Austempering (Fig. 9) starts with heating the part (A-B) to a specific temperature in the austentite range (1,500 to 1,750 degrees F; 815 to 955 degrees C) and holding it there (B - C) for a sufficient time to saturate the austentite with carbon. The part is then cooled (C - D), at a rate sufficient to avoid formation of ferrite or pearlite, to the appropriate austempering temperature (450 to 750 degrees F; 230 to 400 degrees C), held there (D - E) for a time sufficient to achieve the desired properties, and cooled (E - F) to room temperature.

Twice the Tensile and Yeild Strength

The strength level (or grade) of ADI (Fig. 11) is determined by the austempering process, not by the as-cast grade of the ductile iron.

• A higher austempering temperature produces a strong part with excellent dynamic properties.
• A lower austempering temerature produces a part with higher strength and increased wear resistance.

Alloy is added to the iron only when required to through harden a thicker section. Figure 10 shows a typical relationship between the hardness and the tensile/yield strength. For a given level of ductility, ADI will have over twice the tensile and yield strength of the standard grades of ductile iron.

Take a Closer Look

The only "raw material" required for ADI is good quality ductile iron. It's that simple, because the same characteristics that produce excellent as-cast properties in ductile iron are also critical to producing optimum performance in ADI. Misconceptions still exist, however. Some believe the ADI must be "specially alloyed" or have certain microstructural differences that separate it as a "special" material. In fact, what is required is a consistant ductile iron with a predictable chemical analysis, a reasonably consistent pearlite to ferrite ratio and a commercially acceptable level of both graphite nodularity and count. A commercially available ductile iron when properly austempered, results in the high-performance Ausferrite matrix in ADI.