On the Origin of the White Spot Defect in Vacuum Arc Remelted Ingots

Felix Shved
Laboratory of Special Metallurgy Co.

International Symposium on Liquid Metal Processing and Casting - Presentation at VAR Process Session

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Abstract

Proceedings of the International Symposium on Liquid Metal Processing and Casting. TMS. Santa Fe (New Mexico, USA), September 23-26 2009, pp. 85 - 93.

The main currently known peculiarities of white spots are discussed and explained. A number of proofs are provided that the most probable mechanism of white spot formation includes splashing of molten metal on the side surface of an electrode. Subsequent fall to the liquid pool of spatters, entraining slag from the pool surface, leads to arising in the ingot of randomly located metal impregnations, accompanied by strings of nonmetallic inclusions. Depletion in niobium of white spot area may be explained by the process of isothermal solidification via atoms exchange between spatter residue and surrounding liquid in the deep layers of the pool.
An attempt is made to explain the origin of the defects, observed by a group of investigators in Alloy 718 ingot, remelted at abnormally long arc gap. It is shown that exogenous defects in the case considered are rather not of white spot but of a crown type.

References

1. K. O. Yu et al., Microsegregation in ESR and VAR Processes, JOM, v.38 (1986) № 1, pp.46-50.

2. A. Mitchell, White Spot Defects in VAR Superalloy, Proceedings of the Vacuum Melting Conference on Specialty Metals Melting and Processing, Warrendale, PA, 186, 1986, pp.56-61

3. J. F. Wadier, G. Raisson, J. Morlet, A Mechanism for White Spot Formation in Remelted ingots, Proc.Vacuum Metallurgy Conf. on Specialty Metals Melting and Processing, Pittsburgh, PA, June 11-13, 1984, Bhat and Lherbier eds., Iron and Steel Soc. 1985, pp.119-126.

4. F. I. Shved , An Analysis of the Mechanism of White Spot Formation, JOM, v.46 (1994), № 1, p.36

5. L.A. Jackman , G. E. Maurer, and S. Widge, White Spots in Superalloys 718, 625, 706 and Various Derivatives, Ed. E.A. Loria, TMS, 1994, pp. 153-166.

6. X.Wang , M. D. Barratt, R. M. Ward, and M. H. Jacobs, The Effect of VAR Process Parameters on White Spot Formation in Alloy 718, Proc. of the International Symposium on Liquid Metal Processing and Casting, Еds.: P.D. Lee, A. Mitchell, J.P. Bellot, and A. Jardy, Nancy, France, 2003, pp.99-106.

7. F. J. Zanner , R. L. Williamson, and R. Erdmann/ On the origin of defects in VAR ingots, Proc. 2005 Symposium on Liquid Metal Processing, publ. ASM, New York, 2005, pp. 13-20.

8. F. J. Zanner, Metal transfer during vacuum consumable arc remelting, Met.Trans. B, v.10B, 1979, pp. 133-142.

9. Problems of vacuum arc remelting regime, Zhupakhin A. D. et al., Stal, 1978, № 1, pp. 33-37 (Rus.).

10. F. I., Shved, Vacuum arc remelting – review of the results of some Russian process investigations, Proceedings of the 1994 International Symposium on Liquid Metal Processing and Casting, Eds. A. Mitchell, J. Fernihough, Santa Fe, New Mexico, 11-14 Sept 1994, pp.112-129

11. Felix I. Shved, Ingot of vacuum arc remelting, 428 p., (Rus.), 2009 (to be published), Tatyana Lourie publ. house, Chelyabinsk.