Cast Iron Grades
For most modern automotive and consumer applications, cast iron falls into 4 separate categories:
- Gray Iron (this page)
- Ductile Iron (this page)
- Austempered Ductile Iron (page 2)
- Compacted Graphite Iron Page 2)
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)
Graphite Comparison - Photomicrograph Studies
Cast iron is an alloy of iron-carbon-silicon containing more than 2% carbon that is poured into a mold containing a negative of the desired final shape. The mechanical properties and material behavior are dependant on the graphite morphology, which is formed in the iron matrix by process thermal controls and elemental additions to obtain the desired graphite form. All cast irons solidify by means of a complex eutectic reaction that involves Fe, C and Si. There are also influences from Cu, Mn, S and P.
Gray Iron
Gray cast iron is the simplest, most common and lowest cost form of cast iron in the world. The iron microstructure is characterized by the “Flake Graphite” qualities in the forms of: a.) Type, b.) Size and c.) Martrix.
Gray cast iron has excellent castability and machinability qualities, making it a high value for parts that can be heavy. In addition, gray iron has superior damping and thermal heat transfer, making it excellent for automotive brake systems. While it is strong, it is also brittle.
Flake Graphite Types
Gray cast iron is by far the most common of the cast irons. Next to steel, it is the most widely used engineering alloy. In the 1st century of the of the automobile industry, it was the material of choice for cylinder blocks, heads and many other power-train components. The name derives from the appearance of the fracture surface, which is gray. In gray cast iron, the graphite solidifies as interconnected flakes, as illustrated above in the 3D microscopy with a scanning electron microscope. The clustered shapes of the graphite has been compared to potato chips being glued together at a central location. The central point is the original graphite nucleus. Each cluster of flakes defines a eutectic cell in the gray iron. Eutectic cells are somewhat analogous to grains in other metals. The strength of the iron is improved with finer cell sizes.
The form of graphite in gray cast iron is an important factor in determining the properties of the alloy. Graphite shape and the size can vary markedly due to the cooling rate and the alloy content. The most common form, as described in the preceding paragraph, is is referred to as Type A. The five types of graphite in gray cast iron are classified b ASTM.
Type A Flake Structure
Type A graphite has a uniform distribution and a random orientation. It generally produces the best properties and is typically specified for powertrain components.
Type B Flake Structure
Type B graphite appears as rosettes with a random orientation. It occurs most often in near-eutectic alloys that are improperly inoculated and contain very few graphite nuclei. Type B graphite is often found at the casting surface, where it is otherwise Type A.
Type C Flake Structure
Type C graphite occurs in hypereutectic irons (CE greater than 4.3) in which graphite forms as the primary phase. This primary graphite is called Kish graphite. It’s presence reduces tensile properties, by may be desireable for some specialist applications.
Type D Flake Structure
Type D graphite occurs as a result of interdendritic segregation in rapidly cooled irons. It consists of small, randomly oriented flakes between the austenite dendrites. Type D may lead to higher tensile properties, but may be difficult to heat treat because of the segregation.
Type E Flake Structure
Type E graphite is similar to type D, but with oriented flakes between the austenite dendrite arms. It most often occurs in irons with very low carbon equivalent.
Flake Graphite Size
In addition to graphite type, ASTM has established specifications for graphite size. This is determined by comparing the number of flakes in a 100X magnification photomicrograph with a standard chart.
Gray Iron Mechanical Properties
Generally, gray iron has low strength and very low ductility. The graphite flakes act as tiny internal cracks which create stress intensification. This makes it very easy for cracks to propagate through the material, which inhibits strength, ductility and impact strength (fracture toughness). Gray iron is specified by a class number, which corresponds to the nominal tensile strength of the alloy. For example, a class 30 gray iron, which was typical for an engine block, has a nominal tensile strength of 30,000 psi (207 MPa). Because gray iron has very low ductility, yield strength and percent elongation, these properties are rarely measured or specified. With the YTS and UTS being so close (less than 1% elongtion), gray iron is classified as a “brittle material”. Another way of specifying gray iron is by the hardness of the material. The Society of Automotive Engineers (SAE) published a material standard for Cast Iron, SAE J431, to account for both tensile and hardness properties. In addition, each automotive OEM has their own set of standard to govern gray iron properties and specific test methods required to measure the material quality.
Strength and Hardness
The strength and hardness of gray iron is varied by at least four main factors:
- Carbon Equivalent
- Alloy Content
- Innoculation
- Solidification Rate
The tensile strength decreases rapidly with the increasing Carbon Equivalent.
Carbon Equivalence (CE)
For cast iron the Carbon Equivalent (CE) formula is used to understand how alloying elements will affect the casting behavior. It is used as a predictor of strength in cast irons because it gives an approximate balance of austenite and graphite in final structure.
CE = %C + 0.33 (%Si) + 0.33 (%P) – 0.027 (%Mn) + 0.4 (%S)
CE < 4.3%, hypoeutectic behavior during solidification
CE = 4.3, eutectic behavior during solidification
CE > 4.3%, hypereutectic behavior during solidification
Alloy Content
The most effective elements for Gray Iron are Carbon and Silicon. When the objective is to cast the iron at the eutectic to prevent iron carbides from forming, it is possible to use ladle additions of Silicon to modify the molten alloy so that a eutectic equivalent is always achieved. Using the CE calculation and confirming the appropriate amount of Ferro-silicate additions, a wedge block is used to confirm the gray iron is at the desired CE level.
Carbon & Silicon
Compared to common steel grades, the carbon content in gray iron is about ten times higher. With scrap steel being a large part of the melt charge for an iron foundry, carbon usually has to be added at some point in the process, either in the main charge or after the iron is in the molten state. Since there is very little carbon in the scrap steel charge materials, the metallurgist needs to take into account all of the metallic charge materials (steel, scrap iron, pig iron) in the main furnace. Carbon raising additions depend heavily on the melting method (cupola melting with coke will elevate carbon), the amount of silicon used and the availability of low-cost graphite. However, as the iron foundry industry transitions from cupola to inductive melting with similar high additions of steel scrap in the charge, alternative ladle, in-stream or in-mold additions to achieve Type A flakes in Gray iron are required.
Inoculation Methods
With the switch to inductive melting, foundries carefully charge theire induction ladles with carefully weighed amounts of scrap steel, scrap iron and the more expensive pig iron. Today, higher amounts of lower carbon steel can be accommodated by adding a ferrosilicon (FeSi) inoculants. The inoculation process involves an addition of between 0.05 to 1% of a specialized FeSi alloy containing controlled amounts of one or more carefully selected elements to further refine the graphite morphology. The explanation of how these carefully selected elemental additions, including Al, Ca, Ba, Sr, Ce, La, Mn, Bi, S, O, and Zr can be found on Table 1.0 Structural Effects of Elemental Additions to Cast Iron, shown below. In addition to raising the Si level, the inoculant provides nucleation sites that promote graphite precipitation and growth, together with iron solidification based on a stable Fe-C system.
When Carbon needs to purchased for addition, the addition material is generally in the form of graphite. Graphite additions frequently come from carbon electrodes, previously used in steel arc melting furnaces.
Purpose of Inoculation
The purpose of inoculation is to promote heterogeneous graphite nucleation by introducing elements that form suitable substrates that will act as nuclei and initiate the desired graphite formation. By promoting a stable eutectic solidification, inoculation enables the C to come out of solution in a favorable form of graphite and not as iron carbide.
With careful control, the use of an inoculate addition will help:
- Avoid formation of carbides (cementite)
- Promote the formation of graphite
- Reduce segregation
- Reduce shrinkage
- Improve machinability
- Promote a homogenous structure
- Increase ductility
The effect of inoculation is presented in the figure below (Inoculation Effect), where the cooling curves for an un-inoculated iron are indicated with a black-dotted line and an inoculated iron are indicated as a blue solid line.
Inoculation can take place either at tapping, in the ladle, in the stream during casting, or even inside the mold. Inoculating alloys are available in granular form, packed in a wire, or cast/pressed into various shapes. The size is adjusted based on the point of addition and the time and temperature available for dissolution into the molten iron. As a rule, additions can be reduced when inoculation takes place as close as possible to the pouring of iron into the mold, which is why “in-stream” inoculation with FeSi granules is the most common method of adding.
Inoculation Effect
In the cooling curves for an un-inoculated iron (black-dotted line) and an inoculated iron (blue solid line), The affect is shown as a reduction in the degree of undercooling before the graphite forms (red arrow). Inoculation also prolongs the formation and growth of graphite, thereby increasing the solidification time (green arrow).
In the commercial production of cast iron, both grey and ductile iron are inoculated, but gray iron generally requires smaller inoculating additions, depending on the iron alloy composition, melting method and charge make-up. Ferrosilicon inoculation transforms the structure from undercooled graphite (Type E graphite in gray iron) to fully flake or spheroidal graphite, as shown below.
Other Elements
Since the strengths of cast irons depends on content of ferrite against the pearlite content, alloying elements that suppress the formation of ferrite and increase the amount of pearlite are added for increasing the strength. To study the affects, a number of elements have been studied and general descriptions provided. Alloying elements such as chromium (Cr), molybdenum (Mo), and tungsten (W) are used for this purpose. These elements promote carbide formation and will increase the iron hardness. Please refere to Table 1.0 Structural Effects of Elemental Additions to Cast Iron, shown below, for additional information.
Ductile Iron
Ductile (also called Nodular and Speroidal Graphite) Iron is stronger than gray iron, is tougher and is much less expensive to cast complex parts than steel forming. The iron microstructure is characterized by the “Nodularity” (roundness) quality, nodule size and nodule density (nodules per mm²).
Ductile iron is considered a tough material for chassis parts (like steel) when the morphology is ferritic. While the strength can be increased by using alloy additions to promote pearlite around the nodules, the resulting % Elongation is reduced.
Summary
Ductile cast iron is stronger and more ductile (tougher) than gray cast iron. It is formed by treating a relatively high carbon equivalent iron with a nodularizing agent such as magnesium (most common) or cerium to cause graphite spheres that grow during solidification. The most common automotive applications are in parts that require high strength and toughness when undergoing stresses related to thermo-cycling or impact. The part types include: crankshafts, camshafts, exhaust manifolds, steering knuckles, suspension arms, differential gear carriers, spring buckles and the like. In the plumbing and piping industry, using ductile iron was a big achievement in providing piping, joints and valves that were less expensive than the malleable iron alternative. With all of these applications, ductile iron is growing in applications, often displacing gray iron designs so that weight savings can be achieved with the improved mechanical properties.
Historical Background
In the 1948 AFS annual conference a new iron morphology was introduced to the conference attendees: Ductile Iron. The creator of this new material is generally credited to Keith Mills, however 3 people are listed on the original 1949 Patent assigned to the International Nickel Company (INCO): Keith Dwight Millis, Albert Paul Gagnebin and Norman Boden Pilling. In their patent (US2485760A) the inventors are credited for inventing a “Cast Ferrous Alloy for Ductile Iron Production via Magnesium Treatment”.
As the patent holder for Ductile Iron using a magnesium treatment, INCO promoted the benefits of the material properties and introduced Ductile Iron to designers and engineers by distributing technical literature and conducting seminars. As knowledge of the properties and economies of Ductile Iron spread, the usage increased dramatically throughout the fifties and early sixties. After the termination of INCO’s promotion of Ductile Iron in 1966, Ductile Iron market growth continued to outperform other ferrous castings, but as the engineers and designers who benefited from the early promotional efforts of INCO retired they were replaced by a new generation that wasn’t familiarized about the process technology from their academic training. For this reason the Ductile Iron Society (DIS) was established to provide training and close the knowledge gap with new generations of metallurgists, design engineers and manufacturing engineers.
Desulfurization
The processing of ductile iron is of extreme importance in the determination of its properties. The initial step in the production of ductile iron is to remove excess sulfur from the molten iron. Sulfur is sometimes added to gray iron because it promotes the formation of graphite flakes. For this reason, it must be nearly eliminated from the ductile iron melts. There are some foundries that process both gray and ductile iron from the same charge material, so any internally recycled scrap gray iron would have sulfur to remove. Removal of Sulfur involves the addition of CaO or some other agent. It should be mentioned that foundries that specialize in only processing ductile iron may have the opportunity to avoid extensive desulfurization procedures, but they should always be monitoring for it when measuring the composition chemistry.
Nodularization using Magnesium Conversion
The unique step in the processing of ductile iron is the nodularization of graphite. This is the step where magnesium is added to the molten alloy to create a residule Mg level of approximately 0.03-0.06%, which is the amount necessary to cause graphite to form spheroids. Unfortunately, the Mg vaporizes at a temperature well below the melting temperature of nodular iron, so innovative conversion methods have been developed to achieve the conversion:
Open Ladle Conversion
- Mg conversion recovery = 20-25%
- Desulfurization of base alloy prior to conversion is important
- Reaction is violent and not recommended for safety
- Very smoky during conversion
Sandwich Ladle Conversion
- Mg conversion recovery = 40-45%
- Desulfurization of base alloy prior to conversion is important
- Reaction is violent but reduced because the ladle is deeper & has a treatment pocket
- Still smoky during conversion & needs a vent hood
Tundish Ladle Conversion
- Mg conversion recovery = 60-65%
- Desulfurization of base alloy is less critical
- Tundish cover contains the reaction
- Smoke is reduced by 90% from open ladle conversion
George Fischer Converter Method
- Mg conversion recovery = 70% (magnesium chips used)
- Desulfurization of base alloy is generally not necessary
- Enclosed conversion vessel contains the reaction
- Safe for large ladle volumes
In-mold Conversion
- Mg conversion recovery = 70% (magnesium chips used)
- Desulfurization of base alloy is less critical
- Sand mold contains the reaction
- Generally combined with an auto-pouring ladle to regulate flow
Magnesium Conversion
Tundish ladle conversion is the most common conversion method and considered safe:
60-65% Mg recovery, low fumes, reduced C loss.
There are other conversion methods with varying degrees of safety and magnesium recovery efficiency: Open Ladle Conversion (20-25% Mg recovery), Sandwich Ladle Conversion (40-45 Mg recovery), George Fischer Converter (70% Mg recovery), In-Mold Conversion (75% Mg recovery)
Magnesium Fading
Another issue with nodularization is the fading of Mg over time, which is the tendency of the Mg to gradually evaporate (or oxidize) out of solution when the ladle surface is in contact with air. When the Mg fades out of the alloy composition, the nodularity is reduced and the mechanical properties will be unexpectedly different. To prevent fading in an open ladle, the alloy must be poured within a fixed period of time (generally 10-12 minutes). However, with an enclosed auto-pouring station, this time limit can be extended to an hour if an inert cover gas is used to prevent melt contact with air, thereby preventing Magnesium oxidation.
Inoculation
The final step before pouring is the inoculation, which is also done by the same methods as gray iron. As described in the gray iron section, Ferrosilicon alloys are commonly added to the melt just prior to pouring to provide locations for graphite nucleation. Better and later inoculation methods will produce a finer nodule distribution and high nodule density (nodules per mm²). Nodule counts of 200 or more are often specified for thinner sections, but these higher counts are hard to achieve in thick or heavy sections.
Ferrite vs Pearlite Graphite Structures
Because of the high elongation rates achievable for as-cast ferritic grades of ductile iron, automotive designers responsible for safety critical chassis structures like to specify ductile iron. To the automotive designer, ductile iron is considered a high-value material options because it is lower in cost than alternatives such as forged steel, forged aluminum, or even solution heat treated aluminum castings. With high elongation and toughness properties associated with as-cast ductile iron structural parts, vehicle chassis designers have a low cost production process that enables their designs to pass strict crash testing requirements. However, when these same designers are considering how to reduce overall vehicle weight, they’re now starting to specify ducile iron grades that are higher in strength. However at the same time, the designers are also demanding high ductility, as measured by % Elongation with routine production tensile testing. Therefore, we now have iron grades that rely on some amounts of pearlite in the iron morphology to increase the YTS, but still maintain > 8% Elongation. Therefore, careful analysis of iron morphology must be combined with mechanical property testing, so that the alloy chemistry can be tightly controlled to yield expected results after specimens are extracted from as-cast parts.
Metallographic Analysis
The ability to assess ductile iron microstructures in the foundry is particularly important with using alloy compositions where Tin (Sn) or Copper (Cu) are being added to the alloy for the purpose of elevating pearlite formations around the graphite nodules. Interpretations of these types of photomicrographs, and their correlation to the mechanical properties are very important to establishing meaningful alloy chemistry limits as the iron is melted and poured into the production line.
Image Credit for above microstructure images:
Stefanescu, Doru M., et.al, “The Metallurgy and Tensile Mechanical Properties of Thin Wall Spheroidal Graphite Irons”, International Journal of Cast Metals Research, 2003, Vol. 16 Nos 1-3.
https://www.researchgate.net/publication/260037434_The_Metallurgy_and_Tensile_Mechanical_Properties_of_Thin_Wall_Spheroidal_Graphite_Irons
Pearlitic Ductile Iron
When elongation properties aren’t as important as the YTS and UTS, controlled additions of Cu and Sn are commonly used to promote the formation of pearlite around the graphite nodules. The amount of formed pearlite has been found to be proportional to the improved tensile strengths, with an exponential decay in the % Elongation.
Image credit:
Ductile Iron Society figure 3.16
https://www.ductile.org/didata/Section3/Figures/pfig3_16.htm
Industry Specifications
In the chart shown below, the Society of Automotive Engineers (SAE) publishes a nice chart where alloy additions are used to modify the structure from 100% Ferrite to increasing levels of pearlite in the iron morphology. SAE classifies the materials by the UTS (for a minimum expectation) and the expected % Elongation. As illustrated in the DIS figure 3.16, the % Elongation decays exponentially, while the mechanical properties increase logarithmically.
Image credit for chart above:
Ductile Iron Society – SECTION XII. SPECIFICATIONS
https://www.ductile.org/didata/Section12/12intro.htm
Tensile Property Testing
A ductile iron foundry needs to be able to properly test tensile specimens more carefully than a gray iron foundry. Assessment of ductile iron to any governing engineering specification will require measurement of YTS, UTS, %Elongation, Charpy Impact specimens and Brinell hardness. So, in addition to performing spectrographic and metallographic analysis, the foundry must have an accurate tensile tester.
Determination of YTS (Yield Tensile Strength)
With the demand for high tensile strengths and minimal loss in ductility, as measured by % Elongation, careful attention must be given to tensile testing as a part of the production process. We now have iron grades that rely on some amounts of pearlite in the iron morphology to increase the YTS, but still maintain > 8% Elongation. Therefore, careful analysis of iron morphology must be combined with mechanical property testing, so that the alloy chemistry can be tightly controlled to yield expected results after specimens are extracted from as-cast parts. With this in mind, determining the YTS based on a 0.2% offset method must be based on the use of accepted software-based interpretation. Furthermore, all testing labs associated with the measurement of tensile properties for a given product category should be comparing results with “round-robin” forms of testing. All labs involved should demonstrate reasonable correlation between facilities and operators.
Supporting Information:
Table 1.0 Structural Effects of Elemental Additions to Cast Iron
Element
Type
Effect During Solidification
Effect During
Eutectoid Reaction
Aluminum
.
Antimony
.
.
Bismuth
.
.
Boron ≤ 0.15%
.
Boron > 0.15%
.
Chromium
.
.
.
Copper
.
Manganese
.
Molybdenum
.
Nickel
.
Silicon
.
Tellerium
.
.
.
Tin
.
.
Titanium < 0.25%
.
Vanadium
Strong Graphitizer
.
Little Effect in amount used
.
Carbide promoter, but not carbide former
.
Strong graphitizer
.
Carbide stabilizer
.
Strong carbide former. Forms complex carbides which are very stable
.
Mild graphitizer
.
Mild carbide former
.
Mild carbide former
.
Graphitizer
.
Strong Graphitizer
.
Very strong carbide promoter, but not stablizer
.
Little effect with amount used
.
Graphitizer.
.
Strong carbide former
Promotes ferrite and graphite formations
.
Strong pearlite stabilizer
.
.
Very Mild pearlite stabilizer
.
.
Promotes graphite formation
.
Strong pearlite retainer
.
Strong pearlite former
.
.
.
Promotes pearlite formation
.
Pearlite former
.
Strong pearlite former
.
Mild pearlite former
.
Promotes ferrite and graphite formation
.
Very mild pearlite stabilizer
.
.
.
Strong pearlite retainer
.
.
Promotes graphite formation
.
Strong pearlite former