Material Characterization of Ductile Irons for Industrial Applications

Abstract

Ductile iron, also known as spheroidal graphite cast iron or nodular cast iron, is an alloy of essentially the same composition as gray cast iron but with the graphite in form of spheroids instead of flake graphite found in gray cast iron. Recent interest in the development and utilization of ductile cast irons has resulted in considerable study of the physical metallurgy and mechanical properties to obtain meaningful design data for these materials. Equally important is the identification of process control and quality assurance factors to achieve the desired properties successfully 3nd consistently. In this study, aspects of the physical metallurgy, production process and properties have been, investigated. Results of properties of ductile cast irons from tests performed on cast specimens of a variety of compositions and micro structures are presented. The test specimens were produced from gray cast iron scrap using various magnesium additions in order to obtain alloys with varying amounts of residual magnesium contents and different micro structures. The chemical composition, mechanical properties and microstructura1 properties were analyzed. Tension properties were measured in accordance with ASIM E fi bl test procedures, fracture toughness was measured using viii the compact specimens in accordance with ASTM E 399-81 test procedures and the micro-structural properties were measured as per the ASTM E 112 procedures. The variables studied include: nodule count, nodularity, matrix micro-structure, specimen section size, residual magnesium content, alloying elements,matrix grain size, and inter-nodule spacing. After failure, the fracture surfaces were observed visually and by moans of the scanning electron microscope, so as to relate micro-structure and crack morphology. The properties were observed to improve as the residual magnesium content increased from 0.004% for gray cast iron to 0.043% for the best ductile iron that was produced. The tensile strength increased from 247 MN/m2 to 378 MN/m2 while the average fracture 9/2 3.'2 toughness, K , increased from 24 MN/m to 53 MN/m Q for the annealed specimens. The nodule count and nodularity were found to increase with increasing magnesium levels. Annealing heat treatment was found to transform pearlitic matrix structures into ferritic matrix structures from which the highest toughness values were obtained. The ferritic specimens were observed to fracture with ductile tearing while those with pearlitic matrix microstructures fractured with intergranular and partly quasi-cleavage fracture.