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The remarkable properties of ADI are developed by a closely controlled heat treatment operation (austempering) which develops a unique matrix structure of bainitic ferrite (60%) and retained (high carbon) austenite. The retained (H.C.) austenite is thermally stable to extremely low temperatures but is work hardenable and will locally transform to martensite under suitable conditions of stress. Advantage of this feature of ADI is taken by allowing the service loading stresses to work harden the load bearing services. Alternatively, surface stresses can be deliberately imposed prior to service, e.g. by shot peening of gears or fillet rolling of crankshafts in order to achieve significant improvements in wear resistance and fatigue life. Presently there are no accepted standard specifications for ADI, but proposals for five grades of ADI have been made which form the basis for discussion and material selection between designers and foundrymen. |
The heat treatment necessary to produce ADI is essentially a two-stage operation:
Stage 1 Austenitizing in the range 1500-1700°F (815-920°C). The specific austenitizing temperature selected is related to the subsequent austempering temperature and the grade of ADI required. Once selected, the austenitizing temperature must be closely controlled (±10°F). |
