Influence of rare earths on shrinkage porosity in thin walled ductile cast iron

Abstract

Ductile cast iron has been cast in test bars with thickness from 2 to 10 mm. The rare earth elements La and Ce have been added to some of the castings to evaluate their influence on microstructure and shrinkage tendency. Both La and Ce increased the graphite nodule count, especially for thickness of 6 mm and below. La gave the best nodule size distribution with many small nodules. La had less shrinkage tendency than Ce in the 10 mm test bars. This tendency was less pronounced for the 6 and 4 mm test bars and other factors may have a large influence at these thicknesses. Increasing the temperature T 1 , which is controlled by the growth of off-eutectic austenite dendrites, increased the shrinkage tendency.

Thermal Analysis of Ductile Iron Casting

“…Copper is a strong pearlite promoter; its addition up to 1% converts ferritic structure into pearlitic. Arsenic, tin and antimony promote pearlite and carbides and are hence kept to lower limits; their effect can be counteracted by cerium additions [2]. Achieving the desired microstructure in thin castings necessitates controlling chemical composition and cooling rates during solidification.…”

“…In ductile iron castings, the carbon equivalent should be high (hypereutectic) in the case of very thin-walled components to avoid formation of primary carbides [2]. Thin walls cool faster due to the larger surface area available for heat transfer through mould, influencing the microstructure and mechanical properties of the casting.…”

Optimización microestructural de fundiciones grises dúctiles no aleadas, con matriz ferrítica, empleadas en la fabricación de bujes de aerogeneradores

“…Cerium and lanthanum tend to increase the number of graphite "counts" per unit area, producing an increase in the undercooling required for the nucleation of graphite (Onsoien et al, 1999;Sheikh and Iqbal, 2007). Lanthanum is assigned an additional advantage; namely that of reducing the presence of possible shrinkage micro-cavities and improving the "sphericity" of small-sized graphite nodules (Pedersen and Tiedje, 2009). The origin of the fracture leading to fatigue failure tends to be located in areas in the vicinity of a graphite nodule whose sphericity has "degenerated", or in areas adjacent to micro-cavities (Shiraki et al, 2015).…”