ductile iron nodularity

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    June 18, 2006





  • DIS Research Project No. 37

    Page 2 of 34




    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.

  • DIS Research Project No. 37

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    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. Whats 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.

  • DIS Research Project No. 37

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    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

  • DIS Research Project No. 37

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    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 Grade Sample Quantity


    Tensile El. Modulus Metallography UT

    Velocity HB

    D4018 33 Five 5 x 6 5 x 2 5 x 2 5 x 6 5 x 6

    D4018 Ann. 33 Five 5 x 6 5 x 2 5 x 2 5 x 6 5 x 6

    D4512 33 Five 5 x 6 5 x 2 5 x 2 5 x 6 5 x 6

    D5506 33 Five 5 x 6 5 x 2 5 x 2 5 x 6 5 x 6

    D7003 33 Five 5 x 6 5 x 2 5 x 2 5 x 6 5 x 6

    D9002 33 Five 5 x 6 5 x 2 5 x 2 5 x 6 5 x 6 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 re


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