Austempering of low manganese ductile irons

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Abstract
X-ray diffraction and optical microscopy were used to follow the stage I and stage II kinetics of a low Mn, Cu iron and a low Mn, Ni–Cu iron during austempering at 300, 370, and 440°C. Nickel and Cu are shown to delay the bainitic transformation without the undesirable features displayed by Mn and Mo and to influence the stage I kinetics through their effect on the C content of the austenite. The rate of transformation varies with the driving force for the transformation. In contrast, the rate at which the stage II reaction proceeds depends on the total alloying content through its effect on the nucleation and/or growth of the ferrite and carbide phases. The concept of the processing window is discussed and definitions using mechanical property measurements and microstructural and X-ray observations are described. It is shown that the processing window defined from the microstructural observations is not coincident with the optimum mechanical properties. Modifications to the microstructural criteria are discussed. MST/1742

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