Ricardo C. Dommarco

Advances in the Development of Carbidic ADI

Carbidic ADI (CADI) is a new type of Austempered Ductile Iron containing free carbides in the microstructure, providing a particular combination of wear resistance and impact toughness. In this work, four CADI variants were evaluated, in which carbides were promoted by alloying with chromium. Tests performed under the low stress abrasion condition imposed by the ASTM G65 standard show that CADI can increase the wear resistance up to 100 % when compared with conventional ADI austempered at the same temperature. The carbide content must be higher than 10 % to promote a considerable reinforcing effect. However, at this carbide content level, the impact toughness varies between 7 and 11 J/cm2 for unnotched samples. These values are much lower than those of conventional ADI, but higher than those of other abrasion resistant materials, like white irons. Some CADI variants were also evaluated in field tests, producing abrasion under either low stress or high stress conditions. For this purpose, two CADI prototype parts were studied: screw segments for animal food extruders (low stress abrasion) and wheel loader bucket edges (high stress abrasion). The results gathered showed that CADI behaves satisfactorily under low stress abrasion, but the performance is not so good under high stress conditions. To analyze the differences in the abrasion response, scratch tests were performed in order to evaluate the interaction between the abrasive tip and the microstructure.

Wear Behavior of Carbidic Ductile Iron with Different Matrices and Carbide Distribution

Similar to other mechanical properties, wear resistance is entirely dependent on a materials microstructure, which, in turn, is related to the chemical composition and solidification rate, which controls the type of phase, size, amount, and dispersion. Depending on the tribosystem, the abrasive wear resistance of ductile iron (DI) may be improved by heat treatment as well as by reinforcing the matrix with hard particles such as carbides, typically obtained by alloying with elements such as chromium. The solidification rate mainly depends on wall thickness and mold characteristics. In DI, the solidification rate affects microstructural characteristics, such as nodule size, nodule count, carbide size and distribution, and matrix refinement, also including the last to freeze (LTF) amount, size, and distribution. This study evaluates the influence of the wall thickness (12.5, 25, 50, and 75 mm) on the abrasion resistance and impact toughness of DI with different matrices reinforced with carbides. Carbidic structures were obtained by alloying the melt with Cr, and the different types of matrices such as pearlitic, martensitic, and ausferritic (CADI) were obtained under as-cast conditions or by heat treatment. The results reflect the influence of cooling rate on the microstructural characteristics and its relationship with the mechanical properties, particularly the abrasive wear resistance. It was demonstrated that, under the present experimental conditions, the highest carbide content and matrix hardness, obtained from the 12.5-mm-thick part with a martensitic matrix, resulted in the highest abrasion resistance.