ABSTRACT

This study involves evaluation of using bitumen-palm kernel oil blend as quenching media during the heat treatment of two grades of steel (0.56%C-Steel and 0.76%C-Steel) and ductile cast iron. The purpose is to eliminate high handling cost of molten salt and reduce time wastages on single quenchant during foundry processes. The chemical composition of the steel and ductile cast iron was analyzed using X- ray fluorescence spectroscopy method. The two quenchants palm kernel oil and bitumen were taken in a known proportion (15:85) and blended using a magnetic stirrer. The factors evaluated were austenitizing temperature (A) and holding time (H) while the responses were ultimate tensile strength (u.t.$), hardness (h), impact strength (i), percentage elongation (e), percentage reduction in area (r.i.a). Multi- response multi-factor mathematical models were estimated which were visualized in 3-D response surface and 2-D contour plots using response surface methods. Final numerical optimization was conducted using desirability function approach provided by MINITAB 18 to determine the optimal settings of the response and factors after adequacy of the models to approximate the measured data had been established at 0.10 confidence level. The result showed that the optimum conditions for austenitizing temperature was 9 16.5°C and 45 mm while the predicted values of quenching hardness, ultimate tensile strength, impact strength, %elongation, %reduction in area were 1327.29MPa, 29.36J, 72.77%, 86.79% and 175.59HRC for 0.56%C-Steel; 1407.97MPa, 20.04J, 50.80%, 49.17% and 196.13 for 0.76%C-Steel; 1235.OIMPa, 32.42J, 60.39%, 55.99% and 234.99HRC for ductile cast iron. From the analysis, it was established that all the factors investigated influenced the steel performance significantly and it was concluded that as the austempering time and holding time increases, that the mechanical properties under investigation were affected. From the cost analysis carried out, quenching of 0.56%C, 0.76%C-steel and ductile cast iron at the optimal settings using B-PKO can save about one thousand naira (N1000.0O) which is approximately 28.57% profit gained than when palm kernel oil or other quenching media are used. The microstructural investigation carried out showed that there was no attack on the surface of the steel quenched. Palm kernel oil-bitumen blend from the study was found to have the required properties as a quenching media for austempering of steel and ductile cast iron. Hence, adoption of the optimal blend ratio and application of the developed empirical models as well as exploitation of this natural resource are recommended in some of the local industries and heat treatment operations.