Active Mg Estimation Using Thermal Analysis: A Rapid Method to Control Nodularity in Ductile Cast Iron Production
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HOUGH graphite is the most common crystalline phase of solid carbon, its nucleation and growth during production of ductile cast irons (SGI) is a complex process. Processing variables as carbon equivalent, inoculation, and Mg content strongly affect the shape and distribution of graphite particles, though the amount and nature of other minor alloying elements also play an important role.[1–7] Regarding graphite nucleation, most of the small inclusions found in ductile irons are compounds related to Mg treatment and inoculation (e.g., MgS, CaS, MgOÆSiO2, and 2MgOÆ SiO2) which may act as nucleation sites.[8] However, particles with similar composition have been found both inside and outside the nodules showing that not all of these compounds become nucleation sites for graphite particles.[9] This is possibly because some of them do not fulfill the requirements to be efficient nuclei[10] and certainly because very little of the nucleation sites are effectively activated as shown for aluminum alloys.[11] RAMON SUA´REZ, Manager R&D of Metallurgical Processes, is with Ingenierı´ a, I+D y Procesos de Fundicio´n, IK4-Azterlan, 48200, Durango, Bizkaia, Spain, and also with Veigalan Estudio 2010, 48200, Durango (Bizkaia), Spain. Contact e-mail: [email protected] JON SERTUCHA and PELLO LARRAN˜AGA, Project Leaders, are with Ingenierı´ a, I+D y Procesos de Fundicio´n, IK4-Azterlan. JACQUES LACAZE, Senior Scientist, is with CIRIMAT, Universite´ de Toulouse, 31030, Toulouse Cedex 4, France. Manuscript submitted February 2, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
This blocking of the grain refiner action has been rationalized in the case of aluminum and magnesium alloys,[12–14] and this certainly applies to cast irons as well. Although a definitive understanding about graphite growth is not available at present, the effective control of Mg, O, and S in cast iron alloys is well known to be critical for ensuring the formation of correct nodules. As a matter of fact, Mg is the most employed element among known nodularizers (Ca, Mg, Ce, La), and it is commonly added by means of FeSiMg master alloys. As Mg easily combines with S and O, raw materials with low S contents are required to minimize costs due to Mg consumption during nodularization treatment. Also, it is accepted that the comparative high C and Si contents usually found in cast irons drastically reduce the amount of available O dissolved in the melt.[15,16] In foundry plants, determination of the S content in cast irons is made by optical emission spectroscopy or combustion analysis, while measurement of the O content needs techniques that are usually not available in foundries. The common technique used for determining Mg content in normal production is also optical emission spectroscopy, though control of Mg content in cast parts can also be occasionally made by induction coupled plasma technique. However, both methodologies give the so-called ‘‘residual’’ or total Mg content, while it is well known that only the not combined or free Mg acts for obtaining rounded graphit
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