Influence of casting heterogeneities on microstructure and mechanical properties of austempered ductile iron (ADI)

Abstract

An investigation was carried out to examine the influence of as-cast heterogeneities on the resulting microstructure and mechanical properties of an austempered ductile cast iron (ADI). A commercial hypereutectic ductile iron alloy with 3.59% C, 2.68% Si, 0.24% Mn and 0.46% Cu (in wt. %) was produced in a coreless induction furnace. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The results of this investigation indicate that as-cast heterogeneities have a detrimental effect on the transformed microstructure and mechanical properties of ADI. During austempering, non uniform distribution of graphite nodules and segregation of alloying elements at cell boundaries resulted in transformation gradients throughout the metallic matrix. Intercellular regions displayed inhomogeneous microstructures containing eutectic carbides and martensite. Yield strength, tensile strength and hardness decrease for increasing austempering temperature. Fracture surfaces showed a rapid transition from ductile to cleavage mode at intercellular regions, containing solidification carbides.

Crosshead Speed Effects on Ferro Casting Ductile (Fcd) Tensile Strength and Fault Morphology

“…Cleavage can be found inside the particles of crystallography area. Cleavage fracture changes its direction when crossing the intergranular fracture and graphite nodules, similar to a river pattern [26]. Cleavage fracture generally occurred in high microstructure strength (with low plasticity) and tended to take place in the inclusion around the eutectic cell boundary rather than in the distance between nodular interfaces [27].…”

Microstructural Evaluation of an Austempered Cast Iron Alloy

“…Eltaggaz et al 7 argues that its unique ausferrite microstructure contributes to several properties. This resulting ausferrite (acicular ferrite in the middle of high carbon austenite) results in high values of mechanical strength, ductility, impact resistance and wear resistance 2,8 . Thus, the material provides great flexibility in the design and manufacture of parts [9][10][11][12] .…”

“…According to Bendikien et al 8 , higher austempering temperatures produces gross acicular ferrites. Also, the increasing of austempering temperature leads to a higher % volume of retained austenite, contributing to the hardness decrease 2,4 .…”

“…For lower austempering temperatures, for example, 280 °C, result in finer ferrite fractions with greater volume and greater flow resistance. Finally, the authors concluded that yield strength, tensile strength and hardness decrease alongside the increase of austempering temperature 2 .…”

“…Boneti et al 2 analyzed the microstructure and mechanical properties of an ADI, where austempering was performed in melted metal baths through a range of temperatures of 300 °C and 370 °C, for 30 minutes. The study shows the microstructure found in these materials was ausferrite microstructure (acicular ferrite + retained austenite).…”