Surface blow holes

Characteristic features Individual or groups of cavities. Mostly large with smooth walls.Incidence of the defect Gases entrapped by solidifying metal on the surface of the casting which result in a rounded or oval blow hole as a cavity. Frequently associated with slag or oxides. The defects are nearly always located in the cope part of the mould in poorly vented pockets and undercuts. The formation of blow holes is more intense with grey iron castings than with SG iron. Possible causes Resin-bonded sand  Core venting not good enough  Release of gas from core too great  Moisture absorption by the cores too great  Too low gas permeability of the core sand Clay-bonded sand  Moisture content of sand too high, or water released too quickly  Gas permeability of the sand too low  Sand temperature too high  Bentonite content too high  Too much gas released from lustrous carbon producer Moulding plant  Compaction of the mould too high Gating and pouring practice  Casting temperature too low  Metallostatic pressure too low when pouring Remedies Resin-bonded sand  Improve core venting, provide venting channels, ensure core prints are free of dressing  Reduce amounts of gas. Use slow reacting binder. Reduce quantity of binder. Use a coarser sand if necessary.  Apply dressing to cores, thus slowing down the rate of heating and reducing gas pressure.  Dry out cores and store dry, thus reducing absorption of water and reducing gas pressure. Clay-bonded sand  Reduce moisture content of sand. Improve conditioning of the sand. Reduce inert dust content.  Improve gas permeability. Endeavour to use coarser sand. Reduce bentonite and carbon carrier content.  Reduce sand temperature. Install a sand cooler if necessary. Increase sand quantity.  Reduce bentonite content. Use bentonite with a high montmorillonite content, high specific binding capacity and good thermal stability.  Use slow-reacting lustrous carbon producer or carbon carrier with higher capacity for producing  lustrous carbon. In the last instance, the content of carbon carriers in the moulding sand can be reduced. Moulding plant  Reduce compaction of the moulds. Ensure more uniform mould compaction through better sand   distribution. Gating and pouring practice  Increase pouring temperature, if necessary increase pouring speed.  Increase metallostatic pressure by changing the gating systems. If possible raise the cope flask. Background information The occurence of gas bubbles is dependent on the gas volumes present and their pressure. If it is not possible to discharge the gases from the mould cavity they can be trapped in the liquid metal. There is a great danger af surface pitting an cores because they are surrounded by liquid metal and the gaseous reaction products are primarily removed through core prints. Gas bubbles are more frequently observed with smaller cores. It is recommended to use coarser sands and a corresponding application of mould dresssings [1]. Cores with an unfavourable shape should contain waste gas channels. The necessary cross-sections of gas discharges from cores in relationship to core binders and geometry are thoroughly investigated in [2]. Obstruction of gas discharge results in bubbles being trapped in the metal. This fault also accurs with large gas discharge cross-sections when using phenolic resins. Hygroscopic binders like waterglass require large cross-sectians for gas discharge. Contrary to this, the occurrence of gas bubbles can assist drying af the cores. Use of cold cores in hot moulds can lead ta water adsorp­tian with hygroscopic binders. These can explosively vapo­rize during pouring and lead to defects. With bentonite sands, gas bubbles also primarily occur through the formation of water vapour [3]. This can be countered by reduction of the pouring rate and avoidance of impingement of the metal flow an the mould wall. When this defect occurs tbe gas permeobility of the sands should be high but the water content as bw as possible. All water absorbing materials like inert dust, bentonite and carbon carriers should be as bw as possible. Under certain circumstances this necessitates the use of clays containing large percentages of montmorillonite as well highly active carbon carriers. lt is also recommended ta develop the moulding sand as weil as possible. Well developed  sands require less water and release this slower during heating up. The occurrence of condensed water should be avoided. There should be no temperature differences between cores and moulds. Water can also precipitate on chaplets or chills and lead to gas defects on account of the absence of gas permeability. Frequently used chills can exhibit hairline cracks, in which capilbary condensation of water vapour  can occur and lead ta gas defects during pouring. It is important to avoid too high compaction in the moulding plant. With high compactian it should be checked whether the compacting pressure has to be reduced. References [1] Walter, Ch.; Gärtner, W.; Siefer, W. Analyse der Putzkosten bei Stahlguß Gießerei 73, 1986, S. 612-620 [2] Schlesiger, W.; Winterhalter, J.; Siefer, W. Zur Gasabführung aus Kernen Gießerei 74, 1987, S. 76-84 [3] Levelink, H. G.; van den Berg, H. Gußfehler aufgrund zu harter Formen Disamatic Tagung 1973, Vortrag 4, Kopenhagen Further references [4] Levelink, H. G.; Julien, T. P. M. A.; De Man, H. C. J. Gasentwicklung in Form und Kernen als Ursache von Gußfehlern Gießerei 67, 1980, S. 109-115 [5] Bauer, W. Einfluß der chemischen Zusammensetzung und des Formstoffes auf Gasblasenfehler im Gußeisen Gießerei-Rundschau 31, 1984, S. 7-13 Giess.-Prax. 1984, S. 198-205 [6] Kulkarni, A. R. Einfluß von Hinterfüllsand auf die Gußstückqualität Indian Foundry J. 26, 1980, S. 36-38 (engl.) [7] Hofmann, F. Einflüsse der Zusammensetzung und des Aufbereitungsgrades von Form- und Kernsanden auf Eisen-Formstoff-Reaktionen und andere Fehler bei Gußeisen mit Kugelgraphit 4. Int. Tagung der Lizenznehmer für das GF-Konverterverfahren, Schaffhausen 1981 Vortrag Nr. 8, 19 S. [8] von Nesselrode, J. B. Gußfehler in Gußeisen mit Vermiculargraphit, die beim Furanharzformen mit Phosphorsäure entstehen können Giess.-Prax. 1984, S. 37-39 [9] Tot, L.; Nandori, G. Verringerung gasbedingter Fehler in Gußstücken Sov. Cast Technol. 1988, S. 4-7 (engl.) Litejnoe proizvodstvo 1988, S. 6-7 (russ.) [10] Nikitin, V. G. Gasporenbildung in Gußstücken unter Einwirkung des hydraulischen Schlages in der Gießform Litejnoe proizvodstvo 1976, S. 28-29 (russ.) [11] Ramachandra, S.; Datta, G. L. Gasentwicklung aus Form- und Kernsanden Indian Foundry J. 21, 1975, S. 17-21 (engl.) [12] Orths, K.; Weis, W.; Lampic, M. Einflüsse von Formstoff und Form, Schmelzführung und Desoxidation auf die Entstehung verdeckter Fehler bei Gußeisen II Giess. Forschung 27, 1975, S. 113-128 [13] Kolotilo, D. M. Gasbildungsfähigkeit und Bildung verkokten Rückstandes der organischen Formkoponenten beim Gießen Litejnoe proizvodstvo 1976, S. 27-29 (russ.) [14] Probst, H.; Wernekinck, J. Zur Gasabgabe und Blasenbildung beim Erstarren gashaltiger Metallschmelzen Giess.-Forsch. 29, 1977, S. 73-81 [16] Perevyazko, A. T.; Nikitin, B. M.; Lozutov, V. N.; Yamshchik, I. I. Untersuchung der Ursachen für Gasblasen in Gußstücken Litejnoe proizvodstvo 1986, S. 6-7 (russ.) [17] Pant, E.; El Gammal, T.; Neumann, F. Einfluß der Schmelzweise und des Formstoffes auf die Gasblasenbildung bei Stahlgußstücken Gießerei 75, 1988, S. 238-245 Fig.24 Formation of a large gas bubble in the top of a grey cast iron radiator. Fig.25 Low alloyed grey iron casting. Formation of surface bubbles in the top part. Abb.26 Grey iron casting. Section through a surface bubble. The cut out segment is on top of the rest of the casting. Hardly any bubbles on the surface.