What is the difference between Cast Iron and Ductile Iron?

Cast iron is a generic name for any high carbon molten iron poured as a casting. When used to refer to pipe, cast iron (sometimes called gray iron) is a specific type in which the free graphite (Carbon) is in the shape of flakes. Cast Iron pipe were introduced into the United States in . 
 

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Ductile Iron is a specific type of cast iron in which the free graphite is in the shape of nodules or spheroids. (Other names for ductile iron are nodular iron or spheroidal graphite iron.) Ductile Iron Pipe were introduced to the market in . 

Although nearly identical chemically, the two irons are quite different metallurgically. The now obsolete standard for Cast Iron Pipe (ANSI/AWWA A21.6/C106) required an iron strength of 18/40 (18,000 psi Bursting Tensile Resistance and 40,000 psi Ring Modulus of Rupture.) Although tensile testing was not a requirement of this standard, a tensile test of gray cast iron pipe would give a test result of approximately 20,000 psi Ultimate Tensile Strength, with no measurable Yield Strength or Elongation. 

The current standard for Ductile Iron Pipe (ANSI/AWWA A21.51/C151) requires a minimum grade of 60-42-10 (60,000 psi Ultimate Tensile Strength, 42,000 psi Yield Strength, and 10% Elongation.) In addition, Ductile Iron Pipe manufactured under this standard are required to meet a minimum of 7 ft lbs impact resistance by the Charpy test. (Compare Gray Iron Pipe with an impact resistance of approximately 2 ft lbs or less.) 

The difference in the physical properties of these two materials is attributable almost entirely to the difference in the shape of the free graphite. The shape of the graphite is determined at the instant of solidification and is made nodular by the addition of magnesium to the molten iron bath. Although Cast Iron was the best engineering material available for pipe production for nearly five hundred years, the development of Ductile Iron Pipe provides a far superior product.

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It's been a fascinating 3-decade run for me serving the water and sewer industry. Beginning with 14 years at Griffin Pipe Products as their National Product Engineer, then developing that role further over the last 20+ years with the McWane organization, also a National Product Engineer, I have been blessed to see the entirety of North America and much of Canada time and again for too many engaging scenarios ever to list.

I've met and worked with countless wonderful people across all aspects of utility design, manufacture, construction, troubleshooting, and professional development. Through the years, many of them have become personal as well. I've loved every day of it, even the bad ones, because those are the ones where your character, communication skills, and desire to productively assist are not only required but tested. That's what makes it fun&#;stepping in when others are compelled to step back. 

Outside of work, I enjoy quiet time with friends and family, sports of all kinds, and my greatest assignment ever ... being a Grandpa, courtesy of my son Kyle and his wonderful wife, Erin.

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