JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1992, VOL. 7 545

Analysis of Heat-treated Steels by Spark Excitation and Glow Discharge Optical Emission Spectrometry

Dezs6 Demeny Institute of Inorganic and Analytical Chemistry, Lajos Kossuth University, H-4010 Debrecen, Hungary

High-alloy steels, heat treated in different ways, were studied with spark excitation and plane cathode glow discharge methods. The cathode sputtering rate of the steels increases linearly with decrease in hardness of the samples. The influence of the metallographic structure on the intensity of the 165.70 nm line of carbon was considerably reduced by increasing the temperature of the sample to 400-500 "C. It was possible to construct a joint calibration graph for each element with the types of steel examined. The relative standard deviation of the concentration for an unknown heat-treated steel sample was 1 -5%, and was not substantially influenced by whether intensities corrected according to the sputtering rate or the relative intensities were used. Keywords: Steel analysis; spark excitation; glow discharge lamp

A proven method for the emission spectrometric analysis of steels involves the application of high-voltage spark excita- tion. The most widespread procedures use a high repetition rate or high-energy pre-burn spark (HEPS) discharge. The popularity of the method can be explained by the relative simplicity of sample preparation and the rapidity of analysis. The measurements are reproducible and the method operates within low limits of error. The matrix effects that occur can be eliminated, in most instances, without problems by electronic signal detection and com- puterized data processing.

In recent years there have been considerable advances in the optical emission spectroscopic analysis of metals' through the introduction of plane cathode sputtering sources, especially the Grimm glow discharge lamp.2 The material of the sample is removed by cathode sputtering and there is no thermal evaporation. Sputtering from layer to layer with respect to the original surface gives the possibility of obtaining depth concentration profiles. As a consequence of excitation in a low-pressure discharge, the half-width of the spectral lines is small. The sputtering and excitation processes are reproducible, so the analytical error and matrix effect are small. As a consequence of these advantageous features, the method has been adopted in several fields of metal analysis, including the analysis of surface layers, multi-component metal alloys and high-alloy steels.

The aim of our work was to compare the analytical performances of two methods of excitation, viz., spark and glow discharge excitation, in the analysis of high-alloy steels of various compositions and heat treated in different ways. Owing to their different metallurgical backgrounds (differ- ent ways of heat treatment and degrees of forming) the samples have various metallographic structures and hence different hardnesses. The quality and amounts of the possible inclusions may be different even if the compo- sitions of the samples are identical. The different properties may, in some instances, considerably influence the analyti- cal results. In order to elucidate the effects induced and reduce them, the measurements reported in this paper were carried out.

Experimental Apparatus The spark excitation studies were performed with an ARL Model 34000 spectrometer. Increased energy pre-burn and reduced energy signal integration were applied (Table 1).

The Grimm glow lamp operated with a stabilized discharge voltage. Argon was used as the working gas. The

Table 1 Conditions of excitation in spark discharge

Parameter At pre-burn At integration Operating voltage/V 550 450 Capacity/pF 30 10 Induction/pH 20 120 Frequency/Hz 100 100 Resist ance/Q 1 2.1 Ignition volt age/kV 11 11 Sparking time/s 15 5

Table 2 Experimental conditions with glow discharge

Discharge power

Parameter Normal Increased Vol tagelV 1000 1600 Current/mA 100 140 Argon pressure/mbar* 6 8 Burn spot diametedmm 7.5 7.5 Integration timeis 10 10

*1 bar= 1 x lo5 Pa.

selection of the spectral lines was carried out with a Hilger- Watts E 600 fluorite prism vacuum spectrometer and an RSV SPN 500 Ebert spectrometer. The measuring system applied has been described by Kruger et aL3 The glow discharge lamp was operated at normal and increased discharge energies (Table 2).

Samples In order to construct the calibration graphs, a series of standards from the Bureau of Analysed Samples (BAS) of high- and low-alloy steel with a homogenously distributed fine-grain metallographic structure were used.

Measurements were also performed, depending on the heat treatment, with tool and high-speed steels, which represent extreme compositions among steels. Samples were taken from the same piece of steel, but were heat treated in different ways (annealed, hardened, tempered). Thus, a sample series was obtained from steels of identical composition but with different metallurgical structure and hardness (Table 3).

Results and Discussion In the formation of the structure of steels an essential role is played by carbon. When carbon is determined using the

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546 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1992, VOL. 7

Table 3 Characteristic features of heat-treated samples

Sample Type code

Tool steel, Kl-A C 2.1 1, Si 0.32, Mn 0.41, Cr 12.19, V 0.12, W 0.73, Mo 0.15 and S 0.01 5%

KI-B

KI-C

Kl-D

High-speed steel, C 0.96, R-A Si 0.21, Mn 0.26, Cr 4.05, V 1.83, W 1.83, Mo 4.99, and S 0.12%

R-C

R-D

Heat treatment Annealed

5 min at 550 "C, 20 min at 980 "C, 50 "C cooling in oil

As for Kl-B, but tempered in air, 2 h at 180 "C

As for Kl-B, but tempered in air, 2 h at 550 "C

Annealed

5 rnin at 550 "C, 5 min at 850 "C, 5 rnin at 1050 "C, 200 s at 1200 "C, cooling in oil, tempered :in air

twice in air As for R-C, but tempered

Vickers hardness

(HV) 274

829

878

634

29 1

800

906

Structure Basic structure perlite,

carbide grains of irregular distribution and various grain size

Basic structure martensite, rough carbide segregates of irregular distribution

As for Kl-B

As for Kl-B

Basic structure perlite with meshed carbide segregation

Basic structure martensite, with carbide mesh greatly increased

As for R-C

~~

Table 4 Relative standard deviation of carbon determination, Sr(C), al. mean concentration E with spark excitation; n= 10

Relative concentration Type of steel range (O/o) E (O/O) S,(C) (Oh)

Tool steel Cr-Ni steel Cast iron Low-alloy steel

0.70-1.70 0.05-0.75

1.9-4.55 0.0 1-1.50

0.95 3.3 0.2 1 9.8 3.2 3.5 0.46 2.4

spark excitation method, basically different calibration graphs are obtained for each type of sample (Fig. I), as is well known from previous work. For the calibration graphs a linear regression analysis was performed (Table 4). Data were calculated using a program developed by Klocken- kamper and B ~ b e r t . ~ The reproducibility of the method can be expressed by the standard deviation (SD) or the relative standard deviation (RSD) obtained from five replicate measurements on a sample. The quality of the calibration graph can be given by the standard error of the estimate, S, (residual scatter):

r n 1.

where n is the number of calikration samples, Ii the measured relative intensity and Ii the relative intensity obtained from the calibration graph.

The Nalimovs correlation allows an estimate to be obtained for the SD of the mean concentration ratio for an unknown sample:

where b is the slope of the calibration graph, n the number of calibration samples and k the number of measurements of the sample to be analysed.

The estimate of the RSD at the mean concentration of carbon determination, Sr(t), with the spark excitation

0 1 2 3 4 Relative concentration (%)

Fig. 1 Calibration graphs for carbon (193.09 nm) with spark excitation from various types of steel: A, cast iron; B, low-alloy steel; C, tool steel; and D, high-alloy Cr-Ni steel. Reference line Fe 271.40 nm

method is within the range 2-10%, depending on the steel type (Table 4). During heat treatment carbon segregates, depending on the conditions of sample cooling. When cooling is done rapidly, iron carbides with a small grain size will segregate, whereas in the annealling procedure mixed carbides of large grain size are formed. These may be preferential points for sparks,6 which may affect the amounts of volatilized carbon, iron and chromium and, naturally, may lower the accuracy of the measurement. In

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1992, VOL. 7 547

I C 165.70 nm I - < Fe271.40nm

Voltage r u) c .-

P 178.28 nm Ar 294.29 nm

I I 1 I I

0 20 40 60 80 100 Timels

Fig. 2 Intensity versus time graphs for a tool steel sample (Kl-A) with glow discharge; U=constant = 1000 V

I Tempered (2 h, 180 '6 ) I .""

2.5 5.0 , 7.5 HardnesdkN mm-'

Fig. 3 Dependence of cathode sputtering rate of heat-treated tool steel samples (Kl) on hardness; U= constant = 1000 V

general the effect of the original structure disappears after 10- 15 s when, as a consequence of increased energy pre- bum, microfusion of the sample takes place at a depth of 0.05 mm in the spark spot.

The sample is introduced into the glow discharge by cathode sputtering. There are no thermal volatilization effects and the sample is cooled during the analytical process by a water-circulation cooler. In spite of this, the intensity versus time graphs of high-alloy and heat-treated tool steels (Fig. 2) indicate the slow time course of the process. This can be primarily observed in the changes in the line intensities of carbon and iron. Carbon is an

500 -

B /x~~x-x-x-x-x-x

+--+-+-+-+-+-+c D

0 30 60 90 120 Time/s

Fig. 4 Temperature dependence of the steel sample (KI-A) with and without cooling at different glow discharge powers: A, 225 W non-cooled; B, 225 W cooled; C, 100 W non-cooled; and D, 100 W cooled

important component of various types of structural ma- terials containing a significant proportion of carbide. These specific phases are of different hardness even within a single sample, so their rates of sputtering are also different. The line intensity of carbon begins to be stable only after 100 s; this variation means that in the course of sputtering the amount of carbon removed from the sample changes with time. The present investigations show that there is linear decrease in the rate of sputtering with increase in the hardness of the sample (Fig. 3).

As the rate of decomposition of the structural elements of steels is highly temperature dependent, the temperature changes of the sample during the cathode sputtering process were measured (Fig. 4). Temperature was measured through a blind hole in the rear side of the steel sample with a digital contact thermometer at a distance of 1 mm from the bum spot. The mass of the sample of 40 mm diameter was 80 g.

It was established that if the rear side of the sample is cooled with a water-circulation cooler and a discharge power of 100 W is applied, then the temperature of the sample settles at 150 "C near the burn spot. On increasing the power to 225 W, a temperature of 250 "C is found. Without cooling, the temperatures recorded were 200-220 "C at 100 W and 500 "C at 225 W. It can be assumed that on the surface of the burn spot, where dissipation of heat energy actually takes place, the temperature may be much higher.

From the metallurgy of steels it is known that the tempering process of heat-treated tool steels begins at

~~~ ~ ~~ ~ ~~ ~ ~~

Table 5 Standard deviation (SD) of the C 165.70 nm spectral line intensity ( I ) as a function of heat treatment state (A, B, C, D)* at various sample temperatures

PowerIW 100 100 22 5 225 225 Pre-burn time/s 30 120 60 60 60 Sample temperature/"C 140 160 260 450 450

Vickers I (arbitrary units)

Kl-A 274 16965 17071 15740 14338 12715 KI-B 829 14853 16151 15195 14500 12678 Kl-C 878 14343 16077 15067 14069 12516 Kl-D 634 14613 16258 15218 14150 12551

Sample code? hardness (HV)

Mean I SD RSD (Yo)

15194 16389 15305 14264 12615 1199 46 I 298 193 97 7.89 2.8 1 1.94 1.36 0.77

*Parameters of heat treatment and matrix structure of the samples are given in Table 3. ?Intensities corrected according to the sputtering rate of the sample.

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548 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1992, VOL. 7

Table 6 Relative standard deviation of spectral line intensities as a function of heat treatment state (A, B, C, D)* using different methods

RSD (%) for 16 A, B, C, D samples RSD (O/O) for 17 A, B, C, D samples --

C Cr S C Cr S Method 193.09 nrn 267.72 nm 180.73 nm 193.09 nm 267.72 nm 180.73 nm HEPS 1.2 0.7 3.5 3.9 2.9 24.7

GDMS 8.2 4.9 5.2 4.2 4.2 4.8 GDOEST 1.4 1.8 4.0 1.8 3.2 2.2

*Parameters of heat treatment and matrix structure of the samples arc: given in Table 3. ?Discharge power 225 W, sample temperature 450 "C.

~~

Table 7 Regression data for the calibration graphs obtained using the glow discharge (P= 180 W, U=constant 1500 V): reference line Fe 27 1.40 nm; t=mean concentration; cL= detection limit; S,(E)=estimate of the RSD of the analysis using sputtering rate-corrected line intensities; S,(c,)lc,)=estimate of the RSD of the analysis using relative line intensities; n = 10

Analytical Concentration S r ( q (O/o) s r o (O/O)

Element linehm range (Oh) c (Yo) cL (O/O) [Z,=f(c,)] [zdzR=ficdcR)l C 165.70 0.018- 2.1 1 1.0 0,011 4.6 3.8 Mn 257.6 1 0.180-1 8.50 2.2 0.008 0.8 0.9 Cr 267.72 0.158-23.50 8.6 10.039 0.8 0.8 Ni 225.39 0.150-13.10 4.1 0.014 2.9 3.9

around 150 "C with the decomposition of martensite. In the 150-300 "C range the residual martensite is converted into austenite. Between 300 and 400 "C recrystallization takes place, cementite grains begin to grow, then granulation occurs above 400 "C. This process is regulated by the diffusion of the carbon atoms, with the progress of which the hardness of the steel decreases. Experiments were performed, as described above, to see how the line intensity of carbon changes as a function of structure when the cathode sputtering of the sample takes place at higher electric power without cooling, and can thus reach the temperature necessary for the transformation of the structu- ral elements (Table 5).

In analyses at higher temperatures, the effect of structure on the intensity of the carbon line decreases. With a line- intensity correction corresponding to the sputtering rate of the sample, the scattering of line intensity due to structure can be reduced to below 1%. One can see that with a slight increase in the temperature of the sample the line intensi- ties increase, then decrease again at 450 "C, which can be accounted for by the temperature dependence of cathode sputtering.

The dependence on structure of the line intensities of several elements using HEPS excitation, glow discharge optical emission spectrometry (GDOES) and glow dis- charge mass spectrometry (GDMS) is shown in Table 6.

80

.$ 60 C al c .- p 40

= 20

.- c - al

I I I 1

0 0.5 1 .o 1.5 2.0

Relative concentration (%I

Fig. 5 Calibration graph for carbon with glow discharge for various types of steels: + , high-alloy Cr steels; 0, high-alloy Cr-Ni steels; W, low-alloy steels; 0, high-speed steels; 0, austenitic manganese steels; and 0, tool steels

One can see that with the exception of sulfur, the scatter of' line intensities as a function of the state of heat treatment is of the same order of magnitude with the HEPS and GDOES methods. As has been mentioned, this is due to the fact that whereas a spark discharge decomposes the original

I

0 0.5 1 .o 1.5 Relative concentration (%)

Fig. 6 Calibration graph for Mn with glow discharge for various types of steel of low concentration of Mn: +, high-alloy Cr steels; (I), high-alloy Cr-Ni steels; , low-alloy steels; 0, high-speed steels; and 0, tool steels

80

). 4- .-

60 al c .- .- $ 40 c - al tz

20

0 Relative concentration (%)

Fig. 7 Calibration graph for Mn with glow discharge for a 'concentration range of 18% Mn: a, low-alloy steels; and 0, austenitic manganese steels

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1992, VOL. 7 549

120

100

2 80

r I-' .- 9) 4-

*: 60 > - % 40 9) u

20

.-

0 5 10 15 20 Relative concentration (%)

Fig. 8 Calibration graph for Cr with glow discharge for various types of steels: +, high-alloy Cr steels; e, high-alloy Cr-Ni steels; U, low-alloy steels; 0, high-speed steels; and 17, tool steels

structure during the increased energy pre-burn, with a glow discharge, at a high temperature of the sample, the structural elements undergo transformation. With a glow discharge the scattering of the line intensities can be further decreased with the correction of intensities corresponding to the sputtering rate of the According to metallographic studies, the high-speed steel sample con- tains a considerable number of sulfide inclusions. Despite this condition, the determination of sulfur using the glow discharge method is not affected by the heat-treatment state to such a degree as with the spark excitation method. The explanation is that with cathode sputtering none of the thermal volatilization processes that are characteristic of spark excitation take place. Using GDMS, the sample sputtering takes place at room temperature and the detec- tion of the elements is independent of the excitation conditions of the glow discharge plasma. The greater scatter of the intensities of the elements in comparison with the other two methods shows the real effects of sample structure on the sputtering processes.

The calibration graphs were subsequently studied with the glow discharge method. Stabilized voltage (1500 V) conditions were applied. In this instance the line intensities are determined by the amount of sputtered matter, and hence the discharge current. The sample was not cooled, and its temperature reached about 500 "C during the analysis.

A single joint calibration graph was fitted to the types of steels examined (low-alloy, high-alloy, Cr, Cr-Ni, tool and high-speed steels) and heat-treated steels (see Table 3) for the elements given in Table 7. The calibration graphs were constructed in two ways: either the relative intensity was plotted as a function of relative concentration, or the line intensities, corrected according to the sputtering rate of the samples, were used. The data in Table 7 show that the two methods of calibration result in similar precisions.

The calibration graphs are shown in Figs. 5-9. The estimate of the relative standard deviation of the mean

80

5 60 .- fA

9) c .E 40 0

z 20 a

.- c,

0 2 4 6 8 10 12 Relative concentration (%)

Fig. 9 Calibration graph for Ni with glow discharge for various types of steels; a, high-alloy Cr-Ni steels; and ., low-alloy steels

concentration for an unknown heat-treated steel sample is 1-5%, and is not markedly influenced by whether intensi- ties, corrected according to the sputtering rate, or relative intensities are applied for calibration. The quality of the results is comparable to that obtained with an ordinary glow discharge by Human et aZ.,'O where the reproducibility of measurement for all elements in terms of concentration was typically 3%.

D.D. expresses his gratitude to the Alexander von Hum- boldt-Stiftung for sponsorship, to the Institut fur Spektro- chemie und angewandte Spektroskopie, Dortmund, to the Spectrometer Laboratory of DIMAG-RT, Miskolc, and to the Scientific Research Foundation of Hungary, OTKA 1644/9 1 , for the support of this work.

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2 3

4

5

6

7 8

9

10

References Dogan, M., Massman, H., and Laqua, L., Spectrochim. Acta, Part B, 1971, 26, 631. Grimm, W., Spectrochim. Acta, Part B, 1968, 23, 443. Kruger, R. A., Bombelka, R. M., and Laqua, K., Spectrochim. Acta, Part B, 1980, 35, 589. Klockenkamper, R., and Bubert, H., Spectrochim. Acta, Part B, 1982, 37, 127. Nalimov, V. V., The Application of Mathematical Statistics to Chemical Analysis, Pergamon Press, Oxford, 1963. Paksy, L., in Proceedings of the 30th Hungarian Annual Conference on Spectral Analysis, Debrecen, I98 7, ed. Zimmer, K., GTE, Debrecen, p. 589. Jager, H., Anal. Chim. Acta, 1972, 58, 57. Radmacher, H. W., paper presented at the XXII Colloquium Spectroscopicum Internationale, Tokyo, 198 1 . Butterworth, A., Report No. T/CS/552/3/78/c, British Steel Corporation, Port Talbot, 1978. Human, H. G. C., Strauss, F. A., and Butler, L. R. P., Spectrochim. Acta, Part B, 1980, 35, 207.

Paper 1/02332B Received May 17, 1991

Accepted December 17, 1991

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