Rapid M e t h o d of Determining Minute Quantities of Carbon in Metals J.
K. STANLEY
AND
T. D. YENSEN
Westinghouse Research Laboratories, East Pittsburgh, Pa.
A rapid method for the determination of minute amounts of carbon (under 0.017') is described. Its advantages are rapidity of analyses, (about PO minutes), great sensitivity (Imm. = 0.000ly0 carbon), and an accuracy of *0.00053;6. The factors which affect the accuracy and reliability of the method are: preheating the sample, temperature and time of oxidation, effect of oxygen pressure on oxidation, formation of carbon monoxide, effect of sulfur, adsorbed carbon dioxide, accumulation of carbon dioxide on standing, and carbon in the residues. Both milled and thin strip samples can b e used, precautions necessary in their preparation are discussed. Results for various samples of materials are presented.
and Murray and Ashley, General Electric Company, have reviewed their procedure (4). Murray and Niedrach have since simplified their apparatus by substituting stopcocks for mercury cutoffs ( 5 ) . While improvements in technique have been made, the basic principles, as devised by Pensen ( 7 ) in 1920 are the same in all present apparatus : Evacuation of system before heating the sample. Heating and burning the sample in pure oxygen. Freezing out water and possibly sulfur trioxide with dry ice, and carbon dioxide with liquid air (oxygen or/and nitrogen). Determination of the carbon by heating the frozen out carbon dioxide to room temperature and letting the gas expand into an evacuated known volume and observing the pressure.
P
ERHAPS in no other field has the accurate determination of carbon, below O.Ol%, been of such commercial importance as in the study and use of soft magnetic materials. It is well known that impurities in soft magnetic materials, such as oxygen, sulfur, nitrogen, and particularly carbon, must be kept low to obtain low hysteresis loss and high maximum permeability ( I , a). I n order to control the amount of carbon and to study its effect on magnetic properties, methods of analysis had to be developed that are much more accurate than conventional methods in which an accuracy of only *O.Ol% can be claimed. The first improved method was developed by the senior author ( 7 ) and detailed improvements were made by Ziegler (9). Other developments have included separation of sulfur dioxide from the carbon dioxide in a somewhat modified apparatus (8) and the use of high-frequency heating (6). Very recently Gurry and Trigg (a), U. S. Steel Corporation, have discussed the precision and accuracy of the method described by Wooten and Guldner (6), Bell Telephone Laboratories,
The present paper shows how a complete, accurate analysis can be made in 20 minutes instead of requiring 35 minutes to 2 hours with the apparatus used by the above authors (3-6). This presupposes a single apparatus operated by a single operator including weighing and blank determinations. (Murray and Ashley state that by using four separate apparatus a single operator with the assistance of one laboratory technician can analyze at the rate of one sample every half hour during an 8-hour day. This means one man-hour per analysis. Murray and Niedrach state that by their simplified apparatus they can make 10 to 15 analyses in one 8-hour day, which means 35 to 48 minutes per analysis.) APPARATUS
The authors' latest apparatus for determining carbon is shown schematically in Figure 1 and a photograph is shown in Figure 2. The apparatus can be considered as comprising the oxygen puri-
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TO MERCURY PUMPS
LIQUID AIR
TRAP
CALIBRATED VOLUME
;'CLASS TUBING
TWO W*" VACUUM
ANALYZING
Figure 1.
Apparatus for Determining Carbon
699
SYSTEM
INDUSTRIAL AND ENGINEERING
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noation system, me unit, analyzing mation. Not shown are the high-frequency oscillator and the vacuum pump.
OXYQENP U ~ C A T I SYSTEU. OU The oxygen puri6est;on ay:!.atemin a h o m outlined by d n i i n d dash line3 at the left in Figure 1. Its purpui~ is to remove iniourities such an cnrhon mouuuide. carbon dioxide. methane. snd water from the OXY: ~
bon monoxide ahd hydr&arbons tobsrbon dioxide) into an evacuated reservoir, whence it itr slowly bubbled through liquid oxygen in a trap, oooled with liquid air (or nitrogen) into evacuated and flamed storage reservoirs to a pressure of about half a n atmosDhere. Thc orypn.is then ready to use. That theoxywen is eulticiriitl.v pure is shown hy the fait l h a t no niemur~lili* uutniiit of mrhon dioxidc is ohtninrd . when the ususl m o u n t of oxygen is passed througg the heated furnace and analyaed-i.e.. the amount of carbon is less thnn 0.001 mg. in the oxygen necessmy far one analysis. No increase in the blanks hns been noted even when the storage oxygen has been allowed t o stand for several weeks. ComnsnoN SYSTEM. The combustion tube is of a simple construction. A 5-em. (zinc)?) outside diameter satin-finished quart5 tube 18 sunvorted in a horizontal oosition and a comer tub;, coil for high-freque&y heating is slib6ed ~~~~
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4+ "... .... Inside the qusrta tuhe, undor the hi%h-frrqucncy
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coil, i9 l o e n t d a pure nickel tube nhwh i9 usud tn hent t1.r ruekc1 h a t eontninine thc s m v l e on a bed of fused (100-mesh) al&in&. T h i nickel tube is approximately 15 cm. (6 inches) long and 2.2 cm. ('/. inch) in inside diameter and has a 0.16-0m. l ' l ~ i n c hw~all. The tube. before it can be sstisf&to&y used, is carefully decarburized by annealing the tube i n wet hydrogen at 1200' C. fur n b u t 30 hours. (Hdrogcn w i u a t e d with moisture nt room temprmturc, 3 to 4 7 by volume, is satid.~ctory.) The nickel bo31 (supplied by Pisher Scientific Coml,nny, Pi~tsbureh.I'a ...whirl~~*eveurunllv Glled with IllO-meshAlunduni sand (Ai;ddum for carbon analysis furnished by the Norton Company, Worcester, Msss.), is likewise conditioned by preliminary heating in wet hydrogen. ANALYZING SYSTEU. The analyzing unit, shown at the righthand side of F i m e 1. consists of a device for measurine the DESewe ni the p.w in a csbbrntcd w l u m e . After 1l.e carbon dihxide i s lrnnsfrrrrd from tho li
