On the microstructure and mechanical properties of automotive parts made of ductile iron Juan José Ramírez-Natera 1, Rafael Colás 1, Sergio Haro-Rodriguez 2, Patricio Gil-Ramos 3 1 Universidad Autónoma de Nuevo León 2 Universidad Autónoma de Zacatecas 3 Black Hawk de México, S.A. de C.V.
The changes taken place in the microstructure and mechanical properties of an automotive part manufactured with a ferritic-pearlitic ductile iron was investigated. Demoulding time at 2, 4, 6 and 8 hours after pouring at 1400 C analyzing three thicknesses from 25 to 53 mm Se estudiaron los cambios que se presentan en la microestructura y propiedades mecánicas en piezas automotrices fabricadas a partir de un hierro dúctil ferríticoperlítico. El metal se vertió a 1400 C en un molde desmoldeando a las 2, 4, 6 y 8 horas analizando tres 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 its continuity for achieve with good ductility. Fig. 1. Carbon and silicon ranges for cast irons and steels [4].
Carbon. Average value between 2.5-4.5%. It decrease yield strength and hardness and increased elongation and impact resistance. Silicon. Average value between 1.8-2.8%. increase nodule counting and decrease tendency to form carbide. Increase yield strength and decreased hardness and elongation. Trace elements influence Sulfur. Decrease magnesium effect, high quality irons have less 0.02% wt. Phosphorus. Decrease ductility and toughness. But small quantities refine pearlite increasing yieldtensile strength ratio [6]. Manganese. Av value between 0.5-0.7% increase tensile strength and yield strength while decrease 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 is relatively ductile and soft. Hardness varies from 140-200 HB. In ductile irons the ferrite is around the graphite nodule and it can be extended to the grain boundaries. Pearlite. Is a mix of laminate ferrite with cementite. It has a good strength but poor ductility and the hardness varies form 200-300 HB. Pearlite content depend of the 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 is due the precipitation of two phases: austenite and graphite in the same way of gray iron. Nucleation and growth of these two phases determine the mechanical properties and they are affected by the chemical composition (mainly carbon and silicon) inoculation and finally the cooling rate. There are two theories about solidification: Solidification Phenomena (I) The melt theory, proposing that graphite forms in the melt simultaneously with austenite dendrites, which rapidly encapsulate graphite in an austenite shell. (II) The dendritic growth theory, maintaining that austenite dendrites precipitate from the melt, enriching it in carbon and that later graphite grows from 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 1 2 2 4 3 6 4 8 25, 40 and 53 mm for microstructure 53 mm for tension and hardness test 25, 40 and 53 mm for microstructure 53 mm for tension and hardness test 25, 40 and 53 mm for microstructure 53 mm for tension and hardness test 25, 40 and 53 mm for microstructure 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 The results indicate that the demoulding time and crosssection thickness have a great influence on the microstructure and mechanical properties of the part. Solidification time, influenced by the thickness section, determines the size and number of graphite nodules and the pearlite to ferrite ratio of the matrix, with the consequent increase in ductility and tensile strength of the casting. Conclusión Los resultados indican que el tiempo de desmoldeo y el espesor de la sección transversal afectan significativamente a la microestructura y propiedades de la parte. El tiempo de solidificación, afectado por el espesor de la sección, determinan el tamaño y número de los nódulos de grafito y la relación entre ferrita y perlita de la matriz, con el consecuente incremento de ductilidad y de la resistencia a la tensión de la pieza.
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