Post-TMT transformation
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Post-TMT Transformation Structure of Ferrite in Pearlite
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J. G. BYRNE and PO-WE KAO Microhardness, X-ray particle size, X-ray microstrain, and positron Doppler shape factor measurements were used to characterize the ferrite resulting from thermomechanical treatments (TMT) of eutectoid steel. The TMT involved cold-rolling at room temperature, short-time transformations to austenite, and retransformation to pearlite by air-cooling. 14 This TMT restores the original cementite platelets in their original locations and very nearly ] in their original thicknesses and intedamellar spacing. This is possible because the very short times necessary for complete austenization do not disperse the carbon away from high concentration locations sufficiently to prevent the carbides renucleating at their original locations upon cooling below the A~ temperature. The advantage of this is that the carbide phase, may be first oriented by heavy cold-rolling, the hardness of the ferrite removed by recovery, recrystallization, and then complete eutectoid transformation to austenite, then retransformation to pearlite but with carbide platelets which again are oriented parallel to the rolling plane and with a soft ferrite matrix. There are advantages to this product, especially in fatigue resistance ] and formability. The alloy was consumable electrode vacuum arc remelted (CEVAR) eutectoid steel with 0.85C, 0.77Mn, 0.002P, 0.008S, 0.17Si, 0.2Mo, 0.002V, 0.02Ni, 0.02Cu, 0.002Sn, and 0.005A1 in wt pct and was provided by the Bethlehem Steel Co. The as-received material was austenitized at 1090 ~ for 1 hour, then isothermally transformed at 675 ~ for 16 hours and furnace-cooled. Material of thickness 0.4 cm was cold-rolled 75 pct to a thickness of 0.1 cm and up-quenched to 740 ~ for various times from 10 to 300 seconds. Earlier, Querales and Byrne ~ found that the microhardness of this alloy after these same TMTs fell to a minimum value for a 10-second upquench to 740 ~ went through a maximum for times between 10 and 20 seconds, then decreased to a plateau by 30 seconds at 740 ~ These data are plotted in Figure 1. To understand better this hardness behavior, several additional experimental techniques have now been utilized. X-ray particle size and microstrain values for the ferrite were obtained by Fourier analysis of the (110) and (200) diffraction peak shapes. Filtered Co K~ radiation was used and the Fourier coefficients were corrected for instrumental broadening5 and doublet broadening. 6 The corrected cosine crefficients from the (110) and (200) profiles were used for particle size and microstrain separation by applying a singleprofile analysis technique. 7'8 Positron Doppler broadening measurements were made as described elsewhere. 9 The X-ray line broadening studies of the ferrite diffraction peaks for the 740 ~ TMT gave the particle size and microJ. G. BYRNE is Professor and Chairman, Department of Metallurgy and Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. PO-WE KAO is Associate Professor and Director,
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