What is Grey Cast Iron?

What is Grey Cast Iron?

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.

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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.

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:

• Gears
• Hydraulic components
• Automotive suspension components
• Plow shares
• Pumps
• Linkages
• Stove parts
• Steering knuckles
• Tractor parts
• Valves
• Truck suspension components
• Other truck parts
• Wind turbine housings.
• Weights and counter-weights
• Machinery bases

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.

What are the Material Properties vs. Casting Processes?

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.

What is the Composition of Gray Cast Iron?

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]

What is the Microstructure of Grey Cast Iron

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.

Classification of Grey Iron Classes

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.

Standard Specifications

Below is a reference chart of other ASTM standards affecting grey iron.

• ASTM A395 & ASME SA395: ferritic ductile iron pressure-retaining castings for

use at elevated temperatures

Contact us to discuss your requirements of oem grey iron sand casting manufacturers. Our experienced sales team can help you identify the options that best suit your needs.

• ASTM A439: austenitic ductile iron castings
• ASTM A476 & ASME SA476: ductile iron castings for paper mill dryer rolls
• ASTM A536 & SAE J434: ductile iron castings
• ASTM A571 & ASME SA571: austenitic ductile iron castings for

pressure-containing parts suitable for low-temperature service

• ASTM A874: ferritic ductile iron castings suitable for low-temperature service
• ASTM A897: austempered ductile iron castings

Is there a Difference Between Grey Iron Castings and Gray Iron Castings?

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, (as accessed from http://www.afsinc.org/files/images/iron alloys.pdf on 23 January )

What is Gray Cast Iron?

Gray Cast Iron, also known as gray iron, is a popular type of iron used in castings. The composition of gray cast iron is 2.5%&#;4% carbon, 1%&#;3% silicon, and some additions of manganese ranging from 0.1% to 1.2%. Gray iron contains graphitic microstructures, giving the iron it&#;s gray color. This iron is easily cast but it cannot be forged or worked mechanically, either hot or cold. While the tensile strength and impact resistance of gray iron is weaker than most other metals, the compressive strength is equivalent to low-carbon and medium-carbon steel. For most applications, gray cast iron typically has acceptable ductility, tensile strength, yield strength, and impact resistance. 

Gray Cast Iron Properties:

Hardness (Vickers): 161-321

Tensile Strength: 7 tons per square inch on average

Tensile Strength, Ultimate: - psi (115-700 MPa)

Tensile Strength, Yield: - psi (65.5-420 MPa)

Compressive Yield Strength: - psi (572- MPa)

Melting Point: - degrees Celsius 

Gray Cast Iron Benefits

There are many unique benefits of using gray cast iron. One of the most notable is gray iron&#;s enhanced lubrication. The graphite flakes in gray iron allow for better lubrication creating less wear on the cast iron. This makes gray cast iron a great choice for parts like brake rotors. Gray cast iron also has the ability to dampen vibrations making it an excellent choice for housing applications or industrial applications such as machinery bases. Additionally, gray iron has high thermal conductivity allowing it to move heat more easily through the metal.

Gray cast iron can withstand thermal cycling, meaning the iron can go through high and low temperature changes without strain. Thermal cycling can create stress and premature failure in other forms of metal casting, however gray cast iron has continuously proved its ability to withstand thermal cycling. Gray iron is very resistant when it comes to oxidation. Gray cast iron iron develops a protective film or scale on the surface, making it more resistant to corrosion than wrought iron or mild steel.

Gray Cast Iron Disadvantages

One of gray cast iron&#;s biggest pros is also its biggest con. The graphite flakes that give gray iron its added lubrication and resistance to wear, also allows areas of weakness in the metal where fractures can begin. These fractures can cause splits and breakage. Fracturing is why gray iron has low tensile and impact strength.

Gray Cast Iron Applications

Gray cast iron has many applications and is often used across every industry such as automotive, construction, and manufacturing. 

Examples of gray cast iron applications:

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• Gears
• Hydraulic components
• Automotive suspension components
• Plow shares
• Pumps
• Linkages
• Stove parts
• Steering knuckles
• Tractor parts
• Valves
• Truck suspension components
• Other truck parts
• Wind turbine housings.
• Housing columns
• Weights and counterweights
• Machinery bases