Preparation of Spectrographic Standards of l o w Boron Content for Determination of Boron in Iron J. C. SHYNE and E. R. MORGAN Scientific Laboratory, Ford M o t o r Co., Dearborn, M i c h .
Spectrographic determination of boron in steel has been hindered by the lack of reliable standards in the low boron range. By using vacuum melting techniques and careful ingot processing, precise boron standards can be made which contain as little as 1 p.p.m. of boron.
BoRoxJ
when present in steel in minute amounts, produces an important improvement in hardening characteristics. Five parts per million is the usual minimum boron' content specified for commercial boron-treated steels. Lately, studies of the effects of boron in steel have been concerned with boron present in even smaller amounts. With increased interest in very low boron contents, improved spectrographic methods for boron determination have been developed (6). Unfortunately, the accuracy of boron determination in the range below 6 p.p.m. has been questionable, because of the lack of reliable spectrographic standards for boron contents of that level. This paper describes a method by which spectrographic standards have been prepared containing as little as 1 p.p.m. of boron. The method of preparation involved the addition of known amounts of boron to iron melted in a vacuum. By the use of careful vacuum melting practice and ingot processing it is believed that 100% recovery of the boron additions was realized. MELTING TECHNIQUE
Vacuum melting was used for the preparation of these alloys, because of the degree of control of the oxygen and nitrogen contamination afforded by the vacuum melting process. Experience has shown that careful melting in air results in only a slight loss of boron (3); however, even the best air melting techniques were judged unreliable for the preparation of low boron alloys of precisely known boron contents. Melting in air would risk the formation of boron oxide and possibly boron nitride and their subsequent loss by interaction with the slag or with the refractory crucible.
cation in the mold eliminated any segregation of boron from the top to the bottom of the ingots. Some slight radial segregation was noted, but subsequent steps in the processing eliminated any compositional errors which could have been caused by this condition. I t is not possible to give any quantitative data concerning the radial segregation. Radial segregation was never examined in the actual standards, as thi- Ivould have required the destruction of large portions of the ingots. The radial segregation of a similar ingot containing a nominal 0 . 0 0 0 6 ~ 0boron was examined. It was found in this 2.5-inch diameter ingot that a surface layer, about 0.25 inch deep, was low in boron, whereas the rest of the ingot cross section was uniform in boron concentration. Segregation from top t o bottom was nil. Likewise, tests on the actual standards showed no measurable longitudinal segregation. PROCESSING OF INGOTS TO SPECTROGRAPHIC PINS
The ingots were not given the prolonged high temperature anneal which is the common practice. This type of treatment was avoided because it has been shown that a t high temperatures boron steels are very susceptible t o progressive boron loss by oxidation a t the surface in much the same way that steels are decarburized. Deboronization has been shown to occur much more readily than decarburization; indeed, it is impossible t o heat-treat boron steels a t a high temperature and avoid deboronization in any reasonably attainable atmosphere other than a vacuum (6).
1
I l l
0 2497.733 FE2496.991
NEW STANDARDS x SECONDARY STANDARD
,0008 .0006
Fifteen-pound heats were made using electrolytic iron with 0.10% carbon added. Upon melting in vacuum (1 to 5 microns of mercury) the carbon combined with the initial oxygen impurity and was drawn off as carbon monoxide. Oxygen contents were reduced in this way from an initial value of 0.0275, to 0.002%, whereas nitrogen, which is readily removed in vacuum, was reduced to about 0.00005%. These remarkably 10x5 residual oxygen and nitrogen contents could not have caused any loss of boron from the melts. It has been observed that boron pickup from refractory materials during melting may be as important as the loss of boron, especially a t low boron levels ( 4 ) . hZagnesia crucibles are especially troublesome in this respect; as much as 0.001% boron has been absorbed by pure iron during melting in magnesia. High purity stabilized zirconia crucibles were used in the present m-ork to prevent boron pickup. Detectable amounts of boron were not picked up by pure iron melted in such crucibles if the melts were held in the crucible less than 30 minutes. Each alloy was held molten for 10 minutes to allow the removal of oxygen and nitrogen before adding boron. The boron additions were in the form of Bureau of Standards spectrographic standard No. 830, containing 0.019% boron. This addition technique was used, because it was the most reliable Ttay of adding a precisely known amount of boron. The heats were held molten for an additional 5 minutes after adding the boron to ?"sure complete solution and mixing of the boron into the liquid iron. Ingots 2.5 inches in diameter were then cast in a watercooled copper mold in the vacuum chamber. Spectrographic analysis showed that the very rapid solidifi-
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.0002
.0001 1.5
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1.7
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LOG OF INTENSITY RATIO
Figure 1. Analytical curve showing ratio of intensities of 2497.733 A. boron line to 2496.991 A. iron line Boron content of secondary standard established by National Bureau of Standards boron steels ( 5 )
The problem of eliminating radial segregation by means of high temperature annealing was avoided by rolling the entire ingot down to the final size. Thus, the average boron content of t h e final product was the same as that of the ingot and any radial segregation pattern in the ingot was simply retained in the proces1542
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
