Burnt-on sand

haracteristic features
Thin sand crusts firmly adhering to the casting. The defect occurs to a greater extent in the case of thick-walled castings and at high casting temperatures.
Incidence of the defect
Where there is a heavy section casting, but also in the proximity of the gate and at high casting temperatures, as a result of low thermal resistance the moulding sand sinters on the casting as a thin crust. The tendency of the molten metal to penetrate into the sand pores results in the firm adherence of the crust to the surface of the casting. It is difficult to remove, even by shotblasting, and usually has to be ground off.
Explanations
The high temperature to which the sand is subjected causes sintering of the bentonite and silicate components. In addition, the always present iron oxides combine with the low melting point silicates to form iron silicates, thereby further reducing the sinter point of the sand. Sintering and melting of the impurities in the moulding sand enable the molten iron to penetrate even faster, these layers then frequently adhere to the casting surface.

Possible causes
Clay-bonded sand
Lustrous carbon content too low
Proportion of low melting substances too high
Oolitization too high
Moulding plant
Uneven mould compaction
Gating and pouring practice
Uneven distribution of inflowing metal with resultant local overheating
Temperature of liquid metal too high

Remedies
Clay-bonded sand
Increase proportion of lustrous carbon producer. This increases the amount of coke as well as the amount of lustrous carbon, which then results in positive separation between mould and metal.
Use purer silica sands or, if necessary, add new sand. Reduce dust content. If necessary reduce the amount of bentonite.
Reduce oolitization by adding new sand.
Moulding plan
Ensure uniform compaction. If necessary, increase heat removal from the moulds.

Gating and pouring practice
Even out incoming metal flow
Reduce pouring rate
Reduce liquid metal temperature.

Background information
Adhering sand layers primarily form when the lustrous carbon producing capacity of the moulding sand is too low. With grey iron castings the lustrous carbon content in the sand should lie between 0.2 and 0.6%, according to other authors between 0.2 and 0.4% (1). Due to the difficulty in precisely determining the lustrous carbon in the sand the "active carbon content" is measured and should be between 0.35 and 0.65%.
If sand adherence is experienced this can be eliminated either by using a higher proportion of or a more "active" lustrous carbon producer.Improved coke formation will likewise reduce the formation of adhering crusts but not as much as increasing the lustrous carbon production.
It is important to limit impurities in the moulding sand. Silicates and oxides can lead to excess consumption of lustrous carbon producers due to oxidation (2). Lowering the sintering point of the sand also increases the risk of burning-on, with simultaneous penetration of metal into the adhering layer.
Likewise, intensified burning-on of sand to grey iron castings has been observed with the use of more highly oolitized moulding sands. It is therefore recommended to add an appropriate amount of new sand to that in circulation. According to S&B Industrial Minerals's previous experience the added amount should not exceed 100 kg of new sand + core sand per t of molten iron.
Russian authors report that, when pouring molten steel into sodium silicate bonded moulds, burning-on is drastically reduced when the surface tension is increased through the use of additives. Increasing the AFS number by using finer new sands similarly reduces adherence of sintered crusts because the casting surface is smoother.
The moulds should be well and uniformly compacted. There is a greater risk of metal penetration at locations where compaction is low and thus the formation of adhering crusts.

References
[1] Wirkung von Sorption und Glanzkohlenstoffbildung tongebundener Formstoffe auf Gußstückeigenschaften
Institut für Gießereitechnik GmbH, Abschlußbericht zum AIF-Forschungsvorhaben Nr. 5405, April 1985
[2] Winterhalter, J., Siefer, W.
Zur Wirkung von Feinanteilen und Glanzkohlenstfoffbildnern im Formstoff auf die Gußstückeigenschaften
Gießerei 74, 1987, S. 633-639
Additional references
[3] Grochalski, R.
Gießereiformstoffe, 1955, S. 22
[4] Disamatic-Application "Gußfehler", S. 78 - 84
[5] Berndt, H.
Die Überwachung von Verschleißerscheinungen an einem Sandumlaufsystem
Gießerei 55, 1968, S. 441-453
[6] VDG-Merkblatt F 252, "Bestimmung der Anschnittelemente"
[7] Holzmüller; Kucharcik
Atlas der Anschnitt- und Speisertechnik für Gußeisen 2. Aufl. 1975, S. 17
[8] Boenisch, D.; Lorenz
Modellversuche über das Formkastenfüllen von Naßgußsanden, Dissertation 1988, TH Aachen

[9] Onillon, M.; Rebaudieres, J.
Physikalische und chemische Vererzung bei Gußeisen
Fonderie 31, 1976, S. 209-216 (franz.)
[10] Paskeev, I.
Untersuchungsverfahren zur Bildung von Anbrennungen an Gußstücken
Litejnoe proizvodstvo 1977, S. 26-28 (russ.)
[11] Aymard, J.-P.; Leger, M.-T.; Lageal, B.
Metall-Formstoff-Reaktionen von Manganhartstahlguß (12% Mn) und Chromstahlguß (13 bis 25 % Cr)
Fonderie 31, 1976, S. 265-273 (franz.)
[12] Ivanov, N. Ch.; Skljarova, V. N.
Formstoffmischungen mit Dibutylphthalat zur Herstellung von penetrationsfreien Gußstücken aus Gußeisen
Litejnoe proizvodstvo 1976, S.18-19 (russ.)
[13] Sarma, A. K. D.
Vererzen von Formsanden
Indian Foundry J. 18, 1972, S. 167-170