Some paradoxical observations about spheroidal graphite degeneracy

Lacaze, Jacques; Sertucha, Jon

doi:10.1007/s41230-018-8166-3

Cited by 9 publications

(4 citation statements)

References 16 publications

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“…This is in fact what is observed in Fig. 3 where are compared the microstructures of a compacted graphite and of a lamellar graphite cast irons that have been solidified in the same conditions (thermal cup) [34] . It is seen that the graphite lamella are much coarser and much less branched in compacted graphite than in lamellar graphite material.…”

Section: Growth Ratesupporting

confidence: 76%

Effects of impurities on graphite shape during solidification of spheroidal graphite cast ions

2019

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This is an author's version published in: http://oatao.univ-toulouse.fr/25345 To cite this version:Lacaze, Jacques and Connétable, Damien and Castro-Román, Manuel Jesus Effects of impurities on graphite shape during solidification of spheroidal graphite cast ions. (2019) Materialia, 8. 100471. ARTICLE INFO ABSTRACT Keyword.s: Cast iron Solidification microstructure Graphite morphology Graphite degeneracy DFTSince the discoveiy that magnesium and cerium (and more generally rare earths) added at low level to cast iron melts lead to spherodized graphite, it is lmown that some other elements are detrimental even when present as traces. In ail practicality, it bas soon been recogniz.ed that adding rare earths to the melt helps counteracting the effect of these detrimental elements. Accordingly, only few works have been devoted to studying the effect of trace elements in melts without any rare earths. This is the first aim of the present work to review those studies as they contain the material to understand the mechanism for spheroidal graphite degeneracy. From this review, three types of degeneracy have been defined which show up when the critical level of any particular element is exceeded. These results are then discussed to show that ail degeneracies certainly proceed in the same way. To substantiate this discussion, the growth of compacted graphite as obtained by low level treatrnent of cast iron melt with magnesium is also presented. Finally, a mechanism is suggested for describing the action of trace elements on spheroidal graphite degeneracy. This mechanism is partly substantiated by first principles calculations which showed that ail elements can strongly adsorb on the prismatic planes which are the planes on which carbon atorns add on during graphite growth. • Corresponding author. E-mail address: jacques.lacaze@ensiacet.fr (J. Lacaze). scription of the impurity effect without RE addition, i.e. concerned with describing and understanding this effect The impurities listed in the patent [4] are As, Bi, Pb, Sb, Se, Sn and Te, and it is worth stressing that most of them are also harmful to non spheroidized lamellar graphite iron melts as reviewed by Rey naud [7]. In an extensive study aimed at controlling the charges for melt preparation, Thielemann [8] proposed the following quality index S b for spheroidal graphite cast irons where Al and Ti appear but not Se and Te which were not considered: S b = 1.6 • w Al+ 2. 0 · w A s + 3 70 · We; + 2 90 · WPb + 5 . 0 · Wsb +2.3 . Wsn +4.4 . WTi (1) in which W; is the weight fraction of i element in the charge used for preparing the melt This quality index has been obtained with spheroidal graphite cast irons containing 0.04 to 0.08 wt. % Mg that were cast with wall thickness in between 8 and 45 mm. When S., is lower than 1 no action is necessary while if it is higher RE should be added to avoid spheroidal graphite degeneracy.What is seen in the Thielemann's factor and which reflects foundry practic e is that bismuth and lead have a much higher coefficient than the other ...

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Section: Growth Ratesupporting

confidence: 76%

Effects of impurities on graphite shape during solidification of spheroidal graphite cast ions

2019

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“…(a) (b) It has been reported previously that the growth rate of compacted graphite cells is comparable though slower than that of lamellar graphite cells [23]. That it is slower at the start of growth may be in relation with the low capability of compacted graphite to branch [24], and thus to the increased distance between graphite worm-like flakes as compared to the flakes in LGI. At the end of solidification, the slowing down of the growth rate of the compacted graphite cells could possibly be related to the accumulation of magnesium rejected by austenite, which has been suggested to leading to austenite closing up around graphite precipitates [25].…”

Section: Discussionmentioning

confidence: 90%

Microstructure Changes During Solidification of Cast Irons: Effect of Chemical Composition and Inoculation on Competitive Spheroidal and Compacted Graphite Growth

et al. 2019

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Amongst the most important graphite shapes, nodules and compacted particles are of particular interest as they can coexist in castings with relevant changes in properties. In this context, it has been often reported that the compacted shape is an intermediate form between nodules and graphite lamellas that may be seen as a degeneracy from the nodular one. The present work shows the microstructure evolution of an initially ductile iron alloy with a silicon content of about 2.4 wt.% when reducing progressively the magnesium content by holding a large melt batch in a nitrogen-pressurized pouring unit for 8 hours. Thermal cups with and without inoculant were cast at a regular time interval together with a sample for chemical analysis. Interestingly, the thermal records of the inoculated samples show no significant changes with time while the structure evolved from fully spheroidal to half spheroidal half compacted graphite. Conversely, the thermal curves of the non-inoculated samples showed two arrests, one at nearly the same temperature as for inoculated alloys and a second one at a temperature decreasing with holding time until being below the metastable eutectic temperature. Microstructure observations showed the presence of a limited number of compacted cells which decreases as well with holding time. These observations suggest that these cells start developing during the temperature interval between the first and second arrests, leading to a bulk eutectic transformation either above or below the metastable eutectic temperature. These results support the view that a fully compacted structure can be obtained only with a controlled inoculation which should not be too high to avoid too high nodularity.

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“…As a main outcome, we identified a microstructural transition between decoupled-and coupled-growth as a function of the solidification velocity. Qualitative features that 1274 (2023) 012033 IOP Publishing doi:10.1088/1757-899X/1274/1/012033 2 were thus revealed in a model transparent system shed new light on previous observations in metallic irregular-eutectic alloys, such as austenite/graphite eutectic in cast irons [4] and Ni-Ni 3 Sn 4 alloy [5]. In this work, we present observations of the growth dynamics in thin AMPD-SCN samples during rotating directional solidification (RDS) experiments.…”

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confidence: 76%

In-situ investigation of the solidification dynamics in an irregular eutectic alloy

Mohagheghi

Bottin-Rousseau²,

Şerefoğlu³

2023

IOP Conf. Ser.: Mater. Sci. Eng.

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We present an in-situ experimental investigation of irregular eutectic growth dynamics observed optically in real time during directional solidification (DS) and rotating directional solidification (RDS) of a model transparent alloy, namely Amino-methyl-propanediol-Succinonitrile (AMPD-SCN). The SCN-rich solid grows with a nonfaceted solid-liquid interface, while AMPD forms thin whisker-like faceted crystals. With RDS, we observed simultaneously four distinct growth dynamics along the solidification front thanks to the rotation induced velocity ramp. At low velocity, AMPD crystals grow individually, with their leading tip ahead of – at a higher temperature than – the SCN-liquid interface which presents a linear undercooling change with V ramp. This regime is defined as a quasi-steady decoupled growth. During RDS, AMPD crystals are found to rotate with the sample in this regime. When they can reach the unsteady coupled-growth regime at higher V, since increasing V favors the formation of AMPD-SCN-liquid trijunctions, they start to branch noncrystallographically. At higher velocity, impurity effects lead to the formation of two-phase fingers. Further increase in the velocity results in the formation of primary SCN dendrites and a fine interdendritic eutectic microstructure. DS experiments confirmed the existence of those regimes at different velocities.

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Some paradoxical observations about spheroidal graphite degeneracy

Cited by 9 publications

References 16 publications

Effects of impurities on graphite shape during solidification of spheroidal graphite cast ions

Effects of impurities on graphite shape during solidification of spheroidal graphite cast ions

Microstructure Changes During Solidification of Cast Irons: Effect of Chemical Composition and Inoculation on Competitive Spheroidal and Compacted Graphite Growth

In-situ investigation of the solidification dynamics in an irregular eutectic alloy

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