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Analysis of graphitized cast irons by optical emission spectroscopy: matrix effects in the glow discharge and the spark excitation
SPECTROCHIMICA ACTA PART B
Spectrochimica Acta Part B 51 (1996) 863-876
ELSEVIER
Analysis of graphitized cast irons by optical emission spectroscopy: matrix effects in the glow discharge and the spark excitation Zden6k Weiss 1 LECO lnstrumente Plzeh, s.r.o., alej Svobody 56, 32318 Plzeti, Czech Republic
Received 24 June 1995; accepted 18 December 1995
Abstract
Graphite has a substantially lower sputtering rate in glow discharge than have the other structural components of graphitized cast irons, which leads to a structure-related matrix effect, consisting of an increasing relative surface coverage by graphite of the sample surface during the initial stage of the GD-OES analysis, and, consequently, to an increasing carbon signal intensity. This effect exists inherently in any multicomponent system with different sputtering rates of the components and should be taken into account in GD-OES quantification. A simple theory is presented to describe quantitatively the changes in relative contributions of different phases to the flux of the sputtered material entering the discharge and a formula is presented, expressing elemental intensity changes as a function of sputtering rates and stoichiometry of the structural components. After reaching the steady state, there are no substantial differences in the GD-OES signal response of the analyzed elements between the graphitic and the white cast irons. To reach this steady state, long preburn times and high sputtering rates have to be used. In the spark atomization/excitation, there are very strong and complex structure-related matrix effects, which make the analysis of graphitic cast irons by spark excitation impossible. Keywords." Glow discharge; Graphitized cast iron; Matrix effect; Spark excitation; Sputtering rate; Structural effect
I. Introduction
Elemental analysis of cast iron is one of the most frequent routine industrial applications of optical emission spectrometry. Both the spark and the glow discharge spectral sources have been used for this purpose for decades, with a methodology based on empirical observations. Analysis of graphitized cast irons, however, is still an t Present address: LECO Corporation, 3000 Lakeview Ave., St. Joseph, MI 49085-2396, USA. Elsevier Science B.V. P H S0584-8547(96)01461-9
interesting topic from the scientific point of view, because of the complex and not yet very well understood structure-related matrix effects. Cast irons represent a wide group of materials with a complex structure, dependent substantially on their metallurgical history. Cast irons are essentially ternary alloys of F e - C - S i , of near eutectic composition. There are four main types of cast irons, (1) white, (2) gray, (3) nodular and (4) malleable. A good concise summary of the chemistry and microstructure of these cast irons is presented in Chapter 18 of Ref. [1]. White cast
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Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
iron has a structure corresponding to the metastable equilibrium of the F e - C - S i system and consists of cementite (Fe3C: the primary cementite in case of hypereutectic composition, the eutectic cementite and the secondary cementite) and of pearlite, resulting from the eutectoid transformation of austenite. For white cast iron to be formed a sufficiently high cooling rate of the melt is essential. Unlike white cast iron, the other types of cast iron mentioned above contain free carbon in the form of graphite. The matrix of most graphitic cast irons consists most frequently of pearlite, ferrite or of their mixture. Solidification of graphitic cast irons and the resulting microstructure is controlled by many factors, the most important of which are the chemical composition and the cooling rate. The shape and size of graphite particles is very important: in normal gray cast iron there are usually graphite flakes (see Fig. 1). In nodular (ductile) cast iron, graphite exists in the form of spherical grains (spherulites). Such a shape of graphite is achieved by adding magnesium to the melt. Finally, malleable cast irons are produced by heat treatment (tempering) of white cast iron, resulting in conversion of cementite into ferrite and graphite. Compositions typical for the different forms of graphitic cast irons are schematically shown in Fig. 2. In addition to the above listed basic types of cast iron, there are also cast irons with high chromium (up to 30% Cr),
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with high nickel, N i - C r - C u cast irons ("Ni Resist"), C3Si2Ni5Cr8 ("Ni Hard") alloys, etc. Spectrochemical analysis of white cast iron is relatively easy and the white chill cast iron is the only form of cast iron which can be directly analyzed by spark spectrometry [2,3]. Analysis of graphitic cast irons with glow discharge excitation has been reported [4,5], but the nature of the atomization/excitation processes is not well described in those papers, and some of the methodology-related conclusions presented there are misleading. This paper aims to describe and explain the structure-related matrix effects in the excitation of graphitized cast irons by the glow discharge and the spark spectral sources.
2. Atomization/excitation by glow discharge 2.1. Experimental
Fig. 1. Microstructure of a gray cast iron with high phosphorus: graphite flakes (black), pearlite matrix (dark) and white regions of steadite (a ternary eutectic Fe3C-FeaP-ferrite ).
To describe the typical behavior of cast irons of different structures in the GD-OES analysis, the following measurements were made: eight samples of chill cast (white) iron (2KD 232-239 ((~KD Res. Inst., Prague, Czech Republic) were measured together with four samples of graphitic cast irons 21G-24G (Brammer Standard Company, Inc., Houston, USA), which have been used as reference materials for analysis of graphitic
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Fig. 3. Carbon and silicon signal response for white and graphitic cast irons: 1100 V, 40 mA, preburn time 60 s (not the optimum conditions). Crosses, white cast iron; squares, graphitic cast irons; abscissae, intensities ratioed to the intensity of the Ar 415.86 nm line.
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z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
cast irons. These samples were prepared by heat treatment of white cast iron, so they have not the true "as cast" structure. It is well known that during sample preparation (grinding) graphite flakes may be partially torn out from the surface, leaving the surface layer depleted in carbon. To avoid this, metallographic papers grit 400 and 600 were used for sample preparation and the resulting sample surface was checked by optical microscopy. All the measurements were made using the LECO SDP-750 glow discharge spectrometer, described elsewhere [6]. The first series of experiments was carried out under conditions which normally are not used for the analysis of cast iron, to make the structure-related differences more apparent: 1100 V and 40 mA (constant voltage, argon pressure controlled by a feedback loop to keep the discharge current constant) with a standard Grimm-type 4 mm d.c. glow discharge lamp; the preburn time was 60 s and integration time was 10 s. Signal intensities were ratioed to the intensity of the Ar 415.86 nm line, to correct for small fluctuations of the excitation conditions: no systematic structure-related change in the Ar 415.86 nm intensity was observed. The resulting calibrations for carbon (156.14 nm) and silicon (288.16 nm) are shown in Fig. 3. In all the calibrations presented (including Fig. 8), the calibration curves are defined by the CKD samples only; points corresponding to the graphitic samples were excluded from the calculations. For graphitic cast irons, the carbon intensity is substantially lower than for the white cast iron. For the other elements, the differences between both groups are very small, if any (see, for example, the silicon calibration). If a higher sputtering rate and a longer preburn time is used, the different signal response for carbon disappears (see Fig. 4; 1300 V, 55 mA, 180 s preburn time). Under these conditions, there are basically no observable differences in the behavior of both groups of samples - see the sulfur calibration (180.73 nm) in Fig. 4. To demonstrate the behavior of a true grey "as cast" sample, the intensity-versus-time depth profiles of carbon, silicon, sulfur and iron of the sample from Fig. 1 are presented in Fig. 5 together with the profiles of the "white" sample CKD-239.
867
These measurements were made at 700 V and 20 mA. The sputter-induced surface roughness was investigated on that grey sample by scanning electron microscopy, showing that the graphite flakes are removed much more slowly than the other phases of the matrix (Fig. 6). 2.2. Discussion o f the atomization mechanism
From the experimental evidence presented above, the following conclusions can be drawn: (1) Graphitized cast irons, unlike white cast irons, exhibit a lower carbon signal at the beginning, which gradually increases, until the stationary state is reached after some time of sputtering. This effect is not caused by selective graphite removal during sample preparation. (2) Within the accuracy of the measurements, this stationary state carbon signal is equal to the carbon signal of a non-graphitic (white) cast iron with the same carbon contents. (3) Signal responses of elements other than carbon exhibit only minor differences for graphitized samples in comparison with white cast iron. (4) The graphite flakes are sputtered off much more slowly than the other phases of the matrix. To explain these effects, a simple model of the atomization process can be adopted. It is based on the assumption that the sputtering rate at each point of the erosion crater is determined by the local phase composition. The development of the erosion crater bottom in this model, resulting from computer simulation, is shown in Fig. 7. Here a section through a hypothetical two-phase material is shown in a plane perpendicular to the original surface. The original surface is represented by the straight line on the top of each picture. The dark spots represent the phase with a smaller sputtering rate than the matrix. The erosion crater bottom is displayed at regular time intervals from the start of sputtering. This simulation was done only to clarify the derivations below; other models are also possible. However, this model is a good enough approximation to explain the observed phenomena. Let the analyzed material consist of two phases, A and B, with the sputtering rates qA, qB in the
868
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
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glow discharge conditions used. Let the volume concentration of the phase A be a. Then the relative coverage of the sample surface by the phase A at the beginning of the analysis will also be equal to a, as is demonstrated in the Appendix (statement T1 applied to the sample surface as the plane mentioned there). During sputtering, however, the relative surface coverage of the analyzed spot by both phases gradually changes as a result of the roughness development due to different sputtering rates of both phases. The resulting relative surface coverage by both phases at the stationary state of sputtering can be derived in the following way. Let us consider an arbitrary point at the analyzed spot and straight line perpendicular to the original sample surface going through this point. The dashed line in the bottom picture in Fig. 7 shows such a line. As is proved in the Appendix (statement T2), the relative coverage of this line by both phases in the sample will also be equal to a. Because the erosion crater
bottom proceeds into the depth, the average dwelling time of the crater bottom in both phases along this line will be inversely proportional to their sputtering rates. Consequently, the ratio ( of the total dwelling times tA, ta of the crater bottom in both phases along that line will be _ tA
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Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
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This effect generally results in changes of the total (average) sputtering rate and also in different intensities of the analyzed elements. To derive a formula for the elemental intensity change, it is assumed that the emission intensity of each element is directly proportional to the flux of the atoms of that element entering the discharge, i.e. to the product of the concentration and the sputtering rate. If cAx, Cax are concentrations of an element X in both phases, then the factor x(X), by which its emission intensity changes as a result of the above described effect, would be
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Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
Fig. 6. Sputter-induced surface roughness caused by different sputtering rates of graphite and of the matrix (sample from Fig. 1). Graphite flakes are sputtered much more slowly than the other phases present in this sample. The sample surface is sputtered at a relatively high argon pressure, at which strong redeposition occurs. The slopes adjacent to the graphite flakes consist of redeposited material (white in the micrograph).
typical grain size of the phase having a different sputtering rate from that of the matrix. To estimate the magnitude of the change of carbon signal intensity in the analysis of graphitized cast irons, resulting from the roughness development, an idealized graphite-pearlite cast iron with 3.5% C can be considered, having the approximate graphite volume concentration a = 0.09, the sputtering rate ratio (estimated) r/= qpearlite/ qgraphite ~- 4, and 7c = CC,pearlite/ CC,graphite ~7.5 x 10-3. For such cast iron, relative coverage of the sample surface by graphite will increase from about 9% to about 28% and the carbon signal will increase by a factor of x ( C ) = 2.6. This value is in a reasonably good agreement with the observed ratio of the minimum and the steady-state carbon intensity for the analyzed gray cast iron, which was found to be equal to 3.1 (see Fig. 5(a)).
3. Atomization/excitation by spark discharge The same calibration as described in Section 2.1 was measured using spark atomization/ excitation. This experiment was carried out using
the ARL 34000 spectrometer. The operating parameters for pre-sparking were 10 #F, 0 f~, 20 #H, 550 V, 100 Hz, 25 s, and for integration they were 10 #F, 2.2 f~, 120 #H, 400 V, 100 Hz, 5 s. To suppress the effect of "diffuse discharges" [8], this cycle was repeated three times and only the integrated intensities from the last cycle were used. In Fig. 8, calibration curves of carbon (193.09 nm), silicon (390.55 nm) and sulfur (180.73 nm) are shown, corresponding to calibrations in Figs. 3 and 4. As is common in spark spectrometry, intensities ratioed to the intensity of the iron 271.44 nm line are presented. The "as cast" gray iron sample from Fig. 1 was exposed to the spark discharge (analyzed) under the same conditions as described above, and after that, it was cut perpendicularly to the surface at the spark discharge spot (sparked area). The structure of the surface-near layer of this sample in the sparked area is shown in Fig. 9. By microhardness and X-ray diffraction measurements, the white surface layer in Fig. 9 was identified as cementite Fe3C. To reveal a prospective redistribution of elements in the surface layer caused by the spark discharge, the sparked area was depth profiled by glow discharge spectrometry, similarly to the sample with a fresh surface (Fig. 5(a)) and the sample CKD-239 (Fig. 5(b)). The resulting signal intensity versus time depth profile is shown in Fig. 10. From the results, it is evident that even if carbon is not taken into account, in the spark spectrometric analysis of cast irons there are dramatic structure-related matrix effects for other elements, which, unlike the glow discharge excitation, make analysis of graphitized cast irons with the spark excitation extremely difficult. It is difficult to describe in detail mechanisms of the observed matrix effects. At the center of the sparked area, the sample surface is re-melted (see the cementite layer). In this remelted area, the sample surface is enriched in carbon and depleted in sulfur (compare Figs. 5(a) and 10). This is most likely due to preferential evaporation, because of the different saturated vapor pressures of both elements: sulfur has a saturated vapor pressure of 10 Pa at 420 K, whilst carbon reaches the same saturated vapor pressure at about 3000 K [7]. At the outer part of
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
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Fig. 7. Computer simulation of erosion crater development, section perpendicular to the original surface. Dark spots represent grains of the phase with a smaller sputtering rate than the matrix. The ratio of the sputter rates in this simulation was 0.8. Continuous lines represent crater bottom shape at regular time intervals from the beginning of sputtering.
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Fig. 9. Structural changes caused by spark excitation in the surface layer of gray cast iron (the sample from Fig. 1). The surface layer was melted by the spark discharge and then allowed to cool quickly by heat conduction into the depth. As a result, a thin layer of cementite (white) was formed on the surface. Cracks in this layer are caused by mismatch in the thermal expansion coefficients of cementite and of the matrix.
Analysis o f graphitized cast irons by glow discharge optical emission spectrometry is subject to a structure-related matrix effect, due to a significantly different (lower) sputtering rate o f graphite compared to those o f the other structural c o m p o n e n t s existing in cast irons. This effect is demonstrated by a lower carbon intensity for the graphitized samples in the initial stages o f the analysis. However, as sputtering goes on, the relative coverage o f the sample surface by graphite increases, until a steady state o f sputtering is reached, at which there are only negligible differences in the signal intensities from those o f the white cast iron, for virtually all the elements analyzed. By using sufficiently long preburn times with high sputtering rates, it is possible to optimize the analytical conditions to suppress this matrix effect up to the level at which the analysis o f graphitized cast irons is possible
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using a calibration based on the white chill cast irons. A similar behavior can be expected with any structurally inhomogeneous material with significant differences of sputtering rates of the components (e.g. hypereutectic AI-Si alloys). This effect violates the hypothesis of matrix-independent emission yields [9,10] and should be generally taken into account in the GD-OES analysis quantification. To reach the steady state with an altered su:face coverage by different phases present in the sample takes typically tens to hundreds of seconds of sputtering, which is typically at least three orders of magnitude longer than the time in which the sputtering equilibrium, with respect to preferential sputtering for homogeneous alloys [11], is reached. In the spark spectrometric analysis of cast irons, complex matrix effects have been observed, making the analysis of graphitic cast irons extremely diffi-
cult. In addition to the selective sampling of graphite by the "diffuse discharges", a compositional change was observed in the re-melted part of the sparked area, caused most likely by selective evaporation of the elements and resulting from different saturated vapor pressures. Unlike the glow discharge atomization/excitation in which only the carbon signal is substantially affected by the structure, signals of all the analyzed elements in the spark atomization/excitation were unstable and significantly different from the white chill cast samples.
Acknowledgments The author wishes to thank Karel Bi~ovsk2~, BIJO s.r.o., Prague, Czech Republic, for providing partially graphitized cast iron samples; Petr Zilka,
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
875
//.// Fig. A1. Geometry considered to derive the formula (A2).
S K O D A Corp., Plzefi, Czech Republic, for making possible the spark spectrometric measurements; Zden6k Kubeg and Dagmar Jandov~, University of West Bohemia, Plzefi, Czech Republic, for assistance in microscopic investigations.
Appendix To derive the formula (2), expressing the change in relative surface coverage by both phases present in the sample due to sputtering, the following theorem was used: (Tl) For any plane within a two-phase material with a random distribution of both phases, the fraction as of the area in which this plane intersects the grains of one phase, relative to the total area considered, is equal to the volume concentration of this phase in the material. (T2) For any straight line in a two-phase material with a random distribution of both phases, the fraction al of its length in which this line intersects the grains of one phase, relative to
the total length considered, is equal to the volume concentration of this phase in the material. (T3) The above two statements hold for average values of the quantities discussed. First, this theorem is proved for the case that the phase A consists of spheres with equal radius r. Let a be the volume concentration of the phase A and as, al the quantities defined above. Then the number of the spheres of the phase A present in a volume unit of the material is 3a n -- 47rr 3
(A1)
Each sphere, the center of which is at the distance of y, y < r from the plane mentioned in the first part of the theorem, contributes to the area as by a circle with the radius of v/'(r 2 - x2). Hence, the coverage of the area unit of the plane by the phase A is as = 2
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which is equal to a according to Eq. (AI). In
876
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
the material can be derived and shown to be equal to a. al = I'0 2 V/( r 2 - y2)27ryndy = ~Trr3n 4
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In this integral, 2 ( x / ~ - y2) is the length of the line within the sphere, and 2pyn d y is the average number of the sphere centers in the space between the coaxial cylinders with the radii o f y and y + dy, the axis of which is the discussed line (see Fig. A2). To include a more realistic model of a twocomponent solid than the random distribution of equal spheres of one phase in a matrix of another phase, the above formulas can be generalized in a straightforward way to spheres of different size, by considering separately each group of spheres having approximately the same radius. Such a system would be quite a good structural model for nodular (ductile) cast iron. Generalization to irregularly shaped grains can be made by sophisticated mathematical methods [12].
References
dy Fig. A2. Geometry considered to derive the formula (A3).
this integral, 7r (r 2 - y 2 ) is the circle area, ndy is the average number of the sphere centers within the layer defined by the distance-from-the-plane interval (y, y + dy), and the factor 2 reflects the fact that integration is done both below and above the plane (see Fig. A1). Similarly, by direct integration, the relative coverage al by the phase A of a straight line in
[1] R. Heine, C. Loper and P. Rosenthal, Principles of Metal Casting, McGraw-Hill, New York, 1967. [2] K. Slickers, Spectrochim. Acta Part B, 28 (1973) 441. [3] K. Ohls, Europ. Spectrosc. News, 61 (1985) 10. [4] M. Fujita, J. Kashima and K. Naganuma, Anal. Chim. Acta, 124 (1981) 267. [5] H.W. Radmacher and M.C. de Swardt, Spectrochim. Acta Part B, 30 (1975) 353. [6] Z. Weiss, Surf. Interface Anal., 18 (1992) 691. [7] American Institute of Physics Handbook, McGraw-Hill, New York, 1972. [8] K. Laqua, Emissionspektroskopie, in Ullmanns Encyklop/idie der technischen Chemie, Band 5, Verlag Chemie GmbH, Weinheim, 1980, pp. 441-500. [9] Z. Weiss, J. Anal. At. Spectrom., 9 (1994) 351. [10l Z. Weiss, J. Anal. At. Spectrom., 10 (1995) 891. [11] Z. Weiss and J. Suba, Czech. J. Phys., 42 (1992) 539. [12] J. Weiss, personal communication (1995).
Spectrochimica Acta Part B 51 (1996) 863-876
ELSEVIER
Analysis of graphitized cast irons by optical emission spectroscopy: matrix effects in the glow discharge and the spark excitation Zden6k Weiss 1 LECO lnstrumente Plzeh, s.r.o., alej Svobody 56, 32318 Plzeti, Czech Republic
Received 24 June 1995; accepted 18 December 1995
Abstract
Graphite has a substantially lower sputtering rate in glow discharge than have the other structural components of graphitized cast irons, which leads to a structure-related matrix effect, consisting of an increasing relative surface coverage by graphite of the sample surface during the initial stage of the GD-OES analysis, and, consequently, to an increasing carbon signal intensity. This effect exists inherently in any multicomponent system with different sputtering rates of the components and should be taken into account in GD-OES quantification. A simple theory is presented to describe quantitatively the changes in relative contributions of different phases to the flux of the sputtered material entering the discharge and a formula is presented, expressing elemental intensity changes as a function of sputtering rates and stoichiometry of the structural components. After reaching the steady state, there are no substantial differences in the GD-OES signal response of the analyzed elements between the graphitic and the white cast irons. To reach this steady state, long preburn times and high sputtering rates have to be used. In the spark atomization/excitation, there are very strong and complex structure-related matrix effects, which make the analysis of graphitic cast irons by spark excitation impossible. Keywords." Glow discharge; Graphitized cast iron; Matrix effect; Spark excitation; Sputtering rate; Structural effect
I. Introduction
Elemental analysis of cast iron is one of the most frequent routine industrial applications of optical emission spectrometry. Both the spark and the glow discharge spectral sources have been used for this purpose for decades, with a methodology based on empirical observations. Analysis of graphitized cast irons, however, is still an t Present address: LECO Corporation, 3000 Lakeview Ave., St. Joseph, MI 49085-2396, USA. Elsevier Science B.V. P H S0584-8547(96)01461-9
interesting topic from the scientific point of view, because of the complex and not yet very well understood structure-related matrix effects. Cast irons represent a wide group of materials with a complex structure, dependent substantially on their metallurgical history. Cast irons are essentially ternary alloys of F e - C - S i , of near eutectic composition. There are four main types of cast irons, (1) white, (2) gray, (3) nodular and (4) malleable. A good concise summary of the chemistry and microstructure of these cast irons is presented in Chapter 18 of Ref. [1]. White cast
864
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
iron has a structure corresponding to the metastable equilibrium of the F e - C - S i system and consists of cementite (Fe3C: the primary cementite in case of hypereutectic composition, the eutectic cementite and the secondary cementite) and of pearlite, resulting from the eutectoid transformation of austenite. For white cast iron to be formed a sufficiently high cooling rate of the melt is essential. Unlike white cast iron, the other types of cast iron mentioned above contain free carbon in the form of graphite. The matrix of most graphitic cast irons consists most frequently of pearlite, ferrite or of their mixture. Solidification of graphitic cast irons and the resulting microstructure is controlled by many factors, the most important of which are the chemical composition and the cooling rate. The shape and size of graphite particles is very important: in normal gray cast iron there are usually graphite flakes (see Fig. 1). In nodular (ductile) cast iron, graphite exists in the form of spherical grains (spherulites). Such a shape of graphite is achieved by adding magnesium to the melt. Finally, malleable cast irons are produced by heat treatment (tempering) of white cast iron, resulting in conversion of cementite into ferrite and graphite. Compositions typical for the different forms of graphitic cast irons are schematically shown in Fig. 2. In addition to the above listed basic types of cast iron, there are also cast irons with high chromium (up to 30% Cr),
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with high nickel, N i - C r - C u cast irons ("Ni Resist"), C3Si2Ni5Cr8 ("Ni Hard") alloys, etc. Spectrochemical analysis of white cast iron is relatively easy and the white chill cast iron is the only form of cast iron which can be directly analyzed by spark spectrometry [2,3]. Analysis of graphitic cast irons with glow discharge excitation has been reported [4,5], but the nature of the atomization/excitation processes is not well described in those papers, and some of the methodology-related conclusions presented there are misleading. This paper aims to describe and explain the structure-related matrix effects in the excitation of graphitized cast irons by the glow discharge and the spark spectral sources.
2. Atomization/excitation by glow discharge 2.1. Experimental
Fig. 1. Microstructure of a gray cast iron with high phosphorus: graphite flakes (black), pearlite matrix (dark) and white regions of steadite (a ternary eutectic Fe3C-FeaP-ferrite ).
To describe the typical behavior of cast irons of different structures in the GD-OES analysis, the following measurements were made: eight samples of chill cast (white) iron (2KD 232-239 ((~KD Res. Inst., Prague, Czech Republic) were measured together with four samples of graphitic cast irons 21G-24G (Brammer Standard Company, Inc., Houston, USA), which have been used as reference materials for analysis of graphitic
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
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Z. Weiss I Spectrochimica Acta Part B 51 (1996) 863-876
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z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
cast irons. These samples were prepared by heat treatment of white cast iron, so they have not the true "as cast" structure. It is well known that during sample preparation (grinding) graphite flakes may be partially torn out from the surface, leaving the surface layer depleted in carbon. To avoid this, metallographic papers grit 400 and 600 were used for sample preparation and the resulting sample surface was checked by optical microscopy. All the measurements were made using the LECO SDP-750 glow discharge spectrometer, described elsewhere [6]. The first series of experiments was carried out under conditions which normally are not used for the analysis of cast iron, to make the structure-related differences more apparent: 1100 V and 40 mA (constant voltage, argon pressure controlled by a feedback loop to keep the discharge current constant) with a standard Grimm-type 4 mm d.c. glow discharge lamp; the preburn time was 60 s and integration time was 10 s. Signal intensities were ratioed to the intensity of the Ar 415.86 nm line, to correct for small fluctuations of the excitation conditions: no systematic structure-related change in the Ar 415.86 nm intensity was observed. The resulting calibrations for carbon (156.14 nm) and silicon (288.16 nm) are shown in Fig. 3. In all the calibrations presented (including Fig. 8), the calibration curves are defined by the CKD samples only; points corresponding to the graphitic samples were excluded from the calculations. For graphitic cast irons, the carbon intensity is substantially lower than for the white cast iron. For the other elements, the differences between both groups are very small, if any (see, for example, the silicon calibration). If a higher sputtering rate and a longer preburn time is used, the different signal response for carbon disappears (see Fig. 4; 1300 V, 55 mA, 180 s preburn time). Under these conditions, there are basically no observable differences in the behavior of both groups of samples - see the sulfur calibration (180.73 nm) in Fig. 4. To demonstrate the behavior of a true grey "as cast" sample, the intensity-versus-time depth profiles of carbon, silicon, sulfur and iron of the sample from Fig. 1 are presented in Fig. 5 together with the profiles of the "white" sample CKD-239.
867
These measurements were made at 700 V and 20 mA. The sputter-induced surface roughness was investigated on that grey sample by scanning electron microscopy, showing that the graphite flakes are removed much more slowly than the other phases of the matrix (Fig. 6). 2.2. Discussion o f the atomization mechanism
From the experimental evidence presented above, the following conclusions can be drawn: (1) Graphitized cast irons, unlike white cast irons, exhibit a lower carbon signal at the beginning, which gradually increases, until the stationary state is reached after some time of sputtering. This effect is not caused by selective graphite removal during sample preparation. (2) Within the accuracy of the measurements, this stationary state carbon signal is equal to the carbon signal of a non-graphitic (white) cast iron with the same carbon contents. (3) Signal responses of elements other than carbon exhibit only minor differences for graphitized samples in comparison with white cast iron. (4) The graphite flakes are sputtered off much more slowly than the other phases of the matrix. To explain these effects, a simple model of the atomization process can be adopted. It is based on the assumption that the sputtering rate at each point of the erosion crater is determined by the local phase composition. The development of the erosion crater bottom in this model, resulting from computer simulation, is shown in Fig. 7. Here a section through a hypothetical two-phase material is shown in a plane perpendicular to the original surface. The original surface is represented by the straight line on the top of each picture. The dark spots represent the phase with a smaller sputtering rate than the matrix. The erosion crater bottom is displayed at regular time intervals from the start of sputtering. This simulation was done only to clarify the derivations below; other models are also possible. However, this model is a good enough approximation to explain the observed phenomena. Let the analyzed material consist of two phases, A and B, with the sputtering rates qA, qB in the
868
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
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glow discharge conditions used. Let the volume concentration of the phase A be a. Then the relative coverage of the sample surface by the phase A at the beginning of the analysis will also be equal to a, as is demonstrated in the Appendix (statement T1 applied to the sample surface as the plane mentioned there). During sputtering, however, the relative surface coverage of the analyzed spot by both phases gradually changes as a result of the roughness development due to different sputtering rates of both phases. The resulting relative surface coverage by both phases at the stationary state of sputtering can be derived in the following way. Let us consider an arbitrary point at the analyzed spot and straight line perpendicular to the original sample surface going through this point. The dashed line in the bottom picture in Fig. 7 shows such a line. As is proved in the Appendix (statement T2), the relative coverage of this line by both phases in the sample will also be equal to a. Because the erosion crater
bottom proceeds into the depth, the average dwelling time of the crater bottom in both phases along this line will be inversely proportional to their sputtering rates. Consequently, the ratio ( of the total dwelling times tA, ta of the crater bottom in both phases along that line will be _ tA
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Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
869
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This effect generally results in changes of the total (average) sputtering rate and also in different intensities of the analyzed elements. To derive a formula for the elemental intensity change, it is assumed that the emission intensity of each element is directly proportional to the flux of the atoms of that element entering the discharge, i.e. to the product of the concentration and the sputtering rate. If cAx, Cax are concentrations of an element X in both phases, then the factor x(X), by which its emission intensity changes as a result of the above described effect, would be
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870
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
Fig. 6. Sputter-induced surface roughness caused by different sputtering rates of graphite and of the matrix (sample from Fig. 1). Graphite flakes are sputtered much more slowly than the other phases present in this sample. The sample surface is sputtered at a relatively high argon pressure, at which strong redeposition occurs. The slopes adjacent to the graphite flakes consist of redeposited material (white in the micrograph).
typical grain size of the phase having a different sputtering rate from that of the matrix. To estimate the magnitude of the change of carbon signal intensity in the analysis of graphitized cast irons, resulting from the roughness development, an idealized graphite-pearlite cast iron with 3.5% C can be considered, having the approximate graphite volume concentration a = 0.09, the sputtering rate ratio (estimated) r/= qpearlite/ qgraphite ~- 4, and 7c = CC,pearlite/ CC,graphite ~7.5 x 10-3. For such cast iron, relative coverage of the sample surface by graphite will increase from about 9% to about 28% and the carbon signal will increase by a factor of x ( C ) = 2.6. This value is in a reasonably good agreement with the observed ratio of the minimum and the steady-state carbon intensity for the analyzed gray cast iron, which was found to be equal to 3.1 (see Fig. 5(a)).
3. Atomization/excitation by spark discharge The same calibration as described in Section 2.1 was measured using spark atomization/ excitation. This experiment was carried out using
the ARL 34000 spectrometer. The operating parameters for pre-sparking were 10 #F, 0 f~, 20 #H, 550 V, 100 Hz, 25 s, and for integration they were 10 #F, 2.2 f~, 120 #H, 400 V, 100 Hz, 5 s. To suppress the effect of "diffuse discharges" [8], this cycle was repeated three times and only the integrated intensities from the last cycle were used. In Fig. 8, calibration curves of carbon (193.09 nm), silicon (390.55 nm) and sulfur (180.73 nm) are shown, corresponding to calibrations in Figs. 3 and 4. As is common in spark spectrometry, intensities ratioed to the intensity of the iron 271.44 nm line are presented. The "as cast" gray iron sample from Fig. 1 was exposed to the spark discharge (analyzed) under the same conditions as described above, and after that, it was cut perpendicularly to the surface at the spark discharge spot (sparked area). The structure of the surface-near layer of this sample in the sparked area is shown in Fig. 9. By microhardness and X-ray diffraction measurements, the white surface layer in Fig. 9 was identified as cementite Fe3C. To reveal a prospective redistribution of elements in the surface layer caused by the spark discharge, the sparked area was depth profiled by glow discharge spectrometry, similarly to the sample with a fresh surface (Fig. 5(a)) and the sample CKD-239 (Fig. 5(b)). The resulting signal intensity versus time depth profile is shown in Fig. 10. From the results, it is evident that even if carbon is not taken into account, in the spark spectrometric analysis of cast irons there are dramatic structure-related matrix effects for other elements, which, unlike the glow discharge excitation, make analysis of graphitized cast irons with the spark excitation extremely difficult. It is difficult to describe in detail mechanisms of the observed matrix effects. At the center of the sparked area, the sample surface is re-melted (see the cementite layer). In this remelted area, the sample surface is enriched in carbon and depleted in sulfur (compare Figs. 5(a) and 10). This is most likely due to preferential evaporation, because of the different saturated vapor pressures of both elements: sulfur has a saturated vapor pressure of 10 Pa at 420 K, whilst carbon reaches the same saturated vapor pressure at about 3000 K [7]. At the outer part of
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
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Fig. 7. Computer simulation of erosion crater development, section perpendicular to the original surface. Dark spots represent grains of the phase with a smaller sputtering rate than the matrix. The ratio of the sputter rates in this simulation was 0.8. Continuous lines represent crater bottom shape at regular time intervals from the beginning of sputtering.
872
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Fig. 9. Structural changes caused by spark excitation in the surface layer of gray cast iron (the sample from Fig. 1). The surface layer was melted by the spark discharge and then allowed to cool quickly by heat conduction into the depth. As a result, a thin layer of cementite (white) was formed on the surface. Cracks in this layer are caused by mismatch in the thermal expansion coefficients of cementite and of the matrix.
Analysis o f graphitized cast irons by glow discharge optical emission spectrometry is subject to a structure-related matrix effect, due to a significantly different (lower) sputtering rate o f graphite compared to those o f the other structural c o m p o n e n t s existing in cast irons. This effect is demonstrated by a lower carbon intensity for the graphitized samples in the initial stages o f the analysis. However, as sputtering goes on, the relative coverage o f the sample surface by graphite increases, until a steady state o f sputtering is reached, at which there are only negligible differences in the signal intensities from those o f the white cast iron, for virtually all the elements analyzed. By using sufficiently long preburn times with high sputtering rates, it is possible to optimize the analytical conditions to suppress this matrix effect up to the level at which the analysis o f graphitized cast irons is possible
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using a calibration based on the white chill cast irons. A similar behavior can be expected with any structurally inhomogeneous material with significant differences of sputtering rates of the components (e.g. hypereutectic AI-Si alloys). This effect violates the hypothesis of matrix-independent emission yields [9,10] and should be generally taken into account in the GD-OES analysis quantification. To reach the steady state with an altered su:face coverage by different phases present in the sample takes typically tens to hundreds of seconds of sputtering, which is typically at least three orders of magnitude longer than the time in which the sputtering equilibrium, with respect to preferential sputtering for homogeneous alloys [11], is reached. In the spark spectrometric analysis of cast irons, complex matrix effects have been observed, making the analysis of graphitic cast irons extremely diffi-
cult. In addition to the selective sampling of graphite by the "diffuse discharges", a compositional change was observed in the re-melted part of the sparked area, caused most likely by selective evaporation of the elements and resulting from different saturated vapor pressures. Unlike the glow discharge atomization/excitation in which only the carbon signal is substantially affected by the structure, signals of all the analyzed elements in the spark atomization/excitation were unstable and significantly different from the white chill cast samples.
Acknowledgments The author wishes to thank Karel Bi~ovsk2~, BIJO s.r.o., Prague, Czech Republic, for providing partially graphitized cast iron samples; Petr Zilka,
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
875
//.// Fig. A1. Geometry considered to derive the formula (A2).
S K O D A Corp., Plzefi, Czech Republic, for making possible the spark spectrometric measurements; Zden6k Kubeg and Dagmar Jandov~, University of West Bohemia, Plzefi, Czech Republic, for assistance in microscopic investigations.
Appendix To derive the formula (2), expressing the change in relative surface coverage by both phases present in the sample due to sputtering, the following theorem was used: (Tl) For any plane within a two-phase material with a random distribution of both phases, the fraction as of the area in which this plane intersects the grains of one phase, relative to the total area considered, is equal to the volume concentration of this phase in the material. (T2) For any straight line in a two-phase material with a random distribution of both phases, the fraction al of its length in which this line intersects the grains of one phase, relative to
the total length considered, is equal to the volume concentration of this phase in the material. (T3) The above two statements hold for average values of the quantities discussed. First, this theorem is proved for the case that the phase A consists of spheres with equal radius r. Let a be the volume concentration of the phase A and as, al the quantities defined above. Then the number of the spheres of the phase A present in a volume unit of the material is 3a n -- 47rr 3
(A1)
Each sphere, the center of which is at the distance of y, y < r from the plane mentioned in the first part of the theorem, contributes to the area as by a circle with the radius of v/'(r 2 - x2). Hence, the coverage of the area unit of the plane by the phase A is as = 2
J'o
7r(r 2 - y 2 ) n d y = 4 7 r r 3 n 3
(A2)
which is equal to a according to Eq. (AI). In
876
Z. Weiss / Spectrochimica Acta Part B 51 (1996) 863-876
the material can be derived and shown to be equal to a. al = I'0 2 V/( r 2 - y2)27ryndy = ~Trr3n 4
I I I
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In this integral, 2 ( x / ~ - y2) is the length of the line within the sphere, and 2pyn d y is the average number of the sphere centers in the space between the coaxial cylinders with the radii o f y and y + dy, the axis of which is the discussed line (see Fig. A2). To include a more realistic model of a twocomponent solid than the random distribution of equal spheres of one phase in a matrix of another phase, the above formulas can be generalized in a straightforward way to spheres of different size, by considering separately each group of spheres having approximately the same radius. Such a system would be quite a good structural model for nodular (ductile) cast iron. Generalization to irregularly shaped grains can be made by sophisticated mathematical methods [12].
References
dy Fig. A2. Geometry considered to derive the formula (A3).
this integral, 7r (r 2 - y 2 ) is the circle area, ndy is the average number of the sphere centers within the layer defined by the distance-from-the-plane interval (y, y + dy), and the factor 2 reflects the fact that integration is done both below and above the plane (see Fig. A1). Similarly, by direct integration, the relative coverage al by the phase A of a straight line in
[1] R. Heine, C. Loper and P. Rosenthal, Principles of Metal Casting, McGraw-Hill, New York, 1967. [2] K. Slickers, Spectrochim. Acta Part B, 28 (1973) 441. [3] K. Ohls, Europ. Spectrosc. News, 61 (1985) 10. [4] M. Fujita, J. Kashima and K. Naganuma, Anal. Chim. Acta, 124 (1981) 267. [5] H.W. Radmacher and M.C. de Swardt, Spectrochim. Acta Part B, 30 (1975) 353. [6] Z. Weiss, Surf. Interface Anal., 18 (1992) 691. [7] American Institute of Physics Handbook, McGraw-Hill, New York, 1972. [8] K. Laqua, Emissionspektroskopie, in Ullmanns Encyklop/idie der technischen Chemie, Band 5, Verlag Chemie GmbH, Weinheim, 1980, pp. 441-500. [9] Z. Weiss, J. Anal. At. Spectrom., 9 (1994) 351. [10l Z. Weiss, J. Anal. At. Spectrom., 10 (1995) 891. [11] Z. Weiss and J. Suba, Czech. J. Phys., 42 (1992) 539. [12] J. Weiss, personal communication (1995).