Bainitic transformation in austempered ductile iron with reference to untransformed austenite volume phenomenon
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ILI AHMADABADI, Assistant Professor, is with the Faculty of Engineering, Tehran University, P.O. Box 11365-4563, Tehran, Iran. Manuscript submitted July 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Optical micrograph of austempered ductile iron with UAV in the intercellular region.
Table I.
Chemical Composition of 1 Wt Pct Mn Ductile Iron
C
Si
Mn
P
S
Cr
Mo
Mg
Cu
3.61
2.82
0.96
0.01
0.01
0.02
—
0.05
0.07
reaction phenomenon’’[6] and ‘‘diffusion-controlled transformation.’’[7] In the case of incomplete reaction phenomenon, it is assumed that the bainitic reaction stops well before the austenite achieves its paraequilibrium carbon concentration, as given by the Ae'3 curve on the phase diagram. Bhadeshia and Edmonds[6] defined a T0 temperature based on a proposal by Zener[8] that stress-free austenite and ferrite of the same composition (with respect to both the interstitial and substitutional alloying elements) are in metastable equilibrium. Thus, any displacive transformation involving a full supersaturation of carbon can occur only below the appropriate T0 temperature. They also introduced two other terms, T'0 and Ae"3 . These are modified versions of T'0 and Ae'3, respectively, in which the strain energy of bainitic transformation, 400 J/mole,[9] is taken into account. In alloyed ADI, matrix composition is different in different regions; therefore, the first step to calculate phase boundaries (Ae'3, T0 and T'0) is calculation of the chemical composition of a matrix as a function of intergraphite distance. The calculation procedure and variation of alloying elements as a function of intergraphite distance are explained elsewhere.[10,11] In this study, the calculated chemical composition of the matrix, after full austenitization, was used for the calculation of phase boundaries. Figure 2, as an example, shows the calculated profile of carbon content in 1 wt pct Mn ductile iron with a nodule count of 125/mm2 for different austenitizing temperatures. This figure shows segregation of the carbon content in the intercellular region. Segregation of carbon is due to the segregation of Si in the vicinity of graphite, which increases the activity of carbon, and the segregation of Mn in the intercellular region, which decreases the carbon activity.[10,11] Calculation of the T0 curve was carried out by using an equation first introduced by Aaronson et al.[12] This equation was subsequently corrected by Shiflet et al.[13] and later modified by Bhadeshia.[6] The no-substitutional element VOLUME 28A, OCTOBER 1997—2159
Fig. 2—Prediction of intercellular segregation of carbon in specimens with three different austenitization temperatures.
partitioning curve and (Ae'3) was calculated based on the equation introduced in Reference 14. Figure 3 shows the result of calculated Ae'3 , Ae"3 , T0, and T'0 curves for specimens austenitized at 900 7C and austempered at 375 7C, and also the carbon content of austenite after austenitization at 900 7C for 90 minutes (C0 curve).
Fig. 3—Calculated phase bounda
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