Ductile and SG Iron Castings
Cast iron is an alloy of iron, carbon and silicon with carbon levels of about 3.2% and silicon at about 2.2%. If molten cast iron is allowed to cool normally the carbon forms flakes of graphite which run through the iron matrix, hence the term flake graphite iron.
These flakes are at the microscopic level, the ends of which form stress points in the cast iron. If cast iron is subject to a compressive load these stress points are not particularly detrimental and flake graphite cast iron is excellent under compressive load. However, tensile loading above the natural tensile strength of the cast iron can cause rapid tensile failure as cracks propagate rapidly out from these stress points.
The result of this is that cast iron has virtually no elongation, is a brittle material and is therefore limited in its use in tensile and shock loading applications. For years foundrymen and metallurgists tried to develop a new type of cast iron that would withstand bending and shock loading and would have the characteristics more of malleable cast iron but could be produced at the lower cost of grey cast iron.
In 1943, in the International Nickel Company Research Laboratory, Keith Dwight Millis made a ladle addition of magnesium (as a copper-magnesium alloy) to cast iron – the solidified castings contained not flakes, but nearly perfect spheres of graphite. Ductile iron was born. The advantage of ductile iron is that the spheres of graphite don’t act as stress raisers but as crack arresters and are what give ductile iron its ductility. This new form of cast iron immediately found uses where malleable iron, forgings, cast steel or steel fabrications would have been used. From this start, ductile cast iron has grown into a world-class material offering cast solutions at a competitive price compared to traditional alternatives.
At Durham Foundry we produce our ductile iron by the addition magnesium, in the form of ferro silicon magnesium, to the melt as it is tapped from the furnace to the casting ladle. The magnesium is used to remove as much sulphur from the molten metal as possible in the form of magnesium sulphide. In a normal grey iron, sulphur is controlled at a level of about 0.06 to 0.08%. Sulphur is a surface active material, which means it tends to come out of solution at surface boundaries during the cooling process. In cast iron, these boundaries are between the graphite and the ferrite/pearlite matrix. The presence of sulphur at these boundaries also lowers the surface tension of the graphite as it comes out of solution which allows it to form an irregular shape, in this case, flakes. By removing the sulphur from the molten metal the graphite forms with a much higher surface tension which “pulls” the graphite into a tighter shape, in this case, a sphere.
Ductile iron can be alloyed with small amounts of copper to produce increasingly stronger irons as the copper levels are increased. These copper alloyed ductile irons can also be heat treated, being particularly suited to induction and flame hardening. With the addition of nickel up to 30% and chrome at smaller levels, a range of austenitic ductile irons can be produced which have improved properties at elevated temperatures and in aggressive atmospheres and environments.
The advantages of ductile iron which have led to its success are numerous, but they can be summarized easily – versatility and higher performance at lower cost. Other members of the ferrous casting family may have individual properties which might make them the material of choice in some applications, but none have the versatility of ductile iron, which often provides the designer with the best combination of overall properties. This versatility is especially evident in the area of mechanical properties where ductile iron offers the designer the option of choosing high ductility, with grades guaranteeing more than 18% elongation, or high strength, with tensile strengths exceeding 120 ksi (825 MPa). Austempered Ductile Iron (ADI) offers even greater mechanical properties and wear resistance, providing tensile strengths exceeding 230 ksi (1600 MPa).
In addition to the cost advantages offered by all castings, ductile iron, when compared to steel and malleable iron castings, also offers further cost savings. Like most commercial cast metals, steel and malleable iron decrease in volume during solidification, and as a result, require attached reservoirs (feeders or risers) of liquid metal to offset the shrinkage and prevent the formation of internal or external shrinkage defects. The formation of graphite during solidification causes an internal expansion of ductile iron as it solidifies and as a result, it may be cast free of significant shrinkage defects either with feeders that are much smaller than those used for malleable iron and steel or, in the case of large castings produced in rigid moulds, without the use of feeders. The reduction or elimination of feeders can only be obtained incorrectly designed castings. This reduced requirement for feed metal increases the productivity of ductile iron and reduces its material and energy requirements, resulting in substantial cost savings. The use of the most common grades of ductile iron “as-cast” eliminates heat treatment costs, offering a further advantage.
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