DIS RESEARCH PROJECT NO.37 NODULARITY, ITS MEASUREMENT, AND ITS CORRELATION WITH THE MECHANICAL PROPERTIES OF DUCTILE IRON June 18, 2006 BY RICHARD B. GUNDLACH STORK CLIMAX RESEARCH SERVICES 51229 CENTURY COURT WIXOM, MICHIGAN 48393 DIS Research Project No. 37 DIS RESEARCH PROJECT NO. 37 NODULARITY, ITS MEASUREMENT, AND ITS CORRELATION WITH THE MECHANICAL PROPERTIES OF DUCTILE IRON ABSTRACT The relationship between nodularity and mechanical properties in the various SAE grades of ductile iron were determined in this study. In addition, a correlation between nodularity and ultrasonic velocity was also developed and an evaluation of this relationship was performed. A large number of heats (26) were procured for this investigation. Six grades of ductile iron were produced, including D4018 Annealed, D4018 As-cast, D4512, D5506, D7003 and D9002. Test bars were produced at various levels of nodularity with nodularities ranging from 95% to 43%. The test castings consisted of keel blocks and rounds of 1-inch section size. The results of the study showed that both tensile strength and elongation decrease with nodularity. When achieving the minimum properties of each SAE grade, the acceptable nodularity level varied with the grade of iron. As strength increased, the degradation in properties began at higher nodularity values. The correlation of nodularity and ultrasonic velocity was loose, and the correlation of mechanical properties and ultrasonic velocity was poor. In addition to the strong influence of nodularity, ultrasonic velocity was also affected by graphite volume and matrix structure. Page 2 of 34 DIS Research Project No. 37 BACKGROUND INFORMATION The mechanical properties of ductile iron are tied directly to nodularity. Castings with poor nodularity will display lower tensile elongation and often do not meet minimum tensile strength and/or impact strength requirements. Degenerate graphite particles are stress risers and can also reduce the fatigue strength of ductile iron. Consequently, industrial specifications usually establish the minimum acceptable percent nodularity allowed in a part. The amount of degradation that occurs with a given deviation from 100% nodularity can vary with the ductile iron grade. The high-strength grades are more susceptible to the presence of degenerate graphite than the low-strength, high-ductility grades. Industrial standards do not necessarily reflect this fact. The objective of this investigation was to determine the relationship between nodularity and mechanical properties of ductile iron. The need for developing more quantitative data on the correlation of nodularity and properties in ductile iron castings is growing. There is a need to better define what an acceptable microstructure, and or properties, should be. Both the producer and buyer of ductile iron castings need to know this correlation, particularly when the criterion of acceptance is based on these properties. A more precise method of measuring nodularity is also needed to properly referee casting quality. In consideration of current times, this has become more and more important in light of the liability with litigation of suspect castings. While it would appear that the correlation of nodularity and mechanical properties is well understood, the literature provides very little quantitative data on this subject. All of the data uncovered in a preliminary search compares properties with nodularities that were determined by visual estimates. The estimation of nodularity by individuals has been shown to be quite subjective, particularly as nodularity decreases (it is easier to recognize 95 to 100% nodularity). Several investigators have shown a correlation between nodularity and sonic properties but, again, the correlations are based on visual estimates of nodularity. With the advent of modern analytical tools it seems appropriate to revisit this subject. With the utilization of ductile iron castings in applications requiring high ductility and toughness and high reliability, various quality assurance techniques have been developed to ensure that high nodularity has been achieved in the casting. Both resonant frequency and ultrasonic velocity measurements have become routine in the foundry. Because of the importance of the correlation between mechanical properties, nodularity and these NDT properties, this investigation makes an attempt to develop correlations between mechanical properties, nodularity and ultrasonic velocity. To improve the repeatability and precision of the nodularity measurement, nodularity was determined by image analysis. What’s different in this proposal are the methods being used to evaluate the graphite structure. Instead of visual estimations of the nodularity and graphite shape, the structures are being evaluated more quantitatively using automated image analysis. Automated image analysis allows the operator to evaluate many more fields to better obtain an average rating of the microstructure. Image analysis will also minimize the variability of the measurement due to operator bias. Page 3 of 34 DIS Research Project No. 37 Furthermore, the correlation of graphite structure with mechanical properties will be extended to include the most common grades of ductile iron. HISTORY AND THEORY Historical investigations1-4 have shown that properties vary significantly with nodularity in ductile iron. Several papers have been published, primarily by BCIRA and in AFS Transactions showing the correlation between nodularity and tensile strength, yield strength and impact toughness. Tensile properties degrade more quickly in high-strength irons than in lower-strength irons as nodularity decreases. The currently available correlations cover only two grades (ferritic and pearlitic); this investigation addresses six grades of ductile iron. The powerful influence of nodularity on strength is due to the effects of increased stress concentration associated with non-nodular graphite particles. As the graphite structure becomes more degenerated, some dimensions of the graphite particles increase in size. The larger particles produce an increase in stress intensity around each particle and thus reduce the critical stress for crack propagation. The stress concentration increases in proportion to the square root of the major dimension of the particle. This change in the shape of the graphite particles also changes the elastic modulus and sonic properties of the material, thus, making it possible to evaluate change in structure by NDT techniques. Several investigations1,2,4 have shown such correlations. EXPERIMENTAL PROCEDURES The experimental procedure consists of producing many sets of test bars required for this study. The test bars were supplied by the Casting Laboratory of DIS member DaimlerChrysler, which greatly reduced the cost of this project. More than 26 heats were produced to obtain four as-cast grades with a wide range of nodularities. While it was not particularly difficult to produce castings with high nodularities, the production of the four grades with specific levels of low nodularity was quite difficult. Several trial heats were produced that could not be used due to duplication. Table 1 contains a list of the desired materials and test matrix for this study. There are various methods that could be employed to produce low nodularity in ductile iron, including reduced Mg treatment levels, holding the treated iron to achieve fade, or adding tramp elements to spoil the graphite structure. For this investigation, the nodularity was varied by controlling the Mg treatment level and also by adjusting nodularity with small additions of S to the treated metal. Both double-coupon keel blocks and round test bars were poured for this study. The chemically bonded sand molds for the keel blocks were produced by Wescast Industries and supplied to DaimlerChrysler for casting. Numerous castings were poured due to the number of test bars required for the investigation. For the annealed ferritic grade D4018 “Annealed”, certain heats were selected based on nodularity and on compositions which were expected to best respond to a ferritize annealing heat Page 4 of 34 DIS Research Project No. 37 treatment. Up to six bars from each heat were subjected to a subcritical anneal. The test bars were heated to 1325oF, held for 4 hours, and furnace-cooled. For the quenched and tempered grade D9002, additional heats were selected based on nodularity and on compositions which were expected to through-harden upon quenching from the austenitizing temperature. Up to six bars from each heat were subjected to a heat treatment by heating to 1650oF, holding 1.5 hours and quenching in oil. The test bars were subsequently tempered at 950oF for 2 hours and furnace-cooled. Table 1. Material and Test Matrix Test Matrix Tensile 5x6 5x6 5x6 5x6 5x6 5x6 El. Modulus 5x2 5x2 5x2 5x2 5x2 5x2 Metallography 5x2 5x2 5x2 5x2 5x2 5x2 UT Velocity 5x6 5x6 5x6 5x6 5x6 5x6 HB 5x6 5x6 5x6 5x6 5x6 5x6 Grade Sample Nodularity Quantity Levels 33 33 33 33 33 33 Five Five Five Five Five Five D4018 D4018 Ann. D4512 D5506 D7003 D9002 Nodularity As each group of test bars was submitted for evaluation, a test bar was sampled to determine nodularity and microstructure. This screening process was used to determine whether the test bars fit the material matrix in Table 1. The metallographic sample used to determine nodularity was cut from a location well away from the cast end of the test bar to avoid any abnormally high nodularity readings associated with end effects. Percent nodularity and nodule count were measured using computer-aided image analysis. The method was identical to that used in developing the DIS nodularity wallchart “Graphite Rating in Ductile Iron”. The method of determining nodularity follows ASTM A 247-67(1998) practices, where percent nodularity was based on area fraction nodules versus total graphite area. The criterion (shape factor) used for distinguishing nodules from other graphite inclusions was "compactness”. More details about the measurement are given in the Metallography section of this report. Page 5 of 34 DIS Research Project No. 37 Ultrasonic Velocity Measurement The ultrasonic velocity was determined for all tensile bars. Flat and parallel surfaces were machined in each test bar and the thickness of the test bar was determined for the machined faces. Thickness was determined to the nearest 0.0001 inch. Subsequently, the thickness was determined using a Panametrics Model 22DLHP Ultrasonic Thickness Gauge and a 5 MHz transducer. The ultrasonic thickness gauge was calibrated with reference gauge blocks in the range of 0.20 to 1.50 inches. The ultrasonic velocity of each specimen was then calculated using the physical thickness reading, the UT thickness reading and the known ultrasonic velocity of the reference gauge blocks. All measurements were made in the middle (mid-length) of each test bar. To verify the measurement technique, numerous samples were prepared for testing and shipped to Randy Hunt of Citation - Brewton for measuring ultrasonic velocity. Velocity measurements were taken at each end of the test bars using an American NDT AX-9 Ultrasonic Velocity Test Machine. The test fixture (Figure 2) utilizes two transducers, which perform thickness and velocity measurements without contacting the part directly. Calibration was performed using a 304 stainless steel block @ 0.2264 in/µsec. Figure 4 demonstrates the manner in which parts were placed into the fixture for measurement. The samples were returned to CRS for measurement with the technique described above. The test results are listed in Table 2, and they show that the two labs compared quite favorably. On average, the CRS readings were 0.0006 in/µsec lower than the Citation readings. The lower readings may in part be due to the fact that CRS made the measurement in the middle rather than near the ends of the bar. Figure 2. Photograph of the American NDT AX-9 Ultrasonic Velocity Test Machine. Page 6 of 34 DIS Research Project No. 37 Table 2. Results of Ultrasonic Velocity Measurements at Citation – Brewton and CRS Sample ID 2-A-1 2-A-2 2-A-3 2-B-1 2-B-2 2-B-3 2-B-4 2-BA-1 2-BA-2 2-BA-3 2-BA-4 2-BA-5 2-BA-6 3-A-1 3-A-2 3-B-1 3-B-2 4-A-1 4-A-2 4-A-3 4-C-1 4-C-2 4-C-3 7-1-1 7-1-2 7-1-3 7-1-4 7-1-5 7-2-1 7-2-2 7-2-3 7-2-4 7-2-5 7-3-1 7-3-2 7-3-3 7-3-4 7-3-5 Citation Velocity @A 0.2217 0.2216 0.2216 0.2221 0.2217 0.2217 0.2223 0.2211 0.2211 0.2212 0.2211 0.2222 0.2211 0.2211 0.2207 0.2232 0.2234 0.2213 0.2214 0.2215 0.2238 0.2240 0.2239 0.2226 0.2223 0.2223 0.2224 0.2224 0.2219 0.2219 0.2222 0.2221 0.2222 0.2210 0.2207 0.2208 0.2211 0.2207 Velocity @C 0.2216 0.2218 0.2216 0.2221 0.2217 0.2215 0.2224 0.2212 0.2208 0.2212 0.2211 0.2223 0.2211 0.2212 0.2196 0.2231 0.2234 0.2216 0.2211 0.2216 0.2240 0.2240 0.2240 0.2223 0.2221 0.2225 0.2228 0.2225 0.2220 0.2220 0.2219 0.2219 0.2222 0.2207 0.2207 0.2214 0.2208 0.2207 CRS Velocity Middle 0.2221 0.2214 0.2218 0.2219 0.2214 0.2215 0.2223 0.2208 0.2203 0.2208 0.2204 0.2215 0.2206 0.2211 0.2210 0.2235 0.2232 0.2215 0.2218 0.2220 0.2231 0.2237 0.2233 0.2232 0.2234 0.2234 0.2230 0.2229 0.2225 0.2222 0.2224 0.2226 0.2227 0.2208 0.2206 0.2216 0.2214 0.2210 Sample ID 5-1-1 5-1-2 5-1-3 5-1-4 5-1-5 5-1A-1 5-1A-2 5-1A-3 5-1A-4 5-1A-5 5-1A-6 6-1-1 6-1-2 6-1-3 6-1-4 6-1-5 6-1-6 6-2-1 6-2-2 6-2-3 6-2-4 6-1A-1 6-1A-2 6-1A-3 6-1A-4 6-1A-5 6-1A-6 6-2A-1 6-2A-2 6-2A-3 6-2A-4 6-2A-5 6-2A-6 Citation Velocity @A 0.2201 0.2205 0.2203 0.2202 0.2202 0.2201 0.2198 0.2202 0.2203 0.2204 0.2201 0.2191 0.2211 0.2210 0.2209 0.2209 0.2209 0.2191 0.2193 0.2192 0.2195 0.2207 0.2210 0.2212 0.2209 0.2206 0.2209 0.2196 0.2194 0.2193 0.2195 0.2195 0.2195 Velocity @C 0.2204 0.2201 0.2204 0.2203 0.2203 0.2201 0.2198 0.2201 0.2203 0.2204 0.2197 0.2193 0.2212 0.2209 0.2206 0.2209 0.2204 0.2191 0.2195 0.2192 0.2193 0.2207 0.2210 0.2212 0.2210 0.2209 0.2209 0.2195 0.2195 0.2194 0.2195 0.2193 0.2194 CRS Velocity Middle 0.2196 0.2195 0.2196 0.2186 0.2188 0.2188 0.2189 0.2174 0.2193 0.2192 0.2185 -0.2201 0.2194 0.2197 0.2200 0.2195 0.2173 0.2180 0.2178 0.2176 0.2194 0.2197 0.2198 0.2198 0.2196 0.2196 0.2172 0.2174 0.2175 0.2181 0.2180 0.2178 Page 7 of 34 DIS Research Project No. 37 Mechanical Testing A single round tensile test specimen was machined from each keel block leg. The tensile specimens were machined with a gauge section measuring 0.50 inch diameter by 2.0 inches long and with a shoulder radius of 0.50 inch. The specimens were loaded in tension at a rate of 0.3% per minute to 1% strain, and then loaded with a controlled crosshead speed of 0.20 inch per minute to failure, in accordance with ASTM standard E8-03. In the test, 0.2% yield strength, ultimate tensile strength, and tensile elongation were determined. The elastic modulus was also determined for several test specimens. Five to six tensile test specimens were tested for each grade and nodularity level. The complete tensile property data are presented in the Appendix. A summary of the results of mechanical testing is shown in Table 3. Metallography Selected test bars were chosen for metallographic evaluation. The samples were compressionmounted in thermosetting resin and polished using standard mechanical techniques using silicon carbide abrasives in accordance with ASTM standard E3-01. The mounted specimens were final-polished using colloidal silica media with a 0.05 um particle size. The microstructures were photographed in the as-polished condition and after etching with 2% nital. Representative photomicrographs are shown in the Appendix in Figures A1 through A11. Percent nodularity and nodule count were measured using computer-aided image analysis. Twenty-five (25) fields at 100X magnification were analyzed for a total area of 27 mm2. The method of determining nodularity follows ASTM A 247-67(1998) practices, where the criterion used for percent nodularity was based on area fraction nodules versus total graphite area. The criterion (shape factor) used for distinguishing nodules from other graphite inclusions was "compactness", using a value 0.70, and particles less than 10 µm were excluded from the calculation. Nodule count was determined and, once again, nodules smaller than 10 µm were excluded from the measurement. Percent ferrite was also measured using computer-aided image analysis. The samples were heavily etched in 2% nital. Twenty-five (25) fields at 100X magnification were analyzed for a total area of 27 mm2. The graphite was ignored in the measurement and the reported values represent % ferrite as a fraction of the metallic matrix, only. The specimens were inspected for intercellular carbides, but none were observed. The results of all metallographic analyses are presented in the Appendix. RESULTS AND DISCUSSION Numerous heats were produced to obtain a series of ductile iron materials with varying nodularity for six grades of ductile iron. Several heats were set aside because of duplication in microstructures. The range in nodularity for the heats in each grade is shown below in Table 4. In general, the tensile strength and tensile elongation decreased with decreasing nodularity, as expected. The correlations of tensile strength with nodularity for the annealed ferritic grade D4018 and for the pearlitic grade D7003 are shown in Figure 1. Page 8 of 34 DIS Research Project No. 37 Table 3. Summary of Test Data for the Six Grades of Ductile Iron Test Bars UT Ferrite Yield Strength Series Nodularity Velocity Content ID % % in/µs MPa ksi 1B <99 95 0.2208 245 35.6 2BA <99 94 0.2207 267 38.8 1A <99 89 0.2205 281 40.8 2A <99 86 0.2199 289 41.9 M <99 73 0.2181 284 41.1 D4018 Ann S <99 70 0.2179 297 43.1 51A <99 77 0.2187 285 41.3 61A <99 68 0.2197 293 42.5 62A <99 58 0.2177 288 41.8 L <99 43 0.2136 264 38.2 1B 90-95 95 0.2211 252 36.6 2B 70-75 94 0.2218 297 43 D4018 1A 70 87 0.2205 289 42 51 77 0.2192 294 42.6 2B 70-75 93 0.2215 291 42.2 2A 84 86 0.2216 330 47.8 M 65 79 0.2192 312 45.2 D4512 S 75 70 0.2194 318 46.1 61 65 0.2197 310 44.9 62 68 57 0.2176 305 44.3 L 65 43 0.2142 296 43 3B 10 94 0.2234 404 58.7 71 92 0.2232 440 63.9 D5506 3A 44 90 0.2209 408 59.1 72 6 86 0.2225 440 63.8 73 74 0.2211 421 61.1 3C 5 96 0.2225 442 64 4C 5 94 0.2234 478 69.2 D7003 4A 16 85 0.2213 473 68.6 83 <1 81 0.2224 464 67.3 4C-H <1 94 0.22 942 137 82-H <1 88 0.2174 ND ND D9002 72-H <1 86 0.2174 896 130 83-H <1 81 0.2164 889 129 73-H <1 77 0.2146 854 124 Grade Elastic Tensile Strength Elongation Modulus % Mpsi MPa ksi 396 57.5 24.2 407 59 23.0 22.2 423 61.29 18.3 426 61.72 20.5 408 59.2 15.9 423 61.39 13.7 419 54.49 6.3 22.8 421 61.0 18 23.5 404 58.58 14.7 21.9 366 53.03 10.3 418 60.56 23.0 514 74.48 16.3 23.4 481 69.83 14.5 400 58 5.1 23.1 500 72.52 18.5 528 76.56 11.3 21.5 502 72.85 8.8 473 68.61 14.2 473 68.63 13.0 22.5 440 63.82 8.8 21.1 430 62.42 6.3 739 107.3 6.5 21.2 741 107.3 5.5 22.2 684 99.3 6.5 22.6 743 108 5.7 21.9 683 99.17 4.9 22.6 812 117.7 5.4 852 123.8 6.2 22.9 757 109.7 4.5 21.3 807 117.1 5.4 1142 165.7 3.8 874 126.8 0.2 1025 148.6 1.5 818 118.7 0.5 984 142.7 1.6 ND Not determined due to insufficient plastic strain (less 0.2% plastic strain) Page 9 of 34 DIS Research Project No. 37 Table 4 Materials procured for this investigation (26+ Heats from DCX) SAE Grade D4018 (Annealed) D4018 D4512 D5506 D7003 D9002 Range in Nodularity 95% to 43% 95% to 77% 94% to 43% 94% to 74% 96% to 85% 94% to 74% It is clear that an acceptable level of nodularity in the ferritic grade is much lower than that for the pearlitic grade and that the tolerance for poor nodularity is much greater in the ferritic grade. Figure 1b illustrates the correlation between tensile strength and nodularity in all six grades. The same trend is apparent; the lower-strength, tough grades of ductile iron are more tolerant of low nodularity. The correlation between tensile elongation and nodularity displays a trend similar to that of tensile strength and nodularity, however, the rate of degradation is even more pronounced. Plots of tensile elongation and nodularity are shown in Figures 2a and 2b. The minimum acceptable levels of nodularity required to meet the tensile strength and for tensile elongation were determined for each grade using the plots in Figures 1 and 2, and the results are shown below in Table 5. Table 5. Minimum acceptable nodularity for each grade according to tensile strength and tensile elongation Specification Grade Tensile Strength ksi 60 60 65 80 100 120 % Elongation 18 18 12 6 3 2 % Nodularity required to meet specification Tensile Strength ksi 68 78 60 NA 85 81 % Elongation 74 90 67 80 84 90 D 4018 Ann D 4018 D 4512 D 5506 D 7003 D 9002 Page 10 of 34 DIS Research Project No. 37 140 D4018 Ann D7003 120 Tensile Stength, ksi 100 80 60 40 20 40 60 80 100 Nodularity, % Figure 1a Correlation of tensile strength with nodularity in D4018 Annealed and D7003. 180 D4018 Ann D4018 160 D4512 D5506 D7003 D9002 Tensile Stength, ksi 140 120 100 80 60 40 20 40 60 80 100 Nodularity, % Figure 1b Correlation of tensile strength with nodularity in all grades of this investigation. Page 11 of 34 DIS Research Project No. 37 25 D4018 Ann D7003 20 Tensile Elongation, % 15 10 5 0 20 30 40 50 60 70 80 90 100 Nodularity, % Figure 2a Correlation of tensile elongation with nodularity in tion. 25 D4018 Ann 20 D4018 D4512 D5506 D7003 D9002 Tensile Elongation, % 15 10 5 0 20 30 40 50 60 70 80 90 100 Nodularity, % Figure 2b Correlation of tensile elongation with nodularity in all grades of this investigation. Page 12 of 34 DIS Research Project No. 37 180 160 140 94% Stress (ksi) 120 100 80 60 40 20 0 0 1 2 86% 81% 94% nod 4CH-5 7-2H-3 86% nod 8-3H-1 81% nod 3 4 5 6 7 8 Strain (%) Figure 3 Stress-strain curves for D7003 tensile bars of varying nodularities. While a decrease in nodularity had a strong affect on tensile strength and tensile elongation, it did not significantly affect yield strength. The stress strain curves in Figure 3 illustrate the influence of nodularity on the stress-strain curves of grade D7003 test bars with varying nodularity. Ultrasonic Velocity The literature shows that graphite structure, graphite volume fraction and section size all influence sonic properties. Ultrasonic velocity is routinely used to evaluate nodularity in ductile iron castings. Degenerate graphite particles slow the speed of sound and, thus, ultrasonic velocity is used to determine percent nodularity non-destructively. Studies1-4 of the influence of nodularity on ultrasonic velocity were conducted by BCIRA in the late 70’s and early 80’s. The findings show that ultrasonic velocity decreases linearly with nodularity, as shown in Figure 4. It is interesting to note that the curve for ferritic irons is not identical to that of pearlitic irons and that for the same nodularity the ultrasonic velocity in ferritic iron is higher than that of pearlitic iron. Page 13 of 34 DIS Research Project No. 37 Figure 4. Relationship between visually assessed nodularity and ultrasonic velocity. After BCIRA1,2 When the values for ultrasonic velocity and nodularity for the irons of this study are plotted, the trend is similar, as shown in Figure 5. There is considerable scatter in the data indicating that the prediction of nodularity from ultrasonic velocity is not particularly rigorous. When the correlation of nodularity and ultrasonic velocity is broken out for each grade of ductile iron, the relationship becomes much stronger, as shown in Figure 6. 0.2240 0.2220 UT Velocity, in/µsec 0.2200 0.2180 0.2160 0.2140 0.2120 40 60 80 100 Nodularity, % Figure 5. Correlation of nodularity, by image analysis, with ultrasonic velocity. Page 14 of 34 DIS Research Project No. 37 0.2240 0.2220 UT Velocity, in/µsec 0.2200 0.2180 0.2160 D4018 Ann D4018 D4512 0.2140 D5506 D7003 D9002 0.2120 40 60 80 100 Nodularity, % Figure 6. Relationship between nodularity, by image analysis, and ultrasonic velocity for each of six grades of ductile iron. The correlation of nodularity with ultrasonic velocity appears to be essentially linear for each grade of iron, as shown in Figure 6; but the plot for each grade is shifted to higher or lower values (intercepts) as compared to other grades. The series for D5506 irons displayed the highest ultrasonic velocities, followed by the pearlitic grade D7003. The ferritic grades displayed much lower velocities. Of particular note is the curve for the heat-treated D9002 grade; it displayed the lowest velocities of all the grades. When comparing the results of this study with those of the BCIRA study, it was found that the velocities in the BCIRA study were generally lower, as shown in Figure 7. The ferritic iron series of BCIRA and DIS are not very far apart, but the pearlitic iron series of BCIRA is much lower than the DIS pearlitic iron series. Furthermore, the pearlitic series of BCIRA is below the ferritic series; whereas in this study the pearlitic series is above the ferritic series. Upon close examination of the BCIRA work, it was found that the pearlitic iron series was produced by heat treating (normalizing) the ferritic series. It is surmised that the heat treatment resulted in some growth of the casting, and it is this growth that reduced velocities in the pearlitic samples. It is also opined that this same phenomenon occurred in the heat treatment of the D9002 samples, and that growth contributed to the low velocities found for the D9002 series in this study. Several factors appear to affect the ultrasonic velocity in ductile iron, including graphite volume, graphite shape, density of the matrix, the presence of porosity and the presence of carbides. The Page 15 of 34 DIS Research Project No. 37 2240 Pearlitic Velocity Ferritic Series DIS Data 2220 UT Velocity x 10 , in/µsec 4 2200 2180 2160 2140 2120 20 30 40 50 60 70 80 90 100 110 Nodularity, % Figure 7. Correlations of nodularity with ultrasonic velocity – BCIRA data versus DIS data. density of the matrix varies with the microstructure, with ferrite being more dense than pearlite, and pearlite more dense than tempered martensite. Of course, with a fully ferritic matrix (the most dense microstructure), all the carbon in the alloy is present as graphite and the large graphite volume results in a low ultrasonic velocity. Consequently, it appears that graphite volume and matrix density have similar influences on ultrasonic velocity – both reduce velocity. In general, a low-density pearlitic alloy has significant amounts of combined carbon and thus a lower graphite fraction over ferritic ductile iron. Consequently, as pearlite fraction increases, the graphite volume decreases. With regard to ultrasonic velocity, an increase in pearlite content (and the attendant decrease in matrix density) is partially offset by a decrease in graphite volume fraction. Further examination of the data of this study revealed that the D5506 series, with a pearlite-ferrite matrix displayed the highest ultrasonic velocities for a given nodularity rating. It appears that the D5506 series contained optimum amounts of ferrite, pearlite and graphite volumes such that maximum ultrasonic velocities were achieved. Ultrasonic Velocity and Elastic Modulus It is generally understood that the ultrasonic velocity is related to the elastic modulus of the metal. As the graphite shape becomes more degenerate, both the ultrasonic velocity and the elastic modulus will decrease. The ultrasonic velocity has been plotted against elastic modulus for the irons of this study and the correlation is shown in Figure 8. The correlation between ultrasonic velocity and elastic modulus was found to be very poor. Page 16 of 34 DIS Research Project No. 37 26 Elastic Modulus, Msi 24 22 20 18 0.2160 0.2180 0.2200 0.2220 0.2240 0.2260 Velocity, in/µsec Figure 8. Relationship between ultrasonic velocity and elastic modulus. SUMMARY The results of this study have revealed a number of findings, including the following. 1. Tensile strength and elongation decrease with decreasing nodularity. 2. The level of nodularity that produces acceptable properties varies with the grade of ductile iron. 3. As strength increases, the degradation in properties occurs at higher nodularity values. 4. The correlation of nodularity and ultrasonic velocity is loose, but improves dramatically when correlated within each grade of ductile iron. 5. The correlation of mechanical properties with ultrasonic velocity is poor. 6. In addition to nodularity, graphite volume, matrix structure and carbides strongly affect ultrasonic velocity. The results of this study should reduce the confusion associated with rating microstructures and the expected properties in ductile iron castings. Furthermore, the results of the study should contribute to the ability to characterize the quality of a casting. Page 17 of 34 DIS Research Project No. 37 RECOMMENDATIONS The literature and the findings of this study show that graphite structure, graphite volume fraction and section size all influence sonic properties. It has been proposed that the matrix microstructure (ferrite vs. pearlite vs. martensite) influences ultrasonic velocity through its influence on density. It has also been proposed that graphite volume similarly influences UT velocity. These findings suggest that there may be a good correlation of UT velocity with both nodularity and density. It is anticipated that some additional work will be conducted to further investigate the correlations of nodularity, sonic properties, density, and mechanical properties in some grades of ductile iron. ACKNOWLEDGEMENTS The author wishes to acknowledge the contributions of Phil Seaton and DaimlerChrysler for providing all of the test castings for this investigation. The keel block molds used to pour the test bars were provided by Tony Thoma and Wescast. And, the ultrasonic testing performed by Randy Hunt and Citation-Brewton are gratefully acknowledged. The author also wishes to thank Al Alagarsamy for the technical support he provided during this study. And lastly, the helpful suggestions and support of Martin Gagne, Kathy Hayrynen, Jim Mullins and Phil Seaton who make up the DIS Steering Committee are greatly appreciated. REFERENCES 1. Emerson, P.J., Simmons, W., "Final Report on the Evaluation of Graphite Form in Ferritic Ductile Irons by Ultrasonic and Sonic Testing and on the Effect of Graphite Form on Mechanical Properties", AFS Trans., Vol. 84, p. 109-128 (1976). 2. Fuller, A.G., "Evaluation of the Graphite Form in Pearlitic Ductile Iron by Ultrasonic and Sonic Testing and Effect of Graphite Form on Mechanical Properties", AFS Trans., Vol. 85, p. 509 (1977). 3. Fuller, A.G., "Effect of Graphite Form on Fatigue Properties of Pearlitic Ductile Irons ", AFS Trans., Vol. 85, p. 527 (1977). 4. Fuller, A.G., Emerson, P.J. and Sergeant, G.F. "A Report on the Effect Upon Mechanical Properties of Variation in Graphite Form in Irons Having Varying Amounts of Ferrite and Pearlite in the Matrix Structure and the Use of Nondestructive Tests in the Assessment of Mechanical Properties of Such Irons", AFS Trans., Vol. 88, p. 21 (1980). Page 18 of 34 DIS Research Project No. 37 APPENDIX Table A1. Results of Mechanical Testing, Metallography and UT Testing Yield UT Ferrite Nodularity Velocity Strength Grade Series ID Sample Content % % in/µs MPa ksi 1B2 >98 95 0.2205 245 35.6 1B 1B3 >98 95 0.2210 245 35.6 1B >98 95 0.2208 245 35.6 2BA1 >98 94 0.2208 274 39.7 2BA2 >98 94 0.2203 279 40.5 2BA3 >98 94 0.2208 267 38.7 2BA 2BA4 >98 94 0.2204 262 38.0 2BA5 >98 94 0.2215 247 35.8 2BA6 >98 94 0.2206 275 39.9 2BA >98 94 0.2207 267 38.8 1A1 >98 89 0.2206 280 40.6 1A3 >98 89 0.2198 283 41.1 1A 1A4 >98 89 0.2208 280 40.7 1A5 >98 89 0.2206 281 40.7 1A >98 89 0.2205 281 40.8 2A1 >98 86 0.2199 287 41.6 2A3 >98 86 0.2198 297 43.0 2A 2A6 >98 86 0.2201 283 41.1 2A >98 86 0.2199 289 41.9 >98 0.2174 274 39.8 M1 67 M M2 >98 79 0.2188 293 42.5 M >98 73 0.2181 284 41.1 S2 >98 70 0.2179 299 43.3 S S5 >98 70 0.2178 296 42.9 D4018 S >98 70 0.2179 297 43.1 Ann 51A1 >98 77 0.2188 282 40.8 51A2 >98 77 0.2189 284 41.1 51A3 >98 77 0.2174 280 40.6 51A 51A4 >98 77 0.2193 283 41.0 51A5 >98 77 0.2192 284 41.2 51A6 >98 77 0.2185 281 40.7 51A >98 77 0.2187 285 41.3 61A1 >98 68 0.2194 293 42.5 61A2 >98 68 0.2197 293 42.5 61A3 >98 68 0.2198 293 42.5 61A 61A4 >98 68 0.2198 292 42.4 61A5 >98 68 0.2196 292 42.4 61A6 >98 68 0.2196 294 42.6 61A >98 68 0.2197 293 42.5 62A1 >98 58 0.2172 288 41.8 62A2 >98 58 0.2174 288 41.7 62A3 >98 58 0.2175 289 41.9 62A 62A4 >98 58 0.2181 288 41.7 62A5 >98 58 0.2180 289 41.9 62A6 >98 58 0.2178 288 41.8 62A >98 58 0.2177 288 41.8 L1 >98 43 0.2139 264 38.3 L L6 >98 43 0.2132 263 38.2 L >98 43 0.2136 264 38.2 Tensile Strength MPa ksi 397 57.6 396 57.5 396 57.5 410 59.5 413 59.9 406 59.0 404 58.5 396 57.4 411 59.7 407 59.0 425 61.7 422 61.1 421 61.1 423 61.3 423 61.3 425 61.6 431 62.5 421 61.1 426 61.7 402 58.4 414 60.0 408 59.2 425 61.6 422 61.2 423 61.4 363 52.7 390 56.5 340 49.3 659 52.1 411 59.7 345 50.0 419 54.5 420 60.9 418 60.6 423 61.3 423 61.3 420 60.9 422 61.2 421 61.0 403 58.5 405 58.7 404 58.6 405 58.7 402 58.4 404 58.6 404 58.6 366 53.1 365 53.0 366 53.0 Elastic Elongation Modulus % Mpsi 24 24 24 23 22.4 22 20.7 23 23.8 24 22.2 24 21.2 22 23.1 23 22.2 10 20 22 21 18 22 20 20 20 15 16 16 13 14 14 4.6 23.2 7.3 23.2 3.1 20.9 4.4 23.3 15 23.5 3.5 22.6 6.3 22.8 18 24.1 12 24.3 20 23.1 20 23.7 19 23.0 19 22.5 18 23.5 16 22.0 14 21.4 14 21.8 15 21.9 14 22.0 15 22.2 15 21.9 10 10 10 Page 19 of 34 DIS Research Project No. 37 Table A1 (continued). Results of Mechanical Testing, Metallography and UT Testing Grade Series ID Sample 1B1 1B4 1B5 1B6 1B 2B1 2B2 2B3 2B4 2B 1A2 1A6 1A 511 512 513 514 515 51A 51 2B1 2B3 2B6 2B 2A2 2A4 2A5 2A1 2A2 2A3 2A M6 M S6 S 612 613 614 615 616 61A 61 621 622 623 624 611* 62A 62 L5 L Ferrite Nodularity UT % % in/µs 90-95 95 0.2215 90-95 95 0.2203 90-95 95 0.2215 90-95 95 0.2211 90-95 95 0.2211 94 0.2219 94 0.2214 94 0.2215 25-30 94 0.2223 94 0.2218 89 0.2208 0.2201 70 84 87 0.2205 77 0.2196 77 0.2195 77 0.2196 77 0.2186 77 0.2188 77 77 0.2192 70-75 94 0.2210 70-75 0.2222 91 70-75 94 0.2213 70-75 93 0.2215 84 86 0.2214 86 0.2215 84 84 86 0.2206 84 86 0.2221 84 86 0.2222 84 86 0.2218 84 86 0.2216 65 79 0.2192 65 79 0.2192 75 70 0.2194 75 70 0.2194 68 0.2201 68 0.2194 68 0.2197 68 0.2200 68 0.2195 52 65 0.2197 68 58 0.2173 68 58 0.2180 0.2178 68 54 68 58 0.2176 68 58 0.2175 68 58 68 57 0.2176 65 43 0.2142 65 43 0.2142 Yield MPa ksi 252 36.5 247 35.8 257 37.2 253 36.7 252 36.6 297 43.1 263 38.2 292 42.3 334 48.5 297 43.0 299 43.3 280 40.6 289 42.0 292 42.3 293 42.5 290 42.1 292 42.3 293 42.5 302 43.8 294 42.6 334 48.4 268 38.8 272 39.5 291 42.2 356 51.7 315 45.7 314 45.6 339 49.1 338 49.1 315 45.6 330 47.8 312 45.2 312 45.2 318 46.1 318 46.1 305 44.2 306 44.3 308 44.7 312 45.2 308 44.7 320 46.3 310 44.9 310 45.0 302 43.8 302 43.8 302 43.8 308 44.7 308 44.6 305 44.3 296 43.0 296 43.0 Tensile MPa ksi 412 59.7 407 59.1 431 62.5 420 60.9 418 60.6 510 73.9 433 62.8 501 72.6 611 88.6 514 74.5 479 69.5 484 70.1 481 69.8 411 59.6 418 60.7 364 52.7 387 56.1 399 57.8 422 61.1 400 58.0 603 87.5 447 64.8 450 65.3 500 72.5 581 84.2 503 72.9 503 73.0 520 75.4 557 80.7 504 73.1 528 76.6 502 72.9 502 72.9 473 68.6 473 68.6 468 67.8 465 67.5 473 68.5 480 69.6 468 67.9 486 70.5 473 68.6 452 65.5 443 64.3 446 64.7 439 63.6 449 65.1 412 59.7 440 63.8 430 62.4 430 62.4 Elongation Elastic % Mpsi 24 25 19 24 23 16 23.3 21 23.8 16 22.4 12 24.0 16 23.4 14 15 15 6.0 25.1 6.9 23.7 2.7 23.5 4.0 22.3 4.8 22.7 6.0 21.3 5.1 23.1 13 21 22 18 9.1 14 12 5.8 23.7 12 17.6 15 23.1 11 21.5 8.8 9 14 14 15 23.8 13 23.6 13 22.5 13 22.5 12 23.4 12 19.2 13 22.5 9.0 20.8 10 21.0 11 21.2 10 20.9 9.1 21.8 4.1 20.6 8.8 21.1 6.3 6.3 1B 2B D4018 1A 51 2B 2A M S D4512 61 62 L Page 20 of 34 DIS Research Project No. 37 Table A1 (continued). Results of Mechanical Testing, Metallography and UT Testing Ferrite Nodularity Grade Series ID Sample Content % % 3B1 90 94 3B2 90 94 3B 3B1 90 94 ORIG 3B 90 94 711 92 712 92 713 92 714 92 71 715 92 71A 92 ORIG 71 92 3A1 44 90 3A2 44 90 3A5 44 90 3A 3A1 44 90 3A2 44 90 D5506 3A4 44 3A 44 90 721 6 86 722 6 88 723 6 86 724 6 86 72 725 6 86 72A 6 86 ORIG 72 6 86 731 74 732 74 733 74 734 74 73 735 74 73A 74 ORIG 73 74 Tensile UT Elastic Yield Strength Elongation Strength Velocity Modulus % Mpsi in/µs MPa ksi MPa ksi 0.2235 398 57.7 728 106.0 6.3 19.4 0.2232 402 58.4 730 106.0 6.0 24.0 413 0.2234 0.2232 0.2234 0.2234 0.2230 0.2229 404 439 438 437 442 438 446 0.2232 0.2208 0.2210 0.2207 0.2211 0.2207 0.2209 0.2225 0.2222 0.2224 0.2226 0.2227 440 406 416 404 405 407 408 431 434 439 453 437 445 0.2225 0.2208 0.2206 0.2216 0.2214 0.2210 440 417 418 419 426 418 430 0.2211 421 59.9 58.7 63.7 63.6 63.4 64.2 63.5 64.7 63.9 58.9 60.4 58.7 58.8 59.0 59.1 62.6 63.0 63.7 65.7 63.3 64.5 63.8 60.5 60.6 60.8 61.8 60.7 62.3 61.1 760 739 704 755 747 742 731 764 741 704 710 695 652 662 684 717 736 748 768 727 762 743 664 675 685 685 683 708 683 110.0 107.3 102.0 109.0 108.0 108.0 106.0 111.0 107.3 102.1 102.9 100.8 94.6 96.1 99.3 104.0 107.0 109.0 111.0 106.0 111.0 108.0 96.3 97.9 99.4 99.4 99.0 103.0 99.2 7.2 6.5 4.1 6.2 5.8 5.4 5.2 6.2 5.5 7.7 7.5 7.3 4.6 5.6 6.5 5.1 5.6 6.2 5.5 5.2 6.7 5.7 4.3 4.9 5.2 4.6 5.0 5.2 4.9 20.1 21.2 23.3 21.9 22.5 22.1 23.3 20.1 22.2 22.1 23.1 22.6 21.5 21.1 22.6 23.8 22.3 20.3 21.9 23.8 22.5 22.9 23.1 22.6 20.5 22.6 Page 21 of 34 DIS Research Project No. 37 Table 1 (continued). Results of Mechanical Testing, Metallography and UT Testing UT Ferrite Yield Strength Nodularity Velocity Grade Series ID Sample Content % % MPa ksi in/µs 3C3 95 96 0.2225 445 64.5 3C 3C5 95 96 0.2224 438 63.6 3C 95 96 0.2225 442 64.0 4C1 95 94 0.2231 472 68.5 4C2 95 94 0.2237 477 69.2 4C3 95 94 0.2233 478 69.3 4C 4C1 95 94 483 69.9 ORIG 4C 95 94 0.2234 478 69.2 4A2 84 85 0.2220 495 71.8 4A4 84 85 0.2209 480 69.6 D7003 4A6 84 85 0.2194 471 68.3 4A 4A1 84 85 0.2215 472 68.5 4A2 84 85 0.2218 460 66.7 4A3 84 85 0.2220 461 66.8 4A 84 85 0.2213 473 68.6 83-1 0.5 0.2212 465 67.4 83-2 0.5 0.2213 473 68.6 83-3 0.5 81 0.2202 472 68.5 83 83-4 0.5 0.2218 458 66.4 83-5 0.5 0.2215 470 68.2 83 0.5 81 0.2212 468 67.8 Tensile Elastic Elongation Strength Modulus % Mpsi MPa ksi 814 118.0 3.8 810 117.5 6.9 812 117.7 5.4 852 124.0 7.0 22.9 852 124.0 6.4 22.4 838 122.0 5.5 23.2 864 852 800 756 777 745 733 729 757 602 741 559 466 507 575 125.0 123.8 116.1 109.6 112.8 108.0 106.0 106.0 109.7 87.3 107.4 81.1 67.6 73.6 83.4 6.0 6.2 5.1 4.1 5.7 4.1 4.2 4.0 4.5 1.2 2.8 0.7 0.3 0.5 1.1 23.2 22.9 20.8 22.5 20.5 21.3 Page 22 of 34 DIS Research Project No. 37 Table A1 (concluded). Results of Mechanical Testing, Metallography and UT Testing Yield UT Ferrite Nodularity Velocity Strength Grade Series ID Sample Content % % in/µs MPa ksi 4C-H1 <1 94 0.2182 954 138 4C-H2 <1 0.2193 938 136 4C-H3 <1 0.2190 950 138 4C-H 4C-H4 <1 0.2191 943 137 4C-H5 <1 0.2181 924 134 4C-H <1 94 0.2187 942 137 82-H1 <1 0.2179 922 134 82-H2 <1 0.2168 82-H3 <1 88 0.2175 82-H 82-H4 <1 0.2174 82-H5 <1 0.2172 82-H <1 88 0.2174 72-H1 <1 0.2185 886 129 90 72-H2 <1 0.2172 886 129 72-H3 <1 86 0.2172 893 130 D9002 72-H 72-H4 <1 0.2166 910 132 72-H5 <1 0.2175 903 131 72-H <1 86 0.2174 896 130 83-H1 <1 0.2159 911 132 83-H2 <1 0.2166 83-H3 <1 81 0.2168 83-H 83-H4 <1 0.2168 866 126 83-H5 <1 0.2157 83-H <1 81 0.2164 889 129 73-H1 <1 0.2140 830 120 73-H2 <1 0.2164 869 126 73-H3 <1 77 0.2150 873 127 73-H 73-H4 <1 0.2129 837 121 75 73-H5 <1 0.2146 859 125 73-H <1 77 0.2146 854 124 Tensile Elastic Elongation Strength Modulus % Mpsi MPa ksi 1154 167.4 3.9 1147 166.4 4.2 1137 165 3.0 1155 167.6 3.8 1117 162 4.0 1142 165.7 3.8 957 138.8 0.4 894 129.6 0.2 879 127.5 0.2 839 121.7 0.1 803 116.6 0.1 874 127 0.2 1028 149.1 1.7 998 144.8 1.1 1041 150.9 2.0 1050 152.3 1.6 1007 146.1 1.0 1025 148.6 1.5 959 139.1 0.4 589 85.5 0.1 793 115 0.1 1021 148.1 1.6 728 105.6 0.1 818 118.7 0.5 956 138.6 1.4 1004 145.6 1.6 1021 148.1 2.1 974 141.2 1.7 964 139.8 1.0 983.8 142.7 1.6 Page 23 of 34 DIS Research Project No. 37 As-polished (a) As-polished microstructure of M1 X100 Etched (b) Etched microstructure of M1 X100 Figure A1 Photomicrographs illustrating the nodularity and microstructure of Grade D4018A sample M1, which had 67% nodularity. Page 24 of 34 DIS Research Project No. 37 Figure 1 As-polished (a) As-polished microstructure of 62A3 X100 Figure 2 Etched (b) Etched microstructure of 62A3 X100 Figure A2 Photomicrographs illustrating the nodularity and microstructure of Grade D4018A sample 62A3, which had 58% nodularity. Page 25 of 34 DIS Research Project No. 37 Figure 3 Figure 4 As-polished (a) As-polished microstructure of 1A6 X100 Figure 5 Figure 6 Etched (b) Etched microstructure of 1A6 X100 Figure A3 Photomicrographs illustrating the nodularity and microstructure of Grade D4018 sample 1A6, which had 84% nodularity and 70% ferrite. Page 26 of 34 DIS Research Project No. 37 Figure 7 Figure 8 Figure 9 As-polished (a) As-polished microstructure of 2A4 X100 Figure 10 Figure 11 Etched (b) Etched microstructure of 2A4 X100 Figure A4 Photomicrographs illustrating the nodularity and microstructure of Grade D4512 sample 2A4, which had 86% nodularity and 84% ferrite. Page 27 of 34 DIS Research Project No. 37 Figure 12 Figure 13 As-polished (a) As-polished microstructure of 623 X100 Figure 14 Figure 15 Etched (b) Etched microstructure of 623 X100 Figure A5 Photomicrographs illustrating the nodularity and microstructure of Grade D4512 sample 623, which had 54% nodularity and 68% ferrite. Page 28 of 34 DIS Research Project No. 37 Figure 16 Figure 17 Figure 18 As-polished (a) As-polished microstructure of 3A4 X100 Etched (b) Etched microstructure of 3A4 X100 Figure A6 Photomicrographs illustrating the nodularity and microstructure of Grade D5506 sample 3A4, which had 90% nodularity and 44% ferrite. Page 29 of 34 DIS Research Project No. 37 Figure 19 Figure 20 Figure 21 As-polished (a) As-polished microstructure of 722 X100 Etched (b) Etched microstructure of 722 X100 Figure A7 Photomicrographs illustrating the nodularity and microstructure of Grade D5506 sample 722, which had 88% nodularity and 6% ferrite. Page 30 of 34 DIS Research Project No. 37 Figure 22 As-polished (a) As-polished microstructure of 4A3 X100 Etched (b) Etched microstructure of 4A3 X100 Figure A8 Photomicrographs illustrating the nodularity and microstructure of Grade D7003 sample 4A3, which had 85% nodularity and 16% ferrite. Page 31 of 34 DIS Research Project No. 37 Figure 23 As-polished (a) As-polished microstructure of 833 X100 Figure 24 Etched (b) Etched microstructure of 833 X100 Figure A9 Photomicrographs illustrating the nodularity and microstructure of Grade D7003 sample 833, which had 81% nodularity and 0.5% ferrite. Page 32 of 34 DIS Research Project No. 37 Figure 25 As-polished (a) As-polished microstructure of 72H1 X100 Etched (b) Etched microstructure of 72H1 X100 Figure A10 Photomicrographs illustrating the nodularity and microstructure of Grade D9002 sample 72H1, which had 90% nodularity. Page 33 of 34 DIS Research Project No. 37 Figure 26 As-polished (a) As-polished microstructure of 73H4 X100 Etched (b) Etched microstructure of 73-H4 X100 Figure A11 Photomicrographs illustrating the nodularity and microstructure of Grade D9002 sample 73H4, which had 75% nodularity. Page 34 of 34