Papers by Author: Carlos Sá

Abstract: A tungsten carbide insert is produced during ductile iron foundry practice to increase the surface hardness of the material. The insert is ensured with the help of a critical quantity of a binder, around 3 weight percent of sodium silicate, previously mixed with the tungsten carbide powder. The binder is verified to form a film around the tungsten carbide particles and bridges amongst these enveloped particles; these films and bridges are replaced by the ductile iron melt during the molding cavity filling. After solidification, a sound inserted layer is formed, composed of tungsten carbide particles in a tungsten enriched cast iron matrix. In the absence of the binder addition to the tungsten carbide powder no inserted layer is produced. The surface hardness of the inserted test pieces is close to 500VHN/4.9N and the respective thickness is around 2.5 mm. The characterization of the effect of the sodium silicate in the mixture has been the main purpose of this work

Abstract: Duplex stainless steels (d2s) are intended to present a microstructure composed of similar volume fractions of austenite and ferrite; this is the phase composition leading to the best compromise of mechanical and corrosion properties. To stabilize the austenite phase, nitrogen is added as an alloying element in the so-called second generation d2s; nitrogen is a cheap element (compared to nickel, the most common austenite stabilizer) that simultaneously increases the strength by a solid solution hardening effect; usually nitrogen contents in the second generation d2s vary from 0.15 to 0.25 weight %. The effect of a 0.14 % nitrogen addition to a cast d2s with a nominal composition corresponding to EN 10283 has been studied to evaluate its austenite stabilizing effect and its influence on the partition intensity of the elements contributing to the pitting corrosion resistance (chromium and molybdenum) between the two phases composing the microstructure. Two cast d2s have been prepared for this study: one without any nitrogen addition and another with 0.14% nitrogen; both had the same nominal composition. Specimens of the two cast d2s have been heat treated in a range of temperatures from 1050 to 1400°C, every 50°C; after a 2 hour soaking the specimens have been cooled in water. It is known that increasing the solution temperature increases the volume fraction of ferrite in the cast d2s microstructure. Both the volume fraction of each constitutional phase (austenite and ferrite) and the respective chromium and molybdenum content have been measured. The results of the volume fraction measurements showed an intense austenite stabilizing effect of the nitrogen. The results of the microanalysis of chromium and molybdenum showed that both elements present an ability to concentrate in the ferrite, although the molybdenum partition is clearly more intense than the chromium partition.