Austempered Ductile Iron...Say What?

"It would probably be hard to find a significant piece of off-highway equipment that doesn’t have austempered ductile iron in it", says John Keough, CEO of Applied Process, provider of austempering heat treatment based in Livonia, MI.

A strong statement. One that may beg the question in many minds of “What exactly is austempered ductile iron?” That’s a question that Keough and his colleagues at Applied Process have been on a mission to answer since the heat treater began offering austempering (ADI) as an option in 1984.

The first step in austempering does not differ from many conventional heat treating processes in that the cast piece is heated up to red hot or austenitized. For conventional processes like quench and temper, the pieces are subsequently quenched into a water or oil bath that is generally kept at room temperature. This heating and quenching sequence produces a crystalline structure in the iron called martensite, a hard but brittle material.

The difference with austempering is in the quenching medium. Instead of oil or water, the piece is “cooled” in a molten nitrite/nitrate salt solution that is kept between 230 C and 400 C depending upon the grade of ADI desired. It never reaches the martensite start (Ms) temperature, and therefore the brittle martensite microstructure never develops. Instead, a material called ausferrite is formed.

In conventional heat treating, when the part reaches the Ms temperature, the microstructure transformation is instantaneous. The surface of the component may cool and harden, but the inside could still be red hot. As a result, the component will not transform uniformly which can cause distortion and small cracks in the part, reducing its overall strength.

With austempering, the ausferrite forms over period of many minutes or hours so the part is uniformly transformed. The resulting material is stronger and tougher than conventionally heat treated ductile iron.

“ADI gives us a much higher toughness for a given hardness,” says Keough. “If we quenched and tempered ductile iron to a certain hardness and pulled a tensile bar from it, we might get 2% elongation. If we austempered a bar out of the same ductile iron, it might get 12% elongation. It’s a much tougher structure — not brittle like quench and tempered iron tends to be.”