Optimizing Structure-Property Relationships in Ductile Iron
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The purpose of this research was to identify ways to achieve a higher combination of strength and ductility, by characterizing the relationships between the graphite and metal matrix, alloy content, and tensile properties in ductile iron. The characterization utilized standard physical metallurgical tools including tensile properties, optical microscopy and automated image analysis to measure ferrite content, grain size, nodule count, nodule size and nodularity. In addition to applying standard statistical parameters such as mean and standard deviation, the study employed Pearson correlation coefficient analysis, transmission electron microscopy, and X-ray diffraction.
Identify chemical and matrix characteristics that might be used to optimize the relationship between graphite morphology, matrix, and mechanical properties.
Optimize the control needed to produce the desired graphite and metal matrix relationships in a given section size to produce the desired mechanical properties.
Improve the control and reduce the variation between mechanical properties measured in thin and heavy sections.
The principal investigators initially analyzed representative samples of grades 80-55-06 and 65-45-12 ductile iron; the compositions and tensile properties were measured and supplied by ten commercial foundries. From the original ten foundries, the investigators received additional samples of the two grades from four foundries and from an additional source. A total of 53 samples were subjected to hardness and mechanical testing. A total of 26 samples were subjected to further testing for composition, microstructure, and strain hardening behavior.
The results of the testing showed that the mechanical properties varied as expected with ferrite content. The strength increased as ferrite content decreased and ductility increased as ferrite content increased, and ferrite grains almost always touched graphite nodules. Manual and automated image analysis measurements of ferrite content showed excellent agreement. In addition, final work on selected samples from this study showed that the substructure of ferrite grains, as determined with X-ray diffraction or transmission electron microscopy, does not appear to influence ductile iron mechanical properties.
This study also determined that the combination of high strength (>80 ksi ultimate and >55 ksi yield) and high elongation (>12%) occurred when ferrite content was between 35 and 60%, and the ferrite colonies were discontinuous. Furthermore, the study also correlated the mechanical properties and microstructures to copper, manganese, silicon, and copper + manganese contents.
Subsequently, the authors investigated the desired alloy content relationship for developing these high strength-high elongation properties. An additional key objective was to investigate post solidification heat treatments to further optimize these composition-structure-property relationships.
To characterize the relationships between the graphite-metal matrix, alloy content, and tensile properties in ductile iron, the study utilized statistical tools to compute mean and standard deviation, Pearson correlation coefficient analysis, Student t-testing, and design of experiments (DOE) methodology.
Section size (1 inch versus 3 inch)
Inoculation (ladle versus ladle plus in-stream)
Silicon content (nominally 2.2 versus 2.7 wt-%)
Manganese contents (0.2 versus 0.45 wt-%)
Copper content (0.3 versus 0.6 wt-%)
All mechanical properties, including both strength and ductility, were lower in the 3 in. section size versus the 1 in. section size. Various statistical testing and DOE analysis showed that increasing Cu and/or Mn increased strength and hardness while decreasing ductility and toughness (UT) in both the 1 in. and 3 in. section sizes. In contrast, increasing Si decreased strength and increased ductility. However, varying Mn or Si was less effective than Cu in affecting the properties in the 1 in. section size. The authors recognize that all of these observations are consistent with the inherent knowledge that has developed during the lifetime of ductile iron. However, in this study the authors were able to begin the process of confirming and quantifying the relationships in a well-controlled dataset.
Based on the results of this study, the principal investigators chose to validate the DOE by casting additional 1 in.Y-blocks of one of the DOE heats, which contained 2.6%Si, 0.3%Mn and 0.6%Cu. The results were almost precisely duplicated with equivalent compositions, microstructures, strengths and ductilities.
The Principal Investigators also investigated post solidification heat treatments to refine grain size and further optimize these composition-structure-property relationships. Heat treatment trials were conducted by intercritically austenitizing and subjecting the samples to various cooling rates such as still air cool, forced air cool, and oil quenching, with the latter followed by tempering.
The most promising results were obtained with a starting microstructure of 50% ferrite and 50% pearlite. After heat treatment, the measured mechanical properties far exceeded some of the study goals of >55 ksi YS, >80 ksi ultimate tensile strength (UTS) and >12 % elongation. Even though 12% minimum elongation was not achieved through heat treatment, the quench and temper heat treatments produced much higher strength and equivalent ductility than many of the standard as-cast grades with equivalent ductility. Mechanical properties obtained after intercritical austenitizing followed by either air cooling or quench and temper heat treatment were as follows: 65 to 85 ksi yield strength (YS), 110 to 130 ksi UTS, and 8 to 9% elongation.
Keywordsductile iron ferrite pearlite heat treatment microstructure strength ductility
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