ABSTRACT
Austempering heat treatment practice helps in achieving high strength with good ductility and toughness by evolving a predominantly bainitic or ausferritic microstructure in steels or cast irons respectively by using salt bath. The potentials of using fatty-based vegetable oils as quenching media for austempering of steels, and cast irons have been investigated. Cottonseed, groundnut and shear butter oils were used as hot quenching bath for austempering of these materials. The effects of these oils as quenching media were evaluated in respect of basic mechanical properties and microstructure. Standard tensile, impact and hardness samples were machined from bar of 0.371%C-steel, low alloyed steel, ductile iron and grey cast iron. The samples were austenitized at 9500C, quenched at 2500C in hot cottonseed, groundnut and shear butter oils and austempered for 1to 5hrs. The results of the microstructures obtained showed that hot shear butter and groundnut oils formed bainite and ausferrite structures respectively at different austempering time in carbon steel and ductile cast iron. However, there was partial formation of bainite in austempered low alloyed steel using shear butter and cottonseed oils. The results showed clear disparity in mechanical properties between these oils. The austempered ductile iron and medium carbon steel had the highest tensile strength values of 1041N/mm2 and 1020N/mm2 using groundnut and shear butter oils respectively; while austempered grey cast iron had the lowest values of 631N/mm2 and 684N/mm2 in groundnut and shear butter oils respectively. However, the austempered samples of all the materials investigated gave poor tensile strength in cotton seed oil. The austempered medium carbon steel and ductile iron equally showed high impact values of 101J and 68J in shear butter at 5 and 4hrs respectively while the hardness values of these materials were 353HV and 412HV respectively in hot shear butter oil. Based on the microstructure developed and mechanical properties results achieved in this work, hot shear butter and groundnut oils are effective as austempering quenchants for medium carbon steel and ductile iron up to 4hrs austempering time.