Cast Iron Grades
For most modern automotive and consumer applications, cast iron falls into 4 separate categories:
- Gray Iron (page 1)
- Ductile Iron (page 1)
- Austempered Ductile Iron (this page)
- Compacted Graphite Iron (this page)
In North America, there are 2 industrial societies that take responsibility to organize and govern programs to regulate and improve the process technologies:
- American Foundry Society (AFS)
- Ductile Iron Society (DIS)
Austempered Ductile Iron (ADI)
During the past decade the development and commercialization of austempered Ductile Iron (ADI) has added a new star to the Ductile Iron family. Combining the strength, ductility, fracture toughness and wear resistance of a steel with the castability and production economies of a conventional Ductile Iron, ADI offers the designer an exceptional opportunity to create superior components at reduced cost. Only one factor has detracted from this story of forty years of Ductile Iron technology – the promotion of this material to designers has been a poor second to its technical development.
In the late 1980’s, Ford Motor Company made plans to use an ADI crankshaft for their high horsepower 3.8L Supercharged engine that was designed for the 1990 Thunderbird. However, prior to launching the ADI crankshaft, the company had reservations over the processing risks and chose to use a more expensive steel forging design instead. Despite that setback, austempered ductile iron materials remain an important alternative or applications requiring high strength, ductility, toughness and castability.
The most significant challenges with managing the quality in an ADI part are generally associated with enforcing manufacturing controls in manufacturing plant locations that are used to less oversight in how the operations are governed. In particular, there are tighter tolerances required to control the alloy composition for ADI castings in the foundry, more so that traditional ductile iron controls. In addition, the machining operations will need to consider methods to remove the harder and tougher ADI materials, as compared to the easier machining ductile irons. Lastly, the heat treatment of ductile iron is highly specialized for both process and quality verification. Experienced companies like Applied Process (click here for link) have facilities, furnaces and quality systems specific to the heat treatment of ADI parts.
Austempering is a heat treating process design to produce a strong, tough microstructure while avoiding the potential for distortion and cracking caused by severe quenching. The austempering process is neither new or novel and has been utilized since the 1930’s on cast and wrought steels. In this process, the part is austenitized and then quenched in an elevated temperature bath, usually composed of molten salt. The part is then held at an elevated temperature long enough for the desired transformation then removed to cool down in still air. The austempering process was first commercially applied to Ductile Iron in 1972 and by 1998 world-wide production was approaching 100,000 tonnes annually. Unlike a steel response, Austempered Ductile Iron will have a microstructure that is a mix of acicular ferrite and carbon stabilized austenite.
ADI is produced by an isothermal heat treatment known as austempering. Austempering consists of the following steps as shown below in DIS figure 4.32:
- Heating the casting to an austenitizing temperature in the range of 1500-1700F (815-927C).
- Holding the part at the selected austenitizing temperature for a time sufficient to get the entire part to temperature and to saturate the austenite with carbon.
- Quenching (cooling) the part rapidly enough to avoid the formation of pearlite to the austempering temperature in the range of 450-750F (232-400C). This temperature is above the martensite start temperature (Ms) for the material.
- Austempering the part at the desired temperature for a time sufficient to produce a matrix of ausferrite. Ausferrite is a matrix of acicular ferrite and austenite stabilized with about 2% carbon.
- The final step is cooling the part to room temperature.
As illustrated in DIS figure 4.32, shown below, the austempering temperature is one of the major determinants of the mechanical properties of ADI castings. To produce ADI with lower strength and hardness but higher elongation and fracture toughness, a higher austempering temperature (650-750F (350-400C)) should be selected to produce a coarse ausferrite matrix with higher amounts of carbon stabilized austenite (20-40%) and a hardness ranging 260-320 BHN. Grades 125/80/10 and 150/100/07 would be typical of these conditions. To produce ADI with a higher strength and better wear resistance, austempering temperatures below 650F (350C) should be used. At a lower austempering temperatures, a lower ductility (i.e. toughess) will also result.
Once the austempering temperature has been selected, the austempering time must also be established to optimize properties through the formation of a stable structure of ausferrite. DIS figure 4.33, shown below, schematically illustrates the influence of austempering time on the stabilization of austenite, and shows the hardness of the resultant matrix. At short austempering times, there is insufficient diffusion of carbon to the austenite to stabilize it, and martensite may form during cooling to room temperature. The resultant microstructure would have a higher hardness but lower ductility and fracture toughness (especially at low temperatures). Excessive austempering times can result in the decomposition of ausferrite into ferrite and carbide, which will exhibit lower strength, ductility and fracture toughness. At the highest austempering temperature (750F (400C)) as little as 30 minutes may be required to produce ausferrite. However, at 450F (230C) as much as four hours may be required to produce the optimum properties.
ADI Matrix and Morphology
Ductile Iron and ADI are similar in that the final mechanical properties are primarily determined by the metal matrix. While the matrix in conventional Ductile Iron is a controlled mixture of pearlite and ferrite, the properties of ADI are due to its unique matrix of acicular ferrite and carbon stabilized austenite; called Ausferrite.
ADI is sometimes referred to as “bainitic Ductile Iron”, but correctly heat treated ADI contains little or no bainite. Bainite consists of a matrix of acicular (plate-like) ferrite and carbide. ADI’s ausferrite matrix is a mix of acicular ferrite and carbon stabilized austenite. This ausferrite may resemble bainite metallographically, however it is not because it contains few or none of the fine carbides characteristic in bainite. An ausferrite matrix will only convert to bainite if it is over tempered.
Polishetty, Ashwin & Singamneni, Sarat & Littlefair, Guy. (2008). A Comparative Assessment of Austempered Ductile Iron as a Substitute in Weight Reduction Applications. Proceedings of the ASME International Manufacturing Science and Engineering Conference, MSEC2008. 1. 10.1115/MSEC_ICMP2008-72091.
Compacted Graphite Iron (CGI)
Compacted Graphite Iron (CGI) is a recent addition to the family of cast irons. As shown by the image to the right, the graphite is characterized as a “vermicular” or worm-like structure. This graphite structure produces an iron with most of the properties being in-between the Gray Iron and Ductile Iron families of properties. Simply stated, CGI has nearly the same castability and wear resistance characteristics as gray iron, but with improved strength. Being able to use CGI for engine cylinder blocks allows thinner wall sections and lighter weight.
As history records it, Compacted Vermicular Graphite Iron was developed during the same time period as the more well known Ductile Iron. This lesser-known material, now referred to as Compacted Graphite Iron (CGI), was also patented in the same time frame as a novel material. While ductile iron was further developed and promoted by the International Nickel Company (INCO), becoming a common iron grade to the design engineer, CGI was never seriously utilized. While not quite as strong as ductile iron, compared to gray iron properties, CGI is 75 percent stronger and up to 75 percent stiffer. It is also twice as resistant to metal fatigue as gray iron. However, significantly important for use in engine cylinder blocks, the graphite morphology of CGI was determint to have similar friction and wear resistance characteristics as found in gray iron.
With all of the material properties taken into consideration, engine designers found CGI ideally suited for engine cylinder block manufacturing, where lighter and stronger materials are needed which can absorb more power and have similar resistance to bore wear when compared to gray iron. An assembled automotive engine can be made nine percent lighter with CGI. The engine block weight alone can be reduced by 22 percent. This corresponds to a 15 percent reduction in length and a five percent reduction in height and width.
This weight savings is made possible by decreasing wall sections less than 4.5 mm thick, where the higher strength and slightly higher stiffness of CGI can be taken advantage of. Similarly, the bore walls and bearing block supports can be reduced in mass without losing the characteristics of gray iron as related to machinability, piston ring wear or noise damping.
Over many years of metallurgical development and engine testing, the data suggests that a nodularity range of 0-20% is appropriate for the cylinder bore walls and other structural regions of a cylinder block. By looking at the chart below for an iron with an unknown content of Sulfur, the Mg percentage in the metal should be about 0.007 to 0.017% to keep the Nodularity 0-20%, however this varies with the sulfur content of the base alloy. With any morphology, the presence of flake graphite is inadmissible because the mechanical properties of CGI decrease by 20-25% as soon as flake-type graphite appears in the microstructure. However this condition is avoided by careful control of the %Mg and designing the casting process with a small enough batch size so that Magnesium fade is avoided. It should also be stated that the foundry must also be cautious about pushing the %Mg too high and starting out the ladle where higher nodularity will exist. Although the mechanical properties gradually increase as the nodularity exceeds 20%, thermal conductivity is reduced and castability and machinability become more difficult.
Alloying elements can also be specified to improve selected properties. The pearlite content, which is linearly related to hardness and tensile strength, can be specified to suit the wear and is controlled by %Cu additions. Increasing the pearlite will affect the machinability and high temperature performance requirements of the component.
“Cast Iron Microstructures” video provided courtesy of SinterCast, Dr. Steve Dawson, CEO
Dampening Characteristics of Compacted Graphite Iron
The relative ability of a material to absorb vibration is evaluated as its damping capacity. The suppression of vibration is made possible by converting any mechanical energy into heat, which can be explained by a material’s viscoelastic behavior, as displayed in a hysteresis curve for Loading and Unloading. Damping capacity is very important in structures and in devices containing moving parts and noisy interfaces such as a valve train, pistons, firing pressures, etc. For any material behavior, we can look at the stress-strain curve for both loading and unloading in the elastic range. In a dampened material, while the strain is recoverable, the stress-strain curve is not the same for loading and unloading. Such materials instead exhibit viscoelastic bahavior, involving both elastic and viscous components, which at normal loading and unloading rates leads to hysteresis. A typical hysteresis curve is shown below, and the energy absorbed during one loading-unloading cycle is given by the area within the loop. The shape of the loop depends on the rates of loading and unloading (unlike normal time-independent elasticity).
Components made of materials with a high damping capacity can reduce noise such as chatter, ringing and squealing, and also minimize the level of applied stresses. Vibration can be a significant factor relate to machinery wear patterns, transmitted noises and unsatisfactory operation. In grey iron the array of graphite flakes is interconnected in a cell and absorbs sound (as in an engine block), with the graphite acting as a dislocation blocker in the iron matrix. In ductile iron with separate discrete graphite nodules there is no connectivity between graphite apart from the “steel” matrix, thus no dislocation blocking in the matrix. While the toughness of this material is good, Ductile iron doesn’t have the vibration damping is a characteristic of grey iron, whereas the compacted graphite graphite morphology does. Compacted Graphite Iron retains superior vibration damping characteristics, while providing significantly improved mechanical properties over gray iron.
E. Nechtelberger, “The properties of cast iron up to 500°C”, English edition published in 1980 by Technicopy Ltd, England; German edition published in 1977 by Fachverlag Schiele und Schön GmbH, Germany.
Source credited inside a Sintercast internet publication: Compacted Graphite Iron – Mechanical and Physical Properties for Engine Design, Figure 7.
CGI Foundry Process Control
The Swedish company SinterCast® licenses a foundry technology to accurately and reliably produce CGI. Many OEM and Tier 1 foundry operations have already secured their SinterCast®-CGI licenses, yielding many millions of CGI castings in every region of the earth. Most of the CGI uses in the United States are for diesel and gasoline engine blocks, where the machinability and the production results in interna porosity concerns require close attention to the process control.
The production and operating demands of modern automotive cylinder blocks and heads provide the basis for defining the requirements of Compacted Graphite Iron (CGI) production techniques. Specifically, these demands include:
- Foundry: A stable high volume production with no risk of flake graphite formation. Minimal scrap due to porosity, without resorting to expensive feeders.
- Machining: A consistent hardness, microstructure and machining characteristics to provide stable machining parameters. Minimal after-machining scrap due to porosity.
- Engine: A material with metallurgical and mechanical properties aligned with the assumptions used to design the structure so the hardware will provide excellent durability. Consistent high thermal conductivity to prevent thermal fatigue failures in heads and piston seizing in blocks.
These requirements can only be simultaneously satisfied by producing CGI in the 0-20% nodularity range. Within this consistently low nodularity range, the castability, machinability, heat transfer and low wear resistance are achieved, thereby providing cost-effective production in the foundry and machining operations, and delivering confidence to the end-users.
Nodularity Control to allow for Compacted Graphite
The graphite microstructure of Compacted Graphite Irons is expressed in terms of % Nodularity. As mentioned previously, the control of the metal alloy base composition and the additions of inoculation materials are carefully specified to result in castings that exhibit spheroidal graphite measured as 0 to 20% Nodularity. When assayed on a photomicrograph, the remaining graphite (over 80%) in the structure is expected to be in the Compacted type (also referred to as vermicular or wormlike). No interpretation of Flake type is allowable, because of the sharp degradation of material properties. In the serial production of CGI, the foundry is expected to significantly increase the amount of metallography samples be taken, as opposed to what is traditional with Gray Iron casting. To assist in the interpretation of evaluation results, the SinterCast company publishes a photographic comparison chart to categorize the % Nodularity and ferrite/pearlite matrix structure so reports can be written. This SinterCast microstructure rating technique has been adopted by the ISO 162112 international standard for Compacted (Vermicular) Graphite Iron. SinterCast has also developed software macros for CGI image analysis according to the ISO 16112 rating technique. These macros are compatible with Image Pro Plus image analysis software and are available to all SinterCast customers.
Narrow Range of Magnesium Control Required
In practice, the usable Mg-range is very small (< 0.008%). From experience and practice we know that the active magnesium will fade directly after treatment at a rate of approximately 0.001% Mg every five minutes. Therefore, the initial starting point of the iron must be sufficiently far away from the abrupt CGI-to-grey iron transition to ensure that the iron does not deteriorate to form flake-type graphite before the end-of-pouring. However, we can’t over compensate and risk going to high on nodularity and produce higher scrap, found after machining. In some cases, depending on the base alloy sulfur content, this inability to start casting at the ‘left end’ of the CGI plateau effectively narrows the usable Mg-range by approximately 0.002%.
Affect of Sulfur in the Base Iron
As the sulfur content increases, the Mg-fade rate becomes faster. Therefore, the production of high quality CGI requires that the base iron contain less than 0.020% sulfur. If the base iron sulfur content is higher than 0.020%, the Mg-fade rate can be too fast to enable the entire ladle to be cast before the iron fades below the flake graphite transition, causing grey iron formation in the castings. The presence of higher sulfur also increases the number of sulphide inclusions, increasing the risk of filling defects caused by clogged filters and casting defects caused by dross and inclusions. The demand for low sulfur is more important in CGI production than in the production of high quality ductile iron because ductile iron allows for some over-treatment of magnesium to compensate for the Mg-fade. However, Mg over-treatment cannot be relied upon in the production of CGI because the additional Mg will increase the nodularity beyond the 0-20% specification range and cause the formation of porosity defects.
Although the actual size and location of the stable CGI plateau is different for each product, it generally spans a range of approximately 0.008% Mg. A schematic illustration of the stable range for CGI, for a base iron that contains 0.010-0.015% sulfur, is shown in the figure to the right.
Because the active magnesium fades at a rate of approximately 0.001% Mg every five minutes, the initial starting point of the iron must be high enough to account for time before pouring.
Sintercast internet publication: “Process Control for the Reliable High Volume Production of Compacted Graphite Iron”, Figure 1, 2014.
CGI Process Control Video provided courtesy of SinterCast, Dr. Steve Dawson, CEO
The starting point of any process control technology must be an accurate measurement of the behaviour of the molten iron. In the case of Compacted Graphite Iron, the control measurement must simultaneously determine the Modification, the Inoculation and the Carbon Equivalent. In particular, the measurement must quantify the proximity of the Modification to the abrupt CGI-to-grey iron transition, and to predict the subsequent magnesium fading. With
thermal analysis of the iron after the magnesium and inoculant base treatment reaction has been completed, the foundry can assess the metal quality. In the licensed SinterCast method, a 200 gram thermal analysis sample is obtained in a patented Sampling Cup by immersing the cup into the iron for less than three seconds. The SinterCast Sampling Cup is fabricated entirely from stamped and drawn steel sheet and has a predominantly spheroidal containment area. In comparison to conventional thermal analysis sand cups, the design of the thin-wall immersion sampler ensures a constant sample volume, prevents oxidation of the iron during pour-in filling, provides a more uniform solidification profile and yields a more accurate measurement of undercooling because of the elimination of chill-solidification. These design advantages are a key element of the accuracy of the thermal analysis: the CGI stable window is so small that it is essential that all measured differences in the thermal analysis can be attributed to differences in the solidification behaviour of the iron rather than to variation in the sampling conditions.
As the thermal analysis sample solidifies, the cooling curves are analysed and the Modification, Inoculation and Carbon Equivalent results are presented as dimensionless indices. The Modification and Inoculation indices are sufficient to fully define the solidification behavior and potential microstructure of the base treated iron. The production strategy is to always start casting in the top corner of the specification window in order to ensure that the natural fading of magnesium and inoculant will not result in flake patches or carbides.
Sintercast internet publication: “Process Control for the Reliable High Volume Production of Compacted Graphite Iron”, Figure 8, 2014.
Process Controls by Analysis of Multiple Factors
As part of the thermal analysis output results, the SinterCast process control system calculates the optimum magnesium and inoculant additions for the base treatment of subsequent ladles. This calculation is based on the historical recovery results from previous ladles and automatic input of the sulfur content of the base iron and the actual ladle weight and temperature.
The state-of-the-art CGI foundry has a higher level of metal control than typically used for Gray or Ductile iron control.
Sintercast internet publication: “Process Control for the Reliable High Volume Production of Compacted Graphite Iron”, Figure 9, 2014.
Foundry Management & Technology Magazine, “Diesel Options Drive CGI’s US Market Gains”, Robert Brooks, author, Feb 18, 2013.
VM Motori S.p.A., a diesel engine specialist in Cento, Italy, co-owned by Chrysler parent Fiat S.p.A., builds Chrysler’s 3.0-liter V6 EcoDiesel. The foundation of the diesel engine is a CGI cylinder block cast at the Tupy foundry in Joinville, Brazil. Tupy is a SinterCast licensee, and operates CGI production lines at its engine foundries in Mexico as well as Brazil.
The V6 EcoDiesel has been in production since 2010 for the European version of the Jeep Grand Cherokee, Chrysler 300, and Lancia Thema.
According to SinterCast, Tupy increased series production of the V6 EcoDiesel cylinder blocks and bedplates in the second half of 2012, in anticipation of Chrysler’s plan to add the new option for its North American Jeep and pickup truck lines. It added that it foresees Tupy’s output of CGI for the Chrysler engines will exceed 5,000 metric tons (“100,000 engine equivalents”) during 2013.
Car and Driver Magazine, “Leave the Iron On: Ford Buries New-Age Iron in Its Aluminum-Intensive 2015 F-150”, Don Sherman, author, Roy Ritchie, photographer, April 24, 2014.