«©SinterCast 2012 Introduction As the demand for high torque, low emissions and improved fuel economy continues to grow, engine ...»
Compacted Graphite Iron Mechanical and Physical Properties
for Engine Design
©SinterCast 2012 www.sintercast.com
As the demand for high torque, low emissions and improved fuel economy continues to grow, engine
designers are forced to seek stronger materials for engine block construction. This is particularly true in the
diesel sector where resolution of the conflicting performance objectives requires increased cylinder bore
pressures. The bore pressures in today’s direct injection diesels hover around 135 bar while the next generation of DI diesels are targeting 160 bar and beyond. Peak combustion pressures in heavy duty truck applications are already exceeding 200 bar. At these operating levels, the strength, stiffness and fatigue properties of grey cast iron and the common aluminum alloys may not be sufficient to satisfy performance, packaging and durability criteria. Several automotive OEM’s [1-3] have therefore evaluated Compacted Graphite Iron (CGI) for their petrol and diesel cylinder block and head applications.
Although compacted graphite iron has been known for more than forty years, and several review papers have been published [4-11], the properties of CGI are not yet as well known as those of grey cast iron and the common aluminum alloys. This paper therefore provides more detailed information on the mechanical and physical properties of CGI as a function of the graphite nodularity, carbon content and the influence of ferrite and pearlite. The data presented are primarily from tests conducted at independent laboratories, although literature data is included and referenced where appropriate.
Microstructural Considerations As shown in Figure 1, the graphite in compacted graphite iron (sometimes referred to as vermicular iron) appears as individual ‘worm-shaped’ or vermicular particles. Although the particles are elongated and randomly oriented as in grey iron, the compacted graphite particles are shorter and thicker, and have rounded edges. While the compacted graphite particle shape may appear worm-like when viewed with a conventional light microscope, deep-etched SEM micrographs (Figure 2) show that the ‘worms’ are connected to their nearest neighbours within the eutectic cell. This complex graphite morphology, together with the rounded edges and irregular bumpy surfaces, results in strong adhesion between the graphite and the iron matrix. Ultimately, the compacted graphite morphology inhibits both crack initiation and propagation and is the source of the improved mechanical properties relative to grey cast iron.
Figure 1: Compacted Graphite Iron Figure 2: Deep-etched SEM micrographs show the true three dimensional microstructure with 10% nodularity
The percent nodularity of all test specimens was evaluated by image analysis using the criteria described in Reference . The micrograph presented in Figure 1 has a calculated nodularity of 10%. In addition to the influence of nodularity, the presence of graphite flakes or discrete patches of flake-type graphite also impacts upon the mechanical and physical properties of CGI. In order to allow the data obtained from flake-containing microstructures to be plotted together with data from CGI and nodular microstructures, the present study introduces a nodularity range of 0 to -5% nodularity to represent the amount of flake graphite present in the otherwise compacted microstructure. Specimens containing isolated patches of flake graphite are assigned nodularity ratings ranging from -1 to -5% with increasingly negative values representing increasing severity of flake patch occurrence. A value of 0% nodularity corresponds to a fully compacted structure, while -5% represents a fully flake graphite microstructure. The abrupt step from 0% nodularity for CGI to -5% nodularity for grey iron is intentionally chosen to parallel the rapid transition from a fully compacted microstructure to a flake-containing structure. In practice, this transition can occur due to a decrease of only 0.001% magnesium in the molten cast iron. A flake patch micrograph with an assigned value of -3% nodularity is presented in Figure 3 as a visual reference.
Finally, the pearlite content of the test specimens was evaluated visually with the criteria that pearlite-plusferrite must equal 100%. Cementite (carbide) was not present in any of the test pieces. All microstructure results are obtained directly from the test pieces after the relevant mechanical or physical property has been evaluated. The nodularity and pearlite results represent the average value of at least three fields of view (10 mm total area) subjectively chosen to represent the bulk microstructure of the test piece.
Test Piece Production The data presented in this paper were obtained from standardised test pieces produced in a production foundry. The base iron was melted in 6 tonne coreless medium frequency induction furnaces and held in a 60 tonne coreless line frequency furnace as part of the standard base iron for ductile iron series production.
Base treatment was performed with a commercially available MgFeSi alloy followed by SinterCast analysis and corrective additions of 5 mm magnesium and/or 9 mm diameter inoculant cored wires to obtain the desired graphite shape. Test pieces were poured from a one-tonne production ladle. Specific details of test piece geometry, chemistry and microstructure are provided in each section of the paper as appropriate.
Tensile Properties The test pieces used for tensile testing were machined from as-cast cylindrical test bar samples produced according to the ASTM A 536 standard. Three tensile specimens were machined from each bar. The tensile tests were conducted by the ABB Corporate Research laboratories in Västerås Sweden using a Wolpert 100 kN:s universal testing machine according to the ASTM E 8M (room temperature) and ASTM E 21 (elevated temperature) norms. The individual results presented in this section represent the average of two separate ‘pulls’ for the room temperature data and three separate ‘pulls’ for the data at 100°C and 300°C. A total of 84 tensile tests were performed.
The experimental design of the tensile tests was established to focus on the most relevant microstructures for the production of CGI cylinder blocks and to identify the variation in tensile properties as the microstructure strays beyond the desired range. Thus, the materials studied can be separated into two
1 CGI with fixed (0-10%) nodularity and as-cast matrix structures ranging from 20-100% pearlite, and 2 CGI with fixed (85-100%) as-cast pearlite content and graphite shape ranging from mixed CGI/flakepatch structures up to 90% nodularity.
Eleven different microstructure Groups were evaluated to represent the total range of cast iron microstructures. The microstructure data and chemical composition for the eleven different microstructure Groups are summarized in Table I while the tensile results are presented in Table II.
The change in the ultimate tensile and the 0.2% yield strength of predominantly (85-100%) pearlitic cast irons as a function of nodularity is shown in Figure 4. While the strength gradually increases with increasing nodularity, the presence of even a small amount of flake graphite results in a step-reduction of 20-25% tensile strength. Extrapolation of the data suggests that a fully A-Type flake structure would have a roomtemperature tensile strength of approximately 200 MPa at the carbon content required to produce CGI.
It is apparent from these results that flake graphite must be avoided in CGI castings.
Figure 4: Ultimate tensile strength and 0.2% yield strength of 85-100% pearlitic cast irons as a function of nodularity and temperature The data of Figure 4 also indicate the effect of increasing nodularity which occurs naturally due to increased cooling rates in the thin sections of a casting. The gradual increase in tensile strength with increasing nodularity is validated by extrapolation of the 25°C line which tends toward 750 MPa for pearlitic ductile (90% nodularity) iron. However the 0.2% yield strength was measured to only increase by 5% over the range of 0 to 50% nodularity. Other literature sources report 10%  and 20%  increases in yield strength over the same nodularity range. Regardless of the actual amount, the strength increase from 0 to 50% nodularity is gradual rather than a step function.
The tendency toward increased graphite nodularity and thus increased strength in thin sections may at first appear contrary to the traditional approach to grey iron engine design, which requires graphite homogeneity throughout the casting. With grey iron, thin sections become naturally weaker. However, the increase in the strength of thin-section CGI may be beneficial in many product applications. In the specific example of passenger car cylinder blocks, ‘thin’ means less than approximately 4.5 mm. If the remaining ‘thick’ sections of the block contain 5-15% nodularity, the thin sections may contain 30-60% nodularity depending on the gating system, their location and thus their cooling rate. As the thin walls are typically water jacket housings, crankcase housings and ribs, there are no thermal or machining requirements and the increased strength and stiffness of the higher nodularity material benefits the product. Microstructually inhomogeneous products are not uncommon in the industry and include some cylinder liners, flywheels, glass bottle molds and even the extreme examples of cast iron liners in aluminum cylinder blocks and ductile iron bearing caps or bedplates in grey iron cylinder blocks. Specific materials are used where their properties can best contribute to the performance of the finished component. The natural tendency of CGI toward higher nodularity in thin sections provides opportunities to place thermally efficient low-nodularity CGI in the central portions of a cylinder block and higher strength higher nodularity microstructures in the mechanically loaded areas. Indeed, this approach has been advocated in recent inhomogeneous graphite cylinder block patents and patent applications from Mazda  and Ford .
The influence of pearlite content on the tensile and 0.2% yield strengths of 0-10% nodularity CGI (Figure 5) is linear with R-squared correlation coefficients in excess of 0.95. In consideration of the normal variation in mechanical property test results, and particularly in the visual determination of pearlite content, it appears that a 20% pearlite specification range (for example, 60 to 80% pearlite with a target of 70% pearlite) will result in a 10-15% range of ultimate tensile strength. In the final analysis the pearlite specification for most applications should be based on hardness, wear and machinability considerations rather than tensile properties.
With regard to elongation, the data presented in Table II clearly show the ductile nature of CGI and the fact that elongation increases with increasing nodularity and decreasing pearlite content. The data presented in Table II are in good agreement with data published in the open literature .
Figure 5: Ultimate tensile strength and 0.2% yield strength of 0-10% nodularity CGI as a function of pearlite content As shown in Figure 6, the variation in elastic modulus as a function of graphite nodularity is similar to that observed for tensile and yield strength. The presence of even a small amount of flake patches reduces the elastic modulus of CGI by as much as 20%. While the elastic modulus of conventional pearlitic grey iron ranges from 105-110 GPa, interpolation of the room temperature data in Figure 6 suggests the elastic modulus of a fully A-Type flake structure produced with CGI base iron chemistry would be less than 100 GPa.
Again, flake graphite must be avoided.
Elastic Modulus (GPa)
Figure 6: Elastic Modulus of 85-100% pearlite cast irons as a function of nodularity and temperature The data presented in Table II show that the elastic modulus of 0-10% nodularity CGI is constant in the range of 145-150 GPa for pearlite contents above approximately 50%. The modulus gradually decreases at lower pearlite contents and increases with increasing nodularity.
Another important design consideration regarding the elastic modulus of CGI is that, unlike grey iron, the elastic modulus of CGI remains constant in the presence of applied tensile stress and elevated temperatures . As shown in Figure 7, because grey iron does not elongate when subjected to tensile stress, it immediately experiences a linear decrease in elastic modulus when subjected to tensile stress. In contrast, ductile materials such as CGI and steel iron have a distinct range of proportionality between stress and strain and thus maintain a constant elastic modulus until a certain temperature-dependent stress limit is reached.
The modulus then decreases in a linear manner. The practical significance of this is that the actual elastic modulus (stiffness) of dynamically loaded CGI components can be 50-75% higher than identically designed and loaded grey iron components.
Compression Properties The compressive yield strength of cast irons plays an important role in determining the thermal fatigue life of constrained components, particularly for the valve-bridge area of heavy-duty diesel engine cylinder heads.
The 0.2% compressive yield strength of unalloyed and Cr-Mo alloyed compacted graphite irons was therefore evaluated together with a pearlitic grey cast iron according to the DIN 1691 standard for GG25.
The compression tests were conducted at the Swedish National Testing and Research Institute (Sveriges Provnings-och Forskningsinstitut) in Gothenburg, Sweden. The tests were conducted at room temperature and 400°C according to the ASTM E 9 standard with ‘medium’ length test bars. All test specimens were produced in the manner previously described for the tensile specimens. The microstructure and chemistry details of each test material are presented in Table III while the compressive properties are summarised in Table IV. The results reported for compressive yield strength and elastic modulus represent the average of three individual compression tests.