Effect of different austenitizing temperatures on the impact strength and hardness of ductile iron austempered in sesame oil bath

Abstract

Heat treatment operations that require hardening of components by quenching, those quenching media such as water, brine, oil may sometimes be too drastic, hence resulting to inferior mechanical properties and defects such as cracking and distortion. Such defected components are eventually rejected, resulting to economic waste. However, in order to impart good mechanical properties, especially strength and toughness and simultaneously to avoid cracking and distortion, many researchers suggested that other heat treatment method like austempering in molten salt bath or hot oil bath could be used. But unfortunately, salt bath furnace is known to be very costly. Therefore, this study aimed at the effect of different austenitizing temperatures on the impact strength and hardness of ductile cast iron austempered in hot sesame oil (Sesanum indicum) bath. Samples of ductile cast iron were machined to standard dimensions for impact strength test, hardness test and microstructural analysis prior to the austempering process. Sesame oil was used as hot quenching bath for the austempering process. The samples were normalized and thereafter, austempered in hot Sesame oil bath at 300^oC for periods of 1,2,3,4 and 5 hours from austenitizing temperatures of 850^oC and 910^oC. Mechanical properties (impact strength and hardness) of the test samples were evaluated. The test results obtained showed that the highest hardness value of 403HVN was obtained from samples austenitizing at 910^oC. Maximum impact strength of 157 kJ/m² was also obtained from samples austenitizing at 910^oC. Microstructures of the samples were analysed using scanning electron microscopy (SEM). The microstructure, which consist of graphite nodules embedded in a matrix of ausferrite (acicular ferrite and carbon enriched austenite) was observed.