FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2025 63 Specifications for ADI include information about base iron quality, which typically requires a minimum of 80% nodularity. In addition, a minimum nodule count of 100 per mm2 is recommended. ASTM recognizes five different grades of ADI (Table 1) with different strength and ductility combinations. If a component is properly alloyed, any of the five grades can be produced by adjusting the quench temperature and time. Furthermore, distortion issues are minimized. Because the microstructural transformation is completed in the salt bath, no subsequent tempering is required. Not all steels are suitable for austempering. The steel to be processed must have a Ms low enough to allow for the formation of bainite, sufficient hardenability to avoid the formation of pearlite on quenching to the AUS temperature, as well as a reasonable bainite transformation time. Most steels that are austempered have a minimum of 0.40% C in order to quench above the Ms to form bainite; however, plain carbon steels with >0.60% Mn and boron additions like 10B38 are commonly austempered[2]. Austempering is the preferred process over marquenching when high toughness is needed, particularly at hardness levels greater than 40 HRC. Bainite is more resistant to crack initiation and early propagation, so it has better high cycle fatigue performance than tempered martensite at an equivalent hardness[3]. As such, AUS steel is usually substituted for Q&T or MQ steel when combinations of higher toughness, fatigue resistance, and wear resistance are needed at high hardness levels. Examples of applications include levers, chain links, lawn mower blades, transmission gears, axles, shafts, wrenches, drive sprockets, and connecting rods. Ductile cast iron is also austempered and more commonly known as ADI (austempered ductile iron). Because ductile iron has a much higher Si content than steel, the formation of iron carbides is suppressed, which prevents bainite from initially forming during the isothermal hold at the quench temperature, see Fig. 2. Consequently, a microstructure that consists of predominantly acicular ferrite and high carbon austenite— ausferrite—is created. The high carbon austenite within the ausferrite matrix (Fig. 3) is not the same as retained austenite in steel and should not be referred to as such. Unlike retained austenite, it is carbon enriched and thermally stable at very low temperatures. It is this combination of acicular ferrite and high carbon austenite that gives ADI unique property combinations[4]. Like steel, not all ductile iron can be austempered. Hardenability is important to avoid pearlite formation with alloy content requirements increasing with section thickness to sufficiently through-harden a component. Because ADI has a higher strength than conventional ductile iron, it is more susceptible to the presence of defects within the base iron. TABLE 1 — GRADES OF ADI PER ASTM A897/897M-22[5] Grade Tensile strength, MPa Yield strength, MPa Elongation, % Typical hardness, HBW 1 900 650 9 269-341 2 1050 750 7 302-375 3 1200 850 4 341-444 4 1400 1100 2 388-477 5 1600 1300 1 402-512 Fig. 2 — This schematic I-T diagram illustrates a process for austempering ductile iron to make ADI. Note that the final microstructure that is formed is ausferrite. Fig. 3 — Photomicrograph of the ausferrite microstructure in Grade 1 ADI. This microstructure consists of acicular ferrite and high carbon stabilized austenite. 9
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