No-Bake S-Containing Mold/DI Metal Interactions: Consequences and Potential Applications
The ductile iron liquid metal is very reactive with environmental gases and solid molds after a nodulizing treatment. Mold-metal reactions could alter the spheroidal graphite shape to vermicular or flake. In this research, the metal-mold reactions were enhanced by various sulfur additions to the no-bake mold inserts to study these phenomena and to artificially produce layered dual-graphite structure in the casting. Thermodynamic and solidification modeling together with sets of different experimental techniques were applied to a test casting with internal core passages representing the cylinder head of a diesel engine. The effects of insert surface shape and location in the mold; content and type of sulfur-containing agent; coating; pouring temperature and additional chilling were experimentally studied and discussed. These results could be used to better understand the metal-mold interaction, prevent surface defects and design dual-graphite structure processes.
Thermo-chemistry of Non-metallic Inclusions in DI
Non-metallic inclusions formed during multi-stage ductile iron melt treatment play an important role in solidification, structure, and casting quality. Professor Carl Loper made fundamental studies on the role of inclusions in heterogeneous nucleation of graphite. In this article, a comparison of inclusions in steel and cast iron was done using thermodynamic calculations (FACTSAGE software) and automated SEM/EDS inclusion analysis (ASPEX system). Statistics of non-metallic inclusions (size, shape, composition) were studied in the iron and steel samples collected by quenching after different stages of melt treatment as well as from castings. The suggested methodology of inclusion analysis has the potential to be applied together with adaptive thermal analysis (ATAS) for solving practical problems such as decreasing shrinkage defects in ductile iron. INTRODUCTION This article was specially written for the Carl Loper Cast Iron Symposium. Professor Loper has been doing research and industrial development in metal casting during the last five decades. His scientific interests cover the spectrum of different cast alloys with some of his most significant contributions achieved in the areas of: cast iron, casting solidification, ductile iron (DI), and inoculation. Table 1 presents the number of his published and refereed articles according to Cambridge Scientific Abstract and METADEX data bases, an indication of his prolific writing abilities, outstanding record of contribution and the high regard of that contribution in the industrial and scientific communities. Table 1. Published and refereed articles of Dr. Loper according Cambridge Scientific Abstract and METADEX Topic Cambridge Scientific Abstract (METADEX) Published Refereed Cast iron 200 (182) 313 (285) Solidification 111 (93) 208 (174) Ductile iron 65 (62) 122 (118) Inoculation 33 (33) 53 (53) Ductile iron (DI) treatment dramatically changes the structure and the properties of castings. Deep refining of the melt by Mg transforms the flake graphite to spheroidal graphite. Because liquid iron treated by magnesium has a tendency to undercool resulting in meta-stable cementite formation, DI treatment also includes inoculation by FeSi(X)(where X is Al, Ca, Ba, Sr) additives which create additional graphite nuclei, resulting in a carbide-free structure even in thin wall castings. Therefore, the industrial DI treatment typically consists of two stages – nodulization and inoculation. These processes were studied by C. Loper and coauthors 1-4 as well as other authors 5-7 from different perspectives. In this article, computational-thermodynamic and experimental methods were used to study refining reactions in liquid iron and steel. Non-metallic inclusions formed during these refining reactions can influence heterogeneous graphite nucleation. This paper introduces a method to statistically evaluate the inclusion population in DI, a novel application of the automated inclusion analyzer. PROCEDURE Traditional thermodynamics can be used to determine the equilibrium conditions of separate individual reactions and is often used for analysis of the refining processes in cast irons. However, in actual liquid metal processing parallel reactions take place. The same additive can react with more than one impurity and, at the same time, one impurity can interact with different active additives. The possible changes in the melt composition from the initial stage to the equilibrium state is critical in many practical cases. To get around these problems, computer simulation, based on the minimization of the Gibbs free energy, was applied. This allowed the parallel reactions to be simulated step-by step as the amount of additive was increased. Simulations of the reactions of nodulizing and inoculation additives were carried out using FACTSage for the complex additive compositions and the amounts which are used in industrial practice. For experimental verification of computed refining reactions, specimens of iron-carbon-silicon (3.7 wt.% C, 1.8 wt.% Si) alloys were prepared from pure (99.99 wt.%) charge materials held in alumina crucibles (200 gram size) in a furnace having silicon carbide heating elements under an argon atmosphere. These alloys contained two levels of sulfur content, the first one was a relatively pure alloy with less than 0.005 wt.% S and the second alloy had 0.04 wt.% S. The master alloys used for refining the iron melts were Cu-15 wt.% Ca, Fe-50 wt.% Ce, and Ni-10 wt.% Mg. After the liquid iron treatment, the oxygen activity was measured using a solid electrolyte (zirconium oxide stabilized by yttrium oxide) probe with Mo-MoO2 reference electrode. In addition, DI heats were melted in a 100 lb induction furnace using pig iron and steel stampings. A two step DI treatment included pre-treatment by submerging 0.1 wt. % Ca-La mishmetal in the IF followed by 1.8 wt.% nodulizer (Fe-6%wt.%Mg55 wt.%Si) and 0.2% inoculant (Fe-75wt.%Si) in the ladle. Samples were collected directly from the melt using submerged core samplers with two internal chill plates as well as from the 20 mm casting wall. The total oxygen content, which is present either dissolved in the matrix as "free" oxygen or as oxygen in inclusions, was measured using a Leco TC-500. Non-metallic inclusions were studied using an automated inclusion analyzer system, the Aspex PICA-1020. The automated inclusion analysis is described elsewhere 8 . Briefly, in the Aspex SEM system, a focused electron beam is moved across the specimen in an array of fairly coarse steps, as shown in Fig. 1a. The field is subdivided into smaller fields (Fig. 1b). As the electron beam moves across each field, the brightness or intensity of the back scattered electron detector (BSED) signal is recorded. If the signal is bright (or dark) enough to indicate that an inclusion is present at the position, the software initiates a particle-sizing sequence using a rotating chord algorithm (Fig. 1c). This is a fast process because the instrument only spends time collecting detailed EDS data where inclusions are known to be present. Subsequently, number, size, and shape parameters are computed from the lengths of the chords. After the sample is completely analyzed, the data is evaluated using automated feature analysis software. The inclusions were classified into various classes based on their composition as determined by user-defined rules. (a) (b) (c) Fig. 1 Automated inclusion analysis including a) subdividing the image into fields, b) moving electron beam across field in an array and c) sizing and cataloging of particles detected by back scattering electrons and centering the beam on each particle and obtaining composition by x-ray spectroscopy. 8 THERMODYNAMICS OF REFINING REACTIONS IN LIQUID STEEL VERSUS IRON CALCIUM In liquid low carbon steel (1873K), the calculated sequence of reactions of Ca additions with sulfur and oxygen are depicted in Fig. 2a. Small additions of calcium react first with oxygen, and only after the addition of larger amounts of calcium will it react with sulfur. The amount of calcium required for deep refining depends on the initial [O] and [S] contents in the steel melt. In the case of liquid cast iron (3.7 wt.% C and 1.8 wt.% Si at 1673K), Si and C change the sulfur and oxygen activities and so the sequence of their reactions with Ca is reversed (Fig. 2b). The extent of deoxidization is limited by the CaC2 formation in the liquid cast iron. Solid, dashed and dotted lines show the calculated interactions of Ca for the different initial concentrations of impurities in the melts. The oxygen activities [a[O]] following Ca additions in the liquid cast iron having different initial sulfur contents were measured for experimental confirmation of the calculated sequences of the refining reactions (Fig. 3). The increase of Ca additives was proportional to the decrease in the oxygen activities in the high purity Fe-C-Si irons melted in an argon atmosphere. While the initial melt contained 0.04 wt.% S, there was a negligible influence of small Ca additives on a[O]. The oxygen activity in the melt decreased only after desulfurization with larger amounts of the calcium additives. At the same time, an oxygen activity smaller than 1x10 -4 was not reached. The shape of flake graphite in the cast iron treated by calcium did not change. These experiments confirmed the calculated prediction of the sequences of the refining reactions of Ca in the liquid cast iron. a) b) Fig. 2 (a) The calculated interactions of the Ca with [S] and [O] in steel at 1873K and (b) in cast iron at 1673K. Fig. 3 Influence of Ca addition on a[O] and final sulfur contents in Fe-C-Si alloys with different initial sulfur contents. MAGNESIUM The calculated data showed that a small amount of added magnesium (Mg a ) produced significant deoxidation of the cast iron melts with different initial S levels (Fig. 4a). The two parallel refining reactions took place when Mg a was increased. Unlike Ca, the Mg a reaction with oxygen in the liquid cast iron is not limited by carbide formation. It is generally accepted that 0.02 0.03 wt.% free (dissolved in the melt) Mg f is necessary for transformation of the flake graphite to spherical graphite. The oxygen [O] DI in equilibrium with this Mg f is shown in Fig. 4b by a dashed line. This calculated sequence of the reactions in the magnesium treated cast irons was experimentally confirmed (Fig. 4b). A small amount of Mg a dramatically decreased [aO] in all irons while the reaction with [S] forced deoxidation to occur later in the cast irons having larger initial sulfur contents. The remaining concentration of [S] and [aO] decreased at the same time. After that, when Mg a was sufficient to react with [S] and decrease the [aO] to a value less than 8x10 -5 wt.%, the flake shape of graphite transformed to the spherical shape. The correlation between the measured [Mg f ] and [aO] in the melts, and the resulting graphite structure is sh
Right coronary artery stenosis associated with tricuspid valve ring annuloplasty
A 70-year-old man with severe symptomatic functional mitral valve regurgitation underwent successful mitral valve repair combined with tricuspid valve ring annuloplasty. Pre-operative coronary angiography (CAG) showed no significant stenosis. One-and-a-half years later, the patient presented with an acute exacerbation of heart failure. Repeat CAG demonstrated tight stenosis in the right coronary artery (RCA) with arterial kinking that corresponded to the same point as the stenosis where the RCA is the closest to the tricuspid valve ring. The new lesion probably occurred as a consequence of the tricuspid valve ring annuloplasty.