Why do we get nodular structure when magnesium is added to iron?
Answers
Answer:
Nodular (Spheroidal Graphite) Cast Iron :
Nodular iron is also called ductile iron. The graphite is present as tiny balls or spheroids. Because the spheroids interrupt the matrix much less than graphite flakes, nodular cast iron has higher strength and toughness than gray cast iron. The formation of nodules or spheroids occurs when eutectic graphite separates from the molten iron during solidification. The separation of graphite in nodular form is similar to separation of graphite in gray cast iron except that the additives facilitate the graphite to take nodular shape.
Nodular iron is however a material that contains different kinds of defects, such as inclusions, dross, surface defects, slag stringers and micro shrinkage pores.
Explanation:
Nodular (Spheroidal Graphite) Cast Iron :
Because the spheroids interrupt the matrix much less than graphite flakes, nodular cast iron has higher strength and toughness than gray cast iron. Magnesium or cerium is added to the ladle just before casting. The matrix can be ferrite, pearlite, or austenite.
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Nodular (Spheroidal Graphite) Cast Iron
Nodular iron is also called ductile iron. The graphite is present as tiny balls or spheroids. Because the spheroids interrupt the matrix much less than graphite flakes, nodular cast iron has higher strength and toughness than gray cast iron. The formation of nodules or spheroids occurs when eutectic graphite separates from the molten iron during solidification. The separation of graphite in nodular form is similar to separation of graphite in gray cast iron except that the additives facilitate the graphite to take nodular shape.
Spheroidal graphite (SG) cast iron has excellent toughness; it has higher elongation and is used widely, for example, in crankshafts. Unlike malleable iron, nodular iron is produced directly from the melt and does not require heat treatment. Magnesium or cerium is added to the ladle just before casting. The matrix can be ferrite, pearlite, or austenite. The quality of SG iron is excellent, and X-ray quality castings are regularly produced.
The latest breakthrough in cast irons is when the matrix of spheroidal graphite cast iron is not pearlite but bainite. The chemical composition of the SG cast iron is similar to that of the grey cast iron but with 0.05 wt% of magnesium. This results in a major improvement in toughness and strength. The bainite is obtained by isothermal transformation of the austenite at temperatures below which pearlite forms. The process of graphitization is discussed in some detail in Chapter 12 on heat treatment...