compacted graphite iron range. The accuracy of the oxygen activity measure- ment is discussed in the Appendix.
Experimental Results Ferritic Matrix
The first series of experiments was carried out for a ferritic matrix. The advantage of the ferritic matrix is the high elon- gation which is very susceptible to the graphite form. Figure 5 shows how oxygen activity, tensile strength, proof strength, hardness, elongation, ferrite content and nodularity change while holding melt 070839-S2 in the furnace. The chemical analysis for all melts can be found in Table 1A. The initial oxygen activity of the melt prior to magnesium addition (i.e. time zero) is shown in Figure 5 (and 9) at a negative value of the time. The minimal mechanical specifications for a ferritic matrix as required by ISO are listed in Table 2.
The transition from compacted graphite iron to lamellar graphite iron is reflected in Figure 5 by a sudden drop of elongation and tensile strength. The advantage of the ferritic matrix for research purposes is the drastic change of elonga- tion. This happens once traces of lamellar graphite appear in the matrix. Nodularity is not a good criterion here because in the vicinity of the transition, nodularity often hardly chang- es. The sudden drop in nodularity only occurs when massive lamellar graphite is present in the matrix.
During the transition of ‘bad’ ductile iron with too much ver- micular graphite (about 50 percent nodularity after 2 min- utes) to truly compacted graphite cast iron (14 percent nodu- larity after 15 minutes), proof strength varies little, tensile strength decreases more but elongation drops considerably. Figure 5 also shows ferrite content and hardness which both slowly decrease without presenting a sudden change when lamellae appear.
Table 2. Minimal Mechanical Properties for Compacted Graphite Iron with a Ferritic Matrix
All heats: P 0.014; Mn 0.03 – 0.05; Cr 0.05; Ni 0.07; Ti 0.02; V 0.03; Sn
30 International Journal of Metalcasting/Spring 10
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