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In order to substantiate the literature results with more detailed microstructural data, a series of damping capacity tests were conducted at the Department of Vehicle Engineering of the Royal Institute of Technology (Kungliga Tekniska Högskolan) in Stockholm, Sweden. A total of 21 different as-cast microstructures were produced to represent various combinations of low (3.5-3.6%) and high (3.7-3.8%) carbon contents with low (70-80%) and high (95-100%) pearlite contents. Samples were cast over the entire range of graphite nodularity, including specimens with flake-patches. Conventional pearlitic grey iron (3.25% C) was included in the study as a reference material. All as-cast specimens were machined to provide test beams measuring (750x25x25 mm). The beams were suspended by two thin wires (free-free conditions) and excited near one end by a mechanical hammer. The vibrations were monitored by a 2 gram accelerometer attached near the opposite end of the beam, but on the same face that was struck by the hammer. The data presented in this section represent the average values of five individual measurements at each resonant frequency. The measurements uncertainty was estimated to be 10%.
The damping capacity of a material is evaluated at its resonant frequency. Upon excitation, the 750 mm test beams developed seven different resonances ranging from 200 to 5000 Hz. In order to minimize the effect of microstructural variations in the long test beams, the relative damping capacity results (calculated from the logarithmic decrement) plotted in Figure 14 represent the average of the values measured at each of the seven resonant frequencies.
Figure 14 shows that the damping capacity decreases rapidly as the microstructure changes from Type A flake graphite to flake-patch and thereafter to CGI and higher nodularities. Relative damping capacities are on the order of Grey : CGI : SG = 1.0 : 0.35 : 0.22, although considerable scatter exists. For all practical CGI compositions, the damping capacity is independent of carbon content and pearlite content. Thus, it is concluded that damping capacity varies in relation to elastic modulus, as was also reported by Kurikuma et al . Sergeant and Evans  and Subramanian et al  have reported that the damping capacity can be increased by 5-10% with increasing size or coarseness of graphite, however this factor could not be evaluated in the present study because the dimension and cooling rate of the as-cast test bars was kept constant for all specimens.
While it is clear that CGI has a lower damping capacity than grey iron, this result must be taken in perspective. Relative damping capacity values for wrought aluminum alloys (normalized to grey cast iron) range from 0.004  to 0.04 . More specifically published values of the logarithmic decrement are on the order of 10-3 for cast iron and 10-5 or 10-6 for aluminum. However, the construction or boundary (design) effect even for a flat panel results in a logarithmic decrement on the order of 10-2 . Thus, for commercially available cast irons and aluminum, the construction of the engine is significantly more important than the damping capacity of the material. Relative to identically designed grey iron blocks, modal analyses with fourteen different CGI cylinder blocks show an increase of 8-18% in the resonant frequency modes, which can be attributed to the 35% increase in elastic modulus of CGI relative to grey iron. This positive shift in vibration frequency has lead to 1.0 – 1.5 dB reductions in sound pressure levels of operating engines in semi-anechoic chamber tests .
Thermal Conductivity The thermal conductivity of the graphite phase in cast irons is three to five times greater than that of either ferrite or pearlite . It is therefore intuitively evident that the amount and shape of graphite are the critical factors in defining the thermal conductivity of cast irons. In order to quantify the effects of carbon content (in the practical CGI range of 3.5-3.8%), graphite shape, matrix composition and temperature on thermal conductivity, a series of controlled tests were conducted at the Austrian Foundry Institute (Österreichisches Giesserei-Institut, Leoben). The composition of the specimens, the test technique and the results are presented in this section.
The thermal conductivity specimens were cast as ASTM A 536 Y-block samples according to the method previously described for the tensile tests. Cylindrical samples (25 mm diameter by 25 mm long) were machined from the central area of the Y-block. Each specimen is ‘stacked’ in tight thermal contact between two reference materials of the same diameter and the thermal conductivity is evaluated by a comparative method with stationary axial heat flow. The upper reference specimen was coupled to a heat source while the lower reference was coupled to a heat sink. Radial heat losses were minimized by a guard heater and insulators. The reference material was SRM 8421 electrolytic iron supplied by the US National Institute of Standards and Technology.
Type N thermocouples were embedded at known locations in the cast-iron specimens and the reference materials. The temperature difference along the length of the specimen was approximately 10°C and the average value of the temperatures measured at the two thermocouple points was taken as the bulk temperature of the specimen. All measurements were conducted under argon atmosphere. The total uncertainty of the thermal conductivity results was estimated at 7%.
A total of 18 specimens were examined to determine the influence of carbon content, nodularity and pearlite content on thermal conductivity in the temperature range of 55-400°C. As shown in Table IX, the specimens were grouped into four broad categories of similar carbon (3.5-3.6%C and 3.7-3,8%C) and pearlite (70-80% and 95-100%) contents. The data, which are plotted in Figure 15 (a-d) thus show the change in thermal conductivity as a function of nodularity, pearlite content and temperature. Each line in Figure 15 is identified by its nodularity in parentheses which allows for cross-reference to the raw data contained in Table IX. The thermal conductivity of conventional pearlitic grey iron [4,38,41] is included in each of the Figure 15 plots as a reference.
The trends revealed in each of Figure 15 (a) through 15 (d) show that the thermal conductivity of CGI is approximately 25% less than that of pearlitic grey iron at room temperature and 15-20% less at 400°C. The observation that the thermal conductivity of grey iron decreases with increasing temperature is well known.
However, it is interesting to note that even the presence of relatively small amounts of flake graphite in a predominantly compacted graphite microstructure also causes the conductivity to decrease with temperature.
This is observed in the flake-patch specimens in each of the four microstructure groups. In contrast, the thermal conductivity of CGI and nodular cast irons gradually increases with increasing temperature. This is also in agreement with the literature which sometimes reports that the thermal conductivity of ferritic CGI can exceed that of pearlitic grey iron at temperatures above 200°C [16,42].
The trending of the thermal conductivity data presented in Figure 15 is logical, however the results show that compositional changes within the practical production range of 3.5-3.8% carbon can only influence the thermal conductivity by a maximum of 10%. While the microstructural latitude of CGI may be too narrow to significantly increase thermal conductivity, the data indicate that increases in nodularity from 10-30% cause a further 10% decrease in the thermal conductivity of CGI relative to grey iron. Therefore, it is more meaningful for CGI specifications to focus on avoiding high nodularity (and thus conductivity losses) in thermally loaded areas rather than trying to identify opportunities to increase conductivity. This is particularly true in consideration of the risk of the rapid decrease in tensile properties if flake graphite begins to appear in the CGI.
Summary The graphite shape, carbon content and pearlite content of cast irons each play an important role in determining the mechanical and physical properties of cast irons. While some material properties, such as Poisson’s ratio or thermal expansion, are effectively constant over a wide range of microstructure and chemistry, the present study addresses the effect of microstructural changes on the properties of compacted graphite iron.
The mechanical properties of CGI decrease by 20-25% as soon as flake-type graphite appears in the microstructure. Although the mechanical properties gradually increase as the nodularity exceeds 20%, thermal conductivity is reduced and castability and machinability become more difficult. The data obtained at increased nodularity levels also serve to indicate the expected change in material properties as the nodularity varies between thin and thick sections of a casting, which can be used to advantage in CGI engine design. Although the demands of every component are unique, the data suggest that a nodularity range of 0-20% is appropriate for the cylinder bore walls and other structural regions of a cylinder block. Flake graphite is inadmissible. The pearlite content, which is linearly related to hardness and tensile strength, can be specified to suit the wear, machinability and high temperature performance requirements of the component. Alloying elements can also be specified to improve selected properties.
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