What is ductile iron?

By C.A. Lawton | August 3, 2022
ductile iron Blog Cover

Ductile iron is more impact and fatigue resistant than gray iron due to the spheres of graphite that form with added magnesium.

The science and strength of ductile iron

From a microstructure standpoint, the primary difference between gray and ductile iron is the shape of the graphite. Gray irons have flake graphite, whereas ductile iron has a nodular or spherical shape. This graphite shape drives the primary difference in each from a material properties specifications perspective.

The flake graphite shape in gray iron, which creates fracture planes, is brittle and has less strength. On the other hand, with its nodular graphite shape, ductile iron, as the name implies, has ductility, which allows for elongation under tension and better impact resistance. In the material properties of gray iron, only tensile strength is listed because there is no elongation. For ductile irons, the specifications call out tensile strength, yield strength, and elongation.

When should ductile iron be used?

Gray iron is typically used in applications where strength is not the primary requirement. Due to the graphite shape, it is more effective at damping vibration than ductile iron. Typical part families are compressors, pumps, counterweights, low-speed gearboxes, and machine bases, to name a few.

On the other hand, ductile iron will be used in engineered applications due to its superior strength and elongation. Typical parts include structural or other highly stressed components, high-speed gearboxes, gears, wheel hubs, air/gas cylinders, and other pressurized components.

What is the R&D process?

Just like casting large gray iron components, the cost of research and development of ductile castings can be very high. To achieve a high first-pass yield, we must optimize the mold’s design to ensure consistent, low-cost manufacturability. This starts with open dialogue with our customers early in the design process. Our engineers find out precisely what our customers’ performance requirements are. Then they make adjustments that may include specific chemistry needs, material properties, and special hardness requirements.

What many design engineers don’t realize is that even seemingly simple changes to a gray or ductile iron part design – such as making a wall thinner – can make a casting significantly more expensive to produce. The cost of a large casting can be substantially reduced by eliminating cores, loose pieces, and change pieces. Up-front dialog helps the Lawton sales engineer work through the trade-offs with the customer – what’s essential and what can be eliminated or changed.

How are castings tested?

Technology plays a significant role in testing all of our castings. Our casting engineers rely heavily on flow and solidification software to optimize the part’s design. This software simulates how molten metal flows into a mold. It helps the casting engineer predict and eliminate defects, so when the foundry casts the part, it’s made right the first time.

Ultrasonic testing, which uses high-frequency sound energy to examine and measure castings, is frequently used to detect flaws in ductile iron.

Test bars, a separately casted sample of iron from the same pour as the casting, are used to test castings. The sample piece is machined to the standard specification that will be used to determine the mechanical properties of the iron.

Whatever your ductile or gray casting requirements are, our engineers will work with you to deliver your castings to spec and on time.

Why does The Lawton Standard company continually strive to improve our ductile iron processes? What are we working on next? Stay tuned as we’ll share more here and on our YouTube channel soon. Be sure to follow us on your favorite social media, so you don’t miss a thing.

Check out our Metalcasting Dictionary.

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