Buying Guide

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Revised 7/20/2018
NIBCO is the largest United States manufacturer of pressure rated Ductile Iron Valves. NIBCO Ductile Iron
multi-turn valves are ideal for a wide variety of services: hydrocarbons, chemical, marine, fire protection services, pulp and paper
applications where cast iron and/or cast steel valves could be installed.
Ductile Iron, also known as nodular iron, was developed in 1949 as a substitute for steel. Cast steel contains carbon of less than
.3% by weight, while cast and ductile irons have at least 3% total carbon. This low carbon content in cast steel does not allow the
carbon to form as free graphite resulting in a laminate type of structure. The natural form of carbon in cast iron is the free graphite
flake form. In Ductile Iron, this graphite flake is modified by a specialized treatment process to form tiny spheres or nodules. These
modified graphite nodules provide Ductile Iron with physical properties greater than cast iron and comparable to steel. It is this
nodular microstructure of carbon in Ductile Iron which produces high ductility and shock resistance while the flake form of cast iron
results in no malleability. Optimum ductility is obtained with a ferritic matrix, therefore, all NIBCO Ductile Iron pressure contain-
ing parts are treated with a ferritizing annealing cycle. In Ductile Iron spheroidal nodules also eliminate the crack effect of flake
graphite which is exhibited in cast iron. In microscopic photos of Ductile Iron, cracks can be seen traveling to a graphite nodule and
stopping. These graphite spheroids are know as “crack arresters” in the Ductile Iron industry because of their ability to stop cracks
in their tracks.
In some circles, Ductile Iron is known as the metal that is the “best of both worlds” meaning that Ductile Iron combines the supe-
rior strength of cast steel with the excellent corrosion resistance of cast iron.
Ductile Iron vs. Cast (Gray) Iron
The strength of Ductile Iron when compared to cast iron is overwhelming. Ductile Iron tensile strength is 60k versus cast iron at
31k. Ductile Iron has a yield strength of 40k, whereas cast iron exhibits no yield, only ultimate fracture. Ductile Iron strength-to-cost
ratio offers greater value for a marginal increase in cost over cast iron. (See page 90 for a complete comparison of mechanicals.)
Ductile Iron offers excellent corrosion resistance that is equivalent to cast iron.
Ductile Iron vs. Cast Steel
The strengths of Ductile Iron and cast steel are comparable. Ductile Iron has a higher minimum yield strength at 40k versus cast
steel at 36k. (See page 90 for a more complete comparison of mechanicals.) Ductile Iron has corrosion and oxidation resistance
that surpasses cast steel in most general utility service applications. Because of Ductile Iron’s spheroidal graphite microstructure,
Ductile Iron is superior to steel in its ability to deaden vibration and therefore reduce stresses. An important factor in selecting
Ductile Iron over cast steel is cost. The lower expense of Ductile Iron results from readily available materials, foundry operation
efficiencies and reduced machining costs of Ductile Iron.
Cast (Gray) Iron
Flake Form
Ductile Iron
Spheroidal Graphite Nodules
Cast Steel Form
NOTE: For our metal comparison, NIBCO has chosen to use ASTM A395 Ductile Iron, ASTM A126 Cast Iron and ASTM A216 WCB Cast Steel.
Wherever Cast Iron is listed, we are referring to Gray Iron.
Why Ductile Iron?