V O L U M E 2 7 , NO. 10, O C T O B E R 1 9 5 5 sed pins. Such radial segregation does not affect analytical result if a spectrographic method is used in which a representative crom section of the sample material is consumed during excitation. The ingots were reduced by hot rolling to 0.25-inch diameter rod. In this step the problem of deboronization again had to be reckoned with. I t is known that boron diffuses much more rapidly in the high temperature fxe-centered-cubic modification of iron than it does with body-centered-cubic form ( I , 2 ) . I n pure iron the body-centered-cubic phase is &able below 910" C. Therefore, all rolling and processing were limited to temperatures below 865" C. After hot rolling to 0.25-inch rods, the surfaces of the standards were cleaned by centerless grinding to inrh in diameter. Figure 1 shows an analytical curve made using the three nex low boron standards and one other established standard. The spectrographic data obtained were by the Runge, Brooks, arid Bryan ( 5 )improved method for determination of low boron content. The agreement between these data is another indication of the reliability of standards produced l)y the described method.
1543 Although the analytical curve appears linear, it is not feasible to extrapolate data from high boron standards to low boron levels. Different exposure conditions are required for the accurate determination of high and low boron contents, making reliable low boron standards necessary. LITERATURE CITED (1) Buyby. 1'. E., and Wella, C . , J . Metals. 6, A I M E T r a n s . , 200, 972 (1954).
(2) Busby, P. E., Warga, 11.E.. and Wells. C., .I. Metals. 5 . A I M E Trans.. 197, 1463 (1953). (3) Diggs, T. G., and Reinhart, F. lI.,,I. Research S a t l . B u r . Standards, 39, 1815 (1947). (4) llornan, E. R., and Frey, D. S . , J . Electrochem. SOC,102, 61-13
RECEIVED for review March
24, 1955.
-4ccepted .iugiist 10, 19.55
Spectrographic Determination of Trace Quantities of Boron in Steel E. F. RUNGE, L. S. BROOKS, and F. R. BRYAN Physics Department, Scientific Laboratory, Ford M o t o r Co., Dearborn, M i c h .
.\ quantitative spectrographic procedure has been devised for the determination of boron in steel within the range 0.0001 to 0.0006%. Precision of the method is *lo% at 0.0003% boron concentration. Accuracy is estimated to be within 0.00005% boron. The improved procedure combines the following features : measurement of the 2497.73 A . boron line by means of a Littrow spectrograph crossed with an echelle; recording of only the initial portion of the arcing period in order to improve line-to-background ratio; superimposition of several spectra to obtain adequate over-all spectral intensitp; and quantitative calibration based on materials prepared by diluting a high concentration standard w-ithelectrolj tic iron.
C
OMMERCIAL boron steels have no rigid specifications in respect to boron content since production and inspection control is difficult, and since absolute minimum and maximum effective quantities are not, as yet, well determined. ,4pparently, boron can be helpful in promoting hardenability of steels when present in concentrations roughly behveen 0.0005 and 0.02%. And for this concentration range, considerable successful work has been done in this country with both chemical and spectrographic (4,6, 9, I O ) analytical procedures. Since 1946, the National Bureau of Standards has provided reliable standards ( 7 ) together with spectrographic procedures ( I ) to cover essentially this range. Current metallurgical research in this laboratorv on the mechanism of boron in the hardenability of steel has required extension of analytical procedures to include minimum effective quantities with a fair degree of accuracy. As a result, emphasis has been placed on the concentration range from 0.0006 to O.OOOl% boron. In extending the detectability of spectrographic procedures, several serious difficulties needed to be surmounted. First, it waB considered desirable to use the most sensitive boron line a t 2497.73 A. which is within 0.09 A. of an iron line of comparable intensity. Corliss and Scribner ( 1 ) have suggested that a twofold increase in sensitivity might be realized by using this line,
which is the stronger of the t F o boron lines available in this rrgion of the spectrum. The instrument used to resolve the prcferred line is a Bausch & Lomb echelle attachment crossed with :t Littrow quartz prism spectrograph ( 6 ) . This arrangement provides sufficient dispersion and resolviiig power to separate adequately the desired boron line from it? neighboring iron line. A second difficulty lay in the inherently poor line-to-background inteiisity ratio for boron when conirentional excitation and exposures are used (9). The ert,reme volatility of boron relative to iron, however) will allo~wthe boron line to be recorded before an appreciable background accumulates. Thus the superimposition of spectra from short exposures of separate samples provides a more favorable line-to-background ratio than a spectrum from a single long esposure. The third problem was that of calibrating for a concent,ration range where chemical standardization is impractical. Rather than attempt analyses on a few grams of sample where the absolute error is likely to equal the concentration, it was decided to synthesize calibration materials on a large enough scale to make weighing errors and contamination negligible quantities. This latter procedure is practical for boron steels, provided that high purity electrolytic iron and vacuum melting equipment are employed. APPARATUS
Commercially available spectrographir equipment is employed throughout. A direct current arc source is used with an opencircuit voltage of 250 volts. Two mercury-vapor rectifier tubes produce a fully rectified direct current arc discharge. A Bausch & Lomb echelle grating attachment crossed with a
Table I. Quantities of NBS No. 830 Steel Added to Electrolytic Iron to Produce Standards of Lower Boron Concentration Amount of NBS Steel Added, Grams
Resulting Standard, % Boron
226.6 109.0 34.9
0.0006 0,0003 0,0001
ANALYTICAL CHEMISTRY
1544
large Littrow quartz spectrograph is employed. This optical arrangement results in a reciprocal linear dispersion of 0.31 A. per mm. a t 2500 A. The echelle-Littrow system is illuminated by forming an image of the electrode gap on the echelle slit by means of a spherical crystalline quartz lens. The spectrum is recorded photographically on Eastman Spectrum Analysis KO.I plates which are developed in a thermostatically controlled processing machine. Transmittances of the analytical lines are measured on a recording-type projection microphotometer. PROCEDURE
Preparation of Standards. Three standard steels were synthesized in the following manner. Fifteen pounds of electrolytic iron were melted in a vacuum furnace for each of the standards. The iron selected was examined spectrographically and was found to be free of boron within the limits of sensitivity of this method. To the molten electrolytic iron, accurately weighed amounts of Sational Bureau of Standards steel containing 0.019% boron were added. Quantities used are shown in Table I. The vacuum melted ingots were then rolled to l/r-inch rod a t a temperature below 1600" F., a range wherein the diffusivity of boron is low enough to prevent significant loss by oxidation a t the surface. From the rod, slugs of 7/32 inch in diameter and 1/4 inch in length were machined to serve for the calibration. The diameter n-as selected to conform with conventional rod-type steel samples obtained for production control by either the glass tube sampling method (8) or by the cast pin technique ( 5 ) .
The counterelectrode (cathode) is also made from purified boron-free graphite 11/2 inch long and, 5/18 inch in diameter by reducing the diameter to 6 / 3 ~ inch over a distance of 3/8 inch from the top. The restricted diameter of the counter electrode confines the arc position and thus maintains the necessary optical alignment. Analytical gap width is monitored a t 2 mm. by manual adjustment of the electrode holders. Gap spacing can be maintained to within *O.l mm. by means of a projected image of the gap on a reference L target. EXCITATION
Standards and samples are excited in a direct current arc obtained from a 250-volt regulated power line. A variable transformer provides a current of 10 amperes when the electrodes are short circuited. Presumably, an interrupted direct current arc (9) or an overdamped condenser discharge ( 3 )could have been used with equivalent success, had such units been available in this laboratory.
L-$4 Figure 2. Preformed graphite anode
2 4 9 7 73 B
00037 % E
+
_-
W a
2 4 7 2 Fe SAMPLE NOT PREVIOUSLY ARCED
Figure 1.
SAMPLE PREVIOUSLY ARCED
Effect of arcing on boron line intensity
100'
FIRST
SECOND IO SECONDS
IO SECONDS
Analysis of sections of the cast cylindrical ingot indicated some radial segregation of boron. Therefore the rolling operation, in reducing this cross section, minimizes the effect of segregation on analytical results. Precision data are noticeably better on segments of rod than on segments of the corresponding ingot.
2497;
Figure 3.
THIRD IO SECONDS
Time-intensity characteristics of boron and adjacent iron line
I
Fe
00037% B
Preparation of Samples. 4 representative sample of steel is machined into four cylindrical slugs, each inch long and having a diameter of 7/32 inch. Extreme care must be exercised to prevent the sample from coming into contact with boron-containing materials such as borax cleaners. Precaution should also be taken to avoid the use of metal which has been previously arced or heat treated to the extent of having lost boron by oxidation. Slugs taken from the ends of rods previously arced provide analytical values much lower than slugs taken from the opposite unburned ends of the same rods. The extent of discrepancy between boron line intensities obtained from a sample not previously arced and one which has been previously arced is illustrated in Figure 1. ELECTRODE SYSTEM
The lower, sample-containing, electrode (anode) is formed from a purified boron-free graphite rod 11/2 inch long and 5/18 inch in diameter, by drilling a hole inch in diameter and 1/8 inch deep in one end to receive the metal slug. I n order to concentrate heat in the slug, the graphite electrode is undercut to a diameter of 1/8 inch over a length of inch a t a distance of 6/le inch from the top. Figure 2 illustrates this electrode shape.
Fe
~
CONTINUOUS 210 SECOND EXPOSURE
FOUR SUPERIMPOSED 20 SECOND EXPOSURES
Figure 4. Comparison of a single long exposure and superimposed short exposures
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
1545
1 40t n Fixing Washing
Eastman rapid liquid fixer for 1 minute Running tap water 3 to 5 minutes, distilled water
Drying
Warm air blast for 3.5minutes
rinse
EMULSION CALIBRATION AND PHOTOMETRY
WAVE LENGTH
Figure 5.
Typical microphotometer recording EXPOSURE CONDITIONS
Instrument settings and exposure periods are as folloxs: Spectral region Echelle slit width Littrow slit width Pre-exposure period Exposure period
The emulsion is calibrated, normally only once for each emulsion lot number, from a separate iron arc spectrum. This spectrum provides a group of lines of known relative intensities ( 2 ) which are plotted against their corresponding transmittance values to produce the characteristic emulsion response curve. Transmittance measurements are made with the microphotometer for the analytical line boron 2497.73 A. and the internal standard line iron 2496.99 A. (Figure 5 ) . The iron reference line is chosen to match the boron line as nearly as possible in transmittance as well as in wave length and exitation potential. The exposure conditions provide measurements within the 20 to 80% transmittance range corresponding to the linear portion of the emulsion response curve. Spectral recordings of the boron line obtained from standards containing 0.0006, 0.0003, and 0.0001% boron are shown in Figure 6. Transmittances of the
2200-2850 A. 0.050 mm. 0.4 mm.
None 20 seconds (4 superimposed) 0.0005
2497.733 Fe 2 4 9 6 9 9 1
4c 2497.73 B
V W
7
E
0.0002
W V
5
60
k-
k
0.0001
ul
z K
t-
z w
/i B
K 0
T.5
d T6 T7 TB T9 0 01 LOG INTENSITY RATIO
80
Figure 7 . Analytical curve based on synthesized standards
E n. W
100 00006%
Figure 6.
B
0 0003 % B
0.0001% 8
Recorded spectra from synthesized low boron standards
The selection of the exposure period is based on a study of line intensity versus time relationships of the boron and iron lines. The results of this study indicate that boron tends to vaporize and produce intense lines during the first 10-second period, after which there is a sharp diminishing of the boron spectrum. Figure 3 illustrates the time-intensity characteristics of the boron and adjacent iron line as photographed during the first, second, and third 10-second intervals after striking the arc. It was necessary to superimpose four separate spectra from each period to obtain ample line density. A comparison of boron and iron line intensities for a single long exposure and for a series of superimposed 20-second exposures is shown in Figure 4. It is apparent that a series of short exposures, superimposed, provides more favorable relative line intensities than a single long exposure. PHOTOGRAPHIC PROCESSING
Emulsion Eastman SA No. 1 plates Development Eastman D-19, agitated for 3.5 minutes at 68' F. Stop bath Dilute acetic acid for 30 seconds
analytical lines and the internal standard lines are converted to log intensity ratios by means of the emulsion calibration curve This procedure applied to standards serves to establish an analytical curve relating log intensity ratio to concentration for the pair of lines (Figure 7). Log intensity ratios for analytical determinations are converted to concentrations by referring to the analytical curve. Since 20-second exposures of four separate pieces of sample are superimposed on the plate to produce one echellegram, the log intensity ratio obtained from the reading of sample lines is, in effect, an average of four runs, PRECISION AND ACCURACY
Nidpoint in the concentration range, the 0.0003% boron standard has shown an average deviation of &lo%. The 0.0001% boron standard repeats to within approximately the same absolute value of 2~0.00003% boron, representing a deviation of *30% of the amount present. The 0.0006% boron standard has not yet been tested for precision, but would be expected to repeat within &IO% of the amount present. Accuracy of the results depends largely on the precision of the spectrographic determinations] the care used in sampling, and the reliability of the reference standards. Of these, reliability of the reference standards is thought to be the limiting factor. The amount of boron that may be present in the electrolytic iron is unknown, and, although undetectable by this same nnalyticnl
ANALYTICAL CHEMISTRY
1546 procedure, may exist to the extent of perhaps 0.00005% without being observed above background. ACKNOWLEDGMENT
The authors wish to express their appreciation to E. R. Morgan and J. C. Shyne, Scientific Laboratory, Metallurgy Department, Ford Motor co.,for directing the preparation of the standard steels. LITERATURE CITED
( I ) Corljss, c. H., and Scribner, B. F., J . Research S a t l . Blcr. S t a d Urds, 36,351-64 (1946) (2) Dieke, G..H., “Studs of Standard Methods for Spectrographic
Analysis,” War Production Board Report W-90 (March 20, 1944). (3) Hasler, M. F., and Dietert. H. W., J . O p t . SOC.Arner., 33, 218-22 (1943). (4) Irish, P. R.. Ibid., 35, 226--33 (1945). (5) Irish, P. R.. Steel, 113, 100~-5,127-30 (1943). (6) Kirchgessner, W. G.. and Findelstein, 5 . .i., AS.
