Influence of aluminium on the formation of pinholes in cast irons

Transcription

1 ARCHIVES of FOUNDRY ENGINEERING Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 8 Issue 2/ /2 Influence of aluminium on the formation of pinholes in cast irons T. Elbel*; J. Hampl; R. Vladík; R. Koukal VSB Technical University of Ostrava, FMMI 17. listopadu 15, CZ Ostrava-Poruba *Corresponding author. address: tomas.elbel@vsb.cz Received ; accepted in revised form Abstract Purpose: This study concerns the formation of pinholes in castings by reaction between cast iron with compacted graphite and green sand mould. Methodology: Experiments were done on bar castings moulded in green bentonite mixtures without carbonaceous matters. Moulding sand moisture and aluminium content in the melt were purposely changed. Metal was melted in a 100 kg furnace by remelting the uniform charge of return material. Metal was inoculated all at once in a ladle and modified in a reaction chamber in a mould or as sandwich method in a pouring ladle. Findings: The formation of pinholes was not caused by high moisture of the mould but this defect was sensible to aluminium content in metal. In castings with high aluminium content > 0.2 % the pinholes occurred in high numbers, and namely under low and also high moistures (> 4 %). In previous melts with aluminium contents < 0.02 % defects occurred in small range. Practical implications: Thus the conclusions known from literature about the influence of Al on pinholes occurrence in cast irons were confirmed. Oxygen activity in metal during its flowing and cooling in the mould was also measured but this quantity was not changed too in dependence on pinholes content. Morphological analyses near the defect have indicated that it is a physical type of pinholes caused by hydrogen. Originality: A number of works were aimed at explanation of causes of pinholes formation in lamellar graphite iron castings and spheroidal graphite ones. But this defect is formed in castings from compacted graphite cast iron too and the research was aimed just to this material. The contribution is in such a way an exceptional one. Keywords: Defect of castings; Pinholes; Cast iron with compacted graphite. 1. Introduction Authors of the contribution have met the occurrence of pinholes in experimental iron castings during study of reoxidation processes of metals cast in green sand moulds. Several contributions were published with this topic [1, 2, 3]. They were related to cast irons treated with magnesium to spheroidal and compacted graphite. In technical literature the problem of pinholes formation in iron castings has been thoroughly discussed, and namely for lamellar graphite cast irons (EN GJL) and spheroidal graphite ones (EN GJS). Therefore it can be seemed that the mechanism of pinholes formation in cast irons is sufficiently well-known and this casting defect can be prevented as well as in case of steel castings. In spite of it the pinholes occur in foundries when pouring lamellar graphite cast irons and spheroidal graphite ones too and similarly as in case of steel castings in the past the question is an emergency epidemic occurrence. This study is aimed at an almost unexplored field of ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue 2/2008,

2 the pinholes occurrence in castings from compacted graphite cast iron (EN GJV). The opinion on the pinholes formation in iron casting is not uniform. Some endorse an opinion that the question is the effect of gases of exogenous nature, others argue that the questions are endogenous blow holes. Our research of the defect was therefore based on the analysis of hitherto knowledge and we will try to summarize it here. Formation of exogenous pinholes is often attributed to nitrogen originating from thermal destruction of organic binders from moulding mixtures [4. Carter and co-authors [5] have found out that spheroidal graphite cast iron (GJS) inclines to pinholes formation more than lamellar graphite cast iron (GJL). According to their opinion in GJL castings there are more likely exogenous pinholes which have suppressed secondary oxidation under the influence of carbonaceous matters from moulding mixture and nitrogen from binders in connection with hydrogen can cause those defects. Literary data are consonant with critical limits of pinholes formation in GJL castings from 80 up to 100 ppm N 2 [6, 7]. If hydrogen or CO is added to nitrogen too the exogenous pinholes can be formed also with lower N 2 contents. Exogenous character of pinholes is connected with decomposition of water vapour from the moisture contained in moulding mixture. Opinions about aluminium content are different. It used to be given that liquid GJL should be without aluminium and magnesium [6]. By the reaction of those elements, with high affinity to oxygen, with water vapour its decomposition takes place with formation of atomic hydrogen that is immediately absorbed by the melt. Al contents from 100 ppm up to 1000 ppm can be dangerous [7]. Rahmoeller [8] recommends reducing aluminium to minimum. Carter [5] for suppressing of pinholes recommended on the contrary the Al content up to 200 ppm. Katz [9] in his recent work has proved that Al content in the limits of 100 up to 1500 ppm causes surface gas defects. He connects this phenomenon mechanism with precipitation of aluminium nitride during cooling of cast iron in a mould. It then reacts with slag containing FeO or with mould moisture with formation of hydrogen and nitrogen. If cast iron contains residual Ti in amount above 0.03 %, in such a way it prevents the pinholes formation caused by Al. According to many authors the decrease of pinholes occurrence can be achieved with the aid of carbonaceous matters added in moulding mixtures [4, 5, 6, 7, 10]. In 2006 a study of pinholes formation in GJL castings when reacting with a green sand mould was published in France and it is a most extensive one up to now. Hecht and Castelhano [11], have confirmed that the pinholes occurrence increases with increasing of Al, Ti and Mn contents in cast iron, with casting temperature decrease, with reduction of carbonaceous matters content in moulding mixture and with its increasing moisture. As regards the Al content, the critical area for the pinholes formation is the range of 200 up to ppm. On the contrary with contents of 1500 up to 2000 ppm the occurrence of pinholes can be totally suppressed as a strong continuous layer of Al oxides is formed on the mould-metal interface. Endogenous mechanism of pinholes formation used to be connected with reaction of secondary oxides rich slag with carbon from the melt or from carbonaceous matters in moulding mixture. This reaction results in formation of CO which forms the pinholes. The defect formation is also supported by water vapour [7]. Zych, Glazowski and Krzywoń [12] in their systematic research of pinholes formation in GJS castings, when modifying cast iron in a mould, have concluded that pinholes in studied castings are caused by reaction of metal components with water contained in moulding mixture. They also suppose that an important factor increasing the tendency to the pinholes formation is the metal flow front leaving as the first one the reaction chamber which is oversaturated by elements from the Mg, Al, Ti, and Ca modifier. It is not the aim of this contribution to summarize all knowledge about pinholes but let us to mention in addition to it the work by Riposan [13] who has confirmed that aluminium content non-exceeding the limit of 50 up to 100 ppm is a guarantee of limitation of pinholes occurrence in GJL. Results of the above mentioned works were taken into account by a team of Japanese research workers [14] who has stated that the pinholes formation in castings has no quite unambiguous causes and the particular conditions of casting manufacture will be the decisive ones for the prevailing mechanism. Based on research of defect morphology with the aid of scanning electron microscope SEM and energy dispersion spectrometric analysis EDS these authors have determined five mechanisms of pinholes formation. The mentioned work by Kurokawa [14] has a great import for determination of causes of pinholes formation according to their appearance and morphology what is often an only way in practice how to identify the defect mechanism. 2. Description of proper experiments A comparative experiment was used during which the moisture of moulding mixture, Al content in metal, and the method of filling the mould were changed otherwise with the same conditions (the same composition of charge, of inoculant, of modifier, the same casting temperature). A step-like bar casting weighing 21 kg [2], with length of 1200 mm, width of 40 mm, and thicknesses of 40, 30 and 20 mm was used for the research. Moulds were made of green bentonite mixture. Nitrified bentonite was used as a binder. The mixture did not contain any carbonaceous additive, for increasing of its plasticity dextrin in amount of 0.75 w. p. was added in it. Used quartz sand has mean granularity of d 50 = 0.27 mm. Mixtures differed with water content. Metal for test castings was melted in an electric meanfrequency furnace of 100 kg volume. Metal was cast from a hand ladle. It was inoculated (FeSi 75) in a pouring ladle and modified for GJV in the ladle by sandwich method or in-mould method with the aid of the Bjomet 3 modifier (noduliser). For modification in the ladle the amount of the modifier was 1 % and in the mould it was 0.6 %. A method of planning the experiment (DOE) was used. Amount of pinholes and measured value of oxygen activity were chosen as dependent variables. Oxygen activity in castings was measured with TSO 6 probes from the firm Termosondy Kladno, Ltd., and with sensors derived from those probes. The first set with modification in the ladle was formed by 4 melts; next set with modification in the mould was formed by 7 melts. Composition of metal charge in an induction furnace, basic chemical composition of metal, basic composition of moulding mixture, modifier amount, and casting temperature were chosen as constant parameters. Al content in cast iron (3 levels from up to 0.25 %), moulding mixture moisture (3 28 ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue 2/2008, 27-32

3 levels from 2 up to 5.2 %), and metal rate of flow ε in the mould cavity (2 kinds of gate throat in the mould cavity) were the input factors. Different rate of flow in castings was ensured in such a way that from one slag trap the gates throated alternatively in thicker (A casting) and thinner (B casting) bar section. Chemical compositions of studied melts are given in Tab. 1. and Tab. 2. Table 1. Chemical composition of GJV melts with sandwich modification in a ladle Chemical composition of melts [%] Melt number C 3,47 3,51 3,45 3,53 Mn 0,26 0,18 0,30 0,10 Si 2,71 2,91 2,51 2,67 P 0,018 0,010 0,066 0,010 S 0,005 0,006 0,009 0,009 Cr 0,047 0,040 0,033 0,036 Cu 0,034 0,035 0,043 0,040 Al 0,019 0,039 0,0095 0,008 Mg 0,020 0,020 0,028 0,025 Ti - - 0,016 0,008 O 2 0,004 0, N 2 0,003 0, S c 1,03 1,06 1,00 1,04 Chemical analysis given in Tabs. 1 and 2 was done from samples cut from test castings on a spectrometer of the firm Hilger. Total oxygen and nitrogen contents were determined on the LECO aparatus. Table 2. Chemical composition of GJV melts by modificiation in mould Chemical composition of melts [%] Melt number (casting) C Mn Si P S Cr Cu Al Mg Ti O N S c Results During casting in case of several melts the oxygen activity was measured in test moulds. After casting the pinholes occurrence on upper casting surfaces was evaluated, samples for chemical and metallographic analysis were taken. Pinholes were evaluated visually and counted from magnified photographs. Number of them has been determined in three points of the casting (with different rate of flow) on the area of 16 cm 2. Pinholes occurred fairly equally on the whole surface, mainly on the upper surface, but in lower extent on lower surface and side walls too. In the previous work [2] we have found out that the rate of flow has low influence on the pinholes number and therefore the arithmetic mean of three measurements we have considered a resulting value which served for next calculations. Table 3. Occurrence of pinholes in castings from GJV made by modification by a sandwich method in the ladle Casting No. Water content [%] Loss by ignition [%] Al content [ppm] Number of pinholes per [cm 2 ] 48A B A B A B * A B * Moulding mixture contained sea coal. Following Tabs. 3 and 4 give results of both melt sets. Besides the number of defects per 1 cm 2 the Al content in metal, moisture, and loss by ignition of the moulding mixture are given here too. Results of oxygen activity measurements were given in the previous work [3]. Measured values didn t show any connection with pinholes occurrence and they will not be discussed here any more. Table 4a. Occurrence of pinholes in castings from GJV made by modification in mould Casting No. Water content [%] Al content [ppm] Number of pinholes per cm 2 61A1* A B B A A B B A A B B ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue 2/2008,

4 Table 4b. Occurrence of pinholes in castings from GJV made by modification in mould Casting No. Water content [%] Al content [ppm] Number of pinholes per cm 2 64A A B B A B A B A B A B * Letters A and B mean: The A bar has the gate located in a thicker wall; the B bar has the gate located in a thinner wall. Figures 1 and 2 are serial numbers of pouring (ladles).(moulding mixture without carbonaceous matters, loss by ignition 1.2 %) Results from Tabs. III. and Iv a,b were subjected to the statistic processing. At first a dependence between moulding mixture moisture and pinholes number was searched. Week dependence was determined with the aid of the regression analysis. Therefore the values of both sets were divided into two groups: for moistures < 3.1 % (14 values) and > 3.1 % (10 values). In the first group with lower moisture in six castings no pinholes were determined and the arithmetic mean of pinholes occurrence was 0.55 pcs/cm 2 and in case of higher moisture it was 1.16 pcs/cm 2. In spite of the fact that in the set with higher moisture in one casting the pinholes occurrence was zero one, it can be inferred that with increasing mould moisture the pinholes number grows almost double. For the relation between Al content and the pinholes number the closer relations were determined with the aid of regression analysis. A set of castings made from cast iron modified in a ladle has unambiguously higher number of pinholes and 100 ppm seems to be the limit. Regression equation: NP = (Al) ½ (1) in which: NP number of pinholes per cm 2,Al aluminium content [ppm]; correlation coefficient r = In the set of 24 castings from Tab. IV,V cast with cast iron modification in a mould the pinholes occurrence was lower. The lower limit was 100 up to 150 ppm and the pinholes didn t occur or in a smaller extent only. With high content of Al > 1900 ppm the pinholes we could also observe smaller extent. Regression equation: NP = (Al) ½ (2) in which: NP number of pinholes per cm 2, Al aluminium content [ppm]; correlation coefficient r = 0,834. The result of those relations is given in Fig. 1. Fig.1. Evaluation of pinholes number (NP) on Al content in cast iron for a GJV castings set modified by Mg in a ladle (1) and in a mould (2) Obtained results agree with above mentioned literary data, and namely both for lower, and upper limit. In castings with Al content about 2000 ppm the wrinkling surface of oxidic films was really seen and also Al content was found out here higher then in casting centre or lower surface. The difference in the pinholes occurrence between the melt set modified in the ladle (curve 1) and that one modified in the mould (curve 2) is probably given by the course of reaction of Mg with the melt when in the ladle (with higher modifier content) the reaction courses more vigorously than with modification in the mould (with lower modifier content). It is possible that pinholes formation was simultaneously supported by reaction of Mg with water vapour from the mould. Ti content that, according to Katz [9], can eliminate the Al effect, in case of our melts it was only in tens of ppm while Katz recommends 300 ppm. The effect of nitrogen was taken into account too. In experimental melts it was very low from 20 up to 40 ppm and its influence on pinholes occurrence can be excluded. Tabs. I. and II. also give total oxygen content. Oxygen content ranged from 20 up to 55 ppm and it can be also considered low and non-exceeding the values known for cast irons that are according to Mampaye [15] 30 up to 80 ppm. Explanation was searched in chemical microanalysis of pinholes from some melts. Pinholes were investigated with the scanning electron microscope (SEM) with X-ray energy dispersion areal and spot analysis EDAX for evaluating their morphology. Details of the defects are given in Figs. 2. up to 5. The defect morphology on Fig. 2. (from the casting 48 A) is similar to pinholes that were analysed in spheroidal graphite cast irons [1]. Inside the defect there is separated a thin graphite layer from which dendrite ends get out. In accordance with the work by Kurokawa [14] we have marked the defect as hydrogen pinholes 30 ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue 2/2008, 27-32

5 of exogenous character. Kurokawa ranges those pinholes into the group of physical type arising during forming a casting in liquid phase and the cause can be the decrease of surface tension with the presence of Al and Ti in metal and their reaction with water vapour. Pinholes from the casting 50 A on Fig. 3. has a similar character but inside it the white slag inclusions occur. Figs. 4. and 5. show pinholes from castings made with the aid of in mould modification of GJV (casting 65 A) with high Al content. A graphite layer can be seen here too but it has a flaky character and it contains Fe oxides. It results from observations of pinholes on the SEM and EDAX that morphology of graphite separated in pinholes differs. Differences are probably dependent on Al content in original metal, on moisture of used moulding mixture or on the modification method. Pinholes in GJV castings with low Al content in the melt have the graphite layer crystallized in a regular arrangement (Figs. 2. and 3.). Pinholes in castings with increased Al contents in cast iron have the graphite crystallized irregularly attended usually with reaction products Al-O-X as a consequence of modification reaction and the mould moisture (Figs. 4. and 5.). Fig. 4. Melt of GJV 50 A, modification in the ladle, Al = %, graphite with slag inclusions Al-O-Si in hydrogen pinholes, SEM, EDAX, magnification 650 x Fig. 2. Melt of GJV 48 A, a graphite layer in pinholes with Fe dendrites, modification in the ladle, Al = %,SEM, EDAX, magnification 250 x Fig. 5. Surface analyze of elements concentration in inclusions Fig. 3. Melt of GJV 48A, a layer detail (3500 x) Fig. 6. Melt of GJV 65A,in-mould modification, morphology of the graphite layer in pinholes with ferrous oxides, Al=0.19%, SEM, EDAX, magnification 2000x ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue 2/2008,

6 4. Conclusion The work aimed at the research of pinholes formation in castings made from compacted graphite cast iron when casting in green sand moulds. Experiments were done on laboratory bar castings with a long trajectory of metal flow. Realized experiments have confirmed some literary knowledge about Al influence on pinholes formation in lamellar graphite cast irons and spheroidal graphite ones. It has been confirmed that in GJV too with Al content above 200 ppm there exists a strong probability of pinholes formation, and namely in practical terms without respect to moulding mixture moisture. But great difference between the pinholes amount when modifying cast iron in a mould and in a ladle has been found out. More pinholes were formed when modifying cast iron in the ladle when greater amount of modifier was used and also more reaction products were formed. It was also shown up in different appearance of pinholes when searching them with the SEM. From the defect morphology we have found out that the question is the exogenous mechanism of the so called hydrogen pinholes. Acknowledgement The work was realized with financial support of the Grant Agency of the Czech Republic in the framework of the project of the registration number 106/08/0789. References [1] J. Hampl, T. Elbel, L. Kocián, The formation of pinholes in SG iron castings. In Sborník vědeckých prací VŠB - TU Ostrava, řada hutnická, No. l, vol. IL, 2006, article No. 1271, pp ISBN (in Czech). [2] T. Elbel, J. Hampl, R. Vladík, R. Koukal, Research of pinholes in cast irons. Technological Engineering, vol. IV, 2007, No. 1, pp. 2 3, ISSN (In Czech). [3] T. Elbel, J. Hampl, R. Vladík, R. Koukal, The formation of pinholes in compacted graphite iron castings. Acta Metallurgica Slovaca 2007, vol. 13, NO. 4/2007, pp ISSN (in Czech). [4] International atlas of casting defects. American Foundrymen s Society, Inc. Des Plaines, [5] S.F. Carter et al., AFS Transactions, vol. 87, 1979, pp [6] S. Hasse, Guss- und Gefügefehler. Schiele & Schön, Berlin [7] J Baier, M. Köppen, Handbuch der Gussfehler. IKO-Erbslöh, Marl, [8] K. H. Rahmoeller, Mold Binder Decomposition. Modern Casting, vol. 83, October 1993, pp [9] S. Katz, A New Theory for Aluminium-Induced Gas Defects in Iron Castings and Potential Solutions. AFS Transactions, vol. 114, 2006, pp [10] S. Fujio et al., Prevention of Pinholing in Spheroidal Graphite Iron Castings. Imono, 56, 1994, No. 4, pp (In Japan) [11] M. Hecht, V. Castelhano, The formation of pinholes by reaction between lamellar graphite cast iron and green sand. Fonderie-Fondeur d aujourd hui, Part I, No. 251, janvier 2006, pp Part II. No. 256, juin-juillet 2006, pp Part III, No. 257, aout-septembre 2006, pp [12] J. Zych et al., Pinholes in automotive SG iron castings using Mg treatment in-mold. In: sb. X I. miedzynarodowa konferencja WSPÓLPRACA, Zakopane 2005, pp (In Polish). [13] I. Riposan et al., Role of Residual Aluminium in Ductile Iron Solidification. AFS Transactions, vol. 115, 2007, [14] Y. Kurokawa et al., Observation of Pinhole Defects in Cast Iron Castings by Surface Analysis. AFS Transactions 2002, vol. 110, pp [15] F. Mampaey et al., The use of oxygen activity measurement to determine optimal properties of ductile iron during production. Presented on the WFO Technical Forum, Düsseldorf, June, 13, 2007,proceedings p ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue 2/2008, 27-32