Continuous casting of ductile and gray iron dominates the industry in terms of low material cost, high machinability, and high performance. Both metals are used for a wide range of applications, including construction equipment, pipe fittings, oil field machinery, and even transportation services.
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Each metal has its own advantage due to the differences in carbon levels and is used for different applications according to its strengths.
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Gray and ductile iron, both superior alternatives to steel, each present a different makeup and suitability in the CNC machining industry.
American Iron has the best replacement for steel in the form of V-2 Class 40 Versa-Bar continuous cast gray iron. During solidification, rod-like graphite flakes precipitate into the composition, outlining the difference between gray and ductile iron. Producers utilize grey continuous cast iron for its high density and strength, but its popularity comes from its low cost. Continuous cast gray iron combines affordable prices with quality lightweight metal to create a material ideal for applications such as vibration damping and use in manufacturing equipment frames. Gray iron is also applied in:
Gray iron is often mistakenly described as cheap and dirty metal. Still, its high cutting speeds, superior damping ability, durability, and unique metallurgical makeup make it popular. Part of the gray iron’s success comes from its ability to be easily machined and is used where strength is not a critical requirement, like maintenance hole covers and counterweights. Gray iron is considerably stronger than steel and costs less. You’ll save money in the short AND long run. Consider gray iron the immediate steel replacement. Ductile iron is the next step up.
Ductile iron offers numerous machining advantages over gray iron.
Ductile continuous cast iron has made its name in the machining industry from its high tensile strength and flexibility. Its flexibility makes it great for being shaped into wires and other applications that require a level of elasticity. Instead of rod-like flakes, the ductile iron bar is composed of graphite nodules making it flexible during solidification. This type of iron is often used in areas that require the ability to elongate under tension. It is most often used in:
The many advantageous features of continuous cast ductile iron make it the perfect alternative to steel. American Iron offers two high-quality ductile iron grades, V-3 and V-4. The distinctive metallurgic composition of V-3 65-45-12 iron makes it the easiest to machine out of the three grades of continuously cast metal. V-4 80-55-06 ductile iron is stronger than V-3 and is a popular choice for automobile parts and hydraulic cylinders. Ductile iron is not only more versatile than steel but costs significantly less. Buying Versa-Bar continuous cast ductile iron from American Iron means savings on every project.
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Ductile iron stock is available for sale as a raw material for you to process and finish on your own, or we can finish and machine it into custom continuous cast iron components based on your requirements at our state-of-the-art CNC facility in Waukesha. It’s up to you. We provide you with high-quality ductile iron in whatever form you desire.
Grey cast iron (a.k.a. gray iron castings) is a type of iron found in castings known for its grey color and appearance caused by graphite fractures in the material. Specifically, what makes grey iron “grey iron,” is the graphite flake structure that is created during the cooling process from the carbon that is in the component.
Grey iron is a result of both the materials used and the process used to cast a part. In other words, the properties of the grey iron will change depending on what materials are melted together to pour into the mold as well as the process that is used for casting (it is primarily the cooling part of the process that defines the characteristics of the cast part versus other parts of the process.).
If you use a powerful microscope you can see the graphitic microstructure that makes grey iron so easily identifiable. In a grey iron casting, you can see little black flakes of graphite. These flakes cause fractures and cause the material to have a grey appearance. The mechanical properties of grey iron are controlled by the size and shape of the graphite flakes present in the microstructure and can be characterized according to the guidelines given by the ASTM.
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There are more pounds of grey iron castings produced every year throughout the world than any other type of casting. Just about every manhole cover is from grey iron and the disc brakes on your car are most likely grey iron as well. Other common uses for grey iron include:
The popularity of grey cast iron components (gray iron castings) is because grey iron is one of the cheapest types of iron castings to produce. It has acceptable ductility, tensile strength, yield strength, and impact resistance for most applications. Grey Iron is also excellent in its ability to dampen vibrations making it ideal for machinery bases and as well as many housing applications. Grey iron has high thermal conductivity meaning it moves heat more easily through the metal.
A final benefit of grey iron castings is its ability to withstand thermal cycling well. Thermal cycling is where the component goes back and forth between warmer and colder temperatures. While thermal cycling can create stress and premature failure in some types of metal castings, grey iron has proven to endure the strain of thermal cycling quite well and not stress as easily.
While grey cast iron has less tensile strength and shock resistance than most other castings or even steal, it has compressive strength that is comparable to low- and medium-carbon steel. These mechanical properties are controlled by the size and shape of the graphite flakes present in the microstructure.
Material properties come from the makeup of the materials present. Some of the materials are added to the iron for strength while others are added to improve castability (affecting melting temperatures, etc.). The properties of all metals are also influenced by the way they solidify during cooling. If you change the cooling process, you automatically change the structure of the metal in its final state. How the cast part cools can be affected by how the metal is introduced into the mold, the pouring temperature, and the process and speed of cooling. One of the difficulties in making a quality casting is to account for the way a part will cool in different parts of the mold, accounting for both thick and thin areas of the mold.
Just like all cast iron parts, the primary component of Grey Iron is going to be the iron (no surprise there, right?). Just like most other cast irons, it also has 2.5–4.0% carbon and 1–3% silicon. If you look at Chart 2 you will notice that the composition of the iron is very close to other common cast parts, but with a little more phosphorus.
Iron Family Gray Ductile CGI Malleable White Carbon 2.5-4.2 3.0-4.0 2.5-4.0 2.2-2.8 1.8-3.6 Silicon 1.0-3.0 1.8-3.0 1.5-3.0 1.2-1.9 0.5-2.0 Manganese 0.15-1.0 0.1-1.0 0.10-1.0 0.15-1.2 0.15-0.8 Sulfur 0.02-0.25 0.01-0.03 0.01-0.03 0.02-0.2 0.02-0.2 Phosphorus 0.02-1.0 0.01-0.1 0.01-0.1 0.02-0.2 0.02-0.2[1]
Grey iron has graphite flakes entwined with the rest of the structure. Unhardened grey iron is “fragile” compared to other cast metals. The graphite flakes create areas of weakness in the metal where fractures can begin that will split the metal. This propensity to fracture is why grey iron has low tensile and impact strength. Yet the graphite flakes are what create great wear resistance because as friction occurs the graphite acts as a lubricant. The graphite structure also has great vibration dampening properties because the structure created by the graphite flakes helps cancel out vibrations.
In the United States, the classification for grey iron is usually set using the ASTM International standard A48. ASTM A247 is also commonly used to describe the graphite structure. Grey iron is grouped into classes which correspond with its minimum tensile strength in thousands of pounds per square inch (ksi); (i.e. class 20 grey iron has a minimum tensile strength of 20,000 psi (140 MPa)). Other ASTM standards that deal with gray iron include ASTM A126, ASTM A278, and ASTM A319.
Class 20 grey iron has a ferrite matrix and a high carbon equivalent. Higher strength gray irons, up to class 40, have lower carbon equivalents and a pearlite matrix. To get grey iron above class 40 requires alloying to provide solid solution strengthening. Grey iron can also be heat treated which modifies the matrix giving the cast iron greater strength. Class 80 is the highest class available for grey iron and it makes the iron have high tensile strength but low impact resistance because class 80 parts are more brittle.
Below is a reference chart of other ASTM standards affecting grey iron.
use at elevated temperatures
pressure-containing parts suitable for low-temperature service
No, there is absolutely no difference between Grey Iron Castings and Gray Iron Castings. You will notice we were using both terms interchangeably throughout the article; it is just that some people like to spell the color “gray” and others like to spell it “grey.” Maybe we should have stuck with one spelling, but we wanted everyone to feel at home when they looked up our gray iron castings. Thanks for reading!
[1] Santner, Joseph S. and Goodrich, George M., “Iron Alloys,” Casting Source Directory, 2006 (as accessed from http://www.afsinc.org/files/images/iron alloys.pdf on 23 January 2016)
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