ANALYTICAL CHEMISTRY
1636
he omitted. However, the practice of analyzing a sample, of NBS No. 4 steel after each sample has been continued because it gives B good indication of the reliability of the results. ACKNOWLEDGMENT
The assistance given by many members of the Metallurgy Research Department, General Electric Co., is acknowledged. I n particular, thanks are due t o R. L. Hadley for preparing the "1% oxygen" zirconium and A. U. Seybolt for metallographic examination of the "1% oxygen'' zirconium. L. 8. Bronk of the Analytioal ChemiPtry Unit performed the carbon analyses.
(3) IXorton, W.S.,and Rrady, J., Zbid., 25, 1891 (1953). (4) MoGeary, R. K.. Stanley, J. K., and Yensen. T. D., Trans. Am. SOC.Metals,42,900 (1950). (5) Mallett, M. W., Ihid.,41,870 (1949). (6) Naughton, 3. J., and Uhlig. H. H., IND. ENO.CHEM.,ANAL.ED.. 15,750 (1943). (7) Sloman. H. A,. Iron Steel Inst. (London), Spec. Rept. No. 9. 83 (1935). (8) Slornan. H. A,, J . Zmt. Metals, 71, 391 (19415). (9) Sloman, H. A,. and Harvcy, C . E.,Zbid., 80 , 391 (1951). (10) Smith, W.H.. ANAL.CXEM.,27,1636 ( 1 9 W I (11) Stanley, J. K..yon Hoene, J., and Wienei, "., l y m . , (1951). (12) Thompson. J. G.,Vachcr. ri. C . . atid Bri, Natl. Bur. Standards. 18, 2% (1937). (181 Walt,ar. n. ILANAL.C a e ~ .22. . 297 11950
LITERATURE CTTED
(1) Derge, G..J . Metetala, 1,31 (October 1949). (2) Guldner, W. A., and Beach. A. L., ANAL. CHEM.,22, 366 (1950).
10. 1954. Accepted M w 24. 1955. PreAnalytical Chemistry a t the 126th Meeting . C ~ T Nev Y , York, N. Y. of the Anaasrcm C x ~ ~ i c aSr O
R JCEIVED for review Deoernber sented before the Division of
Yacuum Fusion 1Analysis by 1:he iron Bath Technique W. H. SMITH General Electric Research Laboratory, Schenectady, N. Y, A n investigation of the reason for the increased riscosity of the iron bath used in fusion analysis reveals that the cause is precipitation of graphite as a solid phase. The iron bath beromes semisolid within 2 hours at 1800" C. A I tion of graphite is propo
CDONALD, Fagel, and Bails ( I , nave mown mar a major murce of error in v&cuum fusion analysis by the iron bath technique is the tendency of the bath to become very viscous, or solidify. I n an attempt to determine the cauEe for this behavior the investigation outlined below was undertaken. To a graphite crucible previously outgassed a t 2300" C. were added 20 grams of lov-carbon, low-oxygen iron. The temnem.t.ure WR.R raised and~ held ~ for 2 hours. then cooled ~ . ~ . . to ~1800" ~ C.~ to room temperature. A second experiment waa performed in the same manner. except that a temperature of 2000' C. u'aa used. Bath experiments were carried out in vacuum. It re quired about 7 minutes for the melt to cool to a temperature of 1135' C.. where solidification should be comnlete. Macrh graphs and' micrographs of sections through each inrot are shown in Figures 1 to 5. ~~~
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Examination of these photographs reveals a high ooiicent.rstion of large graphite flakes near the top of both ingots mith much higher concentration occurring in the 2000' C.melt. The finer graphite flakes are believed to have been formed during cooling of the melt, There is no evidence of any iron carbide in any of the micrographs of either heat. The very heavy concentration of graphite flakes near the top of the melt would cause thip region to become very viscous or pasty. Such a condition at the top of the bath would not dlow dropped samples t o penetrate into the bath, The presence of the entrapped gas bubble in the 2000" C. melt is further evidence of the high viscosity near the top of the bath. It is possible to account for this particular distribution of graphite a8 follows: when the iron is heated in contact wit.h graphite, the bath becomes saturated with carbon. Because of thermal gradients in the bath, loss of iron by evaporation, and radiation lasses, some free graphite is either precipitated or carried to tho top of the bath. Once formed, these particles can continue to grow, as thermal gradients will supply more carbon to the growing graphite flakes.
The importance of not holding the r o n bath a t hlgb temperatures for long periods of time pan thus be understood As it has been shown by Sloman, Harvey, and Kubusehevski ( 4 ) and
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
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Figure 4. Micrograph of center section of iron ingot held in a graphite crucible Two hours at 2000' C. showing fine graphite, X25
Figure 2. Macrograph of section through iron ingot melted in graphite crucible Held2htiur:ursataOMl°C.. X 4
Figure 5. Micrograph of bottom section of iron ingot held in graphite crucible T w o hours at 2W0' C., X30
Outgassing of the initial iron bath can easily be done in a short time at 1800' C., especially if iron of low oxygen content is used a8 a charge. LITERATURE CITED
Figure 3.
Micrograph of top section of iron ingot held in a graphite crucible
Two hours at 2000' C. ehowing large graphite R a k e , x25
.
(1) McDonald. R. S., Fagel, J. E., Jr., and Balis. E. W.. ANAL C n a ~ .27,1632 , (1955). (2) Mallett, M. W.,and Griffith, C. B.. Trans. Am. Sac. Metals, 46. 375 (1954). (3) Siornan. H.A,, Harvey, C. A,. and Kubusohevski, 0..J . Znnat. Melds, 80,391 (1952). R E C E I ~ Sfor D review December 10,1954. Acoepted May 24.1955
