How To Tell Ductile Pipe From Cast Iron Pipe

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Ductile iron pipes are lighter, stronger, and more durable than cast iron. The main difference between ductile iron and cast iron lies in the form of graphite present. Ductile iron features spherical graphite, while cast iron contains flake graphite. This nodular graphite in ductile iron is achieved by introducing an inoculant (most commonly magnesium) to the molten iron during production. To accurately determine the composition of ductile iron, a spectrograph analysis can be performed in a laboratory setting, or a tensile test can be conducted.

Step 1

Inspect the pipeline for weathering. Ductile pipe was introduced in the 1940s and commercially produced in the following decade, with most installations occurring in the late 20th century. If the pipe is older than this, it is likely cast iron. Cast iron pipes have been in use for over 150 years in North America, and ductile pipes eventually took their place. Generally, cast iron pipes exhibit a smoother surface finish than ductile pipes, which often have a characteristic "pointed" pattern if struck.

Step 2

Look for labels on the bell flange surface. Ductile pipes typically have markings, such as "DI" or "ductility," cast onto the surface. These identifiers are absent on cast iron pipes.

Step 3

Check for factory-grade markings on the exterior of the pipe. Ductile and cast iron pipes are labeled with grade marks during manufacturing. Cast iron receives the classification number "20." The malleability classifications for ductile iron have evolved, initially ranging from "2" to "6," and later expanding to include thickness designations, with markings between "50" and "56." In modern times, ductile iron pipe pressures are rated based on psi, typically between 150 psi and 350 psi depending on the wall thickness.

Step 4

A spectrograph can determine the exact composition of the materials. The ultimate tensile strength of cast iron is about 20,000 psi, while ductile iron surpasses 60,000 psi, with a yield strength of 42,000 psi. Ductile iron pipes will generally elongate by about 10% before breaking during tensile tests, with the sequence often recorded as 60-42-10.

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