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Metal Casting Technologies : March 2009
TECHNICAL FEATURE For furnaces and kiln applications Nodular (Ductile) Silals are preferred to the fine flake types since, with a tensile strength of 400N/mm2 combined with 2% Figure 2. Microstructure of Silal: undercooled eutectic graphite in a ferrite matrix (x200) The use of silicon to produce silals Increasing the Silicon content to 4-8% can reduce oxidation and growth by improving the protective nature of the oxide film and by stabilizing a ferrite matrix. At a level of 5%Si the ferrite ? austenite change is raised to 950o fine undercooled graphite structure. This structure has a tensile strength of around 200N/mm2 The alloy can perform in a satisfactory manner up to 900o C. For Silals in which the graphite is not in a nodular form the %Carbon is limited to 2.5 to avoid the formation of primary flake graphite and a small amount of Titanium (about 0.2%) is usually added to ensure a C when temperatures remain more or less constant and thermal shock is unlikely to occur. However, due to their inherent brittleness at ambient temperatures, castings must be handled carefully and in furnace part applications they must heated slowly from ambient temperature. and a hardness of 200-300Hb. elongation, they have better mechanical properties and are more resistant to thermal stresses. They are produced in the same way as unalloyed Ductile irons using suitable Magnesium treatment and inoculation. Small additions of Molybdenum are used to improve creep resistance for applications such as exhaust manifolds and turbocharger casings. Typical compositions are: 3.2%C- 4%Si - 0.5%Mo and 2.6%C- 6%Si -0.5%Mo with the %Manganese level usually limited to below 0.3% in order to minimize formation of intercellular carbides. The oxidation resistance of Silals can be better than that of the lower Si high Ni Austenitic irons (known as Ni Resists) but the latter tend to be preferred for many applications because they are less brittle and more resistant to thermal shock. Typical microstructures  of a Silal and a Ni-Resist are given in Figures 2 and 3. High Si irons with 10-16 %Si and 0.8- 1.2%C are not suitable for heat resistance but are designed for resistance to corrosion by oxidizing mineral acids in chemical plants and for anodes in cathodic protection systems. 3(a) General microstructure showing normal flake and undercooled forms of eutectic graphite in a matrix of austenite. (x100) 3(b) View of cored austenite dendrites with interdendritic Cr rich carbides (x500) Figure 3. Microstructure of a flake graphite grade of Ni-Resist cast iron  38 www.metals.rala.com.au