Nodule count in ductile iron: theoretical model based on Weibull statistics

• Published in: International journal of cast metals research (Online) 2005-03, Vol.18 (3), p.156-162
• Main Authors: Fraś, E., Wiencek, K., Górny, M., López, H. F.
• Format: Article
• Language: English
• Subjects: HETEROGENEOUS NUCLEATION; NODULAR IRON; WEIBULL STATISTICS
• ISSN: 1364-0461; 1743-1336
• DOI: 10.1179/136404605225023009

In this work, a model is proposed for heterogeneous nucleation on substrates (sites) whose size distribution can be described by Weibull statistics. It is found that the nuclei density N can be given in terms of the maximum undercooling ΔT m by N = N s exp[(−b/ΔT m )Γ(1 + 1/n) n ]; where n is a positive integer, N s is the overall nucleation site density in the melt and b is the nucleation coefficient (b>0)(Γ denotes the gamma function). In the case when nucleation occurs on all the possible sites, the graphite nodule density N V after solidification equals N. Consequently, measurements of N V values were carried out on experimental nodular iron castings processed under various inoculation conditions. In addition, maximum undercooling ΔT m was measured by thermal analysis and volumetric nodule count (nodule density).N V was estimated from area densities N A on cast iron surface sections using stereology. The experimental results indicate that nucleation can be described by the proposed expression for n = 1 (i.e.N = N s exp[−b/ΔT m ]). Moreover, N s and b values were experimentally determined. Finally, the proposed model suggests that the site size distribution is exponential in nature.