The analysis of tungsten carbides by optical emission spectrometry

THE ANALYSIS SPECTROMETRY

OF

A. C. KNOX-l-.

K. KINSON

The hhkcrl

Proprkrtrry

(Reccivcd

Ililt

29th October

TUNGSTEN

AND

CARBIDES

BY

OPTICAL

EMISSION

C. B. HEL.Ct-fER

Co. Ltd..

C’cvr~rtrl Rrsccmlr

Lahorctfnric~s.

Slttrrrkmct,

N.S. W. ,7_?07 (Ammrlicc)

I97 1)

Tungsten carbide hard metals constitute a group of industrialiy important alloys applied in industries such as engineering. agriculture, appliance. textile and construction, where the cxtrcmc hardness and wear resistance of these materials can be used to advantage I. Development and maintenance of optimal propertics in the alloys requires precise testing and control of composition. to ensure best performance and avoid wastage of the expcnsivc materials involved in manufacture. The literature on the analysis of tungsten carbide hard metals and powders is rclativcly sparse Classical chemical methods” -s have been described for Al, C. Co, Cr. Fe, Nb, Mn, MO. Ni, Si, Ta. Ti, and W; Shanahan’ rcmovcd tungsten by chlorination bcforc the chemical determination of aluminium, calcium and silicon. Molecular spcctrophotomctry6 has been used for cobalt. iron and titanium,and atomic absorption spcctrophotometry for cobalt’ and irons’. X-ray fluorescence spectrometry has been applied to the determination of the major alloying elements in ccmcntcdg*rO, and powdered carbidcs’c, and solutions* ‘. Maeda et ctf.*O discuss the difficulties caused by surface in~?omo~eneities and the complex inter-element effects which require cxtensivc suites of standards for each alloy type. Klintor et of.” described an optical emission spectrographic solution method for the dctcrmination of W, Co, Ti. Ta. Nb. Fe, Ni, Cr. Mn, Mg and Ca in hard metals. Spark excitation and the rotating disc electrode were used, the coefficients of variation ranged from 3-7 ‘x,,, and the detection limits were adequate for all clcmcnts excepting niobium; the method of sample solution preparation and the excitation technique are time-consuming. Adcquatc d.c. and a.~. arc tcchniqucs ’ 3 - 1’ have been presented for the analysis of tungsten metal and tungstic oxide, These methods have been developed for trace clement analyses and much attention has been given to suppression of the complex tungsten spectrum by inducing carbide formation in the heated electrode. However, poor reproducibility is obtained when spark excitation is used16 because of erratic “in situ” formation of tungsten carbide. The manufacturing process, which is briefly outlined below. r-4,

C

1 wo3-

4

w

1

Co, TaC, NbC

Tic,

pressure heat

VC, etc. I

-WC

+

-

powders

Blended carbides

I +-

Sintered b hard metals solids Aural.Chim.

Actn, 59 (X972)

A. C.

120

KNOTT,

K. KINSON,

C. 13. ISELCHER

requires rapid analyses of oxide, carbide and intermediate powders as well as residues, rejects and the final cemented carbide products. The physical form of some sintered samples such as chips and fracture pieces, makes thcrn unsuitable for direct analysis. However, these samples can bc oxidized”*13*17- I9 and thercforc a method for oxide analysis is rcquircd; an oxide method would also bc suitable for controlling the input tungstic oxide and as a calibration method for ccmcntcd and powdcrcd carbide refcrcncc standards. Rapid direct reading optical emission spectromctric methods have been investigated and devclopcd for all of thcsc products. EXPERIMENTAL

Jarrcll-Ash wavclcngth

ranges

2-m direct-reading model 66-105; dispersion, 0.42 175-210 and 220-420 nm; prcssurc 0.65 N m-‘.

nm mm-

1;

High voltage spark; 23 kV peak (briqucttcs), 27 kV peak (sintcrcd solids); 40,~H (briquettes), residual PH (sintcred solids); 5.5 r.f.A (briquettes), 9 r.f.A (sintered solids); 7.5 nF; 30 ohms primary; < 1 ohm secondary; 300 discharges see- i. Electrode system Graphite 6.5 mm dia. ASTM CSA tip ; gap 4 mm; argon flow 7.5 1 min - ’ ; sample preparation, hand rubbed on 400 grit Sic (briquettes), machine finished on 60 grit Sic (sintcrcd solids). Exposure conditions Pm-flush 5 set; prc-spark 10 set (briqucttcs), 30s~ (sintcrcd solids); integration 25 sec. terminated on La 379.08 nm (briquettes), and background (sintercd solids). Entrance slit, 25; W, 75,400.88;Al, SO, 396.15; La, 35379.08 ; background, 100, 363.05; Co, 50, 345.35; Ni. 75, 341.48; Ti, SO, 324.20; Nb, 35, 319.50; V, 75. 310.23; Cr. 75, 267.72; Ta, 50, 263.69; Fc. SO, 259.84; Si. 100, 251.61. Reugen ts Graphite powder. briquetting grade; lithium nitrate, mixture (64 ‘x, lithium tetraborate-36 ‘YOlanthanum tetraborate).

anhydrous;

fusion

Recommended procedure Briquetted oxides. Hand-crush solid and chip carbides to -c 500 pm in a tungsten carbide mortar and further crush to < 125 /cm in a lo-cm3 tungsten carbide grinding barrel on a Siebtechnik swing mill. Grind tungstic oxides to < 63 Ctrn in a tungsten carbide grinding barrel and dry at 1 IO”. Weigh 85 mg (carbides) or 100 mg (oxides) into a 95/5, 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. Pm-heat tungsten carbide mixes at 800° for I5 min to convert carbides to oxides, and then fuse all samples at 950” for 10 min. Cool, recover the fused bead, weigh the bead and add graphite powder (1.5 x recovered bead weight). Mill the And. Chim. Acta, 59 (1972)

ANALYSIS

OF TUNGSTEN

121

CARBIDES

fusion bead and graphite powder for 3 min in a IOO-cm3 tungsten carbide grinding barrel. Press the milled powder to a 25mm dia. briquette at 45 kN. Briquetted powder ctcrhicies. Weigh 200 mg of carbide powder samples or calibration standards into a phiai containing 2 g of fusion mixture and 3.3 g of graphite powder. Mix by shaking, transfer to a iOO-cm3 tungsten carbide grinding barrel, mill for 3 min on a Sicbtechnik swing mill and press to a Z-mm dia. briquette at 45 kN. Excite the oxide or carbide briquettes, or solid samples under the indicated experimental paramctcrs. and calculate the results. RESULTS

AND

I~1SCUSSlC)N

National standard samples of tungsten carbide to cover the range of oemcntcd and powdered carbides produced (50-90 ‘x, WC, 4-25 ‘x, Co. O-20 ‘x, Tic, 2--l 2 “/;:TaC. O-.4 ‘x, NbC, O-O. I ‘:/; VC, t&O.5 ‘2, Fc and < 0. I I!<,Cu. Mn, Ni, and Cr) are not available and it was ncccssury to manufacture suitabic refcrcnce standards. Synthetic oxide calibration standards made by prccisc weighing and fusion of pure materials and which contain some thirty major and tract clcmcnts arc adcquatc for the briqucttcd oxide tcchniquc. Howcvcr, the preparation of powdcrcd and ccmcntcd carbide standards required a major effort, A small-scaic commercial tcchniquc was used to blend and homogcnizc the appropriate carbides and tract metals to give six powdered carbide standards with a complctc covcragc for eleven major and minor clcments. In turn, these powders wcrc compacted to 33 mm dia. pieces and sintered to yield ccmcnted standards. These powdered and cemenicd standards were unaiyscd by the briqucttcd oxide tcchniquc by X-ray fluorcsccncc and optical emission spectrometry, and by atomic absorption spcctrophotomctry, to confirm that the results prcdictcd from the blended weights had been achicvcd and to assign a standard result. Wybcnga” indicated that a variety of compositional changes can occur in the outer fayers of cemented carbide during the vacuum sintcring process, especially cobalt within 0.2 mm of the surface. Since homogeneity is csscntial in cemented carbide standards several checks were carried out. First, 0.5 mm was removed from all surfaces in 0. l-mm layers and it was confirmed that a constant Co/W ratio had been re-establishcd. Electron probe microanalysis scans wcrc then carried out at high and low resolution; inhomogcncitics were evident bctwccn 4 pm (dia.) areas which arc below the size of the original blended particles, but no inhomogencitics wcrc dctectcd between 100 pm (dia.) arcas along sections 4 mm deep. The briqucttcd oxide technique described in the recommended procedure has its origin in the work of Hasier” and subsequent developmcnts2’ -24. Factors which were considcrcd in establishing a suitable briquctted oxide composition included sample-to-flux ratio. type of fusion mixture, choice of internal standard clement and a requirement that the briquettes bc suitable also for X-ray fluorcscencc spectrometry. A sample-to-flux ratio of 1 :20 was considered to be a suitable compromise between adequate sensitivity for minor elements and sufficient dilution to minimize matrix effects. The investigation of several fusion mixture formulations indicated that a lanthanum-containing flux would serve as a heavy element absorber in X-ray fluorescence analysis and lanthanum could be used as the internal standard element in optical emission spectromctry. Platinum-gold crucibles arc superior to graphite crucibles because “memory” contamination is much less, in-fusion oxidations can be carried out, and no elemental Anal. Chim. Acta,

59 (1972)

122

A.

C:. KNOTT,

K. KINSON.

C. 1%. J3ELCHER

crucibles must be protected against reducing reductions take placcZ ‘. Platinum-gold conditions and this requirement leads fogicatfy to the USCof in-fusiun tc~hniqucs to oxidize carbides and sufphidcs. Of the various in-fusion oxidizing agents which have is prcfcrrcd to sodium nitratezz and barium been rccommcndcd, lithium nitrate” because of maximal oxygen ~~v~il~~bility per unit weight, and because the peroxide”‘, cxccss of nitrate is destroyed, and no extraneous clcmcnt is added to the flux. Although the briquettes arc tctraboratc in character compared with the mom basic mctaboratc of many workers, nevertheless it was observed that a long-tcrrn tendency to pick up water and czlrbon dioxide was oxhibitcd. This cffixt resulted in a gradual drift in calibration curves derived from calibration briquettes which had been cxposcd to the atmosphcrc, compared to curves obtained with new briqucttcs. The cffcct could be avoided by storing briquettes at 1 10” and surfitcing freshly before each group of excitations. No difficult& were cxpericnccd with the preparation and excitation of briqucttcd powder carbides. The cemented carbides are extremely hard (> 1200 D PH) and the rcIative abrasion resistance dccrcascs from SiC to BC’ to diamond’. Quktfity control situations involving large numbers of solid sampfcs would rcquirc spccializcd surfacing cquipmcnt, such as diamond grinding wheels or electrolytic polishing. For limited sample throughput. satisfalctory surfaces were obtained by using commercially available silicon carbide floor sanding discs. The hard surface rcquircd a more energetic discharge (9 r.f. A) and a rclativciy long pm-burn (30 see) to ensure that adequate burn pcnctration was achieved. The major elements prcscnt in tungsten carbide hard metal aifoys, particularIy tungsten. titanium and cobalt, possess complex atomic emission spectra. and conscqucntly lint intcrfcrcnccs arc almost incvitablc, The major clcmcnts arc not seriously affected by intcrfcring fines, but significant intcrfcrcnccs exist For many wavelengths normalty used for the determination of minor elements. and corrections must be applied. The w~lvclcngths used in this i~~vcstig~tion rcprescnt the best choice of the available lines with a gcneraf-purpose spectrometer used for analyses ofvarious metals and oxides. For briqucttcd oxides and powder carbides, under the conditions outlined in the rccommendcd praccdurc. I ‘,9$ of the interfering clcmcnts stated caused the following positive interferences: with Ta, O.OOG’i~ Si, 0.017’2; Fe, and 0.004’%, N b : with \I?r, 0.00 I “y;, Si. 0.035 ‘!;; Cr and 0.0002’!.,, V : with Co, 0,001 “;, Ca. 0.~6~~~, Ni; with Nb, 0.0007‘%; Al. Typical calibration graphs for detcr~~n~~tions of the more important cicments in tungsten carbide hard mctats arc shown in Fig. I and a comparison of reprodueibilities obtained by the three methods is shown in Table I. Grcatcr eicmcntal coverage and highest precision wcrc attained by means of the oxidation-fusion proccdurc but results obtained on unfused briqucttcs and sintercd solids arc quite satisfactory for comrncrcial analysts. While published literature indicates that X-ray fluorescence is the preferred method for direct reading analysis of tungsten carbides, this investigation has shown that optical emission spcctromctry can be applied succcssfulfy. Appreciation is expressed to the Broken for permission to publish this work. A?rczl. Chime Actor, 59 (1972)

Hill Proprietary

Company

Limited

ANALYSIS

OF TUNGSTEN

CARIXIT)ES

i23

l!ioOa .-s jj

C (3

loo&

6l 2 2

!500-

a

1

I

25

0 ZOOO-

0

%W

4 % Ti

8

(324-2

w------

0

% Nb

Fig. 1. Calibnttion sintcrcd solid.

L

50 75 (400.8 nm)

(319-5

,

100

I

I

0

5

% Co (345.3

1 16

12

I

t

0

4

% Ta

nm)

nm)

gr:lphs for tungsten

6

8 carbide

;lnalysis.

I 0

I

15

20

12

16

nm)

t

8

(263.5nm)

0.1 0.2. % V ( 310.2

(0)

,

10

Fused oxide.

(I)

1

0.3 ----z4 nm)

unfused

powder.

(A)

And. Chim. Acta, 59 (1972)

124 TABLE

C. R. RELCWER

f

~~r’I~~~~U&frj~LiTY ..-_..._--

DATA KIR ONE STANDARI> .---I 1.-.. ~----.-..------_------..I-.-

Mc~liloll

--..-_.

K. KINSON,

A. C. KNOTT,

W

_ . - _.-.--_- -I--..”

13riquctlcd oxides

E

Is *%

sriqucttcd

ii

pOW&r

8s‘

CitrhidCS

s,

Ccrncntcd carbides

rf Is s, __..-._______......- ...._..

Ti

CO

THHiiE METIIODS _I.__.__. ---_----...--.-

-.._ _-“-_ I..,.. -

Til

Nh

V

Ni

-..-

.__.-

.-_-

Cr

Fc

.____.__. _____

Si

AI

__.--.._ - .__- -- __..._..I “._I.11,.” ._~._..._...._ -_ .-_ .__l.-.“~“,“l” .,,_..-.-_.-.- ..-. .._,_ I_-_.._._____,

0.1 0.28 0.005

0.3 0.1 I 0.011

0.1 0.21 0.014

0.4 a.19 0.018

0.04 0.031 0.025

0 0.005 0.071

0.01 O.ao6 0.105

0.01 0.009 0.087

0 0.01 I 0.033

0 0.005 0.035

0 0.011 0.26

0.4 0.54 O.OI 0

0.3 Cf.17 0.0f7

0 Q-20 0.013

0.2 0,2fi 0.024

0 o.mii am2

0 o.rlOs a071

nd

iid

nd

nd

nd

0.3 0.26 0.017

0.3 a34 0.03 I

0.06 0.068 0.056

O.Ol 0.005 nd 0.07 1

nd

nd

nd

nd

.._”

..,._...

I.22

Cf.07

0.3 0.2 0.49 0.26 0.009 0.02s - ..-._-..-_ --_.-

.-

.F C~~lfihGlfiXl 55.4 10.3 IS.1 rchult __._ ____,__._ _..__, __ _._.._ ._.,,_.____ ..__. _ .._._,.I........--_-,

‘* d = Deviation.

HY

s, = Is/z. Ttlc d~v~~~i~~lls and mxff3

..-.... ““... ,I

10.8 . ..

_ _.“*... . .._...

0.07

__._.“II . __..-_-., . ...._...-.. ,

ml= 5 = Mcatt. to 12 results,

Not

0.12

.

.

0.33

_,._.,_______.__

0.14

0.04

.__.I_.- -.-....___._...._.._.-__.

dctcrminod.

rcfcr

SUMMARY

Three direct reading, optical emission spcctromctric techniques which use spark excitation arc described for the analysis of tungsten carbide alloys in powder and compacted forms. The techniques covcroxidized samples which arc fused with lithiumlanthanum tetraboratc and briquctted with graphite, powdered carbide samples which are briquctted with lithium-lanthanum tctraboratc and graphite, and ccmentcd carbide metal samples. The preparation of reference standard samples and spectral interferences are described. The relative standard deviations arc 0.005-0.031 for major and 0.02-0.26 for minor eIemcnts, and the best precision is obtained with oxide briquettes.

On d&-it trois techniques spectromdtriqucs d’Cmission pour l’anafyse de carbures de tungst&ne, soit en poudre, soit sous formc compacte, On indique la propagation d’&hstntitlons de rSrence et Ies intcrf&rences spectrafes. Les dCviations standards relatives sont 0.005-0.031 pour les 6lements majeurs et 0.02-0.26 pour tes elements mineurs. ZUSAMMBNFASSUNG

Es werden drei ~m~ss~onsspektromet~is~he Verfahrcn im optischen Bereich mit Funkenanregung und direkter Ablesung beschrieben ftir die Anaiyse von Wolframcarbid-Legierungen in Putver- und kompakter Form. Die Verfahren verwenden oxidierte Proben, die mit Lithium-Lanthan-Tetrabor~~t geschmofzen und mit Graphit gepresst werden, gepulverte Carbid-Proben, die mit Lithium-Lanthan-Tetraborat

ANALYSIS

OF TUNCXTEN

CAKIHDES

125

und Graphit gcpresst wet-den. oder gesinterte Carbid-Metall-Proben. Die Hcrstellung der Vcrgleichs-Stundardproben und die spcktralen Stiirungen werden bcschricbcn. Die relativen Standardabweichungcn sind 0.005-0.03 1 fiir Elemente in griissercn Mcngcn und 0.02-0.26 fiir Elcrnentc in geringeren Mengcn : die bate Reprodu~ierb~irkcit wird mit Oxid-Presslin~en erzielt.

REFERENCES

Ard.

C/rim

Actcr. 59 (1972)