International PhD Foundry Conference

Transcription

1 Brno University of Technology Czech Foundrymen Society CFS International PhD Foundry Conference 3 rd June 2009 Optimizing the inoculation of a ductile cast iron using thermal analysis Alexis VAUCHERET Philippe JACQUET Frederic ROSSI Patrick LAGARDETTE Arts et Métiers ParisTech Cluny, LaBoMaP ECAM, Material Sciences Departement Fonderie Vénissieux SAS, Venissieux

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3 In the 1960s, the development of robust, reliable, inexpensive electronic recorders and the introduction of inexpensive Croning sand analysis cups led to a spurt of growth in the thermal analysis of cast iron. Most of the world s foundries acquired analysis systems of this type, which can determine the chemical composition of a cast iron and a few key temperatures as its cools. At the beginning of the 2000s, the remarkable technological progress in computing made possible a new type of thermal analysis. The thermal analysis software programs now on the market claim that they can determine - in addition to the key temperatures - the probable defects of the cast iron, its mechanical properties, and the number of nodules in a ductile cast iron, for example. The Arvin Meritor foundry in Vénissieux was one of the first foundries in France to acquire a system of this type; this article examines a part of the potential of this tool, applied to tracking the quality of a spheroidal graphite cast iron. After a few general theoretical reminders concerning thermal analysis, we will see how this technique is used in practice to measure the beneficial (or harmful) contributions of inoculants added during the preparation of an SG cast iron. 1. Principles of thermal analysis in iron casting. A thermal analysis curve (see Fig. 1, cooling curve), like its first derivative (see fig. 2, cooling rate) can be broken down into several parts, each defining a particular physical phenomenon: solidification of the first constituant, solidification of the eutectic, end of solidification, etc. In each domain, temperatures, temperature ranges or parameters revelant to the phenomena considered are then defined. The work of D. Sparkman [1] includes a good inventory of this various parameters. The bases of thermal analysis were described remarkably by M. Hecht at the beginning of the 1980s [2], then by M. Van Der Perre [3].The first studies in which the capabilities of this instrument were applied date from the end of the 1980s [4] with the determination in particular of the carbon equivalent. Later, there were studies in greater depth. Among them, for example, were the prediction of shrinkages cavaties [5], [6] or the visualization of the influence of the hold time on the evaporation of certain light elements [7]. With the help of thermal analysis, the effects of an addition (such as a recarburizer or an inoculant) can be visualized on a sample taken upstream of the casting part; this makes it possible to learn about the consequences (beneficial or harmful) of these substances on the metallurgy whithout waiting for the end product. Whithout thermal analysis, this impact is revealed after the fact, very often by a destructive examination, which can add significantly to costs [8]. Rec T Emin T Emax T sol PAE Temperature ( C) dt( C) /dt VPS Time (s) Time (s) D1 Tsol Fig.1 Cooling curve Fig.2 Cooling rate 3 / 9

4 2. Parameters and ranges of variation Thermal analysis records and determines many parameters. This study focuses on nine parameters from Proservice s ITACA software that are shown in Fig. 1 and 2 and defined in the section below. We will also describe their influence on the quality of the metal and indicate for each an optimum range of variation, based on our experience. T Emin: minimum temperature of the eutectic reaction. This provides information about the suitability of the cast iron to form carbides. In particular, it can be used to make sure of the effectiveness of an inoculant. The desired value is T Emin>1140 C T sol: solidification temperature. This is the temperature at which solidification ends; it is determined with maximum precision on the cooling rate curve (see Fig. 2) T Emax: maximum temperature of the eutectic reaction; this is the maximum temperature recorded in the eutectic range. VPS: angle formed by the cooling rate curve at the solidus. It indicates the thermal conductivity of the cast iron at the end of solidification, which can be correlated whith the formation of porosity. The desired interval ranges from 25 to 45. PAE: length of the eutectic range in second (time between T Emax and T sol). It tells us about the growth of the eutectic cells and therefore about how liable the cast iron is to form shrinkage cavities. The eutectic range should be rather long to guard against the risk of shrinkage cavities and so the PAE should be between 110 and 125 seconds. Rec: this is the recalescence. It is defined as the difference between the maximum and the minimum temperatures of the eutectic reaction: Rec = T Emax-T Emin. The recalescence is directly related to the graphitic expansion (thrust on the mould) and therefore its value influences the shrinkage cavities prediction. The recalescence should be between 2 and 6 C. D1 Tsol: the cooling rate at the solidus. Like VPS, it reflects the thermal conductivity of cast iron. It too therefore tlls us something about the possible formation of porosity. It is essential to have D1 Tsol<-3 C/s Ceq (Carbon equivalent): it is defined by the relation Ceq = C + 1 / 4 %Si + 1 / 2 %P (Eq. 1). It tells us something about the composition of the cast iron. Inoculation index: it is related to the reduction of undercooling by inoculation. It is the ratio between the undercoolingwhitout and whith inoculation, where the undercooling is defined by the relation: T = T Egrise-T Emin whith T Egrise = %Si.The inoculant must increase T Emin; the goal is an inoculation index between 1.5 and Test of repeatability The first stage of our approach was to make sure of the repeatability of the measurement provied by the thermal analysis software used. These tests are necessary because they serve to determine the precision and the variability of the measurements. If the measurements are not repeatable, no study is possible because it would be impossible to ascribe the observed variations to the effectviness of the products tested. For this purpose, we used the four acquisition lines available with the software. We cast the same cast iron from the same ladle in the cup of each acquisition line. We repeated this test three times. For a repetition, we averaged the values of each line to obtain the mean and the standard deviation. Then, we compared the mean and the standard deviation obtained for each repetition to know the maximum of dispersion of our measurement system. The results obtained for a repetition are given in the table below: Parameters Test 1 Test 2 Test 3 Test 4 Mean Standard deviation T Emin ( C) Recalescence ( C) PAE (s) VPS ( of arc) Table 1 Assessment of the repeatability tests For the temperature measurements, the standard deviation is about 0.3 C, which correspond to the measurements uncertainty of the manufacturer of the cup-thermocouple assembly. The results are therefore perfectly reproducible at this level. For parameters PAE and VPS, the standard deviation is larger, of the order of 3 to 4 points. This difference is explained by the fact that these parmeters are more difficult to determine, because they are not well-defined single value but 4 / 9

5 combinations of several values. Moreover, we do not yet have any clear idea of how to influence the precision of these parameters, and so we decided for the moment to treat them as acceptable and reproducible. 4. Qualitative study of various inoculants This first stage consisted in testing 6 different inoculants, with the goal of testing each inoculant which each of the recarburizers used in our foundry, defining the best configurations for each inoculant, and most partucularly identifying two inoculants of which the yield is goog for all the recarburizers used in our production. Before presenting the results, it may be useful to recall the effects of an inoculant on a cast iron. An inoculant can influence the form of solidication in various way [9]. The best-known effect is a reduction of the tendency to form cementite. This effect is easy to observe in the cooling curve, as an increase of T Emin and of T sol (T sol is important for estimating the risk of white cast iron in the core: inverse chill). However, the inoculant also contributes to the nucleation of the cast iron, ans so must in theory reduce the recalescence. It must also reduce the liability of the cast iron to the formation of shrinkage cavities and of porosity: it must therefore increase the length of the eutectic range (PAE) and reduce VPS and the derivative at the solidus to reduce the risk of porosity. To compare the various inoculants, it is therefore necessary to monitor their ability to increase PAE, T Emin and T sol and to reduce the recalescence, VPS and D1 Tsol. However, reducing the recalescence is not automatic, because the recalescence depends not only on the composition of the cast iron but also on its nucleation condition. Some allowance must therefore be made for the effect of the recalescence in the testing of the various inoculants. Procedure To perform this first campaign of tests, we had four acquisition lines; one line was reserved for the cast iron whithout inoculant called the basic cast iron used as point of comparison to judge the action of the various inoculants. In each of the other three cups, the same quantity of a different inoculant was introduced (the quantity normally used in production). This yielded, from a single ladle, the records of the basic cast iron and of three of the six inoculants. The same protocol was repeated on the next ladle of metal in order to produce recordings of the last three inoculants whith a cast iron as close as possible to the one used in the first three tests.this protocol was repeated some fifteen times to guarantee the reproducibility of the results. This protocol was repeated for each of the three recarburaziers tested; we attempted to have the same number of tests for each recarburizers: this also enabled us to compare the recarburizer + inoculant pairs. To eliminate the influence of carbon, it was decided to keep only the eutectic curves: at least seven or height eutectics curves are needed to reveal a trend. We then examined the evolution of each of the parameters of influence for each inoculant: graphs were used to rate the inoculants as a function of the recarburizer with respect to the theorical effects of an inoculant. Here is an example of the type of graph procuded by the analysis of the curves recorded in each test: Fig.3 Variation of the parameters for different inoculants with recarburizer A 5 / 9

6 As a function of the theorical effect of the inoculant, it can be seen here that the best inoculant is the inoculant A, because it increases T Emin and T sol substantially, reduces VPS substantially and increases PAE. For the opposite reasons, it can be seen that inoculant B is the least good of all. After analysis of all the tests, it is possible to produce a summary table of the properties of each inoculant with each recarburizer. This table enables us to identify the two best inoculants, which we subsequently tested more thoroughly. The summary table of results: Test of the various inoculants Recarburizer A Recarburizer B Recarburizer C Comments Inoculant A Very good Good in recalescence compared to recarburizer A Very good The best inoculant Inoculant B Mediocre Mediocre Mediocre Do not use Inoculant C Problem with VPS and the derivative at the solidus: higher risk of shrinkage cavity than with inoculant A To be compared more precisely with inoclant A Inoculant D Good in recalescence and average in VPS Good in recalescence and average in VPS Good Product of quality. Could replace inoculants A and C in case of problem Inoculant E Good in Tsol and average in PAE Average in PAE, Temin et Recalescence. Product works very poorly withe recarburizer B Table.2 Summary of results Good To be used only with recarburizer C Inoculant F Poor Poor Poor Not worthwhile 5. Test of the two best inoculants We compared in more detail the two best inoculants identified in the previous section. The aim of this work was added depth to our tests and confirm the presumed superiority of inoculant A. In this part, we used the same experimental protocol as before, but all tests were counted, including those in which the composition was not eutectic. For this test, we recorded all ladles of one morning s production, whichever recarburizer was used-about fifty tests. The number of tests was sufficient to reveal a trend that we confirmed in a second campaign of tests. The best inoculant was indentified by observing the evolution of each parameter over some fifteen ladles prepared with the same recarburizer, in order to see how each inoculant responded to possible drift in our process. The difference between the basic cast iron and the inoculated cast iron whith each product could also be observed: the effects of the inoculant were then compared directly. Fig. 4 and 5 are two typical graphs of the results obtained. Fig.4 Evolution of the parameter VPS as a function of test number with recarburizer A 6 / 9

7 Fig.5 Evolution of T Emin with recarburizer A compared to an untreated cast iron This type of graph was plotted for each parameter, providing more detail to confirm our choice of the best inoculant. We found in this way that, for our application and with the recarburizers used, inoculant A is in fact the best choice. 6. Quantitative study of inoculant A The above-describes campaigns of tests informed us about the quality of the proposed inoculants (or inoculant/recarburizer pairs). It can be assumed that all foundries tend to over-inoculate to have a sufficient margin on safety on the inoculation. In ordre to optimize this item, we attempted to determine the optimum quantity of inoculant for each casting. The purpose of this new campaign of tests was therefore to vary the level of inoculant in each cup. Since we had four cups, we left one cup whithout inoculant, one whith the level of inoculation normally used in production, one whith inoculation cut in half and one whith inoculation increased by half. We performed some fifteen tests combining these three different levels of inoculation whith each of the recarburizers, then conducted an assessment to see whether a trend could be indentified. The results are in the form of graph, like the one in figure 6 below: Fig.6 Example of variation of parameters as afunction of the quantity of inoculant used for a test 7 / 9

8 It can be seen that there are few difference between the level of inoculation used in production and the tests performed whith half as much inoculant: it was therefore decided to take the comparison further to see whether inoculation in the mould can be reduced. As before, a campaign of a score of tests was performed with the usual level of inoculant and half that level: the evolution of each of the parameters in these tests was then tracked, and in particular the difference between the two evolutions were compared. For example, Fig. 7 compares parameter PAE whith the normal quantity of inoculant and with half that quantity. Finally, following an analysis of the results obtained in this campaign of quantitative tests, it was decided to reduce the inoculation in the mould by one third to achieve performance intermediate between the levels tested while retaining a good margin of safety. Fig.7 Variation of parameter PAE with two quantities of inoculant 7. Determination of the eutectic segment In this last part, we are going to show how the data previously recorded can be processed to yield additionnal information,the eutectic segment, that is the key to a better understanding of our product. For this purpose, the concentrations of carbon, silicon and phosphorus must have been determined in parallel by spectrometric analysis, for each test. It is then possible to calculate the carbon equivalent of the cast iron for each test using Eq. 1. The eutectic ranges found by searching in the curves obtained (with the inoculant used in production at the level used in production) for the eutectic curves, i.e. those curves for which T Liquidus = T Emin. The values of Ceq are then grouped into classes of 0.01 and a histogram of the frequencies of the appearance of the Ceq classes in the eutectic curves is generated. The histogram has a Gaussian profile (see Fig. 8) from which it is possible to deduce the eutectic segment of the cast iron. From now on, an eutectic cast iron will be obtained by aiming at a Ceq between 4.38 and Concretly, this means changing the target values for carbon and silicon. The same protocol is repeated in order to obtain, for the eutectic curves, the distribution of the carbon and silicon contents, which yields the target values for these two elements. 8 / 9

9 Fig.9 Determination of the carbon equivalent 8. Conclusion A small fraction of the potential of thermal analysis has been investigated through this work. Other aspects of its potential may be study in the future: predicting the mechanical properties, the risk of porosity, etc.but this will require a substantial effort to assimilate the tool and adapt it to our particular product and process. Once this step has been taken, as is now the case for us in the domain presented in this study, thermal analysis becomes the more reliable and rapid tool for: testing the effectiveness of new products as they come onto the market, tracking the quality of those products over time. 9. References [1] D.Sparkman, Understanding thermal analysis of iron AFS Transaction, 1994, 06, p [2] M.Hecht, J.C.Margerie, analyse thermique en fonderie de fonte, Fonderie Fondeur d Aujourd hui, n 8, Octobre 1981, p [3] M.Van der Perre, Analyse thermique : pricipes et applications, Electro-Nite, Belgium [4] A.Remy, N.Hueber, M.Barthole, Analyse thermique directe et dérivées des fontes, Fonderie Fondeur d Aujourd hui, n 68, Octobre 1987, p [5] J.Bradley, C.A. Fung, Thermal analysis for shrinkage prediction in commercial ductile iron casting Canadian Metallurgical Quaterly, Vol. 30, n 4, p , 1991 [6] T.Skaland, A new method for chill and shrinkage control in ladle treated ductile iron, 3rd Keith Millis Symposium on ductile cast iron, October 20-23, 2003, Hilton Head, South Carolina [7] E.Huerta, V.Popovski, A study of hold time, fade effects and microstructure in ductile iron, Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois [8] A.Jentsch, Influence of recarburizers in the microstructure and properties of cast components: cost analysis Foundry Trade Journal, 2006, Vol.180, n 3638, p [9] M.Durand-Charre, La microstructure des aciers et des fontes, Genèse et interprétation, SIRPE / 9