Characterization of ductile iron through fractographic study

Abstract

Ductile iron is a special type of cast iron family which differs from other cast iron in the manner of ductility since others are brittle in nature. Ductile iron (DI) is gaining its popularity in many industrial applications due to its strength and considerable amount of ductility which is because of the presence of spheroidal graphite in microstructure. Fracture is very common in almost every industry and field of application and analysis of fracture now-a-days has become essential for optimizing the product life span. The current study is focused no investigating the mechanical properties and fracture characteristics of ductile iron subjected to various heat treatment processes. Tensile and impact specimens are machined from a test block according to ASTM E8 and ASTM D256 standards respectively. Specimens are austenitized at 1000°C, followed by different rate of cooling and quenching. The austenitizing time being 90minutes and quenching media are mineral oil, air and salt bath for tempering, normalizing and austempering processes respectively. Isothermal annealing is also carried out in some specimens to have comparison between mechanical properties and behaviour of the material. The tempering and austempering temperature is 500°C and time being 2hrs and 4hrs respectively. Tensile test has been performed using INSTRON-1195 and Izod Impact test is performed using Izod impact tester. Vickers’s hardness is determined by application of 20 kg with 10sec. dwell time using Vickers’s Hardness Tester. Fracture surfaces of each heat treated and as-cast specimens, after tension and impact test are observed under Scanning Electron Microscope. Tensile strength is found to be maximum for tempered and hardened specimen whereas annealed specimen is having more ductility at the expense of strength. The annealed specimen is found to be ductile in nature whereas the tempered and hardened and normalized specimens have showed mixed mode of failure.