The difference between ductile iron castings and gray iron castings

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

main difference
Ductility – Ductility is determined by a greater percentage of elongation under tension. The addition of magnesium to ductile iron means that the graphite is spherical/spheroidal, giving it greater strength and ductility than flake gray iron. For example, 18% elongation can be easily achieved with ASTM A395 and A536 grade 60-40-18 material.
Tensile Strength and Yield Strength – There are definitely differences in the tensile strength 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 cast iron according to ASTM A48 standard. While gray cast iron does not have a measurable yield strength, the tensile strength ranges from 20,000 psi – 60,000 psi.
Impact – Impact strength, also known as toughness, is a measure of a metal’s ability to resist fracture upon absorption and impact (collision). Ductile iron is more impact resistant, capable of resisting impacts of at least 7 foot-pounds (compared to 2 pounds for gray iron). This means that while ductile iron can be used in critical applications involving impact, gray iron has limitations that prohibit its use for certain purposes.
Thermal Conductivity – Ductile iron has lower thermal conductivity than gray iron. Specifically, the graphite phase in gray cast iron gives it very high thermal conductivity, since the heat transfer takes place through the graphite flakes. The isolated nodules of graphite in ductile iron greatly reduce its thermal conductivity – in fact it is not much more conductive than steel. As the carbon content increases, the thermal conductivity increases. This means that the lower strength gray iron has a higher thermal conductivity. If the main reason for choosing gray cast iron is thermal conductivity, design engineers need to keep this in mind. Gray cast iron has high thermal conductivity and is an excellent choice for components subject to thermal shock, such as brake drums.
Vibration Damping – Internal friction is the way a material absorbs vibrational energy. Gray iron dampens vibrations more effectively than ductile iron because gray iron exhibits inelastic behavior at very low stresses. Higher damping capacity enhances fatigue resistance because the duration of stress at or above the fatigue limit is reduced. This increased resistance to fatigue is the most beneficial aspect of the higher damping capacity. Another major benefit is the reduction of possible vibration and noise from mechanical parts.

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