Influence of Tellurium Addition to Spheroidal Graphite Cast Iron on the Number of Graphite Particles

Influences of magnesium content on the actions of tellurium to graphite nodule count and chilling tendency in spheroidal graphite cast iron are investigated. Casting samples 30 mm in diameter were prepared by lab size induction furnace melting. On castings with two levels (0.020 mass% and 0.035 mass%) of magnesium content, tellurium content was changed from 0 to 45 mass ppm. With additions of a small amount of tellurium, the graphite nodule count increases and the chill ratio decreases. In the 0.020 mass% Mg samples, the deterioration of the spheroidal shape is intensified with excess content of tellurium, and the critical amount is 20 mass ppm. In the 0.035 mass% Mg samples, the chilling tendency become pronounced as the tellurium content is larger than the critical content of 24 mass ppm, even though the degree of spheroidization is sufficiently large. The cooling rate of the 0.035 mass% Mg sample with 10ppm Te decreases at the beginning of the eutectic reaction, and there was no chill in the microstructure with the maximum graphite nodule count. Although it is rarely found by electron probe microanalyzer (EPMA) analysis, inclusions in the center of a spheroidal graphite consist of Mg, Ca, S, and Te.