What is the difference between ductile iron and grey cast iron?

Ductile Iron vs. Gray Cast Iron: Understanding the Differences

Posted by Penticton Foundry

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Gray iron and ductile iron are differentiated by the type and structure of the carbon present in them. The question, of course, is which one is right for your project? Before we look at possible answers, let’s examine some of the key differences between the two cast irons. For a quick summary, scroll down to the bottom of the blog and check out our chart.

Key Differences

1. Ductility: Ductility is determined by a greater percentage of elongation under tension. The addition of magnesium in ductile iron means that the graphite has a nodular/spherical shape, conferring higher strength and ductility as opposed to gray iron which is flake-shaped. For example, 18% elongation can be easily achieved with ASTM A395 and A536 grades 60-40-18 material.
2. Tensile and Yield Strength: There are certainly differences when it comes to tensile and yield strength of gray and ductile iron. Ductile iron has a minimum tensile strength of 60,000 psi and a minimum yield strength of 40,000 psi. Many grades of gray iron as per the ASTM A48 standard do not have a measurable yield strength, but the range for tensile strength is 20,000 psi to 60,000 psi.
3. Impact: Impact strength, also known as toughness, is a measure of a metal's ability to resist fracturing while absorbing an impact. Ductile iron has a greater resistance to impacts, able to resist a minimum of 7-foot pounds of impact (versus 2 pounds for gray iron). This means that while ductile iron can be used in critical applications that involve impact, gray iron has limits that prohibit it from certain purposes.
4. Thermal Conductivity: Ductile iron has a lower thermal conductivity than gray iron. The graphite phase in gray iron gives it very high thermal conductivity as heat transfer is through graphite flakes. The isolated spheres of graphite in ductile iron lower its thermal conductivity considerably - not much more conductive than steel. This makes gray iron a great choice for parts that undergo thermal shock, like brake drums.
5. Vibration Damping: Internal friction is how materials absorb vibrational energy. Gray iron dampens vibrations more effectively than ductile iron because gray iron exhibits non-elastic behavior at very low stresses. A higher damping capacity enhances fatigue resistance, reducing the time stress is at or above the fatigue limit, and decreases vibrations and noise emitted by components of machinery.

We have included a chart comparing the two irons across a variety of factors. The checkmark in the appropriate column indicates the better choice between the two.

Ductile Iron vs Cast Iron: What's the Difference?

At Urick Foundry, we often promote the advantages of ductile iron vs. cast iron. But what's the difference? Both belong to a class of iron alloys known as cast irons — they are cast in a foundry. Within the industry, the term "cast iron" typically refers to gray cast iron (aka gray iron). Likewise, "ductile iron" is understood as an abbreviation of ductile cast iron.

If you missed our previous blog post where we defined what ductile iron is, here's a quick recap: ductile iron is a unique iron-carbon alloy developed in the mid-20th century notable for its exceptional ductility, or ability to elongate. That is the primary difference between ductile iron and cast iron, but let's elaborate further.

Chemical Composition of Ductile Iron vs Cast Iron

Elemental iron is a lustrous, grayish metal very susceptible to oxidation, or rusting, in damp air — because of this tendency, it is very rarely found in its pure form. Consequently, most iron on Earth exists within iron oxides — minerals such as hematite, magnetite, and taconite. Iron comprises 34.6% of the Earth's mass, more than any other element, representing 90 percent of all metal refined worldwide.

Metallurgists have learned to enhance iron's desirable qualities by formulating alloys, which combine refined iron with smaller percentages of other elements. In all varieties of cast iron, graphite carbon is the second-most prevalent element by mass, making up roughly 3 to 5 percent of the alloy. Iron-carbon alloys with lower percentages of graphite are classified as steel.

Gray cast iron has been in use for centuries and remains the type most foundries produce. Compositionally, it is 96 to 98 percent iron, 2 to 4 percent carbon, and a small percentage of silicon. Ductile cast iron is a relatively new metallurgical innovation, introduced in the 1940s. It consists of about 3.2 to 3.6 percent carbon and 2.2 to 2.8 percent silicon, with elements like manganese, magnesium, phosphorus, sulfur, and/or copper. Magnesium is the most critical as it acts as a nodulizer.

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Physical Properties of Ductile Iron vs Cast Iron

The differentiator in ductile iron vs. gray cast iron is the way the graphite microstructure is arranged. In gray cast iron, graphite flakes are distributed throughout the structure as the metal cools. When under too much tension, these flakes form fault lines that cause the casting to splinter and crack.

However, in ductile cast iron, a nodulizer (magnesium, tellurium, or cerium) influences the graphite to form spheroidal nodules, allowing the casting to bend or pull with a smaller chance of fracturing. Higher concentrations of these nodules contribute to yield strength, tensile strength, and elongation.

Applications of Ductile Iron vs Cast Iron

Compared to ductile iron, gray iron is harder and more brittle. However, its brittleness is not always consequential. Gray iron offers excellent vibration damping and machinability and is more economical to produce. It serves in non-mechanical applications like manhole covers, storm grates, counterweights, and general items like gates, park benches, and railings. In machinery, gray iron's stiffness helps offset vibration in heavy machine bases, brakes, and some engine components.

However, gray iron's lower impact and wear resistance make it a liability in mission-critical situations. Ductile iron compensates beautifully for these shortcomings, making it a trustworthy alternative to steel in many engineered applications with a preferable strength-to-weight ratio. Demanding industries demand ductile iron. At Urick, we create castings for agriculture, heavy truck, rail, recreation, and more. These customers require parts that can withstand extreme forces without breaking or deforming — that is ductile iron's reason for being.

Can Ductile Iron Make a Difference for You?

Now that you know the difference between ductile iron and cast iron, you should have a better idea how ductile iron can make a difference for you. Urick Foundry is renowned for ductile iron castings with the best nodularity rates in the business through its proprietary vertical in-mold inoculation process, creating products primed to perform.

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