The Spheroidal Graphite Cast Iron (SG Cast Iron/SGCI) often termed as Ductile Cast Iron (DCI)/ Ductile Iron (DI), has become the most preferred member of the cast iron family and at times opted over steel as well due to its superior mechanical, tribological, corrosion properties. The Graphite in SGCI is spherical in nature than that of flakes as in Grey & White cast iron, resulting increased ductility of cast iron without compensating the strength with improved machinability characteristics. The spheroidization of graphite is a two-fold technology involving desulphurization and Magnesium treatment to the cast iron melt. The former minimizes the Sulphur content in the melt so as to preventing the Magnesium loss, that occurs due to rection between Magnesium and Sulphur during the Magnesium treatment. On the other hand, the magnesium treatment results in accumulation of Carbon molecules in the liquid melt and thereby forming the spherical shape of graphite in the room temperature microstructure. The as-cast microstructure in SGCI could be ferritic, pearlitic, and ferritic-pearlitic depending upon the molten metal chemistry and pouring temperature as well as time.Typical chemical composition of SG cast iron has a specific range for respective elements that determines the mechanical and physical properties of the specified grade.
|C%||3.20-3.60||3.50-3.78||UTS||500 Mpa||400 Mpa|
|Si%||2.30-2.90||2.80-2.85||YS||320 Mpa||250 Mpa|
Ferritic - Pearlitic SGCI
Heat treatments are generally employed for steels to relieve stresses from the casting and achieve desired properties. The quenching treatment increases hardness and brittleness of steel leading to reduced machinability as a consequence of microstructural transformation to martensite in due process. In order to mitigate the machinability difficulty, post the quenching process tempering is employed to steel to eliminate the stresses as well as increase the toughness and achieve ease of machinability. Tempering involves heating the quenched steel between 125°C to 700°C and holding there for required time span there by allowing the martensite to decompose from iron carbide particles and diffuse into the austenite matrix. The room temperature microstructure of tempered martensite appears as needle surrounded by carbon rich cementite, resulted due to the breaking of larger grains of martensite during the tempering process. The carbon atoms trapped during quenching, starts moving from the space between iron and martensite upon further heating to austenitic range and starts diffusing with the neighbour cementite to form the iron carbide. This results in release of strain in the martensite and consequentially increasing toughness of steel by sacrificing the strength and hardness.
Manganese steel as the name suggests is rich in Mn content ranging from 11 to 15% with up to 1.25% of Carbon. It is uniquely non-magnetic in nature with highly suitable for tribological applications. The surface of Mn-steel can become three times harder upon bombardment of abrasive particles, without compromising the ductility that is generally associated with hardness and thereby retaining its toughness. Addition of 10% Mn to the liquid iron melt results in austenite to become stable at room temperature subjected to proper cooling has been taken care off, thereby resulting in high hardness as well as high ductility at 12% Manganese. However, the case is not similar for low Carbon steels, but for high Carbon steels the effect is significant. Mn-steel has good strength characteristics with UTS ranging from 350 to 900 MPa and even at times it raises to 2000MPa. At the same time the ductility is not compromised and lies around 18-65% depending upon the chemical composition and heat treatment process employed.