Modeling of microstructure and residual stress in cast iron calender rolls
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I. INTRODUCTION
CAST iron “thermorolls” are used in the manufacture of paper for calendering and coating operations. These rolls are extreme in dimensions (reaching up to 1.5 m in diameter and 8 m in length) and are subject to difficult operating conditions including alternating mechanical loads, thermal stresses, and contact with abrasive paper. The proprietary casting procedure used to manufacture them is complex and has been optimized largely by trial and error. In the casting process, the microstructural distribution is one of the key features that has to be controlled to obtain the optimal balance between surface wear resistance and the ability to withstand mechanical loads. Moving from the outer diameter (OD) to the inner diameter (ID) of the roll shell, the microstructure consists of a chill layer of white iron, a transition region of mottled iron, and an inner zone of gray iron. Another important feature to be controlled during casting is the final distribution of residual stress, which has an important influence on the operating limits of the roll. In the present design, a residual state of compression is required at the surface to increase the limit of alternating stress that can be applied in service. The formation of residual stresses in cast iron calender rolls is complex, owing to the combined D. MAIJER, Research Associate, and S. COCKCROFT, Associate Professor, are with the Department of Metals and Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. A. JACOT, formerly with the Department of Metals and Materials Engineering, The University of British Columbia, Vancouver, BC, Canada, V6T 1Z4, is now Senior Scientist with Calcom SA, PSE-EPFL, CH-1015, Lausanne, Switzerland. Manuscript submitted June 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
influence of microstructural evolution and inelastic deformation. A mechanistic understanding of how these stresses arise in the casting process could possibly lead to improvements in the manufacturing process and service performance. Computer simulations provide one means by which the properties of products can be linked quantitatively to the manufacturing process. However, modeling the casting of thermorolls is challenging because of the numerous physical phenomena that occur simultaneously. These include transient transport of heat, liquid-to-solid and solid-to-solid phase transformations, differential thermal contraction and expansion associated with the different microstructure constituents, as well as elastic and inelastic deformation. Solutions to the heat-flow and thermal stress/strain problems can be obtained with commercial finite-element (FE) software in a relatively straightforward manner, provided that some simplifying assumptions hold and that the necessary boundary conditions and material behavior are known.[1] On the other hand, the prediction of microstructural formation in cast iron presently requires specialized models, since this aspect of the process is less understood. Approaches consisting of co
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