behavior of microstrutural gray iron

Practical Applications for Compacted Graphite Iron

S. Dawson SinterCast Limited, London, England

T. Schroeder SinterCast Inc, Naperville, Illinois

Copyright 2004 American Foundry Society

ABSTRACT

As the industrial awareness of Compacted Graphite Iron (CGI) has evolved over the last decade, design engineers have begun to specify CGI for a variety of cast components. Following an overview of the properties and machinability of CGI, the present paper reviews practical applications for CGI including some case histories. Finally, a review of the status of national and international standards is provided.

INTRODUCTION

March 20, 1948 is well known to iron foundrymen as the date that K. D. Mills, A.P. Gagnebin and N. B. Pilling filed the first patent application for the production of ductile iron. The patent was granted on October 25, 1949 and the world-wide production of ductile iron has since grown to approximately 20 million tonnes per year. In October 1998, the world foundry industry gathered at the Ductile Iron Society/AFS World Symposium to celebrate the golden anniversary of ductile iron and all of the good progress that had been made. Relatively few of the delegates knew that Millis, Gagnebin and Pilling filed, and received, a patent for Compacted Graphite Iron on the same two dates in 1948 and 1949.

Although the initial attention of the foundry industry focused on ductile iron, efforts began in the early 1960s to develop CGI production techniques and product applications. Since then, many CGI applications have been successfully established in series production. However, these production references are typically restricted to products with a wide microstructure specification or to those that can tolerate the use of titanium to increase the stable CGI range. Specific examples include exhaust manifolds, bedplates, brake components, pump housings, flywheels and brackets. According to the AFS Metalcasting Forecast and Trends, 66,000 tonnes of these CGI products were produced in the USA during 2001. In comparison the only two CGI cylinder blocks that were in production at that time, the Audi 3.3 liter V8 TDI and the BMW 3.9 liter V8d diesel engines, accounted for less than 500 tonnes of shipped CGI castings.

The application of CGI to the high volume production of complex components such as cylinder blocks and heads, that require a narrow microstructure specification and cannot tolerate the use of titanium due to machinability requirements, is largely based on the following four requirements:-

Foundry Confidence: The foundries must be able to reliably control the production process, with no risk of flake graphite formation, and at viable metallurgical scrap rates.

Machining Solutions: It must be possible to machine CGI cylinder blocks and heads with similar cycle time to gray iron while providing at least one full shift of tool life.

OEM Familiarity: The design engineers must have sufficient knowledge about the mechanical and physical properties of CGI to be able to optimize their designs.

OEM Need: CGI must provide superior performance levels, lower emissions and smaller package sizes compared to conventional materials such as gray iron or aluminum.

Today, after more than ten years of intensive development involving the foundries and their technology suppliers, the machining industry and the OEM community, these requirements have been satisfied. High volume CGI cylinder block production began during 2003 for both Ford and Audi.

Paper 04-047(05).pdf, Page 1 of 9 AFS Transactions 2004 American Foundry Society, Des Plaines IL USA

MICROSTRUCTURE AND PROPERTIES

As shown in Figure 1, the graphite phase in Compacted Graphite Iron appears as individual worm-shaped or vermicular particles. The particles are elongated and randomly oriented as in gray iron, however they are shorter and thicker, and have rounded edges. While the compacted graphite particles appear worm-shaped when viewed in two dimensions, deep-etched scanning electron micrographs (Figure 2) show that the individual worms are connected to their nearest neighbors within the eutectic cell. The complex coral-like graphite morphology, together with the rounded edges and irregular bumpy surfaces of the compacted graphite particles, results in stronger adhesion between the graphite and the iron matrix thus inhibiting crack initiation and growth and providing superior mechanical properties.

Fig. 1. CGI microstructure containing 10% nodularity.

Fig. 2. Deep-etched SEM micrographs show the complex coral-like graphite in three-dimensions.

Compacted Graphite Iron invariably includes some nodular (spheroidal) graphite particles. As the nodularity increases, the strength and stiffness also increase, but only at the expense of castability, machinability and thermal conductivity. The microstructure specification must therefore be chosen depending on both the production requirements and the performance conditions of the product. For example, the production of CGI exhaust manifolds is typically specified with up to 50% nodularity. For manifolds, the higher nodularity provides increased strength to facilitate supporting the exhaust system and also increases the flow of exhaust heat into the catalyst to achieve early light-off. The higher nodularity benefits the product without increasing the incidence of casting defects or impairing machinability. In another example, DaimlerChrysler [1] has presented that the ductility provided by microstructures with up to 50% nodularity reduces cracking defects in bedplates. In this case the higher nodularity is permissible because machining is limited to milling and short-hole drilling and the product is not thermally loaded. Compacted Graphite Iron can be produced with varying pearlite contents to suit the required application. Exhaust manifolds require more than 95% ferrite to prevent high temperature growth. In contrast, cylinder blocks and heads are typically produced with a predominantly pearlitic matrix to maximize the strength and stiffness while simultaneously producing a more consistent matrix structure and hardness to the machining operation. CGI may also be specified with an intermediate ferritic-pearlitic matrix. Within the range of 60-80% pearlite, CGI has approximately the same hardness (BHN 190-225) as a conventional fully pearlitic gray cast iron. The mechanical and physical properties of CGI are summarized in Table 1. Additional detail can be found in the AFS Iron Castings Engineering Handbook, chapter 6D [2].


Table 1. Mechanical and Physical Properties of 10% Nodularity CGI

Property Temp

(C) 70%

Pearlite 100%

Pearlite Ultimate Tensile Strength (MPa) 25 420 450 100 415 430 300 375 410 0.2% Yield Strength (MPa) 25 315 370 100 295 335 300 284 320 Elastic Modulus (GPa) 25 145 145 100 140 140 300 130 130 Elongation (%) 25 1.5 1.0 100 1.5 1.0 300 1.0 1.0 Rotary-Bending Fatigue Limit (MPa) 25 195 210 100 185 195 300 165 175 Endurance Ratio 25 0.46 0.44 100 0.45 0.44 300 0.44 0.43 Thermal Conductivity (W/mC) 25 37 36 100 37 36 300 36 35 Thermal Expansion Coefficient (m/mC) 25 11.0 11.0 100 11.5 11.5 300 12.0 12.0 Poissons Ratio 25 0.26 0.26 100 0.26 0.26 300 0.27 0.27 0.2% Compressive Yield (MPa) 25 400 430 400 300 370 Density (g/cc) 25 7.0-7.1 7.0-7.1 Brinell Hardness (BHN) 25 190-225 207-255 Ultrasonic Velocity (km/s) 25 5.0-5.2 5.0-5.2 Specific Heat Capacity, (J/g-K) 100 0.45 0.45 Enthalpy (J/kg) 26-200 80,000 80,000 Specific Electrical Resistance (-cm) 20 50 50

MACHINABILITY

With at least 75% higher tensile strength and approximately 45% higher stiffness, it is intuitively evident that CGI will be more difficult to machine than gray iron. As shown in Figure 3, which was initially generated in 1998 using standard gray iron cutting parameters, low speed cutting (100-200 m/min) with conventional carbide tools provides approximately 50% of the tool life of gray iron in milling and turning operations. Similarly, high speed (400-800 m/min) milling operations provides approximately 50% of the gray iron tool life when using polycrystalline cubic boron nitride (PCBN) or ceramic inserts. However, the difference between the machinability of CGI and gray iron is most significant when using PCBN or ceramic inserts in high speed continuous cutting operations, such as turning or cylinder boring. For the example of a typical passenger car cylinder bore, where the bore is approximately 90 mm in diameter and 100 mm long, the cutting insert remains in continuous contact with the workpiece for approximately 100 m as it spirals its way down the bore. Under these conditions, the accumulation of mechanical and thermal stresses, combined with diffusion and oxidation, cause accelerated tool wear.

Recent tooling developments and optimization have resulted in CGI milling operations providing 50-70% of gray iron tool life while CGI drilling and tapping provides approximately 70% of the gray iron tool life. To date, solutions have not yet been developed for high speed continuous cutting (boring/turning) of CGI using PCBN or ceramic inserts. These operations typically revert to the superior wear resistance of carbide tools and employ multiple inserts allowing for an increased feed rate that compensates for the lower cutting speed. For example, a CGI cylinder boring tool may have six-to-ten carbide inserts rather than one PCBN insert. The cutting speed is reduced from approximately 800 m/min (PCBN) to 100 m/min


(carbide), however, the feed rate is increased by a factor of six-to-ten times to achieve the same overall cutting time. These techniques are described in more detail in reference [3].

Fig. 3. Comparative tool life for different tool materials in interrupted (milling) and continuous (turning/boring) cutting of pearlitic CGI and Gray Iron.

Today, virtually every major tooling supplier has CGI solutions as a part of their standard product portfolio. In addition to the differences in tool wear, the increased strength and stiffness of CGI results in higher cutting forces. Because of this, CGI machining operations may also require 20-30% higher spindle power and more robust fixturing, than similar gray iron operations. While CGI may be more difficult to machine than gray iron, it is easier to machine than ductile iron. Product conversions from ductile iron to CGI will therefore realize machinability cost benefits in addition to the improved castability and mold yield.

It is well known that the narrow stable range for CGI production can be extended toward higher magnesium levels by alloying with 0.1-0.2% titanium. While the titanium increases the stable range, and thus allows foundries to add extra magnesium in order to stay safely away from the risk of flake graphite, it simultaneously results in the formation of titanium carbide and carbonitride inclusions. As shown in Figure 4, these inclusions, which are harder than many tool materials, significantly increase the abrasive wear. Titanium contents are therefore restricted to 0.015% in the latest OEM specifications for CGI castings that require significant machining.


Fig. 4. The effect of titanium additions on tool life during continuous carbide cutting (turning) of pearlitic CGI.

PRODUCT DEVELOPMENT RESULTS

As an intermediate material between gray iron and ductile iron, the most obvious use of CGI is in applications where the mechanical properties of gray iron are insufficient or where those of ductile iron are in excess of requirement. This type of approach was used in the early application of CGI for high speed train brake disks where gray iron experienced surface cracking (crazing) failures while the high elastic modulus and low thermal conductivity of ductile iron led to excessive warpage. As shown in Figure 5, CGI provided a good intermediate balance between cracking and distortion, and was successfully applied to the brake disk application.

Fig. 5. The properties of CGI are ideal for many applications with simultaneous thermal and mechanical loading.


Relative to conventional gray cast iron, CGI provides opportunities for:-

Reduced wall thicknesses at current operating loads Increased operating loads at current component design Reduced safety factors due to less variation in as-cast properties Reduced brittle failure in handling, assembly and service due to higher ductility Reduced incidence of hot cracking during shake-out Higher strength without resorting to alloying Shorter thread engagement depth and therefore shorter bolts

Relative to ductile iron, CGI can provide:-

Improved castability for more complex or nearer-net-shape components Up to 20% improved mold yield due to better feeding Less accumulated stress due to higher thermal conductivity and lower elastic modulus Improved heat transfer Improved machinability

Although CGI is often specified in simple applications where the strength of ductile iron is overkill, the real future for CGI is in demanding applications that simultaneously require strength, castability, thermal conductivity and extensive machining. The remainder of this section therefore focuses on these demanding applications and provides a review of CGI engine development results where the properties of CGI provide the opportunity for simultaneous power increase and weight reduction.

During the mid-1990s, much of the CGI development activity was focused on weight reduction. The data in Table 2 provide a summary of weight reduction results obtained in design studies conducted by various foundries and OEMs. The percent reduction values in parentheses for some engines were obtained from publications by the OEM. Although these cylinder blocks were never produced in gray iron, the publications stated that the extra mass would have been required to satisfy durability requirements if the blocks were produced in conventional gray iron. Each of the cylinder blocks denoted by xx.x in Table 2 are in series production. While comparisons of weight reduction potential are dependent on the size and weight of the original block, the data presented in Table 2 indicate that a weight reduction of 10% is a reasonable target for any CGI conversion program.

Table 2. Weight Reduction Results for CGI Vs. Gray Iron Cylinder Blocks

Engine

Size (liters) Engine Type

Gray Weight (kg)

CGI Weight (kg)

Percent Weight Reduction

1.6 I-4 Petrol 35.4 25.0 29.4 1.8 I-4 Diesel 38.0 29.5 22.4 2.0 I-4 Petrol 31.8 26.6 16.4 2.5 V-6 (Racing) 56.5 45.0 20.4 2.7 V-6 Diesel xx.x xx.x (15) 3.3 V-8 Diesel xx.x xx.x (10) 3.8 V-8 Diesel xx.x xx.x (20) 4.0 V-8 Diesel xx.x xx.x (15) 4.6 V-8 Petrol 72.7 59.6 18.0 9.2 I-6 Diesel 158 140 11.4

12.0 V-6 Diesel 240 215 10.4 14.6 V-10 Diesel 408 352 14.2

Since 1997, the emphasis in CGI engine development has gradually shifted from weight reduction toward increased power density. This shift has occurred in parallel with the strong market trend toward passenger car diesel engine performance in Europe. In 1997, European passenger car diesel engines were operating at 135 bar peak firing pressure and a specific performance of approximately 40kW/liter. The engines that were released in 1999 operated at 160 bar and 50kW/liter and the generation of diesel engines that will be released for Model Year 2004 will operate at 170 bar and approximately 60 kW/liter. In parallel with this development, the market share for diesel engine passenger cars in Europe has increased from 20 to 40%. Similar increases in peak firing pressure have evolved in the worldwide commercial vehicle sector where peak firing pressures are currently targeted at 220 bar.


The ability of various materials to support the increasing trend in peak firing pressure and specific performance is illustrated qualitatively in Figure 6, developed by the engine design consultancy firm AVL of Austria. The upper limit for each material is generally dictated by the fatigue strength, together with the requirements for durability and package size. It is evident that CGI provides more potential than the conventional materials or even a compound block design of iron-plus-aluminum. Figure 5 also demonstrates the difference in the durability of in-line and V-blocks. An in-line four cylinder engine has five main bearings to support the loading from four cylinders. In contrast, a V-6 block has only four main bearings to support the loading from six cylinders. Additionally, the V-configuration results in a more severe mechanical (tug-of-war) load pattern due to the opposing motion of the cylinders. These differences explain why the first production applications for CGI cylinder blocks have been in V-type engines.

Fig 6. As the trend toward higher peak firing pressures continue, stronger materials are needed to satisfy durability and package size criteria.

Another consideration in CGI engine design is the ability to withstand cylinder bore distortion. In the combined presence of elevated temperatures and combustion pressures, cylinder bores tend to expand elastically. As CGI has superior strength and stiffness to gray iron and aluminum, it is better able to withstand these forces and maintain the original bore size and shape. Table 3 shows comparative bore distortion results for four gray iron and CGI engines with the same design.

Table 3. Cylinder Bore Distortion for CGI vs. Gray Iron

Engine Size

(liters) Engine Type

% Improvement CGI vs Gray

1.8 I-4 Petrol 18 1.8 I-4 Diesel 20 2.2 I-4 Petrol 28 4.6 V-8 Petrol 22

The improved cylinder bore distortion results allow for reduced ring tension and thus reduced friction losses. The stability of the CGI bores also contributes to reduced oil consumption and better emissions performance. From the design perspective, these results indicate the potential to reduce bore wall section thickness in CGI relative to gray iron.

The increased stiffness of CGI also contributes to NVH performance. Although the specific damping capacity of CGI is lower than that of gray iron, the higher elastic modulus effectively stiffens the block, thus making many webs and ribs redundant. As the vibration frequency is proportional to the square root of the stiffness, the 40% increase in the elastic modulus of CGI causes a positive shift in the resonance frequencies which, in turn, increase the separation between the combustion firing frequency and the resonant frequencies of the block. The net result of this increased separation is that the engine operation becomes quieter. The positive shift in the first torsional frequency mode and the reduced noise level of several CGI engines tested in semi-anechoic chambers are shown in Table 4, both for passenger car and commercial vehicle engines.


Table 4. NVH Results for Identically Designed CGI and Gray Iron Engines

Engine

Size (liters) Engine Type

First Torsional Frequency Shift

Sound Pressure Level (dBA)

13.8 I-6 Diesel +8 Not Tested 12.0 V-6 Diesel +8 -0.5 to -1.0 5.8 V-8 Petrol +18 Not Tested 4.6 V-8 Petrol +12 Not Tested 2.4 I-4 Diesel +9 -1.0 to -1.5 2.2 I-4 Petrol +16 -1.0 to -1.5 2.0 I-4 Petrol +7 -1.0 to -1.5 2.0 I-4 Petrol +8 -1.0 to -1.5 1.8 I-4 Diesel +12 Same

DESIGN CONSIDERATIONS

When specifying gray or ductile iron, design engineers know that uniform graphite shape is critical to maintaining mechanical properties. In gray iron, the presence of degenerate graphite forms such as undercooled (Type D) graphite or large kish (Type C) graphite can reduce mechanical properties by 20-25%. Similarly, crab-shaped graphite and exploded nodules reduce the strength and stiffness of ductile iron. Based on these experiences, designers may be inclined to specify a uniform graphite structure in CGI. However, while low-nodularity (0-20%) CGI structures are required in performance-critical sections to optimize castability, thermal conductivity and machinability, higher nodularities may benefit the other structural regions of a casting. The natural tendency of CGI to solidify with higher nodularity in the faster cooling sections may result in the thin outer walls (less than 4 ~ 5 mm) having up to 50% nodularity. In many cases, where the thin sections are not thermally loaded and do not require extensive machining, the higher nodularity only serves to increase the strength, stiffness and ductility of the castings. CGI microstructure specifications therefore focus on performance-critical sections and, whenever possible, realize the benefit from increased nodularity in thin wall areas.

Thermal fatigue failures in gray iron are often rectified by adding material to reinforce strength and stiffness. However, the higher strength and lower thermal conductivity of CGI can cause thermally loaded CGI components to operate at higher temperatures. Therefore, if a CGI component experiences thermal fatigue, particularly in material substitutions using existing gray iron designs, the solution may lie in reducing the wall thickness to improve heat transfer. Todays bench tests for thermal fatigue, particularly for heavy-duty diesel engine cylinder heads, invariably rely on severe thermal cycles to minimize the test duration. However, rapid heating and cooling rates favor materials with high thermal conductivity while higher absolute temperatures and longer holding times favor materials with higher mechanical strength. The design of many bench scale thermal fatigue tests may therefore favor gray iron while the actual in-service duty-cycle would favor CGI. Re-designed CGI components have confirmed this premise. Care must therefore be taken to ensure that short-duration bench tests do not wrongly condemn a CGI component.

Foundrymen and design engineers often state that the weight reduction potential for CGI conversion programs is small because the existing gray iron components are already at the limit of their wall thickness castability limit. However, CGI weight reduction is best achieved by re-designing the relatively thick load-bearing walls of the casting. For example, in an engine block, the reduction of a main bearing wall from 20 mm to 15 mm provides a significant weight reduction, without infringing on foundry process capability. In contrast, a reduction of the water jacket from 4.0 mm to 3.5 mm may exceed process capability and yet only provide a small weight reduction. The higher strength of CGI allows designers to reduce weight by focusing on the relatively thick load-carrying regions of a casting that are not yet limited by molding considerations. Although every kilogram is important in a casting, and thick and thin sections must both be addressed, the most effective design changes are those made to the thick sections, as allowed by the superior mechanical properties of CGI.


CGI STANDARDS

Several national and international organizations have developed and published standards for CGI. These standards specify the CGI grades in terms of the tensile strength and the microstructure, expressed as percent nodularity. The currently available standards are summarized in Table 5.

Table 5. Summary of CGI Standards

Country Issuing Body Number Year Romania STAS 12443-86 1986

China JB 4403-87 1987 USA ASTM A 842-85 1997

Germany VDG W 50 2002 International SAE J 1887 2002

In addition to these published standards, the International Standards Organization (ISO) initiated the development of a new standard for Compacted (vermicular) graphite cast irons during 2002. This CGI standard will be abbreviated by the acronym GJV and is expected to be published during 2006. The CGI subcommittee (SC 7) is a part of the overall ISO Technical Committee for Cast Irons (TC 25).

Beyond the standards issued by national and international organizations, several OEMs have also established their own internal CGI standards. This includes Audi, BMW, Caterpillar, Cummins, DAF Trucks, DaimlerChrysler, General Electric, General Motors, John Deere, Opel, Rolls Royce Power Engineering and Volkswagen, among others.

CONCLUSION

After more than ten years of intensive foundry, machining and product development, Compacted Graphite Iron has entered the realm of high volume series production. While conventional materials have begun to reach durability limits in many applications, CGI provides a new opportunity to satisfy the performance and package size requirements of the next generation of engineered components, particularly in applications with simultaneous thermal and mechanical loading such as cylinder blocks and heads. The successful references provided by the first high volume production engines for Model Year 2004 vehicles will provide the confidence needed for further production commitments and the expansion of CGI to a variety of applications.

REFERENCES 1. Warrick, R.J., Ellis, G.G., Grupke, C.C., Khamseh, A.R., McLachlan T.H. and Gerkits C., Development and

Application of Enhanced Compacted Graphite Iron for the Bedplate of the New Chrysler 4.7 Liter V-8 Engine, SAE Paper pp 99-144, 1999.

2. AFS Iron Castings Engineering Handbook, AFS, Chapter 6D, pp. 171-193, 2003. 3. Dawson, S., Hollinger I., Robbins, M., Daeth, J., Reuter, U., and Schmidt, H. The Effect of Metallurgical Variables on

the Machinability of Compacted Graphite Iron, Presented at SAE International Congress, Detroit, (March 2001).


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