Study of pinholes genesis in iron castings

Transcription

1 A R C H I V E S of F O U N D R Y E N G I N E E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 11 Issue 1/ /1 Study of pinholes genesis in iron castings T. Elbel*, J. Hampl, M. Senkypl Department of Foundry Engineering, Technical University of Ostrava, Tr. 17. listopadu, Ostrava Poruba, Czech Republic *Corresponding author. address: tomas.elbel@vsb.cz unbroken line Received ; accepted in revised form Abstract Purpose: The study concerns the formation of pinholes in castings formed by reaction between a green foundry mould and lamellar graphite cast iron. Great numbers of works have been aimed at clarifying the causes of pinholes formation in iron castings. In spite of this there exists no united opinion on the pinholes formation (genesis) and the authors of this contribution having studied this phenomenon in compacted graphite and spheroidal graphite iron castings were also aimed at lamellar graphite cast iron and they applied for it their knowledge gained in study of reoxidation processes during casting of ferrous alloys. Methodology: Experiments were done on castings of stepped bars moulded in green bentonite mixtures with s graduated moisture and with use of two types of carbonaceous matters. Metal was melted in a 100 kg induction furnace from the same charge. Inoculation was done in a ladle after pouring out from the furnace. Aluminium was dosed in the ladle in some cases and the inoculator kind was changed too. Results: Pinholes were present on castings as small flat pits; on horizontal surfaces sooner singly, on casting edges in clusters. The formation of pinholes wasn t caused by high moisture of moulds but the defect was sensitive to aluminium content in metal. In castings with high aluminium content > 0.01 % the pinholes were present in great numbers, and namely both under low, and also high moistures of moulding mixtures. In melts with low content of Al < 0.01 % the pinholes occurred less extensively only. Practical implications: Conclusions from literature about influence of Al on pinholes occurrence were confirmed in such a way. Study of the defect morphology has shown that the question is a oxidation reaction type of pinholes caused by oxidation of the residual melt between dendrites with formation of CO. Keywords: Casting Defect; Pinholes; Cast iron 1. Introduction In steel castings the pinholes are known as small elongated cavities with smooth surface formed close under the casting surface that lead into the surface with small outlets of diameter 1 up to 2 mm (reminiscent of holes after pin/needle-prick) [1, 2]. Under the surface the cavity widens to about double and the defect length is several times higher than its diameter. The International atlas of casting defects has used for this type the term subsurface pinholes [3]. Such defect appearance need not have been met in iron castings. Pinholes are considered here also shallow surface pits (cavities) with smooth surface or the surface can be filled with a graphite inlay or a layer of oxides for those the term surface pinholes is used. In addition there exists another defect type having many common symptoms with pinholes that is known in German literature as»kommafehler«[4] and that represents shrinkage subsurface cavities leading also into the casting surface. The authors of the contribution were aimed in the past at forming the pinholes in iron castings treated with magnesium to compacted graphite cast iron and spheroidal graphite cast iron [5, 6]. The presented contribution is aimed at extension these knowledge with results of study of pinholes formation in lamellar graphite cast iron and study of the oxygen action in iron castings [7]. For a long time the formation of pinholes in iron castings was imputed to the action of nitrogen dissolved in metal or originating A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

2 from organic binders in a mould or a core. The fact that the pinholes formation hasn t so unambiguous cause has been stated in 3 publications by Hecht and Castelhano [8] that were engaged in pinholes formation in castings from lamellar graphite cast iron made in green moulds. These authors have declared the pinholes as a defect well and (at the same time) little known. They have found out that the pinholes formation depends on three parameters related to chemical composition of metal and the mould (Al and Mn contents in cast iron and the content of carbonaceous matters in the moulding mixture) and on two technological factors (the casting thickness and temperature of metal poured in the mould). Pinholes occurrence in castings considerably increased with increasing the Al content in cast iron to 0.08 up to 0.1 %, i.e. considerably above the critical limit of up to % Al. For raising the pinholes epidemic it was necessary from the point of view of moulding mixture to eliminate from it the carbonaceous matters and to increase the moisture content of the mixture above 3.5 %. In addition to it the pinholes occurrence was supported by low casting temperature and smaller thickness of the casting wall. With increasing the nitrogen content in cast iron from ppm to 100 ppm with Mn content of 1 %, with Al content lower than 0.01 %, under otherwise the same conditions the pinholes occurrence wasn t increased. The C content in the moulding mixture (called by the authors as active carbon ) was on the upper limit of 3.0 % at that. It is not the aim of this contribution to summarize all knowledge about pinholes, notwithstanding let us to mention one more work of crucial import by a team of Japan researchers [9] that has stated that the pinholes formation hasn t quite unambiguous causes and it will depend on particular conditions of casting production which mechanism will be a dominating one. Kurokawa [9] presupposes that pinholes are induced by dissolved gases, oxidation of liquid metal, and by formation of inclusions in the macroscopic zone. For observing the defect interior they used a scanning electron microscope SEM and energy dispersion spectrometric microanalysis EDS that are considered by them very effective methods for determining the defect causes but the evaluating person must be perfect acquainted with these testing processes. A knowledge database of 500 pinholes formed during green casting in bentonite mixtures has been created in the study by them. This database has been formed from observing the defects and classing the examples according to the defect origin. Defects caused by gases are classifiable according to the shape whether the questions are gas blowholes, pinholes and microporosity. The difference between those defects can be defined with difficulty and therefore in the study by Kurokawa the pinholes were considered the gas holes of the size 1 up to 5 mm. Blowholes are cavities exceeding 5 mm and their internal appearance is identical with the casting surface. If the defect interior is of metal appearance differing from the casting surface or it contains inclusions of graphite, carbides or slags then the defect was identified as pinhole one. Microporosity defined as a cluster of small cavities almost invisible to the naked eye has been left out from this study as the mechanism of its origin is different from pinholes. Based on study of the defect morphology the mentioned authors have determined five mechanisms of the pinholes formation without adjoining them to different cast iron kinds. They are as follows: a) Physical type b) Oxidation reaction type c) Dissolution type d) Slag formation type e) Others Recently R. Foret [10] applied the process by Kurokawa for analysis of pinholes in a cover casting from GJS. He succeeded in determination that in the given case it was a defect of the physical type but he hasn t determined its particular technological cause. Notwithstanding he supposes that it could be high moisture of the moulding mixture. 2. Proper experimental works A comparative experiment was used for the research during which under otherwise the same conditions (the same composition of the charge, inoculator, the same casting temperature) the moulding mixture moisture, Al content in cast iron, and the method of mould filling were changed. A stepped bar casting of the length of 520 mm, width of 40 mm, with stepped thickness of 30, 20, and 10 mm was used for the research. Moulds were made from green bentonite mixture. The binder was activated soda bentonite, the base sand was quartz sand Šajdíkovy Humence of medium grain of d 50 = 0.27 mm. Mixtures differed with water content and the addition of carbonaceous additives. In most cases 3.5 weight parts of the addition Ekosimix and in several cases 0.5 weight parts of dextrin were used. The mixture with Ekosimix had the loss by ignition of 3.8 %, whereas for mixtures with dextrin it was 2.2 %. Metal for test castings was melted in an electric mediumfrequency furnace of 100 kg volume. Molten metal was cast in a hand casting ladle. For all melts the inoculation with FeSi75 was used. Checking of an inoculator containing barium Ba (SB5) was interesting. Tapping temperature of cast iron was 1420 o C. Tab. 1. gives a survey of chemical composition from 9 melts of castings. Influence of aluminium content in cast iron, of moulding mixture moisture, casting design, and a degree of through flow of metal were tested on a set of 24 bars. Number of pinholes (NP) was studied as a dependent variable. The composition of metal charge in the induction furnace, basic composition of moulding mixture, the inoculator amount, and casting temperature were kept the same for all castings. The Al content in cast iron (from up to %), moulding mixture moisture (from 3 up to 4.4 %), casting thickness and a relative degree of metal flow rate ε in the mould cavity were the independent variable factors. Different degree of flow rate in castings was ensured in such a way that from one slag trap the ingates led alternately into the thicker (the B casting) and the thinner bar cross-section (the A casting). Number of pinholes was evaluated visually on cast surface, in three casting sections (with different casting thickness and with different degree of flow rate), and namely on lower and upper casting surface. 22 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

3 Table 1. Chemical composition of experimental melts Contents of elements Numbers of melts and castings % S1 1,2,3,4 S21 5,6 S22 7,8 S23 9,10 S24 11,12 S31 13 up to 16 S32 17 up to 20 S41 21,22 C Si Mn P S S42 23,24 Al Ti Sc Pinholes appearance was of shallow depression (pits) on casting surface of diameter 2 up to 3 mm getting in the depth up to 4 mm. Tab. 2. gives numbers (quantity) of pinholes in individual casting sections with no regard to ingate leading including joint casting A+B. Total quantity of pinholes in individual sections is given here from both casting surfaces. Pinholes occurred largely on the upper casting surface 286 defects, whereas 74 defects were identified only on the lower surface. It wasn t able to unambiguously prove the influence of the flow rate degree in individual sections from the done experiments. Notwithstanding as is shown in tab. 2. the higher occurrence of defects was found out in the A casting, i.e. with the ingate leading into the thinnest cross-section. Pinholes were found out in great amounts on casting edges. From 286 defects 173 of them were in the A casting (60.6 %) in comparison with 113 defects in the B one (39.4 %). Castings 21 up to 24 fit in this trend in spite of the fact that they were interconnected on the opposed end from the ingate and the influence of the length (doubling) of the metal route was checked in such a way. It is evident from tab. III. that there is no important difference for them. It was shown that the greatest number of pinholes (125 defects) was found out in the thickest cross-section of 30 mm, in the cross-section of 20 mm there were 96 defects and in the thinnest cross-section of 10 mm 65 defects were found out. Total number of pinholes on upper surfaces of castings from individual melts in dependence on aluminium content is given in tab. 3. together with mould characteristics, i.e. moisture of the moulding mixture and the used addition of carbonaceous additives. As regards the aluminium content in cast iron no unambiguous dependence was determined. In the melt with the lowest Al content of 50 ppm with moulding mixture moisture of 3.3 % no pinholes occurred. Graphical presentation of results of Al content influence [ppm] on number of pinholes (NP) is given on fig. 1. Results of the first 16 castings with use of the FeSi inoculator are represented with the regression line (unbroken line). Table 2. Number of pinholes in individual cross-sections of the casting Cross-section [mm] Sum A bar B bar Total number of pinholes on the upper surface Total number of pinholes on the lower surface A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

4 Table 3. Summary of basic results of experimental castings No Al w Number Casting Carbonaceous ppm % of pinholes type additive A dextrin B dextrin A Ekosimix B Ekosimix A Ekosimix B Ekosimix A Ekosimix B Ekosimix A Ekosimix B Ekosimix A Ekosimix B Ekosimix A Dextrin A Dextrin A Dextrin A Dextrin A* Ekosimix B* Ekosimix A* Dextrin B* Dextrin A+B Dextrin A+B Dextrin A+B Ekosimix A+B Ekosimix Al = aluminium content in cast iron; w = moisture of moulding mixture Number of pinholes is a sum of defects for all 3 casting sections * An inoculator with Ba (SB5) was used for inoculation. Fig. 1. Dependence of pinholes number (NP) on Al content in cast iron In another melt from the same base metal with Al content of 106 ppm, where the moisture of moulding mixture was changed from 2.3 % up to 4.4 %, the number of defects pinholes fluctuated. Fig. 2. A view on a cut-out of the casting No 7 with pinholes Results of melts treated with the SB 5 inoculator are represented with the point above this line and it indicates the increase of pinholes number. Results of the melt on doubled bars and with higher Ti content are situated under the basic line. It seems that with those Al concentrations a maximum became and henceforth the pinholes number will be decreasing (see the point with 1410 ppm Al where 2 defects only were found out). Fig. 3. A defect detail under the electron microscope 24 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

5 Surprisingly the highest number of 28 defects was with the lowest moisture and with moisture of 3.8 % (25 defects). With the highest moisture 4 defects only were found on the casting surface. Fig. 4. A view on a bottom of defect - of a casting No 7 - magnification 140x Kurokawa also found in castings with this type defect the increased Al content and he has concluded that the pinhole formation could be supported by the reaction of Al with water vapour from the green bentonite mixture. It could be the case of the casting No 7 in which the Al content was determined to 353 ppm, i.e. by 70 ppm higher then in the melt sample. When observing the defect on casting No 7 at magnification 140 x (see fig. 4)in the bottom of the cavity the sharp-edged aggregates of aluminium oxide we could find. The composition of this layer confirms results of analyser EDS (30 atomic % of aluminium, 69 atomic % of oxygen). In a sample from casting No 19 results were similar. A defect detail is on fig.5 with the same magnification 40x as in fig. 3. Aluminium oxides were observed in periphery of unmaschined casting surface too as dark layer in right corner of the picture. On the bottom of another pinhole from the same casting sample calcium oxisulphides were determined. From analyser EDS we found out the contents (in atomic %) of Ca= 9,5 %, S= 10,7 %, O=78,8 %. The provenance of calcium can be referable to inoculator with Ba SB5, but the barium in defect cavities was not discovered. 3. Discussion of results Fig. 5. A defect detail from the casting No 19 magnificationí 40x Thus it could be concluded that both the used carbonaceous additive and the mixture moisture have no considerable relation to the number of pinholes. It can be concluded from the done experiments that for the cause of this defect formation must be searched in chemical composition of metal. For checking this hypothesis the defect appearance was analyzed on the scanning electron microscope. The defect appearance on the raw casting surface is on fig. 2. where 3 shallow pits can be seen that were further on studied on the SEM. When studying the surface of the defect cavity on the SEM according to fig. 3. neither slag nor a graphite layer were found. The defect surface was oxidized and if the defect appearance is compared with the description by Kurokawa [9], it can be excluded that the question is a pinhole of the hydrogen type as it was met in castings from GJV [5] and GJS [6]. According to Kurokawa the question may be on the contrary a defect of oxidation reaction type from residual melt of cast iron. Primary crystals are formed as dendrites and in the residual melt between the dendrites the reaction of FeO + C with formation of the CO pinhole is running. Further on Kurokawa specifies that such pinholes can be formed individually or in clusters. It was also our case when on horizontal surfaces the defects occurred individually and on the casting edges, where the dendrites grow more quickly, the defect occurred in clusters. If we compare the results of this work with our results for GJV [5] then primarily the number of pinholes was different: in GJL ca. 0.1 defect per cm 2, in GJV it was from 0.5 up to 1.5 and in one case even 10 defects per cm 2. In GJV the defects occurred in extended colonies. It has been stated that the questions were hydrogen pinholes and as a main cause of these defects formation the higher aluminium content in metal has been indicated [5]. With linear regression analysis between the number of pinholes and moulding mixture moisture a slight dependence was determined only. For the relation between the Al content and the number of pinholes the closer relations were determined with regression analysis. A set of castings made from cast iron modified in a ladle had unambiguously higher number of pinholes and the limit region of Al seems to be 100 ppm. Regression equation: NP = (Al) 1/2 (1) in which: NP number of pinholes per cm 2, Al aluminium content [ppm]. In a set of 32 castings cast with modification of cast iron in the mould the occurrence of pinholes was lower. Lower limit of Al content was 100 up to 150 ppm and with high content of Al > 1900 ppm the pinholes didn t occur at all or in a smaller extent. This set was processed with multiple regression analysis when the independent variables were the Al content and moulding mixture moisture. Regression equation: NP = W (Al) 1/2 (2) in which the NP number of pinholes per cm 2, Al aluminium content [ppm]; W moisture of moulding mixture [%]. In the case of the work results no close relation between the pinholes number and Al content was found but the trend A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

6 from fig. 1. shows that also here the upper limit of Al content is 100 ppm above which the occurrence of pinholes grows and with high Al content above 1000 ppm it decreases again because on the surface of flowing metal a thick layer of Al oxidic films is formed which can suppress the pinholes formation. The effect of increased Ti content in GJL in castings No 21 up to 24 with Ti content of ca ppm is also interesting. According to Katz [12] the Ti content in amount of 300 ppm can eliminate the negative effect of Al on formation of endogenous gas holes. In the case of castings from GJV [5] it has been shown that the melts with Ti addition incline to reoxidation and in this connection the Ti with higher content could support the pinholes formation also in cast irons. According to the trend on fig. 1. the higher Ti content rather suppressed the negative effect of growing Al content. The inclusion of the inoculator with Ba content in this experiment was motivated by the fact that in one Czech foundry with its use for manufacture of castings from GJL the endogenous gas holes appeared. The result of this work has shown that with low Al content in metal (under the critical limit of 100 ppm) with use of the inoculator with Ba the pinholes can occur in higher amounts. In the defects from this set of casting the bariumin EDS was not found but another element with high affinity to oxygen and sulphur was determined calcium which in this kind of inoculant is incorporated. A question of cast iron oxidation in the foundry mould cavity that is a cause of forming the pinholes of the physical type deserves a discussion too. Mocek [13] and Chojecki [14] have determined that oxidation of ferrous alloys is possible and carbon monoxide can be formed on the mould surface only. In the monograph by Zadera [15] it has been stated that oxygen activity in cast irons falls down with inoculation and modification of cast irons, and namely according to the kind and concentration of deoxidizing elements with high affinity to oxygen. Conditions for the formation of pinholes of the physical type from oxidation can be considered real also from the point of view of these works. 4. Conclusion After preceding works by the authors engaged in pinholes formation in castings from GJV and GJS the occurrence of pinholes in castings from GJL was searched in this contribution. The defect was studied on experimental castings of stepped bars under different conditions of casting and properties of the mould and metal. The questions were in particular the content of carbonaceous matters and moisture of moulding mixture and as regards the metal the Al and Ti contents in cast iron and its inoculation. In comparison with castings from GJV the defect appearance was different (shallow pits on the casting surface) and in the case of GJL the occurrence of defects was about 10 times lower than in castings from GJV. The moisture content and the kind of the carbonaceous matter in the moulding mixture didn t influence the amount of pinholes in castings similarly as it was in the case of GJV. As regards the composition and treatment of cast iron the pinholes occurrence was sensitive to the Al and Ti contents and the method of inoculation. With the Al content under 100 ppm when inoculating with FeSi the pinholes didn t occur in castings, the maximum was at about 300 ppm Al and the greatest amount of pinholes was found out in the bar part of the greatest thickness. From analysis of pinholes on the SEM and with the use of the study by Kurokawa it has been stated that the question isn t the physical type of hydrogen pinhole but as defect of the oxidation reaction type. Acknowledgement The paper has been worked out with a financial support from The Grant Agency of the Czech Republic in the framework of the project Graphite nucleation and possibilities of control its morphology in ferrous alloys registration No. 106/08/0789. References [1] T. Elbel et al.: Vady odlitků ze slitin železa. MATECS, Brno (In Czech) [2] P. Levíček,K. Stránský: Metalurgické vady ocelových odlitků. SNTL, Praha, 1984 (In Czech) [3] International atlas of casting defects, Anerican Foundrymen s Society,Inc. Des Plainjes, IL.,1993 [4] S. Hasse.: Guss- und Gefügefehler. Schiele & Schön, Berlin [5] T. Elbel. J. Hampl: Influence of aluminium on the formation of pinholes in cast irons. Archives of Foundry Engineering, vol. 6,issue 2/2008, [6] J. Hampl, T. Elbel, L. Kocián: The formation of pinholes in SG iron castings. Transactions of the VŠB Technical University of Ostrava. Metallurgical Series, vol 49, issue 1/2006, s , (In Czech). [7] T. Elbel, J. Senberger, A. Zadera. J. Hampl: Behaviour of oxygen in cast irons. Archives of Materials Science and Engineering, vol.33,issue2/2008, s [8] M. Hecht; V. Castelhano: La formation des piqures par reaction entre la fonte a graphite lamellaire et le sable vert. FONDERIE Fońdeur d aujourd hui, Part I, No 251, janvier 2006, s Part II. No 256, juin-juillet 2006, s Part III, No 257, aout-septembre 2006, s [9] Y. Kurokawa et al.: Observation of Pinhole Defects in Cast Iron Castings by Surface Analysis. AFS Transactions 2002, vol. 110, pp [10] R. Foret.: Analysis of pinholes in a casting of cover from GJS. Slévárenství, LVII, 2009, č. 7/8, s (In Czech). [11] M. Senkypl: Diploma thesis. VŠB TU Ostrava, (In Czech) [12] S. Katz: New Theory for Aluminium-Induced Gas Defects in Iron Castings and Potential Solutions. AFS Transactions, vol. 114, 2006, s [13] J. Mocek; A. Chojecki: Evolution of the gas atmosphere during filing the sand moulds with iron alloys. Archives of Foundry Engineering, vol. 9, isme 4/2009,s [14] A. Chojecki: Hydrogen in reoxidation process. Technológ, vol. II, special issue,april 2010, s [15] A. Zadera; J.Senberger; T. Elbel: Reoxidation processes at casting from ferrous alloys. CSS, Brno 2009.(In Czech) 26 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,