Modeling Hot Tearing during Solidification of Steels: Assessment and Improvement of Macroscopic Criteria through the Ana
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tearing arises from a complex combination of thermal-mechanical and solidification phenomena. This solidification defect is basically associated with the incomplete liquid feeding and tensile stress generated in coherent regions of mushy zones, more specifically, in areas with a high solid fraction of 0.9 and beyond.[1] Many authors have reported both a strength and ductility drop in a critical interval of a high solid fraction (‘‘brittle temperature range’’ (BTR)). Indeed, interdendritic residual liquid films act as weak zones when the semisolid material is submitted to a mechanical loading. Consequently, the spatial distribution of the liquid phase in between the solid dendritic arms has a first-order influence with respect to crack initiation together with the wetting of the solid phase by the liquid one, which is controlled by surface tension. M. BELLET, Research Group Leader, and Y. CHASTEL, Professor, Head of Laboratory, and Research Group Leader, are with the Centre de Mise en Forme des Mate´riaux (CEMEF) – MinesParisTech, UMR CNRS 7635, BP 207, 06904 Sophia Antipolis, France. Contact e-mail: [email protected] O. CERRI, formerly Postdoctoral Student, Centre de Mise en Forme des Mate´riaux (CEMEF) – Mines-ParisTech, is Professor, Lyce´e Ge´ne´ral et Technologique, 83510 Lorgues, France. M. BOBADILLA, Group Manager of Physical Chemistry Routes Section, is with ArcelorMittal Maizie`res, Research and Development Industrial operations, Voie Romaine, BP 30320, 57283 Maizie`res-le`s-Metz, France. Manuscript submitted February 9, 2009. Article published online September 1, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
A schematic summary of the concurrent phenomena leading to hot tearing is given in Figure 1. Regions with a solid fraction over the coherency point (for which the solid skeleton is continuous and then capable of standing stresses) become less permeable to the liquid phase. The possible cavities created either by solidification shrinkage or by rupture of residual interdendritic liquid films then cannot be compensated by liquid feeding, which leads to crack initiation. This explains , the ductility that for solid fractions higher than gcohe s limit eR decreases. At the coalescence point, more and more solid bridges form and interdendritic liquid pockets are isolated. For higher solid fractions, some cracks may still be initiated, but this phenomenon is limited because of solid bridging and grain coalescence. This explains that for solid fractions greater than gcoal s , both the strength and the ductility increase, as solidification proceeds. All together, those phenomena result in a ductility trough that is often interpreted as a brittleness interval and that can be defined either in terms of critical solid fractions or critical temperatures (the BTR mentioned earlier). Depending on the solidification conditions and materials, hot tearing may lead to surface or subsurface cracks, as well as cracking in the core of cast products. Hot tearing is also a problem encountered in welding processes, for whi
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