Comparative analysis of `hot vegetable oils used as quenchants for austempering of nodular (ductile) and grey cast irons

Although salt bath continues to be the dominant type of basestock for the formulation of austempering quenchants, there in increasing pressure to identify an alternative basestock to address the limitations to their continues use because they are not a renewable basestock and they possess poor toxicity and biodegradability properties. Currently the most often cited alternative basestocks are those based on seed oils that are renewable, biodegradable and non-toxic. In this study, the comparative analysis of four vegetable oilsproduced in Nigeria as alternative quenching media to a salt bath for industrial austempering heat treatment of nodular and grey cast ironswere investigated.The study involved the characterization of physicochemical properties and fatty acid profile of marula seed oil, cotton seed oil, peanut oil and shea butter oil. Samples were austenitized at 9000C, soaked for 1hr followed by austempering: quenching them separately in a hot marula oil, shea butter oil, peanut oil, and cottonseed oil bath maintained at 2500C, 2600C and 2700C for periods of 1, 2, 3, and 4 hrs. The microstructure of austempered samples were examined using optical microscopy. The mechanical properties of the untreated and treated samples (tensile strength, impact strength, hardness, and percent elongation) of the samples were evaluated. Results obtained show the diverse effect on the microstructure and mechanical properties austempered samples in hot shea butter, peanut and marula oil developed to form bainite (ausferrite) structure at different austempering time and temperature. The results obtained show that the oil viscosity-temperature behaviour just as their molecular structures were different. The austempered ductile iron (ADI) had the highest tensile strength of 1140MPa, 1120MPa and 1075MPa using marula, shea butter, and peanut oils respectively, while austempered grey iron (AGI) recorded lower values compared to austempered ductile iron (ADI). Specimens austempered in marula, shea butter, and peanut oils showed a continuous increase in total elongation and impact energy with increase of austempering time while the hardness decreases. The increase in total elongation and impact energy is attributed to the progression of stage 1 austempering reaction which caused austenite to transform to ausferrite. The results show that better properties can be achieve by using hot marula, shea butter, and peanut oils as austempering quenching medium at 2700C austempering temperature for 4hrs.