EVALUATION OF HOT- BITUMEN BATH AS A QUENCHING MEDIUM FOR AUSTEMPERING OF STEEL AND DUCTILE CAST IRON

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Department of Engineering

ABSTRACT
In this research work, the possibility of using hot bitumen as quenching medium for austempering of steel and ductile cast iron was investigated. Standard izod Impact, hardness and tensile samples were machined from 0.55%C-steel, 0.75%C-steel and ductile cast iron bars. The samples were austenitized at 800,850, 900 and 950oC, quenched in hot bitumen at 350oC and austempered for 5minutes, 30minutes, 1, 2, 3, 4 and 5hours respectively. The samples were tested for hardness, impact and tensile strength, microstructures were also examined. The result of the microstructures obtained showed that hot bitumen was able to cause formation of bainite structure in the samples. Austempering in hot bitumen resulted to increase in hardness. The as-received hardness value of 0.55%C-steel increased from 200 to 430HV after austempering, while 0.75%C-steel with asreceived hardness value of 260HV increased to 548HV and the as cast hardness value of ductile cast iron increased from 148 to 410HV. Austempering of ductile cast iron in hot bitumen resulted to improved impact strength which increased from 17J (as-cast) to 29J (austempered). Austenitization temperature was found to have influence on the impact strength of ductile cast iron. The highest impact strength of 29J was obtained at 850oC while the lowest impact energy was 4J at 950oC. At all the austempering times, the impact energy of 0.55%C-steel (21, 22, 37, 33, 29J at 1, 2, 3, 4, 5hrs respectively) was found to be higher than that of 0.75%C-steel (18, 17, 17, 16, 24J at 1, 2, 3, 4, 5hrs respectively). Results of tensile test showed that austempering in hot bitumen resulted to increase in tensile strength, percent elongation and percent reduction in area of ductile cast iron. The investigation carried out showed that hot bitumen can be used as quenching medium in austempering process.