Is gray iron the same as ductile iron?

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

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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. And there is a lot of it — iron comprises 34.6% of the Earth's mass, more than any other element. Because it is so readily available, it represents 90 percent of all metal refined worldwide.

Over the course of several millennia, 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, constituting 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, with a small percentage of silicon. Ductile cast iron is a relatively new metallurgical innovation, having been introduced in the 1940s. In addition to iron, it is about 3.2 to 3.6 percent carbon and 2.2 to 2.8 percent silicon, with the remainder filled out by elements such as manganese, magnesium, phosphorus, sulfur, and/or copper. Of these, magnesium is most critical, as it plays the role of nodulizer (more on that in the next section).

Physical properties of ductile iron vs cast iron

Physically speaking, the big differentiator in ductile iron vs. gray cast iron is in the way the graphite microstructure is arranged. In gray cast iron, the graphite manifests as flakes distributed throughout the overall structure as the metal is slowly cooled. When the resulting casting is under too much tension, these flakes slide into and stack up on one another like minified tectonic plates, forming fault lines along which the casting can splinter and crack. 

Conversely, in ductile cast iron, a nodulizer (often magnesium, but sometimes tellurium or cerium) influences the graphite to form spheroidal nodules (for this reason ductile iron is also sometimes called nodular iron) which are more apt to slip past one another on a microscopic level. What this does is allow the ductile iron casting to be bent or pulled with a much smaller chance of fracturing. When we talk about nodularity in a casting, we're talking about a higher concentration of these graphite nodules that contribute to yield strength (ability to resist irreversible deformity), tensile strength (ability to resist breaking), and elongation (stretchiness).

Applications of ductile iron vs cast iron

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Compared to ductile iron, gray iron is harder and more brittle. But there are many scenarios where its brittleness is not all that consequential. Furthermore, it offers excellent vibration damping and machinability, and is more economical to produce. Gray iron serves in a host of non-mechanical applications, such as hardscape (manhole covers, storm grates, etc.), counterweights, and many other items intended for general human use (gates, park benches, railings, doors, etc.). When it is employed in machinery, the stiffness of gray iron is its winning attribute, helping to offset vibration in the base of heavy machines, in brakes, and in some engine components. 

However, gray iron's lower impact and wear resistance makes it a liability in more mission-critical situations. Ductile iron compensates beautifully for these shortcomings, to the degree that it has become a trustworthy alternative to steel in many engineered applications, with a preferable strength-to-weight ratio. Demanding industries demand ductile. 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, and 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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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.  There are many grades of gray iron as per the ASTM A48 standard.  While gray iron does not have a measurable yield strength, the range for tensile strength is 20,000 psi – 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 (collision). Ductile iron has a greater resistance to impacts, and is able to resist a minimum of 7 foot pounds of impact (versus 2 pounds of impact 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 being used for certain purposes.
4. Thermal Conductivity – Ductile iron has a lower thermal conductivity than gray iron. Specifically, the graphite phase in gray iron gives it very high thermal conductivity as the heat transfer is through the graphite flakes. The isolated spheres of graphite in ductile iron lower its thermal conductivity considerably – in fact it is not much more conductive than steel. As the amount of carbon increases, the thermal conductivity increases. This means that lower strength gray irons will have higher thermal conductivity. The design engineer needs to keep this in mind if the primary reason for choosing gray iron is for thermal conductivity. Gray iron, with its high thermal conductivity, is 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 because the time duration when stress is at or above the fatigue limit is reduced. This enhancement of fatigue resistance is the most beneficial aspect of higher damping capacity. The other major benefit is the reduction of vibrations and noise that can be 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.

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