Statistical Study to Evaluate the Effect of Processing Variables on Shrinkage Incidence During Solidification of Nodular
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SOLIDIFICATION can be considered the most critical step when manufacturing nodular cast irons as final graphite distribution sets up during this transformation, and a significant part of their physical properties is defined by this distribution.[1] Moreover, eutectic growth of graphite leads to an expansion upon solidification of lamellar and nodular graphite irons that could be expected to compensate for contraction due to liquid cooling and crystallization of austenite. Recent experimental investigations[2–4] and earlier works listed in the exhaustive review of Hecht[5] confirm such volume changes during cooling and solidification of small size samples. At the scale of the castings, however, an overall contraction is generally observed that leads to attach risers to the parts in which voids may possibly develop due to liquid flow for compensating early contraction of the metal. According to Hecht’s review,[5] shrinkage observed in cast iron castings may be of three types (in italics the terms that will be used in the present work): –
Primary shrinkage,[6] concentrated macro-shrinkage,[7] or simply macro-shrinkage[8] are large voids that form in risers or in the upper part or in the thermal center of badly designed pieces;
J.M. GUTIE´RREZ, A. NATXIONDO, J. NIEVES, A. ZABALA, and J. SERTUCHA are with the Ingenieri´a, I+D y Procesos Metalu´rgicos, IK4-AZTERLAN, Aliendalde auzunea 6, 48200 Durango, Bizkaia, Spain. Contact e-mail: [email protected] Manuscript submitted August 14, 2016. Article published online January 30, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
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Dispersed macro-shrinkage,[7] dispersed microshrinkage or simply micro-shrinkage,[8] secondary shrinkage,[6] apparent or false shrinkage, are interdendritic voids with a millimeter size found generally in the thermal center of castings; Dispersed micro-porosity that can be detected only under a microscope after polishing a section of the casting.
For given casting conditions and alloy composition, the overall shrinkage is much lower in lamellar graphite irons than in nodular graphite irons. According to Hecht’s review,[5] this has first been related to the fact that the mushy zone is much larger in the case of nodular irons than for lamellar irons, but the difference in the growth mechanism of these structures has been also invoked. Growth of lamellar graphite iron proceeds with both graphite and austenite in contact with the liquid, while graphite is surrounded by austenite during bulk eutectic transformation of nodular irons. Following Heine,[9] the carbon content in austenite could thus be lower in lamellar irons than in nodular irons, and thus the amount of graphite precipitating would be slightly higher. According to The´ret and Lesoult,[10,11] the slight expansion upon solidification of lamellar irons leads to pushing of the liquid outwards thus decreasing the macro-shrinkage. On the contrary, graphite precipitation in nodular iron leads to an expansion of the piece as soon as a continuous solid network has built-up,
VOLUME 48A, APRIL 2017—18
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