DUCTILE CAST IRON INCLUSIONS
Types and causes
Cast iron components are most often produced using disposable sand moulds. These moulds offer a cost-effective and flexible solution for the mass production of cast iron pieces; however, they are also a common source of inclusion defects. According to one review of academic literature on this topic, sand inclusions account for between 30 and 40 per cent of rejected castings. Sand inclusions are exogenous and are caused by loose sand, mould erosion, [and] mould and core wash particles’. Moulding sand may also act as a carrier of contaminant materials, e.g., core residues, slag, alloy from in-the-mould treatment, binder agglomerates and slag coagulants. The quantity of such contaminants will depend on the quality of the sand preparation in the foundry. Despite the prevalence of sand inclusion defects, however, it is ‘usually possible to identify where in the system they come from and so devise remedial action’.
A second common non-metallic inclusion in the metal is slag, which can be found both in the form of small or larger inclusions, and in the form of skins (the so-called dross defect). Dross is a particularly feared type of inclusion defect in ductile iron: due to its shape, it can greatly reduce the local mechanical properties of castings. It belongs to the above-mentioned borderline cases because slag (dross) is predominantly caused by contact between the melt surface and ambient air. The formation of a slag layer on top of the melt is an inevitable result of the nodularisation treatment with magnesium and can be managed through adequate slag separation from the treatment vessel, the metal processing and pouring systems. Several factors may however result in slag skins being entrained in the pour and causing dross defects in the final piece:
? Inadequate slag separation practices.
? Excessive slag formation (e.g., from the use of returns and steel scrap as charge materials).
? The need for high magnesium and/or aluminium additions.
? Slag formation late in the process (e.g., reoxidation of the melt due to turbulence during mould filling).
It must be accepted by the foundry and end customer that dross defects cannot be prevented completely in each area of the casting. In addition, magnesium oxide (MgO) may continue to form during solidification because of continuous enrichment of magnesium in the residual melt, and its reaction with dissolved oxygen. This reaction creates MgO particles, which can be identified as distinct endogenous inclusions. Larger proportions of MgO in the structure can adversely affect the cyclic and dynamic properties of the casting.
CAST STEEL
Types and causes
Inclusions in cast steel are usually small (<0.1mm); however, they may aggregate into larger clusters. It is the quantity of these inclusions that determines the metallurgical purity grade of the steel. The increasing proportion of non-metallic inclusions reduces the static and dynamic toughness of cast steel, especially in heat-treated steels with high strength. As with ductile iron casting, sand moulds are commonly used to cast steel – sand of various types being one of the few materials to withstand the high temperatures involved in casting steel. Sand inclusion defects (as exogenous inclusions) thus present a similar challenge (and with similar solutions) for steel foundries as for iron foundries.
Cast steel can also contain exogenous slag inclusions, which can require considerable repair efforts in cast steel (grinding, welding and heat treatment). These arise from the reaction of elements in the melt with an affinity for oxygen (e.g., Al, Ti, Ca, etc.) with oxygen in the air during melting and mould filling. Particles of refractory material or products of the reaction between refractories and metallurgical slag are possible as well.
ABOUT KEVA
Keva Casting is a professional casting company that focuses on Construction, Machinery, Automobile, Hardware, Pipeline and many other industries.
We provide top quality products with the best service based on our complete production chain including our own mold design, advanced production techniques, and full management of the supply chain. Keva Casting has two factories located in Zhucheng, China's Shandong province.
We use "shell molding", which is a new technique that has been developed in recent years.This new technique has simplified production procedures and reduced air pollution compared with traditional sand casting and investment casting. Shell making and core making are faster to ensure production in enormous quantities. The dimensions and surface finishing are satisfactory for all kinds of industrial requirements. Besides supplying our own formwork & scaffolding systems, our casting products also supply OEM services in various industries.