Effect of Alloying Elements on Thermal Contraction and Crack Susceptibility during In-Mold Solidification
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CRACK formation during solidification is caused by the brittleness of the dendritic front.[1] Cracks are originated in columnar dendritic fronts in which the highest components of thermal stress are directed perpendicularly to the dendrite axis, making them separate. Differently, when the growth is equiaxed, because no prevalent growth direction exists, the effect of thermal stresses is significantly reduced. The external equiaxed zone (so-called chill zone), therefore, acts as a layer exerting good resistance to crack opening and propagation. This mechanism of crack formation is known as ‘‘hot tearing,’’ and several theories have been proposed,[2,3] which put into evidence the factors playing the main roles in the mechanism: solidification interval (temperature difference between liquidus and solidus temperatures), critical or vulnerable region of the mushy zone (region of the mushy zone having solid fraction from 0.9 to 1), and strains induced by thermal and externally applied loads. Crack susceptibility, therefore, is highly dependent on alloy physical properties and process parameters such as cooling history and evolution of shell/mold contact, by which the stress field is generated. Moreover, it is well known that peritectic steels form cracks easier than other steel grades because of the presence of the d-c transformation occurring in the presence of liquid, which induces a strong contraction and generates high tensile strains at the dendrite roots, M.R. RIDOLFI, Senior Researcher, and A. DE VITO, Researcher, are with the Centro Sviluppo Materiali S.p.A., Rome, Italy. Contact e-mail: m.ridolfi@c-s-m.it L. FERRO, Industrial Process Steelmaking Expert, is with Tenaris, Campana, Argentina. Manuscript submitted April 5, 2007. Article published online July 19, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
where hot tears are formed. The peritectic transformation has been extensively studied by means of experimental and theoretical studies.[4–7] In particular, Miettinen and co-workers[8–10] have developed a mathematical model (IDS) for simulating the interdendritic solidification of low-alloyed and stainless steels, allowing description of the phase evolution of peritectic steels depending on the applied cooling rates, and to calculate some basic thermophysical properties as well. Some applications of the model to typical cases of increased crack susceptibility are given in Reference 11, where possible solutions to the problems are proposed in terms of adjustments of the chemical composition. Stress fields arise in the solidifying shell because of the presence of thermal gradients and heat flux nonuniformities due to uneven contact of the shell with the cooling mold wall. The thermomechanical behavior of the solidifying shell has been simulated by means of finite element models in order to investigate the formation of longitudinal off-corner depressions and subsurface cracks by Thomas.[12–14] The models illustrate the series of events leading to the depression: creation of a hot, thin region at the off-corner, ferrostatic
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