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Analysis of cast iron by x-ray fluorescence spectrometry
Analytica Chimica Acta, 169 (1986) 201-207 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ANALYSIS OF CAST IRON BY X-RAY FLUORESCENCE SPECTROMETRY
U. E. SENFF Australian Iron and Steel Pty. Ltd., Central Laboratory, N.S. W. 2500 (Australia)
P.O. Box 1854, Wollongong,
(Received 20th July 1984)
SUMMARY It is shown that a wide range of cast irons can be analysed on one*calibration if the sample surfaces are prepared to a mirror finish with diamond paste. Polishing the samples with any abrasive material results in preferential removal of the softer constituents from the sample matrix. If coarser abrasives than diamond paste are used, the increased depth of the introduced sample inhomogeneity leads to unacceptable error limits for the determination of light elements.
The term cast iron applies to a wide range of ferrous base alloys containing over 2% carbon and an appreciable amount of silicon, usually l-3%. The maximum carbon content at which iron can solidify as a single-phase alloy is about 2%. Thus cast irons by definition solidify as heterogeneous alloys containing a number of microstructures, which have a wide range of hardness. Chemical composition alone is not adequate to designate an iron with particular mechanical properties. The various types of cast irons are thus usually classified into groups (white irons, malleable irons, grey irons, ductile irons and compacted graphite irons) differentiated by their microstructure, which is based on the form and shape in which the major portion of carbon occurs in the iron [l] . The production of a particular type of cast iron depends upon the chemistry and cooling rate of the system. White irons can be produced by rapid cooling or by the addition of traces of bismuth or tellurium which start solidification at lower than normal temperature; the carbon is present as iron carbide, which is dispersed throughout the softer, essentially carbon-free, iron matrix. Malleable irons are produced from white irons which are heattreated to dissociate iron carbide to iron and free graphite, which is then present as irregularly shaped graphite nodules. Grey irons are produced by slow cooling of the molten metal and contain carbon primarily in the form of graphite flakes. In ductile irons, the carbon occurs as graphite spheroids; this structure is promoted by inoculants such as magnesium. Compacted graphite iron has carbon present in the form of blunt flakes which are interconnected within each cell. 0003-2670/85/$03.30
o 1985 Elsevier Science Publishers B.V.
202
Micro-segregation studies [2] on cast irons have shown that chemical heterogeneities on a microscopic scale are much more significant than the effects of macro-segregation. Carbide-stabilising elements, such as titanium, vanadium, chromium, manganese and molybdenum, preferentially dissolve in the iron carbide to various extents whereas graphitising elements, such as silicon, nickel and copper, predominate in the iron matrix. Phosphorus up to about 0.1% is soluble in iron whereas higher concentrations lead to the formation of iron phosphide which is the last constituent to solidify and thus occurs at the grain boundaries. Sulphur is normally present as manganese sulphide which precipitates as separate particles, or as iron sulphide. X-ray fluorescence (x.r.f.) spectrometry can be used to analyse a large surface area of the sample, and by using a spinner to rotate it offers the capability of averaging over any micro-segregation effects. It should therefore be capable of application to all cast irons with only one calibration, provided that the segregation is restricted to these dimensions and matrix effects are taken into account. Because it is essentially the surface of the sample which is examined by this method, care must be taken in sample preparation to avoid or minimise any bias which could be introduced into the structure. Analysis of the full range of cast irons is required in this laboratory, thus the present study was undertaken to establish the applicability and limitations of the x.r.f. method of examining cast irons. EXPERIMENTAL
Samples Fifty international standards (Table 1) were measured to establish calibration lines for iron, silicon, sulphur, phosphorus, manganese, nickel, chromium, titanium, molybdenum, copper and vanadium. Of these standards, forty nine TABLE 1 Standards used in regression Type
Standard
White iron
NBS 1145, 1146, 1147, 1148,1149,1150,1177,1180, 1182;MRDF 1, 2,3,4,5,8 NBS 1140a, 1141a, 1142a MRDF lO,ll, 12,13;SS 666,667,668,669,670 SS 651, 652, 663, 654, 655; BCIRA Al/l, A1/2, Al/3, All4, Al/5 SS 661,662,663,664,665
Ductile iron Nodular iron Malleable iron High-phosphorus engineering iron Low-phosphorus engineering iron Blast furnace (grey) iron Electrolytic iron
SS 656,657,658,659,660 NBS 1143,1144 NBS 1265
203
were cast irons representing most of the previously mentioned types and one was an electrolytic iron, included to give a general low concentration point in the calibration. Cast iron standards are generally all chill cast, so even the non-white iron standards contain iron carbide, as well as some graphite, the formation of which is promoted by the addition of inoculants. Although concentration values for iron are generally not certified on most international standards, values for calibration purposes were obtained by subtracting known concentration values of the remaining elements from 100%. These iron values therefore include the concentrations of undetermined residual elements and would not be of high accuracy. However, the sum of the determined concentrations together with the manually input carbon concentration can serve as a guide to the concentration of undetermined elements when an unknown iron sample is analysed. Procedures Measurements were made at the K, lines of all elements by using a PW1400 sequential x-ray spectrometer. The spectrometer used a chromium target x-ray tube powered at 50 kV and 50 mA. All measured lines were corrected for background and line overlaps if present. Matrix effects on the intensities of the measured lines were corrected for by using influence coefficients obtained from a fundamental parameters program written by the author, which is based on the method of de Jongh [3]. The analytical surfaces of the standards were prepared to two different finishes and regression analyses were done for each type of surface. The first method was to use a 120-grit polish on alumina pads, the standard x-ray preparative method used in this laboratory for solid samples which are not expected to give any smearing problems. The second method was to polish the sample surfaces to a mirror finish using 6-pm diamond paste. Regression analyses were done by using the standard Philips software routines. The line of best fit is determined by minimising a “quality factor” K, which is given by K = u/C1”, where u is the standard deviation of the calculated concentration values from the chemical concentration values, and C is the concentration (%). Sigma (u) is given by u = [XF(C,,, - Ccalc)2] / (n - 2)‘R in the present case because all the influence coefficients were evaluated independently; n is the number of samples included in the regression and varied for each analyte depending on the number of standards which had certified values. Analytical error bars, with respect to the average concentration of each analyte in the set of standards used, expected from the preset counting times alone, are presented in Table 2. Also included are the concentration ranges for each analyte. The total measurement time for one sample was about 5 min.
204 TABLE 2 Cast iron calibration parameters
Cont. range (%) min. max. Ave. cont. (%) Error barss cb (120 grit) ob (mirror)
Fe
Si
S
P
Mn
Ni
Cr
83.0 99.9 93.2 0.10 0.76 0.24
0.01 3.68 1.63 0.006 0.116 0.022
0.008 0.21 0.069 0.0005 0.006 0.005
0.003 1.08 0.19 0.0015 0.020 0.007
0.006 1.64 0.56 0.007 6.014 0.013
0.021 3.01 0.55 0.005 0.020 0.012
0.007 2.56 0.27 0.004 0.007 0.010
aError bars expected from counting times alone, related to the average concentrations above. bStandard deviation of the differences between the chemical and calculated concentrations.
RESULTS
AND DISCUSSION
Standard deviations (a) calculated for some of the analytes in the regression for both surface finishes are included in Table 1. The results show that the standard deviations for iron, silicon and phosphorus approach the error bars associated with wet chemical determination of these elements at concentration levels similar to the average concentrations in Table 2 when the standards were prepared to a mirror finish. When a 12Ogrit finish was used, the standard deviations for these elements were unacceptably high. Calibration quality for the remaining elements showed either a smaller improvement or remained approximately the same for polishing with a 120&t or the diamond paste. The standard deviation for chromium was slightly greater for the mirror-finish calibration than for the 12Ogrit calibration. This could be attributed to counting statistics and instrumental precision. In order to investigate the reason for the low quality of the calibrations obtained on especially silicon and phosphorus when the samples were polished on the 120grit alumina pads, three white-iron, three grey-iron and two steel samples polished to a 12Ogrit finish were evaluated from the mirror-finish calibration. The analyses were then repeated after the samples had been prepared to a mirror finish. Concentrations obtained from these treatments are listed in Table 3. Inclusion of the steel standards, which have low (ca. 0.06%) carbon concentrations and are expected to be homogeneous, gives a check on the variation of analyte line intensity caused by the surface finish alone. Results in Table 3 indicate that all analyte lines show little variation in intensity for the steel standards. Analyte line intensities of the cast iron samples are, however, sensitive to the surface finish, as shown by the change in concentration values obtained. The results for the in-house samples A and B, of predominantly grey iron structure, vary less than those of the chilled cast irons. It is possible to explain these concentration changes by assuming that the softer iron matrix constituent of the samples is somewhat preferentially
205 TABLE 3 Concentrations finisha
(%) found from analysis of samples having a mirror and 120-grit surface
Sample
Finish
Si
S
P
Mn
Ni
Cr
NBS 1145
mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit
0.26 0.22 3.68 3.25 1.22 0.99 2.72 2.16 3.90 3.91 0.71 0.69 1.40 1.40 2.92 2.90
0.204 0.180 0.021 0.021 0.068 0.053 0.025 0.022 0.021 0.018 0.018 0.017 0.058 0.058 0.015 0.017
0.250 0.259 0.566 0.636 0.061 0.052 0.010 0.011 0.194 0.243 0.112 0.143 0.021 0.020 0.029 0.029
0.04 0.04 1.65 1.66 0.82 0.81 0.27 0.26 0.77 0.77 0.78 0.78 1.28 1.26 0.27 0.27
0.565 0.544 2.94 2.83 0.071 0.078 0.119 0.119 0.010 0.005 0.019 0.014 0.120 0.115 0.043 0.037
0.668 0.694 2.54 2.62 0.091 0.096 0.063 0.065 0,007 0.008 0.029 0.034 0.219 0.215 0.021 0.022
NBS 1146 NBS 1150 MRDF 12 A B ss 405 NBS 1134
aSamples NBS 1145,1146 and 1150 are white irons;MRDF 12 is a chill cast nodular iron; A and B are in-house samples having a predominantly grey iron structure; SS 405 and NBS 1134 are low carbon steels.
removed compared to the harder iron carbide fraction in the 12Ogrit polishing technique. This would mean that the concentrations found would be weighted towards the iron carbide constituent of the cast which would then tend to predominate on the surface. The direction of the concentration changes with respect to surface finish indicates that elements such as silicon and nickel, which preferentially dissolve in the soft iron matrix, have decreased concentration levels when polished to a 120&t finish whereas elements which predominate in the iron carbide, such as chromium, give increased concentration levels. Concentration changes are most noticeable for analyte lines of long wavelength because these can only emerge from near the surface of the sample whereas the more energetic lines can emerge from greater depths. These relative changes are largest for iron samples containing a substantial amount of iron carbide. When smaller amounts of iron carbide are present, the sample surfaces are abraded more evenly and the concentration changes in the above elements become similar to those of the steel samples. The concentration levels of phosphorus decrease on 120grit polishing when the amount present is less than about 0.175, because it is then soluble in the soft iron matrix. At higher concentrations, phosphorus forms iron phosphide which is deposited along the grain boundaries and the concentration levels increase as more of these are exposed to the surface. This phenomena will therefore also appear in cast iron samples containing no iron
206
carbide. The concentrations of manganese found are about the same for both surface finishes. Manganese is one of the weakest carbide-forming elements and would scarcely appear in the iron carbide. It also forms manganese sulphide with any available sulphur; this sulphide acts as a nucleus for graphite formation and would thus appear in the soft iron matrix. The relative balance of manganese in the two matrix constituents and the higher energy, compared to Si K, radiation for example, mean that the concentration found is relatively insensitive to the surface finishes studied here. Sulphur is mainly present as manganese or iron sulphides in the soft iron matrix and the measured concentration decreases when this matrix is preferentially removed. The calibration quality for sulphur did not improve significantly when the standards were diamond-polished, as was the case for silicon and phosphorus, because of the smaller concentration range involved here. Improvement in the calibration for iron is due entirely to the influence coefficients being able to correct for “true” matrix effects in the mirrorfinished samples but not in the 12Ogrit finish where sample inhomogeneity is far greater. In order to investigate the type of surface finish necessary to minimise detectable inhomogeneity on the sample surface, a white iron standard was prepared on polishing pads of varying grit sizes and evaluated from the mirror-finish calibration. The results (Table 4) show that the 120~grit finish was the worst case. When the standard was polished on a coarser surface, the abrading action gave a slightly more balanced surface removal. A finer finish decreased the depth of the introduced sample inhomogeneity. However, even a 12OOgrit finish still introduced inhomogeneities of sufficient depth to influence the lighter elements significantly. For elements with more energetic analyte lines, such as nickel, an 800-grit surface finish appears to be sufficient. Conclusion A wide range of cast irons can be analysed successfully on the one x.r.f. calibration provided that no significant sample inhomogeneities are introduced by an injudicious choice of sample preparation. The assumption that the softer iron matrix is preferentially removed when surfaces are polished TABLE 4 Effects of various surface finishes for NBS 1146 white iron on the concentrations
found (%)
Finish
Si
S
P
Mn
Ni
Cr
Mirror 1200 grit 800 grit 320 grit 120 grit 60 grit 40 grit
3.68 3.58 3.50 3.43 3.25 3.47 3.44
0.021 0.020 0.018 0.018 0.021 0.021 0.020
0.56 0.61 0.61 0.62 0.64 0.62 0.62
1.65 1.66 1.63 1.64 1.66 1.67 1.63
2.94 2.95 2.92 2.89 2.83 2.88 2.82
2.54 2.55 2.57 2.55 2.62 2.58 2.57
207
on abrasive pads is supported by correlations between micro-segregation studies and the present results. If samples are polished to a mirror finish, the calibration has acceptable error limits for the light elements over the full range of cast irons. The author acknowledges I. Barrow and G. Mulheron.
useful discussions with K. Ozinga, J. Fittler,
REFERENCES 1 C. F. Walton and T. J. Opar (Eds.), Iron Castings Handbook, Iron Castings Society Inc., 1981. 2 J. Charbonnier and J.-C. Margeiiie, in H. D. Merchant (Ed.), Recent Research on Cast Iron, Gordon and Breach, New York, 1968, p. 389. 3 W. K. de Jongh, X-Ray Spectrom., 2 (1973) 151.
ANALYSIS OF CAST IRON BY X-RAY FLUORESCENCE SPECTROMETRY
U. E. SENFF Australian Iron and Steel Pty. Ltd., Central Laboratory, N.S. W. 2500 (Australia)
P.O. Box 1854, Wollongong,
(Received 20th July 1984)
SUMMARY It is shown that a wide range of cast irons can be analysed on one*calibration if the sample surfaces are prepared to a mirror finish with diamond paste. Polishing the samples with any abrasive material results in preferential removal of the softer constituents from the sample matrix. If coarser abrasives than diamond paste are used, the increased depth of the introduced sample inhomogeneity leads to unacceptable error limits for the determination of light elements.
The term cast iron applies to a wide range of ferrous base alloys containing over 2% carbon and an appreciable amount of silicon, usually l-3%. The maximum carbon content at which iron can solidify as a single-phase alloy is about 2%. Thus cast irons by definition solidify as heterogeneous alloys containing a number of microstructures, which have a wide range of hardness. Chemical composition alone is not adequate to designate an iron with particular mechanical properties. The various types of cast irons are thus usually classified into groups (white irons, malleable irons, grey irons, ductile irons and compacted graphite irons) differentiated by their microstructure, which is based on the form and shape in which the major portion of carbon occurs in the iron [l] . The production of a particular type of cast iron depends upon the chemistry and cooling rate of the system. White irons can be produced by rapid cooling or by the addition of traces of bismuth or tellurium which start solidification at lower than normal temperature; the carbon is present as iron carbide, which is dispersed throughout the softer, essentially carbon-free, iron matrix. Malleable irons are produced from white irons which are heattreated to dissociate iron carbide to iron and free graphite, which is then present as irregularly shaped graphite nodules. Grey irons are produced by slow cooling of the molten metal and contain carbon primarily in the form of graphite flakes. In ductile irons, the carbon occurs as graphite spheroids; this structure is promoted by inoculants such as magnesium. Compacted graphite iron has carbon present in the form of blunt flakes which are interconnected within each cell. 0003-2670/85/$03.30
o 1985 Elsevier Science Publishers B.V.
202
Micro-segregation studies [2] on cast irons have shown that chemical heterogeneities on a microscopic scale are much more significant than the effects of macro-segregation. Carbide-stabilising elements, such as titanium, vanadium, chromium, manganese and molybdenum, preferentially dissolve in the iron carbide to various extents whereas graphitising elements, such as silicon, nickel and copper, predominate in the iron matrix. Phosphorus up to about 0.1% is soluble in iron whereas higher concentrations lead to the formation of iron phosphide which is the last constituent to solidify and thus occurs at the grain boundaries. Sulphur is normally present as manganese sulphide which precipitates as separate particles, or as iron sulphide. X-ray fluorescence (x.r.f.) spectrometry can be used to analyse a large surface area of the sample, and by using a spinner to rotate it offers the capability of averaging over any micro-segregation effects. It should therefore be capable of application to all cast irons with only one calibration, provided that the segregation is restricted to these dimensions and matrix effects are taken into account. Because it is essentially the surface of the sample which is examined by this method, care must be taken in sample preparation to avoid or minimise any bias which could be introduced into the structure. Analysis of the full range of cast irons is required in this laboratory, thus the present study was undertaken to establish the applicability and limitations of the x.r.f. method of examining cast irons. EXPERIMENTAL
Samples Fifty international standards (Table 1) were measured to establish calibration lines for iron, silicon, sulphur, phosphorus, manganese, nickel, chromium, titanium, molybdenum, copper and vanadium. Of these standards, forty nine TABLE 1 Standards used in regression Type
Standard
White iron
NBS 1145, 1146, 1147, 1148,1149,1150,1177,1180, 1182;MRDF 1, 2,3,4,5,8 NBS 1140a, 1141a, 1142a MRDF lO,ll, 12,13;SS 666,667,668,669,670 SS 651, 652, 663, 654, 655; BCIRA Al/l, A1/2, Al/3, All4, Al/5 SS 661,662,663,664,665
Ductile iron Nodular iron Malleable iron High-phosphorus engineering iron Low-phosphorus engineering iron Blast furnace (grey) iron Electrolytic iron
SS 656,657,658,659,660 NBS 1143,1144 NBS 1265
203
were cast irons representing most of the previously mentioned types and one was an electrolytic iron, included to give a general low concentration point in the calibration. Cast iron standards are generally all chill cast, so even the non-white iron standards contain iron carbide, as well as some graphite, the formation of which is promoted by the addition of inoculants. Although concentration values for iron are generally not certified on most international standards, values for calibration purposes were obtained by subtracting known concentration values of the remaining elements from 100%. These iron values therefore include the concentrations of undetermined residual elements and would not be of high accuracy. However, the sum of the determined concentrations together with the manually input carbon concentration can serve as a guide to the concentration of undetermined elements when an unknown iron sample is analysed. Procedures Measurements were made at the K, lines of all elements by using a PW1400 sequential x-ray spectrometer. The spectrometer used a chromium target x-ray tube powered at 50 kV and 50 mA. All measured lines were corrected for background and line overlaps if present. Matrix effects on the intensities of the measured lines were corrected for by using influence coefficients obtained from a fundamental parameters program written by the author, which is based on the method of de Jongh [3]. The analytical surfaces of the standards were prepared to two different finishes and regression analyses were done for each type of surface. The first method was to use a 120-grit polish on alumina pads, the standard x-ray preparative method used in this laboratory for solid samples which are not expected to give any smearing problems. The second method was to polish the sample surfaces to a mirror finish using 6-pm diamond paste. Regression analyses were done by using the standard Philips software routines. The line of best fit is determined by minimising a “quality factor” K, which is given by K = u/C1”, where u is the standard deviation of the calculated concentration values from the chemical concentration values, and C is the concentration (%). Sigma (u) is given by u = [XF(C,,, - Ccalc)2] / (n - 2)‘R in the present case because all the influence coefficients were evaluated independently; n is the number of samples included in the regression and varied for each analyte depending on the number of standards which had certified values. Analytical error bars, with respect to the average concentration of each analyte in the set of standards used, expected from the preset counting times alone, are presented in Table 2. Also included are the concentration ranges for each analyte. The total measurement time for one sample was about 5 min.
204 TABLE 2 Cast iron calibration parameters
Cont. range (%) min. max. Ave. cont. (%) Error barss cb (120 grit) ob (mirror)
Fe
Si
S
P
Mn
Ni
Cr
83.0 99.9 93.2 0.10 0.76 0.24
0.01 3.68 1.63 0.006 0.116 0.022
0.008 0.21 0.069 0.0005 0.006 0.005
0.003 1.08 0.19 0.0015 0.020 0.007
0.006 1.64 0.56 0.007 6.014 0.013
0.021 3.01 0.55 0.005 0.020 0.012
0.007 2.56 0.27 0.004 0.007 0.010
aError bars expected from counting times alone, related to the average concentrations above. bStandard deviation of the differences between the chemical and calculated concentrations.
RESULTS
AND DISCUSSION
Standard deviations (a) calculated for some of the analytes in the regression for both surface finishes are included in Table 1. The results show that the standard deviations for iron, silicon and phosphorus approach the error bars associated with wet chemical determination of these elements at concentration levels similar to the average concentrations in Table 2 when the standards were prepared to a mirror finish. When a 12Ogrit finish was used, the standard deviations for these elements were unacceptably high. Calibration quality for the remaining elements showed either a smaller improvement or remained approximately the same for polishing with a 120&t or the diamond paste. The standard deviation for chromium was slightly greater for the mirror-finish calibration than for the 12Ogrit calibration. This could be attributed to counting statistics and instrumental precision. In order to investigate the reason for the low quality of the calibrations obtained on especially silicon and phosphorus when the samples were polished on the 120grit alumina pads, three white-iron, three grey-iron and two steel samples polished to a 12Ogrit finish were evaluated from the mirror-finish calibration. The analyses were then repeated after the samples had been prepared to a mirror finish. Concentrations obtained from these treatments are listed in Table 3. Inclusion of the steel standards, which have low (ca. 0.06%) carbon concentrations and are expected to be homogeneous, gives a check on the variation of analyte line intensity caused by the surface finish alone. Results in Table 3 indicate that all analyte lines show little variation in intensity for the steel standards. Analyte line intensities of the cast iron samples are, however, sensitive to the surface finish, as shown by the change in concentration values obtained. The results for the in-house samples A and B, of predominantly grey iron structure, vary less than those of the chilled cast irons. It is possible to explain these concentration changes by assuming that the softer iron matrix constituent of the samples is somewhat preferentially
205 TABLE 3 Concentrations finisha
(%) found from analysis of samples having a mirror and 120-grit surface
Sample
Finish
Si
S
P
Mn
Ni
Cr
NBS 1145
mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit mirror 120 grit
0.26 0.22 3.68 3.25 1.22 0.99 2.72 2.16 3.90 3.91 0.71 0.69 1.40 1.40 2.92 2.90
0.204 0.180 0.021 0.021 0.068 0.053 0.025 0.022 0.021 0.018 0.018 0.017 0.058 0.058 0.015 0.017
0.250 0.259 0.566 0.636 0.061 0.052 0.010 0.011 0.194 0.243 0.112 0.143 0.021 0.020 0.029 0.029
0.04 0.04 1.65 1.66 0.82 0.81 0.27 0.26 0.77 0.77 0.78 0.78 1.28 1.26 0.27 0.27
0.565 0.544 2.94 2.83 0.071 0.078 0.119 0.119 0.010 0.005 0.019 0.014 0.120 0.115 0.043 0.037
0.668 0.694 2.54 2.62 0.091 0.096 0.063 0.065 0,007 0.008 0.029 0.034 0.219 0.215 0.021 0.022
NBS 1146 NBS 1150 MRDF 12 A B ss 405 NBS 1134
aSamples NBS 1145,1146 and 1150 are white irons;MRDF 12 is a chill cast nodular iron; A and B are in-house samples having a predominantly grey iron structure; SS 405 and NBS 1134 are low carbon steels.
removed compared to the harder iron carbide fraction in the 12Ogrit polishing technique. This would mean that the concentrations found would be weighted towards the iron carbide constituent of the cast which would then tend to predominate on the surface. The direction of the concentration changes with respect to surface finish indicates that elements such as silicon and nickel, which preferentially dissolve in the soft iron matrix, have decreased concentration levels when polished to a 120&t finish whereas elements which predominate in the iron carbide, such as chromium, give increased concentration levels. Concentration changes are most noticeable for analyte lines of long wavelength because these can only emerge from near the surface of the sample whereas the more energetic lines can emerge from greater depths. These relative changes are largest for iron samples containing a substantial amount of iron carbide. When smaller amounts of iron carbide are present, the sample surfaces are abraded more evenly and the concentration changes in the above elements become similar to those of the steel samples. The concentration levels of phosphorus decrease on 120grit polishing when the amount present is less than about 0.175, because it is then soluble in the soft iron matrix. At higher concentrations, phosphorus forms iron phosphide which is deposited along the grain boundaries and the concentration levels increase as more of these are exposed to the surface. This phenomena will therefore also appear in cast iron samples containing no iron
206
carbide. The concentrations of manganese found are about the same for both surface finishes. Manganese is one of the weakest carbide-forming elements and would scarcely appear in the iron carbide. It also forms manganese sulphide with any available sulphur; this sulphide acts as a nucleus for graphite formation and would thus appear in the soft iron matrix. The relative balance of manganese in the two matrix constituents and the higher energy, compared to Si K, radiation for example, mean that the concentration found is relatively insensitive to the surface finishes studied here. Sulphur is mainly present as manganese or iron sulphides in the soft iron matrix and the measured concentration decreases when this matrix is preferentially removed. The calibration quality for sulphur did not improve significantly when the standards were diamond-polished, as was the case for silicon and phosphorus, because of the smaller concentration range involved here. Improvement in the calibration for iron is due entirely to the influence coefficients being able to correct for “true” matrix effects in the mirrorfinished samples but not in the 12Ogrit finish where sample inhomogeneity is far greater. In order to investigate the type of surface finish necessary to minimise detectable inhomogeneity on the sample surface, a white iron standard was prepared on polishing pads of varying grit sizes and evaluated from the mirror-finish calibration. The results (Table 4) show that the 120~grit finish was the worst case. When the standard was polished on a coarser surface, the abrading action gave a slightly more balanced surface removal. A finer finish decreased the depth of the introduced sample inhomogeneity. However, even a 12OOgrit finish still introduced inhomogeneities of sufficient depth to influence the lighter elements significantly. For elements with more energetic analyte lines, such as nickel, an 800-grit surface finish appears to be sufficient. Conclusion A wide range of cast irons can be analysed successfully on the one x.r.f. calibration provided that no significant sample inhomogeneities are introduced by an injudicious choice of sample preparation. The assumption that the softer iron matrix is preferentially removed when surfaces are polished TABLE 4 Effects of various surface finishes for NBS 1146 white iron on the concentrations
found (%)
Finish
Si
S
P
Mn
Ni
Cr
Mirror 1200 grit 800 grit 320 grit 120 grit 60 grit 40 grit
3.68 3.58 3.50 3.43 3.25 3.47 3.44
0.021 0.020 0.018 0.018 0.021 0.021 0.020
0.56 0.61 0.61 0.62 0.64 0.62 0.62
1.65 1.66 1.63 1.64 1.66 1.67 1.63
2.94 2.95 2.92 2.89 2.83 2.88 2.82
2.54 2.55 2.57 2.55 2.62 2.58 2.57
207
on abrasive pads is supported by correlations between micro-segregation studies and the present results. If samples are polished to a mirror finish, the calibration has acceptable error limits for the light elements over the full range of cast irons. The author acknowledges I. Barrow and G. Mulheron.
useful discussions with K. Ozinga, J. Fittler,
REFERENCES 1 C. F. Walton and T. J. Opar (Eds.), Iron Castings Handbook, Iron Castings Society Inc., 1981. 2 J. Charbonnier and J.-C. Margeiiie, in H. D. Merchant (Ed.), Recent Research on Cast Iron, Gordon and Breach, New York, 1968, p. 389. 3 W. K. de Jongh, X-Ray Spectrom., 2 (1973) 151.