In Situ Measurement and Prediction of Stresses and Strains During Casting of Steel
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DURING casting of steel, mechanically and thermally induced stresses are the source of several problems that continue to plague industry. Mechanically induced stresses are created when the steel contacts another part of the casting system (e.g., a core in shape casting, the mold or rolls in a continuous casting machine), while thermally induced stresses are created by uneven cooling. The stresses generate distortions, which in turn lead to dimensional inaccuracies and defects in the as-cast product. For example, if distortions occur near the end of solidification, hot tears may form, which necessitate the casting to be scrapped. In continuous casting processes, the strand must be carefully cooled to avoid cracking. In shape casting, distortions can lead to a lengthy trial-and-error process of modifying pattern allowances to meet dimensional requirements. Hence, the ability to accurately predict stresses for steel casting can lead to more efficient processes and higher quality cast products. The complexities associated with a casting process (i.e., multi-physics constitutive laws, thermo-mechanical coupling, three-dimensional geometries) provide considerable challenges to efficient and accurate stress modeling. In recent years, however, computational advancements have spurred the development of complex casting deformation models to better predict stresses
DANIEL GALLES, Graduate Research Assistant, and CHRISTOPH BECKERMANN, Professor, are with the Department of Mechanical and Industrial Engineering, University of Iowa, Iowa City, IA 52242. Contact e-mail: [email protected] Manuscript submitted February 10, 2015. Article published online November 16, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
and strains in steel castings. For such modeling, realistic mechanical properties of the steel are needed for the elasto–visco–plastic constitutive law used in a stress analysis. These temperature dependent mechanical properties are normally determined using high-temperature stress–strain data acquired from either tensile[1–3] or creep[4] tests that are performed using reheated steel specimens. Wray[3] comprehensively characterized the mechanical behavior of austenite throughout a range of temperatures [1123 K to 1523 K (850 C to 1250 C)], carbon contents (0.005 to 1.54 pct), and strain rates (6 9 106 to 2 9 102 1/s). Suzuki et al.[4] performed a series of creep tests on austenite at different stress levels (4.1 to 9.8 MPa) from 1523 K to 1673 K (1250 C to 1400 C); the results of the tests were fitted to a time hardening equation. Such tests have provided the crucial data needed to develop constitutive laws for stress modeling in steel castings.[5,6] Anand[5] used the measurements of Wray[2] to determine the parameters of a viscoplastic model for the austenite phase of low-carbon steel. Utilizing the data of Wray[3] and Suzuki et al.,[4] Kozlowski et al.[6] developed four constitutive relations to model the time dependent deformation behavior of austenite; Model III was found to be the best compromise, based on
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