SURFACE REACTIVITY OF THIN WALL DUCTILE IRON. THE EFFECT OF NODULE COUNT AND MICROSTRUCTURE

摘要

Thin Wall Ductile Iron (TWDI) castings are an attractive alternative to light alloys when the strength to weight ratio becomes a key design variable. For conventional sand casting operations, wall thickness reduction implies an increase in the number of graphite nodules (NC) and a decrease in grain size. As TWDI castings have a particularly high surface area to volume ratio, surface properties become relevant.The aim of this paper is to analyze the effect of nodule count and microstructure on surface reactivity on unalloyed ductile iron samples.Ductile Iron (DI) was produced with nodule counts ranging from 260 to 1700 nodules/mm~2. Ferritic, ferritic-pearlitic and ausferritic (ADI) matrix microstructures were obtained by heat treatment. Surface reactivity was characterized by means of electrochemical experiments conducted in a 3.5% NaCl solution at room temperature and at mospheric pressure. Surface attack was evaluated from SEM images of transversal cuts.The obtained results indicate that surface reactivity depends on microstructure and increases with nodule count. Regarding microstructure, ADI samples show the deeper attack.