Comparing Three Equations Used for Modeling the Tensile Flow Behavior of Compacted Graphite Cast Irons at Elevated Tempe
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rve contains two parts—one elastic part and one plastic part. From a simulation and modeling point of view, it is desirable to be able to approximate the flow stress values with equations instead of performing the actual tensile test to retrieve the data. Such an approximation would be less time and money consuming. The elastic part of a tensile test curve can be approximated easily using Hooke’s law, whereas MARTIN SELIN, Ph.D. Student, is with the School of Engineering, Materials and Manufacturing—Casting, Jo¨nko¨ping University, SE-551 11 Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] Manuscript submitted March 16, 2010. Article published online July 7, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
the plastic part is a bit harder to approximate. Some constituent relationships are used to approximate the plastic deformation, for example, Hollomon,[1] Ludwik,[2] Ludwigson,[3] Voce,[4] and Swift.[5] The problem with the approximation of the plastic part is to get a valid approximation for all regions of the curve (i.e., for small plastic strains as well as for near fracture strains). These constituent relationships are general and valid for most types of materials; however, some are suited better for certain materials. Reasons for the difference in validity might be as follows: the material is brittle, there is no strain hardening, or the material forms a necking before fracture. Not much work has been done in applying constituent relationships to the field of cast irons[6–8]; hence, there is an interest in investigating how these equations are applicable to cast irons. This work VOLUME 41A, NOVEMBER 2010—2805
aims to investigate how well the Hollomon, Ludwigson, and Voce equations could approximate the flow stress values for compacted graphite cast irons (CGI) for temperatures between room temperature and 873 K (600 °C). Investigating how these equations are affected by temperature is of great importance to get accurate stress simulations of components that are subjected to elevated temperatures.
II.
EXPERIMENTAL PROCEDURE
A. Material The investigated compacted graphite irons were alloyed with four different amounts of molybdenum according to Table I. The chemical composition was determined by optical emission spectroscopy equipment, ARL 3460 from Thermo Scientific (Suwanee, GA), and the graphite morphology was controlled using the SinterCast process from SinterCast (London, UK).[9] To achieve three different solidification rates, the melts were cast in furan sand molds shaped with the following three different sized cylinders 85 mm, 55 mm, and 20 mm in diameter. The casting took place in an industrial foundry and six molds were cast for each melt. The rate of solidification was not measured, but the highest, intermediate, and lowest rates were denoted A, B and C, respectively. B. Microstructure Compacted graphite particles were classified according to the International Organization for Standardization (ISO 16112:2006) taking into account nodularity and a roundness shape factor (RSF). The RSF was defined as
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