Article

Active Mg Estimation Using Thermal Analysis: A Rapid Method to Control Nodularity in Ductile Cast Iron Production

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  • Fundación Azterlan, Technological Centre
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Abstract

Appropriate nodularity in ductile iron castings is strongly associated with the presence of high enough not combined Mg dissolved in the melt to cast. However, the residual Mg which is commonly measured for production control accounts for both dissolved Mg and Mg combined as oxides and sulfides. To account for the uncertainties associated with such a control, it is quite usual to over treat the melt with the risk of porosity appearance. A new methodology based on thermal analysis has been developed in the present work so as to estimate the amount of free Mg dissolved in the melt ready for pouring. A combination of Te mixture and a new “reactive mixture” composed of sulfur plus a commercial inoculant has been prepared for this purpose. This reactive mixture is able to transform the magnesium remaining dissolved in the melt to combined forms of this element. Experiments performed both during start of production (when Mg overtreatment is usual) and during normal mass production indicate that important variations of free Mg occur without relevant changes in residual Mg content as determined by spectrometry. The method developed in the present work has shown to be highly effective to detect those melt batches where active Mg content is not high enough for guaranteeing a correct nodularity of castings. Selection of proper active Mg thresholds and a correct inoculation process are critical to avoid “false”-negative results when using this new method.

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... wt.% residual magnesium, with 0.02-0.03 wt.% free magnesium left dissolved in the melt [49,52]. In case of over-treatment, namely for 0.10 wt.% Mg and over, spheroidal graphite degeneracy is observed [8,51] that is much alike that obtained with antimony, see figure 6-b. ...
... When magnesium is added to cast iron melts, it first tights oxygen and sulphur, and the remaining which is called free-magnesium may adsorb freely on graphite [25,52]. While the overall or apparent growth direction of spheroidal graphite is along the basal plane direction, the growth mechanism involves carbon atoms attaching to the prismatic planes as previously discussed [56]. ...
Article
Since the discovery that magnesium and cerium (and more generally rare earths) added at low level to cast iron melts lead to spherodized graphite, it is known that some other elements are detrimental even when present as traces. In all practicality, it has soon been recognized 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 all degeneracies certainly proceed in the same way. To substantiate this discussion, the growth of compacted graphite as obtained by low level treatment 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 all elements can strongly adsorb on the prismatic planes which are the planes on which carbon atoms add on during graphite growth.
... The purpose of adding FeS2 into one chamber of the sample cup is to consume the residual magnesium in the molten iron by increasing the sulfur content. With the decrease in residual magnesium content, the graphite will change from spherical to vermicular and then to flaked, as shown in Figure 3 [24]. The cooling curves will also change accordingly. ...
Article
Full-text available
In the production of vermicular graphite cast iron, the allowable range of residual magnesium content in molten iron after treatment is very narrow, amounting to only 0.008%. Therefore, thermal analysis technology was used to quickly evaluate the vermiculation and inoculation level of molten iron at the furnace itself, thus allowing the molten iron to be adjusted in time. The additives in the sample cups play a crucial role in obtaining cooling curves with remarkable characteristics. In this study, either FeS2 or FeSi75 additives were added to one chamber of a double-chamber sample cup made of resin sand, in which the cavities of the double chambers were spherical with diameters of 30 mm. The thermal analysis curves of molten iron in the double-chamber sample cup were acquired using a double channel temperature recorder, and the solidified spherical samples were analyzed quantitatively. The influence of FeS2 or FeSi75 additives on both the cooling curves of molten iron and the graphite morphology were investigated. The experiment’s results indicated that when 0.05% FeS2 is added to one chamber of the sample cup, the cooling curve changes to the solidification pattern of gray cast iron. The continuous increase in the FeS2 additive has little influence on the shape of cooling curves, and the graphite changes form from vermicular to flaked. When the amount of FeS2 is increased from 0.05% to 0.10%, the resulting graphite changes from D-type and E-type to A-type and B-type. When the amount of FeS2 reaches 0.20%, the morphology of graphite is short and thick. With the increase in the amount of FeSi75 additive, the amount of spherical graphite in the sample cup increases gradually, and the vermicularity decreases gradually from 89% to 46%. With the increase in FeSi75 additive from 0 to 0.45%, we observed that the average diameter of graphite decreases from 23 μm to 19 μm and then increases to 22 μm. The eutectic recalescence temperature shows a decreasing trend, and the cooling curve gradually changes from a hypoeutectic to a eutectic pattern. The addition of 0.05% FeS2 or 0.45% FeSi75 to one chamber is more appropriate for a double-chamber sample cup with two spherical cavities with diameters of 30 mm. This lays a foundation for the optimization of additives when using the double-chamber sample cup for thermal analysis of vermicular graphite cast iron.
... The amounts that appear in Fig. VII-8 are the total amounts of Mg or Ce in the casting, but the part that is active is the amount let free in solution in the melt and not tight in compounds. It has been demonstrated that the necessary level of free magnesium for spheroidisation is about 0.020 wt.% [SUA16] and also reported that as little as 0.001 wt.% of free Mg is enough to eliminate flake graphite in compacted graphite castings [DAW03]. Contrary to what is generally considered, solubility of magnesium in liquid iron is far from being negligible and in any case much higher than the above-mentioned limits for spheroidising, see opposite page. ...
Book
This monograph finds its foundation in a simple fact: there is a paradigm with cast irons, which is that these alloys are produced and cast to shape since thousands of years but are amongst the most complicated metallic alloys when considering the formation of their microstructure by solidification and solid-state transformations. In turn, this complexity opens a wide range of possibilities for shaping their microstructure and engineering their service properties. The first cast irons were mostly Fe-C alloys and as such solidified mainly in the metastable system, leading to hard and brittle parts that were heat treated for graphite precipitation to give malleable cast irons. The introduction of silicon into the melt increased the temperature difference between the stable and metastable systems, thus promoting the formation of graphite instead of cementite during solidification. This gave rise to the silicon cast irons that are the subject of this monograph. With the advent of metallographic observations, it was realized that cast iron also often contained phosphides related to the origin of iron ores. A good control of the metallic charge allowed to improving the mechanical properties, in particular by ensuring a minimum elongation before rupture under tensile stress. The essential step, however, was the discovery that it is possible to change the shape of graphite by transforming the interconnected lamellae into discrete spheroids. Cast irons thus became a material for safety parts and were no more restricted to construction. This historical evolution and the research effort during the first part of the 20th century are described in the vast review carried out by Merchant in the 1960s [MER68]. At that time, there was an explosion of research on cast irons with the aim of describing and understanding the formation of graphite during solidification and, to a lesser extent, during heat treatment. As far as solidification is concerned, the review by Lux [LUX70a, LUX70b] of this research effort is an important step that already contained most of the questions and provisional answers that are still referenced in more recent works [STE05]. It is worth mentioning here Zhou's comprehensive literature review on solidification of different types of cast iron [ZHO09, ZHO10, ZHO11]. This monograph is not intended to be an exhaustive review of the literature as those mentioned above, but rather to provide a coherent view of the formation of the microstructure of silicon cast irons. In fact, the authors felt it was very important to present how various aspects of microstructure formation could be related to each other using schemes based on known physical phenomena, and sometimes supported by ad hoc modelling. Consequently, the works that will be referenced first are those that contain information that has proven to be essential for the development of these schemes. Where appropriate, controversies will be mentioned but not discussed, with reference to the works where they are detailed. Instead, emphasis will be on open questions. The main text containing basic information and descriptions appears on odd-numbered pages, while details and more in-depth descriptions are limited to even-numbered pages. All references are listed at the end of the monograph, which also contains a glossary of acronyms and unusual terms and an index of the parameters used in the equations and the values employed for physical parameters. For more than 10 years, our work has certainly benefited from Azterlan's impetus and has greatly benefited from the dynamism of the European Cast Iron (ECI) group. The exchanges within this group, as well as the discussions and controversies that have taken place at its annual meetings have been renewed stimuli. We would like to thank the participants, both academics and industrialists, for their continued contribution to this group.
... Spheroidal graphite is effectively obtained when the final amount of dissolved magnesium is about 0.020-0.025 wt.% [2,3] while compacted graphite corresponds to a level of 0.008-0.017 wt.% [4] in casting sections lower than about 60-70 mm. ...
Article
Full-text available
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... Yet, what controls the graphite shape is the free Mg termed "active Mg". Suarez et al. [32] developed a method of Mg estimation by thermal analysis, which consists of the addition of a well-defined amount of Mg neutralizer mixture (22% S and 78% inoculant) in a Te-containing cup. While Te promotes white iron when gray iron is poured in a Te-containing cup (Fig. 10), no significant undercooling is observed with SG iron, probably because of a Mg-Te reaction. ...
Article
Full-text available
Since its first literature mention in conjunction with cast iron in 1931 by Esser and Lautenbusch, thermal analysis (TA) has journeyed a long way. Today it is an accepted and widely used tool for process control for all types of cast irons. This paper reviews the latest progress in the development of equipment and analysis methods that make TA successful in applications such as the estimation of chemical composition, graphitization potential, and the shape and number of graphite aggregates. The potential and limitations of the prediction of shrinkage defects propensity are analyzed in some details. Examples of attempts at prediction of mechanical properties and shrinkage propensity are also discussed. Several graphs showing the data scattering are presented to convey the reader a better sense of the accuracy of various predictions.
... 22 A further complication appears as Mg fades with time, thus needing strict control during processing. 19,[23][24][25] In solid state, during the eutectoid reaction, austenite transforms into pearlite, ferrite, or a mixture of these phases. 26 The chemical composition and cooling conditions control the fraction of these phases. ...
Article
Full-text available
This work investigates the role of primary austenite morphology on the eutectic and eutectoid microstructures and the ultimate tensile strength (UTS) in a hypoeutectic compacted graphite iron (CGI) alloy. The morphology of primary austenite is modified by isothermal coarsening experiments in which holding times up to 60 min are applied to the solid–liquid region after coherency. The cooling conditions for the subsequent eutectic and eutectoid reactions are similar. Miniaturized tensile tests are performed to evaluate the UTS. The morphological characteristics related to the surface area of primary austenite, the modulus of primary austenite, {M}_{\upgamma} , and the hydraulic diameter of the interdendritic region, DIDHyd {D}_{\text{ID}}^{\text{Hyd}} , increase with the cube root of coarsening time. The eutectic and eutectoid microstructures are not significantly affected by the morphology of primary austenite, thus indicating that the morphology of the interdendritic regions does not control the nucleation frequency and growth of eutectic cells or graphite. UTS decreases linearly with the increasing coarseness of primary austenite for similar eutectic and eutectoid microstructures, demonstrating the strong influence of primary austenite morphology on the UTS in hypoeutectic CGI alloys.
... The production window for CGI is narrow [35] , showing an abrupt transition to LGI [37] . Additionally, it is challenged by the fading of Mg over the time [37][38][39][40] which needs to be monitored [41] . ...
Article
The evolution of primary austenite morphology during isothermal coarsening has been studied in the three main Fe-C-Si alloys used in industry, LGI, CGI, and SGI. The dendritic microstructure increases length scale during coarsening accompanied by fragmentation and coalescence of austenite crystals. The morphological parameters, SDAS, M_γ, D_ID^hyd, and D_γ show a linear relation with the cube root of coarsening time, t1/3, with similar rates for the three different Fe-C-Si alloys. The eutectic microstructures after coarsening of primary austenite in CGI and SGI alloys are not significantly affected by the surface area of primary austenite and the size of the interdendritic regions. Fraction, nodularity, shape distribution of graphite particles and the number of nodules and eutectic cells are similar when studied as a function of coarsening time. These results suggest that the nucleation frequency in CGI and SGI, and the growth of eutectic microstructures in CGI, are not significantly influenced by the morphology of primary austenite.
Thesis
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Despite its significance in ensuring that compacted and ductile cast iron can consistently be manufactured, there are at least two reasons from the sustainability perspective that it is necessary to challenge the status quo in utilizing magnesium. Besides its energy and emission-intensive production process that builds up the whole carbon footprint of the cast iron product, magnesium is classified as a critical raw material by the EU. In this instance, developments in global raw material markets over the past two decades have revealed dependencies in the supply of magnesium products. Consequently, it has recently become evident that supply chain uncertainties result in competitiveness losses. Hence, this work will mainly focus on assessing the role of magnesium in ductile cast iron production, particularly during desulfurization and nodularization, before eventually identifying any possible sustainable alternative. By employing a chill casting approach, high-temperature interaction between magnesium and molten cast iron with different sulfur content can be observed without interference from possible mold material reactions. The results indicate that rapid dissolution, deoxidation, and desulfurization sequentially take place and are complete before the notable boiling process begins. As the reaction progresses, a superheating state at the interface could be expected due to further heat transfer and exothermic reactions related to solid MgO and MgS formation. Depending on the degree of superheating, the distinct mechanism of boiling will be favored, resulting in either liquid fragmentation or physical-chemical reactions at the crack on the pre-existing solid reaction products. Considering the behavior of magnesium during desulfurization, lime was investigated as an alternative that provides a similar result to cast iron. Both laboratory and industrial trials were conducted, concluding that lime is a reliable desulfurization agent for the cast iron industry. Moreover, the reaction mechanism was also explored to control the process optimally. The results indicate that aluminum, silicon, and iron are involved in establishing specific liquid slag systems that favor desulfurization. However, as the reaction continues, the solubility dynamic comes into play, and precipitation of solid calcium silicate is anticipated, thus retarding the further interaction between lime and sulfur. Nonetheless, the results demonstrate that desulfurization continues, allowing the required end-sulfur content to be achieved, in addition to competitive economic and ecological advantages. Another effect of magnesium incorporation associated with nodularization is the change in the graphite precipitation. By examining the development of graphite during the remelting of cast iron scrap with nodular graphite, it is confirmed that two factors are necessary to establish nodular graphite: low sulfur and oxygen concentrations, as well as a sufficient level of dissolved magnesium. It is worth mentioning that the role of irreplaceable magnesium is not only to react with surface-active elements but also to increase carbon activity in the liquid iron, thereby promoting graphite crystallization during solidification.
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The primary objective of this study is to explore the impact of temperature and magnesium addition on the partial oxygen potential in lamellar, compacted, and spheroidal cast irons during the cooling process. The oxygen potential is assessed in large‐scale plant trials with a 1000 kg scale. Thermodynamic calculations are conducted, and the results are compared with the experimental values. There is a reasonable agreement between the experimental and calculated values, facilitating the prediction of oxygen potential in temperature ranges where measurements are challenging. According to the thermodynamic calculations, it is observed that varying amounts of added magnesium result in the formation of different types of inclusions during cooling. This, in turn, influences the temperature dependency of the oxygen potential in the molten metal.
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The narrow production window for compacted graphite iron material (CGI) drastically reduces the possibilities to produce it in small batches outside an industrial environment. This fact hinders laboratory-scale investigations on CGI solidification. This work presents a solution to that issue by introducing an experimental technique to produce graphitic cast iron of the main three families. Samples of a base hypereutectic spheroidal graphite iron (SGI) were re-melted in a resistance furnace under Ar atmosphere. Varying the holding time at 1723 K (1450 °C), graphitic irons ranging from spheroidal to lamellar were produced. Characterization of the graphite morphology evolution, in terms of nodularity as a function of holding time, is presented. The nodularity decay for the SGI region suggests a linear correlation with the holding time. In the CGI region, nodularity deterioration shows a slower rate, concluding with the sudden appearance of lamellar graphite. The fading process of magnesium, showing agreement with previous researchers, is described by means of empirical relations as a function of holding time and nodularity. The results on nodularity fade and number of nodules per unit area fade suggest that both phenomena occur simultaneously during the fading process of magnesium.
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Book
Length-scale in solidification analysis.- Equilibrium and non-equilibrium during solidification.- Macro-scale phenomena - general equations.- Macro-mass transport.- Macro-energy transport.- Numerical Macro-modeling of solidification.- Micro-scale phenomena and interface dynamics.- Cellular and dendritic growth.- Eutectic solidification.- Peritectic solidification.- Monotectic solidification.- Microstructures obtained through rapid solidification.- Solidification in the presence of a third phase.- Numerical micro-modeling of solidification.- Atomic scale phenomena - Nucelation and growth.
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Being able to predict the grain formation process and attendant grain size has been a central topic in solidification. Such an analytical model is presented for constitutional supercooling (CS)-driven grain formation with several simplifications. The model links the nucleation of new grains to the growth of a larger neighbouring grain. The average grain size () is thus determined by two components: the minimum growth (rcs) necessary to establish sufficient CS (ΔTn) for nucleating new grains, and the spatial mean distance () to the most potent available nucleants. Both spherical and planar growth fronts are considered, covering growth curvatures from small to infinite. Two distinct fundamental approaches are used, which result in identical descriptions of , where (D is the diffusion coefficient, v is the growth velocity, Q is the growth restriction factor). The model is compared with literature data produced under various conditions and demonstrated on aluminium alloys as an example.
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A numerical model is presented for the prediction of grain size in inoculated castings and is tested against measured grain sizes obtained in standard grain-refiner tests on aluminium alloys. It is shown that for potent nucleants, such as commercial grain refiners for aluminium, the nucleation stage itself is not the controlling factor. The number of grains is determined by a free-growth condition in which a grain grows from a refiner particle at an undercooling inversely proportional to the particle diameter. With measured particle size distributions as input, the model makes quantitatively correct predictions for grain size and its variation with refiner addition level, cooling rate and melt composition. The model can assist in optimizing the use of existing refiners and in developing improved refiners.
  • Y Liu
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Azzam: 62nd World Foundry Congress
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L. Kozlov, A. Vorobiev, S. Azzam: 62nd World Foundry Congress, April 23-26, Philadelphia, Pennsylvania, 1996, paper 27.
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J. Lacaze, S. Armendariz, P. Larranãga, I. Asenjo, J. Sertucha, and R. Sua´rezSua´rez: Mat. Sci. Forum, 2010, vols. 636-637, pp. 523-30.
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J. Lacaze, N. Valle, K. Theuwissen, J. Sertucha, B. El Adib, L. Laffont: Adv. Mat. Sci. Eng., 2013, paper ID 638451.
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  • T Skaland
T. Skaland: Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, Schaumburg, 2005, pp. 13-30.
  • Y Li
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