Property enhancement of spheroidal graphite cast iron by heat treatment

Abstract

Ductile iron or popularly known as Spherodized graphite iron (SG iron) is a special variety of cast iron having carbon content more than 3% and has graphite present in compact, spherical shapes. These compact spheroids hamper the continuity of the matrix much less than graphite flakes which results in higher strength and toughness with a structure that resembles gray cast iron, thus imparting superior mechanical properties i.e. much higher than all other cast irons and which can be compared to steels. This unique property enables ductile irons to be used for
numerous industrial applications.
Alloying element addition greatly affects the mechanical properties of SG iron with Si, Mn, Cu and Ni being more predominant. While copper helps increasing the tensile strength and hardness causing no embrittlement in the matrix, Ni helps enhancing the U.T.S without affecting the
impact test values. Thus addition of Cu and Ni plays an important role in determining the end properties of SG iron after heat treatment.
The high ratio of performance to cost which they offer to the designer and end user makes them to be used extensively for industrial applications. However, to extend the consistency and range of properties of SG iron castings beyond its properties in as-cast condition, Heat treatment is a necessary operation. Tempering and Austempering are the two most widely used heat treatment operations in SG iron. For example a ferritic iron having 70-80% ferrite has a yield strength of 350Mpa while a similar material which is given a step quenching treatment shows a yield strength of 550-1250Mpa. Recently developed Austemperd Ductile iron (ADI) has good abrasion resistance combined with excellent ductility and toughness.