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One-Minute Mentor: Processing sequence for austempered ductile iron
January 10, 2019
Source: ASM International
The figure shows an isothermal transformation (IT) diagram for a ductile cast iron, together with a processing sequence depicting the production of ADI. In this process, austenitizing is followed by rapid quenching (usually in molten salt) to an intermediate temperature range for a time that allows the unique metastable carbon-rich (~2% C) austenitic matrix (γH) to evolve simultaneously with nucleation and growth of a platelike ferrite (α) or of ferrite plus carbide, depending on the austempering temperature and time at temperature.
This austempering reaction progresses to a point at which the entire matrix has been transformed to the metastable product (stage I in the figure). Then that product is “frozen in” by cooling to room temperature before the true bainitic ferrite plus carbide phases can appear (stage II).
In ductile cast irons the presence of 2 to 3 wt% Si prevents the rapid formation of iron carbide (Fe3C). Hence the carbon rejected during ferrite formation in the first stage of the reaction (stage I) enters the matrix austenite, enriching it and stabilizing it thermally to prevent martensite formation upon subsequent cooling.
Thus the processing sequence shows that the austempering reaction is terminated before stage II begins, and illustrates the decrease in the martensite start (Ms) and martensite finish (Mf) temperatures as γH forms in stage I. The kinetics of stages I and II have been described in detail in the literature.
Typical austempering times range from 1 to 4 hours. If the part is austempered too long, undesirable bainite will form. Unlike steel, bainite in cast iron microstructures exhibits lower toughness and ductility.
Materials Processing and Treatment | Heat Treating
Metals and Alloys | Cast Irons