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On the Microstructure and Mechanical Properties of Automotive Parts Made of Ductile Iron

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On the Microstructure and Mechanical Properties of Automotive Parts Made of Ductile Iron On the microstructure and mechanical properties of automotive parts made of ductile iron Juan José Ramírez-Natera1, Rafael Colás1, Sergio Haro-Rodriguez2, Patricio Gil-Ramos3 1 Universidad Autónoma de Nuevo León 2 Universidad Autónoma de Zacatecas 3Black Hawk de México, S.A. de C.V. • The changes taken place in • Se estudiaron los cambios the microstructure and que se presentan en la mechanical properties of an microestructura y automotive part propiedades mecánicas en manufactured with a piezas automotrices ferritic-pearlitic ductile iron fabricadas a partir de un was investigated. hierro dúctil ferrítico- Demoulding time at 2, 4, 6 perlítico. El metal se vertió and 8 hours after pouring a 1400 °C en un molde at 1400 °C analyzing three desmoldeando a las 2, 4, 6 thicknesses from 25 to 53 y 8 horas analizando tres mm espesores que van desde los 25 mm hasta los 53 mm Introduction • In 2010 94.1 million of tonns of metallic pieces were produced, 25% were ductile iron and continuous increasing still (1). Annual worldwide production is of 12 millions of tonnes and it’s expected that in 10 year get over 20 millions of tonnes. One third of the ductile iron production is for water pipes, another third for automotive parts and another for general castings. It is important to note that automotive sector is the most successful, in addition to the high demand, it has high requirements for quality and low cost with specific properties (2). Ductile iron • Ductile iron is known for its ductility and toughness after special magnesium, cerium, lanthanum treatment. Carbon forms spheroidal particles instead of sheets (Gray iron). Due to the spheroidal shape of the carbon the matrix keeps Fig. 1. Carbon and silicon ranges for cast its continuity for achieve with good ductility. irons and steels [4]. Trace elements influence • Carbon. Average value • Sulfur. Decrease between 2.5-4.5%. It magnesium effect, high decrease yield strength and quality irons have less hardness and increased 0.02% wt. elongation and impact • Phosphorus. Decrease resistance. ductility and toughness. But small quantities refine • Silicon. Average value pearlite increasing yield- between 1.8-2.8%. increase tensile strength ratio [6]. nodule counting and • Manganese. Av value decrease tendency to form between 0.5-0.7% increase carbide. Increase yield tensile strength and yield strength and decreased strength while decrease hardness and elongation. ductility Microstructure and properties The microstructure is based in a matrix of ferrite and a second element of pearlite. Ferrite. It has a BCC structure and it Pearlite. Is a mix of laminate ferrite is relatively ductile and soft. with cementite. It has a good Hardness varies from 140-200 HB. strength but poor ductility and the In ductile irons the ferrite is around hardness varies form 200-300 HB. the graphite nodule and it can be Pearlite content depend of the extended to the grain boundaries. graphitization grade and the cooling rate. Fig. 2. Typical microstructure of ductile iron. Table I The UNS designations for Ductile Irons, cross-referenced to the corresponding ASTM, AMS, SAE and MIL specifications [3]. Solidification Phenomena Solidification is due the • (I) The melt theory, proposing precipitation of two phases: that graphite forms in the melt austenite and graphite in the same simultaneously with austenite way of gray iron. Nucleation and dendrites, which rapidly growth of these two phases encapsulate graphite in an determine the mechanical austenite shell. properties and they are affected by • (II) The dendritic growth theory, the chemical composition (mainly maintaining that austenite carbon and silicon) inoculation and dendrites precipitate from the finally the cooling rate. melt, enriching it in carbon and There are two theories about that later graphite grows from solidification: this supersaturated residual melt, in the interstices amongst the dendrites [11]. Justification In ductile iron production several factors affect the microstructure and the mechanical properties. From the melt preparation, the quality of the charge, chemical composition, type of furnace, nodularization treatment, pouring conditions and others but if we take in account that controlling the cooling time in mold can increase the mechanical properties without any additional cost, it take great importance. This research is carried out in order to identify how much the cooling time in the mold affect on the microstructure and mechanical properties for a No bake mold casting. Objective Evaluate the cooling conditions in mold and its effects on: • Microstructure. • Mechanical properties. For different thicknesses Hypothesis Solidification time affected by the thickness of the transverse section modify the microstructure of the ductile iron. Increasing the cooling time in the mold reduces the tensile strength and hardness of ductile iron. Experimentation Condition Cooling time, h Thickness 25, 40 and 53 mm for microstructure 1 2 53 mm for tension and hardness test 25, 40 and 53 mm for microstructure 2 4 53 mm for tension and hardness test 25, 40 and 53 mm for microstructure 3 6 53 mm for tension and hardness test 25, 40 and 53 mm for microstructure 4 8 53 mm for tension and hardness test Table II Experimental procedure Results Fig. 3. Nodules density according with the demoulding time for different thicknesses. Fig. 4. Nodules size according with the demoulding time for different thicknesses. Fig. 5. Nodules area fraction according with the demoulding time for different thicknesses. Fig. 6. Pearlite and ferrite content for 2, 4, 6 and 8 hours. Fig. 7. Hardness for 2, 4, 6 and 8 hours of demoulding time. Fig. 8. Tensile and yield strength for 2, 4, 6 and 8 hours of demoulding time. Fig. 9. Elongation for 2, 4, 6 and 8 hours of demoulding time. Conclusion Conclusión • The results indicate that the • Los resultados indican que el demoulding time and cross- tiempo de desmoldeo y el section thickness have a great espesor de la sección influence on the transversal afectan significativamente a la microstructure and microestructura y propiedades mechanical properties of the de la parte. El tiempo de part. Solidification time, solidificación, afectado por el influenced by the thickness espesor de la sección, section, determines the size determinan el tamaño y and number of graphite número de los nódulos de nodules and the pearlite to grafito y la relación entre ferrite ratio of the matrix, ferrita y perlita de la matriz, con el consecuente with the consequent increase incremento de ductilidad y de in ductility and tensile la resistencia a la tensión de la strength of the casting. pieza. 1. Alagarsamy, Al. Cast Irons. 2014, págs. 489-581. 2. Karsay, Stephen I.Fundición con Grafito esferoidal I Producción. Frankfurt/Chicago : Talleres graficos Edelvives, 1992. 3. Society, Ductile Iron. Ductile Iron Data For Design Engineers. [En línea] 2013. [Citado el: 28 de Septiembre de 2014.] http://www.ductile.org/didata/forward.htm. 4. ASM International Handbook Committee.ASM Handbook, Volume 1, Properties and Selection:Irons, steels and high-performance alloys. s.l. : ASM International, 2005. 5. Committee, ASM Handbook.ASM Handbook, Volume 15, Casting. s.l. : ASM International, 1992. 6. INC., American Foundryman´s Society.Ductile Iron Handbook. U.S.A. : American Foundryman´s Society INC., 1992. 7. Callister, William D.MAterials Science and Engineering. Utah : John Wiley & Sons, Inc, 2007. 8. Society, Ductile Iron.DUCTILE IRON DATA FOR DESIGN ENGINEERS. Ohio, USA : Rio Tinto Iron & Titanium Inc, 2013. 9. Society, American Foundrymen´s.Ductile Iron Handbook. U.S.A. : American Foundrymen´s Society, 1992. 10. Structure of Spheroidal Graphite in Cast Iron. C. R. Loper, Jr, and K. Fang. 673-682, s.l. : AFS Transations, 2008. 11. Austenite shell evidence in ductile iron solidification. Sá, H. Santos and C. 2004, international journal of cast metal research, págs. 319-320. 12. AUTOMOTIVE DUCTILE IRON CASTINGS. SAE. 2004, SAE. 13. Voort, George F. Vander.Metallography, Principles and Practice. United States of America : ASM International, 1999. 14. Campbell, Flake C.Elements of Metallurgy and Engineering Alloys. United States of America : ASM International, 2008. 15. Karsay, Stephen I.Fundición con Grafito esferoidal I Prodcuccion. s.l. : Talleres graficos Edelvives, 1992. 16. DUCTILE IRON QUALITY INDEX. BV, Ir G.D HENDERIECKX. s.l. : GIETECH BV, 2004. 17. C. F. Walton (ed.), Gray and Ductile Iron Castings Handbook, Gray and Ductile Founders’ Society, Cleveland, OH, 1971..
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