Analysis of tungsten carbides by X-ray fluorescence spectrometry

Talonra, Vol 23. pp 815-818

PergamonPress,1976.Prmted

m Great Bntam

ANALYSIS OF TUNGSTEN CARBIDES BY X-RAY FLUORESCENCE SPECTROMETRY K. KINSON, A: C. KNOTT and C. B. BELCHER The Broken Hill Proprietary Company Limited, Central Research Laboratories, Shortland, N.S.W. 2307, Australia (Received 20 April 1976. Accepted 29 June 1976) Summary-Five sample presentation techniques were examined for the X-ray fluorescence spectrometric analysis of tungsten carbide alloys in powder and cemented forms. Powder samples may be oxidized by air at 600” before fusion (I), or preferably by lithium nitrate during fusion (II); the fusion is effected with lithium-lanthanum tetraborate followed by briquetting with graphite. Powder samples may also be blended with wax and briquetted (III). Cemented carbides are surface-prepared with silicon carbide before analysis (V). Briquettes prepared by blending carbide powder, lithium-lanthanum tetraborate and graphite (IV), give poor reproducibility, however, owing to micro-absorption effects the technique is not recommended. The determination of eight common elements in tungsten carbide is discussed and the relative standard deviations are 0.002-0.004 for major and 0.008-0.01 for minor elements.

mediate and final products; to evaluate sample preparation and presentation methods compatible with point-to-plane OES techniques.’ Five sample preparation/surface treatment techniques were examined: prior (I) or in situ (II) fusion conversion, of carbides into oxides, fusion with lithium-lanthanum tetraborate, blending with graphite and compaction to a briquette; blending of powdered carbides with wax (III) or powdered lithium-lanthanum tetraborate and graphite (IV) and compaction to a briquette; and sintered solids (V) with diamond or silicon carbide grit polishing.

The analysis of tungsten carbides (WC) by optical emission spectrometry (OES), the manufacturing process, the cost benefits of composition control, the preparation of standard samples and a literature sur-

vey have been discussed previously.’ X-Ray fluorescence spectrometry (XRF) of cemented hard metals or briquetted pre-sintered products affords a convenient method for the precise determination of tungsten, tantalum and niobium and is also suitable for titanium and cobalt, the other major alloying elements. Accordingly, tungsten carbides have been analysed by XRF, using the following sample preparation techniques: solutions,2 prior oxidation and fusion with borates, incorporation of barium as a heavy element absorber3s4 pressure-compacted powders5 and cemented hard metals.6*7 The objectives of the present work were: to develop flexible multi-element XRF methods which could be applied to unknown samples; to achieve the standardization of a suite of powdered and cemented reference standard samples for the direct analyses of inter-

EXPERIMENTAL Apparatus

Spectrometer, Siemens automatic vacuum path SRS-1; generator, 4 kW Kristallflex IV; X-ray tubes, Cr 2.6 kW or MO 3 kW; sample holders, sintered carbon, rotated during irradiation; crystals, LiFZZO and Ge,,, ; collimator, 0.15”; detectors, gas flow proportional 90% Ar-10% CH4 (Dz) or scintillation, Tl-NaI (Sz).

Table 1. X-ray fluorescence spectrometric measurement conditions Element Nb Ta Ni co Fe V W Ti W Ti

Method

Line

all (I-V) all (I-V) all (I-V) all (I-V) all (I-V) all (I-V) fusion (I & II) fusion (I & II) unfused (III & IV) & cemented (V)

Crystal

Detector

sz

LiF 220 LiFzzO LiFzzo LtFzz0

MO MO MO MO MO MO MO

Dz Dz Dz Dz sz

Geiii

Cr

Dz

LiFz2a

MO

sz

MO

Dz

J-1Fzzo LiF220 LiFzzO

815

X-Ray tube

sz

K. KINSON, A. C. KNOTTand C. B. BELCHER

816 Measurement conditions

Measurements terminated at a time (4t%2OOsec) adequate to accumulate counts sufficient for the required precision, using the conditions set out in Table 1. Reagents

Graphite powder, briquetting grade; lithium nitrate, anhydrous; fusion mixture (< 500 pm) (64% lithium tetrabora&36% lanthanum tetraborate); cold-bonding paraffin wax powder; Johnson Matthey spectrographically pure oxides heated at 60&900” immediately before use. Recommended procedures Fusion briquettes. Hand-crush solid or chip carbides to 5OOpm in a tungsten carbide mortar and further crush to < 125 pm in a lo-cm3 tungsten carbide grinding barrel on a “Siebetechnik” vibratory disc mill. Grind tungstic oxides to ~63 pm in a tungsten carbide grinding barrel and dry at 1lo”; prior oxidation of carbides can be achieved if desired by heating in air at 600” for 30 min. Weigh 1OOmg (I) (~63 pm oxide) or 85 mg (II) (< 125 pm carbide) into a 95/5x Pt/Au crucible containing 2 g of fusion mixture and 500 mg of lithium nitrate and mix with a platinum wire; also prepare calibration standards from appropriate pure oxides. Oxidize carbide samples in situ by Heating from 400” to 800” (7”/min). Fuse all samples at 980” for 10min. Cool, recover the fused bead, weigh and add graphite powder (1.5 x recovered bead weight). Mill the fusion bead and graphite powder for 3 min in a lOO-cm3 tungsten carbide grinding barrel. Press the milled powder to a 25 mm diam. briquette at 900 kg/cm’ for 20 sec. Prepare a fresh surface for irradiation b; rubbing on P600 &it Sic abrasive paper by hand. Wax-bonded carbides UIIL Weieh 5 e of carbide vowder samples (i 10 pm) or Aalised calibrition standa& into a phial containing 50 mg of cold-bonding paraffin powder and place on a high- peed mixer for 1 min. Add 5 g of boric acid to a press (55 mm ram) to form a backing, add the carbide-wax mixture and press at 5000 kg/cm’ for 20 sec. Cemented hard metals (v). Machine-brush on 60 grit silicon carbide abrasive discs. Use a fresh disc for each re-preparation. Assays are referred to appropriate fused, wax-bonded or cemented solid cahbration standards, using synthetic oxide, or powdered and cemented carbide primary standards as described previously.’

RESULTS AND DISCUSSION The preparation, testing and standardization of a suite of powdered and cemented carbides has been discussed by the authors.’ Other matters discussed were the possibility of elemental inhomogeneity, surface preparation techniques for cemented solid carbides and oxidation and fusion techniques for powdered oxides and carbides. Klyachko and Yakovleva8 state that tungsten losses are significant when tungstic oxide is heated at 900’, whereas 3-hr ignition tests at 750” (Gabler and Petersong) showed that losses are confined to volatile elements, notably molybdenum, lead and tin. Blaas et al3 have advocated oxidation of carbide samples at 450” and this low temperature has particular application to the analysis of samples containing molybdenum. A study has been made of potential losses due to high-temperature oxidation; the oxidation of WC powders was necessary so that standar-

dization analyses could be done against synthetic oxide standards. Isothermal weight-change studies as a function of time were carried out by heating in air a l-g (< 125pm) sample (60% W, 11% Ti, 8% Ta, 1% Nb as carbides + 13% Co). Maximum weight increase was achieved in 15 min at temperatures in the range 600”-900” (loo0 intervals) and was in accordance with stoichiometric oxide formation and elimination of carbon; the stoichiometry of the conversion was not of great significance in view of the known uncertainty in the state of oxidation of ignited cobalt oxide. At 450” oxidation was only 50% complete in 45 min, and after 3.5 hr the sample still contained 0.8% residual carbon. Accordingly a 30-min oxidation at 600” was incorporated in the recommended procedure. The fused oxide/briquette techniques described in the recommended procedure derive from the work of Hasler” and subsequent developments.‘~’ l-l3 The present work required an assessment of the potentialities of the fusion/briquette techniques and the standardization of the prepared samples. Accordingly, 6 calibration mixtures were prepared from pure materials and fused and briquetted to cover the ranges: 2&95x; TiOz, &20x; Co304, TazOs, WOJ, &15%; MOO,, Fez03, SiOa, A1203, &5x; NbzO,, Mn,O,, &2%, and 20 other oxides, including VZOs, < 1%. For major elements, the incremental additions were randomized between the various mixtures and bore no relationship to the carbide compositions being analysed. The concentration ranges of Fe203, SiOl, A1203, Mn304 are not relevant to the analysis of hard metals but were added because a variety of oxide products and residues are analysed by the same technique and it was desired to use a flexible, wideranging system to reveal interelement effects. A further 6 calibration standard briquettes were prepared to approximate to the composition of the 6 standard carbide samples after oxidation. With flux/sample ratio of 21/l and lanthanum oxide/sample ratio of 3.15/l, matrix effects were found to be negligible. The fusion briquette composition represents both increased dilution and lanthanum concentration as compared with the concentrations recommended by Norrish and Hutton.’ ’ However the freedom from interference effects is considered to justify the lower X-ray intensities, because results are obtained without the use of extensive computations. The further availability of the briquettes for optical emission measurements is of great benefit.’ Calibration graphs were linear for the elements measured and correlation coefficients and lower limits of detection are given in Table 2. The lower limits of detection, calculated as the equivalent of three standard deviations of the background, are adequate for present purposes. With this system a notable spectral interference is Ti K, with VK,; however, measurement of the apparent VK,/TiK, ratio in vanadiumfree standards provided a correction factor which was applied successfully to the VK, count-rate for calib-

817

Analysis of tungsten carbides Table 2. Calibration data for the fusion briquette procedures (I and II) Range % (nominal)

Element

W

equivalent ‘% element

cps/lo/ element

2G80 O-15 (rl5 t&15 o-5 G2 (rl G-1

Ta Ti co Fe Nb V Ni

Lower limit of detection*

Background Sensitivity

32 35 740 52 36 200 90 52

Correlation coefficient

% 0.04

0.91 0.81 0.07 0.60 1.28 1.02 0.26 0.60

0.9999 0.9990 0.9990 0.9999 0.9986 0.9992 0.9979 0.9999

0.03 0.002 0.02 0.03 0.01 0.01 0.02

* Calculated as 3 standard deviations of background

standards and assays. With the measurement conditions used, 1% TiOz produced an interference equivalent to 0.024’? V205 and a standard deviation of O.OOSO~ was obtained at the 0.27, VZ05 level. The 6 standard carbide powders were analysed by the proposed fusion techniques (I, II), supported by atomic-absorption spectrophotometry for trace elements, gravimetric techniques for carbon, absorbed water and oxygen, and by optical emission spectrometry for a range of trace elements. Summation of the results obtained (complete analyses) on the 6 powdered samples had a mean value of 99.8%, which is considered satisfactory. Previous OES studies had been made of in situ oxidation-fusion of tungsten carbide powders as compared to air-oxidation followed by fusion.’ However, the greater precision of measurement available with XRF necessitated an extensive study. The in situ oxidation-fusion technique involves a preheating and fusion cycle as compared to a simple fusion; however, a double weighing procedure and potential loss of volatile elements is avoided and calculations are reduced. A comparative set of results is presented in ration

Table 3 and an oxidation temperature of 800” was used to accentuate possible losses. Inspection of the data and application of statistical tests indicate that losses of tungsten oxide must be insignificant at temperatures below 800” and that the techniques give equivalent results. The temperature recommended for air-oxidation however is a conservative 600” and the in situ oxidation is in general the preferred technique and allows better precision for tungsten assays. The standardized analytical data on the powdered carbide materials together with the subsidiary determinations indicated previously, allow the ready calculation of the compositions of the derived cemented products. Both the pre-sintered powders (III) and the cemented products (V) were examined directly by XRF, and calibration graphs were obtained for all major and trace elements. Predictable interelement absorption effects were noted for pre-sintered and cemented carbides; however for practical analysis the usual matrix matching of calibration standard samples to assays, or correction techniques, would be appropriate. Absorption in-

Table 3. Comparison of prior and in situ oxidation procedures (Sample No. SW9) Element prior oxidation (I)

.K % 1 s (n = 4), %

in situ

23 %

oxidation (II)

Nb

W

Ta

co

Ti

0.64 0.005

72.08 0.17 71.98 0.11

5.19 0.01 5.16 0.02

6.46 0.01 6.45 0.02

7.44 0.03 7.46 0.03

0.64

1 s (n = 4), %

0.008

(s = standard deviation). Table 4. Reproducibility

data (1 s, n = 4) for direct carbide analysis methods (cemented)]

Method Carbide powder briquetted with graphite and flux (unfused) (IV) % Carbide powder, wax-bonded (III) ‘$” Cemented carbide, Sic-polished (V) % Cemented carbide, diamond-polished (V) S; Calibration result, % * Not recommended for use. T*L23--11/12--E

[sample No. SW5 (powder)/SW6

Nb

W

Ta

Ni

co

Fe

Ti

0.016 0.004 0.004 0.003 1.22

0.11 0.08 0.10 0.08 55.4

0.03 0.02 0.02 0.01 10.8

0.036 0.001 0.001 0.001 0.07

0.04 0.02 0.05 0.03 10.3

0.064 0.002 0.002 0.003 0.33

0.07 0.03 0.02 0.04 15.1

818

K. KINSON, A. C. KNOTT and C. B. BELCHER

terferences particularly noted were Co with Ta/W, and Ti with Co; spectral interferences were much less extensive than those observed in OES with the same system.’ Calibration curves were of the same form for sets of wax-bonded pre-sintered carbides, and SiCor diamond-polished cemented carbides; no advantage is derived from diamond-polishing. An attempt was made to reduce absorption effects with the direct analysis of pre-sintered carbide powders; the powdered carbides were diluted with lanthanum-lithium tetraborate and graphite (IV) which is analogous to the dilution-buffer process in the oxidation-fusion work. However, matrix effects were not eliminated, as evidenced by deviations from calibration graphs. The associated poorer precision experienced can be attributed to the non-intimate mixing of the lanthanum and carbide particles and accordingly the simple wax-bonded procedure is to be preferred. Reproducibility data for these different techniques are presented in Table 4. The work presented demonstrates that accurate results for major element determinations in tungsten carbides can be obtained by XRF with briquettes which have been prepared by fusion. The direct XRF analysis of pre-sintered carbide powders or cemented products was demonstrated by at least one satisfactory technique; however, interelement absorption effects of the type common in XRF militate against simple calibration procedures and in situations where the precision of measurement is acceptable, OES may be a preferred technique.’ The proposed methods are

suitable for the complete composition control of a tungsten carbide manufacturing plant and similar standard deviations are observed for all the recommended techniques. Acknowledgements-Appreciation is expressed to The Broken Hill Proprietary Company Limited for permission to publish this work and to Mr. J. C. Mills who carried out the experimental work.

REFERENCES

1. A. C. Knott, K. Kinson and C. B. Belcher, Anal. Chim. Acta, 1972, 59, 119. 2. E. Lassner, R. Piischel and H. Schedle. Talanta, 1965, 12, 871. 3. H. Blaas, E. Lassner, A. Leeb, W. Schwartz and C. Zemak, Berg-Huettenmaenn. Monatsh., 1971, 116, 241. 4. W. Schwarz, X-ray Fluorescence Analysis of Master Alloys for Hard Metal Manufacture, XVI Cofl. Int. (Preprints), Heidelberg, 1971, 1, 385. 5. R. Maeda, K. Hino and Y. Hayashi, Bunseki Kagaku, 1968, 17, 1239. 6. F. T. Wybenga, Appl. Spectry., 1965, 19, 193. 7. U. Backmann and B. Lehtinen, J. Phys. E: 1971, 4, 955. 8. Y. A. Klyachko and E. F. Yakovleva, Sb. Tr. Tsentr. Nauchn-lssled. Inst. Chernoi. Met., 1964. 37. 121. 9. R. C. Gabler and M. J. Peterson, Spectiochemical Analysis of Tungsten, U.S. Bureau of Mines R6632, 1965. 10. M. F. Hasler, Spectrochim. Acta, 1953, 6, 69. 11. F. Claisse, Norelco Reptr., 1956, 3, 3. 12. K. Norrlsh and J. T. Hutton, Geochim. Cosmochim. Acta, 1969, 33, 431. 13. A. C. Knott and C. B. Belcher, BHP Tech. Bull.. 1970, 14, 18.