INTRODUCTION
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/mm2. 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 atmospheric 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.
Ductile iron (DI) is a group of iron-carbon-silicon alloys, which microstructure is formed by a metallic matrix that contains spheroidal graphite precipitates. These alloys are candidate materials when manufacturing low cost, recyclable and confident mechanical parts are needed [1]. They also offer an excellent/very good rate of weight per unit of yield strength comparable with more expensive light alloys. In order to reduce cost or in the case of transport industry, also for environmental reasons, wall thickness reduction is needed. For a given chemical composition, the number of graphite nodules/mm2 (nodule count) increases as wall thickness decreases. Differences in the Nodule Count (NC) up to one order of magnitude can be found comparing conventional to thin wall castings (<4mm). The raise in nucleation rate due to higher cooling rate also leads to a decrease in grain size and produces changes in the segregation profile [2]. Mechanical properties of DI can be widely changed by heat treatment. The high ductility ferritic irons provide elongation in the range 18-30 per cent, with tensile strengths equivalent to those found in low carbon steel while Austempered Ductile Iron (ADI), could present tensile strengths exceeding 1600 Mpa. ADI are austempered to obtain a matrix formed by a fine mixture of acicular ferrite and retained austenite. Austempering involves heating to 860 to 930 °C, followed by rapid cooling into an isothermal bath held at 240 to 380 °C, where the material remains usually for 30 to 120 minutes. Different strength grades can be obtained using different austempering temperatures. Even when DI applications are wide spreading, there is scarce information about their performance in corrosive environments[3-8]. On the other hand, as DI is not a single material but a family of materials offering a wide range of properties obtained through microstructure control, it would be very useful to determi ne the influence of microstructure on corrosion behavior. Although the information available on the influence of microstructure and segregation profiles on metallic alloys surface reactivity in general is limited and contradictory, there is agreement in that it should be considered [3-5, 9].
