On Peritectic Reactions and Transformations in Low-Alloy Steels
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PERITECTIC reactions and transformations have attracted the interest of researchers, as they are believed to be a major cause of crack formation during the solidification of many steels. In a peritectic reaction, the melt reacts with ferrite (d phase) to form austenite (c phase), which starts to grow laterally on the surface of d by diffusion of solute atoms through the melt. Fredriksson and Nyle´n[1] proposed that a peritectic reaction could alternatively proceed by the growth of the secondary phase independent of the primary phase. The lateral growth of c continues until the surface of d is completely separated from the melt. c then starts to thicken by growing into d and into the melt simultaneously. The latter growth is referred to as the peritectic transformation,[2] and is believed to be controlled by the diffusion of solute atoms from the melt to d through the c layer.[1] In directional solidification experiments on high-alloy steels, Fredriksson[2] found that the peritectic temperature decreased with the increase in the cooling rate. It was also shown that the peritectic transformation occurred very rapidly without any segregation at very high cooling rates. In a later study on iron-base alloys, Fredriksson and Stjerndahl[3] observed that c stabilizing elements enhanced the peritectic reaction, while d stabilizers promoted a metatectic reaction; a hybrid of both reactions was noticed in some cases. Dhindaw et al.[4] conducted differential thermal analysis (DTA) experiments on medium-alloy steels and H. NASSAR, Postdoctoral Researcher and Lecturer, and H. FREDRIKSSON, Professor Emeritus and Head, are with the Materials Processing Division, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted October 22, 2008. Article published online July 2, 2010 2776—VOLUME 41A, NOVEMBER 2010
observed that the heat released during peritectic reactions was considerably lower than that calculated kinetically. They proposed that the difference in energies was due to creep yielding and vacancy formation at the d/c interface, which accommodated for the difference in molar volume between the two phases. It was also hinted that the peritectic reaction might occur by massive transformation of d to c. Shibata et al.[5] used confocal scanning laser microscopy (CSLM) in directional solidification experiments on Fe-C binary alloys and observed that the rate of the peritectic reaction was much higher than the rate of the peritectic transformation. They proposed that this was caused by massive d-to-c transformation or by direct precipitation of c from the melt during peritectic reactions. They showed also that the growth of c observed during peritectic transformations in Fe-0.42 wt pct C could be predicted by C-diffusion models. However, in some experiments on Fe-0.14 wt pct C, it was noticed that peritectic transformations proceeded at very high rates of up to 5 mmÆs–1, which could not be predicted by diffusion. Similar directional solidification-CSLM experiments by Arai et
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