Effect of tempering temperature & time on impact properties and microstucture of spheroidal graphite iron

Abstract

Spherodized graphite iron (SG iron)or popularly known as Ductile iron is a special variety of cast iron having carbon content more than 3% and has graphite present in spherical shapes. mechanical properties which are much higher than other cast irons and which can be comparable to steels. Thus enables SG irons to be used for numerous industrial applications. Alloying element addition greatly affects the mechanical properties of SG iron with Mn, Cu, Si and Ni being more predominant. While copper helps increasing the hardness causing no embrittlement in the matrix, Ni helps enhancing the ultimate tensile strength without affecting the impact energy values. Therefore addition of Cu and Ni plays key role in determining the end properties of SG iron after heat treatment. The high ratio of performance to cost which it offers to the designer and end user makes it to be used extensively for industrial uses. However, to enhance the consistency & range of properties of SG iron beyond its properties while in as-cast condition, Heat treatment is a required operation. Tempering & Austempering are two most widely used heat treatment operations in Ductile iron. For example a ferritic iron containing 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 Austempered Ductile iron (ADI) has good abrasion resistance combined with excellent ductility and toughness.