Integrated Approach for Prediction of Hot Tearing
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TRODUCTION
HOT tears are cracks that initiate in the mushy zone. These cracks are characterized by intergranular fracture and a smooth fracture surface due to the existence of a liquid phase in the interdendritic region during cracking.[1] Hot tearing is one of the crucial problems encountered during the direct-chill (DC) casting process. The occurrence of hot tears determines the productivity during the process. These solidification defects have been understood for a long time, but a quantitative prediction of their occurrence is still underdeveloped. In general, the solidification processes proceed in four steps that reflect the morphological development and interaction, namely: (1) the nucleated crystals float freely and the macroscopic behavior is close to the liquid behavior; (2) the nuclei are close and tend to attach to each other to form a porous network and the solidification shrinkage strain is easily counteracted by the liquid flow and solid arrangement; (3) the deformation of the solidified body caused by the solidification shrinkage and external strains is not fully counteracted by the liquid flow and solid movement, so that solidification defects such as hot tears and porosity are usually initiated; and (4) the grains are strongly interconnected, so that deformation of the solidified body will not result in further defects. SUYITNO, Associate Professor, is with the Casting and Solidification Technology (CASTEC) Laboratory, Department of Mechanical and Industrial Engineering, Gadjah Mada University, Yogyakarta, Indonesia. Contact e-mail: [email protected] W.H. KOOL, Associate Professor, and L. KATGERMAN, Professor, are with Department of Materials Science and Engineering, Faculty 3mE, Delft University of Technology, 2628 CD Delft, The Netherlands. Manuscript submitted May 13, 2008. Article published online August 19, 2009 2388—VOLUME 40A, OCTOBER 2009
Quantitative prediction of hot tearing is not easy because of the complex interplay between macroscopic and microscopic phenomena. Prediction of hot tearing during DC casting is based on two steps, namely modeling of the thermomechanical behavior during solidification[2–4] and the implementing of hot-tearing criteria into this model.[5–10] The first step uses constitutive equations to describe the thermomechanical modeling, to calculate stresses and strains in the billet. Computed stresses or strains indicate the hot-tearing tendency. In the second step, the results of the first step are used as input into a hot-tearing criterion. Several mechanisms of hot tearing have recently been reviewed[1] and a recent article outlines the requirements for a modern hot-tearing model and a criterion based on this model as well as the future development of hottearing research in terms of mechanisms of hot-crack nucleation and propagation.[11,12] Various criteria that might enable the prediction of hot tears have been proposed.[13–20] These criteria can be classified into those based on nonmechanical aspects such as feeding behavior,[13–15] those based only on mechanical aspects,[16–18
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