ADI: Tough, affordable castings for your most demanding applications

Iron that is nearly as strong and abrasion resistant as steel – and less expensive

What if you could create iron castings that are nearly as strong as steel, and more manufacturing friendly? What if the iron was so strong you could use it to cast complex shapes and redesign multi-part assemblies into a single piece – slashing inventory and assembly costs in the process? And what if you could cut the lead-time of steel in half?

A material with these qualities sounds too good to be true – and expensive, too.

But the truth is that austempered ductile iron (ADI) offers all these advantages at a cost that’s substantially less than steel.

Jesse Milks, president of State Line Foundries, explains: ”It’s a material that not enough OEM engineers are knowledgeable about. Although it’s been available since the 1970s, it’s a relatively new or unknown process to many manufacturers.”

“Honestly, it’s hit or miss,” he adds. “Some OEMs have adopted it very well into their product lines. Others have never used it and know nothing about it.”

What is ADI?

Gray cast iron has a flake microstructure and is not as strong as ductile iron, which has a spherical microstructure. ADI makes ductile iron even stronger by transforming its microstructure.

Austempering starts like a traditional casting process, with parts heated to between 1,550 F and 1,750 F. Then the departure from common casting begins.

With austempering, the heated parts are soaked in a molten salt bath at temperatures between 450 F and 750 F. In the conventional heat-treating process, the bath of oil or water is often at room temperature when the heated parts are soaked, which produces a brittle crystalline structure that prevents expansion. But it can lead to residual stresses, distorted shapes or even cracking.

The austempering bathing process is longer than the traditional method and sometimes lasts for hours. This enables uniform transformation, preventing the distortion that can result from the conventional treatment approach.

ADI parts offer exceptional strength

State Line frequently measures the tensile, yield and elongation properties of iron on-site. Milks is impressed with the performance of ADI. “It rivals certain steels,” he says.

That strength results in excellent wear resistance, which makes it perfect for demanding applications such as agricultural and construction equipment, gears and power trains, mining and forestry machines, off-road trucks, railway components and a host of military products.

John Deere uses ADI in the tractor discs of its tillage tools that muscle through demanding field conditions. “They’re hitting rocks and plowing through abrasive soil. ADI lasts much longer in that environment,” Milks explains. Oshkosh Corp. provides the U.S. Department of Defense with Humvees that feature ADI parts, which hold up well in the demanding climates and terrain where they travel.

How ADI enables part reduction

The improved strength of ADI enables the casting of more complex shapes because the weakest molecular links are still very strong. An ADI castings’ microstructure is quite homogeneous, whereas a multi-part weldment will have stress points from the heating and cooling of the welds. That enables OEMs to reduce the number of parts to make an assembly, and usually results in a product of more consistent quality in the end

It’s hard to overstate the significance of part reduction. One case study conducted by a foundry concluded that converting from multiple parts to a single component reduced inventory by 95 percent and assembly time by 82 percent. The result was a cost savings of 64 percent.

ADI parts have shorter lead times

During the last few years, demand for steel castings has outstripped domestic supply. As a result, lead times for steel castings have reached an all-time high.

“There is more available iron foundry capacity in this country than there is steel capacity,” Milks points out. “Lead times for steel components are usually much longer than for ADI parts. If you need it quick, with consistent, reliable lead times, ADI is an excellent option.”

ADI enables lighter-weight parts

ADI’s strength can also provide significant weight advantages: “Because you can get such high strength, you can engineer parts differently,” Milks emphasizes. Wall thicknesses can be reduced while maintaining the same level of strength. That means less iron – and less weight.

“Engineers have even been able to design ADI castings that weigh less than aluminum castings,” he adds.

How to take advantage of the benefits of ADI

How can you add ADI castings to your lineup of materials? First, it requires a foundry that is adept at meeting its chemical requirements and can shepherd the process effectively.

“If we have a customer considering an ADI casting, we send the prints and models to an outside heat treat partner who is an expert in it,” Milks says. “They’ll send us a quote for their heat treat process and they’ll also send us the part-specific chemistry requirements.”

“Almost every iron foundry is pouring gray and ductile iron,” Milks says. “Less than half are pouring ADI because you need to have better controls and a better understanding of alloys. You also need to understand the characteristics of the iron chemistries that you’re going to pour.”

He points out that alloys are sometimes customized based on the design of the part. For example, a thick-walled casting may have a different chemistry than a thin-walled one.

Even with the added step of an outsourced heat-treating process, ADI is still more economical than steel and can often be delivered quicker.

State Line’s expertise with ADI enables OEMs to get started with it quickly and easily: “OEM engineers and purchasing managers may not have much casting experience, but that’s where we come in. We work hard to make the process easy for you. If you have an application where ADI might fit, we’d love to talk to you,” Milks concludes.