In this research, the corrosion response of heat treated ductile cast iron (DCI) used in the production of engine crank shaft was examined. Five samples were used in this work, one of the samples served as control sample, the remaining four samples were subjected to conventional heat treated operations of annealing, normalizing and hardening. The nodular cast irons were heated to initial austenitic temperature of 900°C, held for one (1hour) in the muffle furnace for homogenization, and samples were now cooled in furnace, air, water and oil respectively. Sodium chloride of 3.5% weight percentage solution was used as environment to study corrosion behaviour of the heat treated ductile cast irons. The microstructural images of the control and heat treated samples were resolved and various developed structures are essentially spheroidal graphite, and pearlitic matrix as in the case of the un-heat treated sample, coarse pearlite matrix for the annealed sample, fine pearlite structure for the normalized sample and partially martensitic and fully martensitic matrix for the oil and water quenched samples. Thus, sample quenched in oil releases an impression of being the materials with the most raised corrosion resistance as a result of more broad scope of passive layers. Oil quenched sample showed best resistance to corrosion in sodium chloride solution with corrosion rate value of 0.0064388mmpy, followed by the air cooled sample with corrosion rate value of 0.008512mmpy. Un-heat treated sample has the highest corrosion rate of 0.2188mmpy. The presence of martensite with less amount of residual improved the corrosion resistance of the ductile iron, also the presence of fine pearlite structure increases the resistance of ductile iron to corrosion in the chosen chloride environment.
In this work, effects of antimony on the corrosion behaviour of antimony modified carbidic austempered ductile iron were studied. Rod-like samples (CADI) were produced by sand casting technique in which sandwich method were used to evaluate the influence of austempering temperatures on corrosion behaviour of antimony modified carbidic austempered ductile iron. Six different alloys with equivalent carbon of hyper-eutectic composition (carbon equivalent of 4.44) were used in order to evaluate the effect of antimony contents ranging from 0.096 to 0.48% at austenitic temperature of 910 o C and austempering temperatures of 300 o C and 325 o C for period of 3hrs. Studying particularly effects of carbide content and graphite shape, besides their stability during the heat treatment with respect to corrosion behaviour of the CADIs. The results show that the microstructures of the as-cast samples mainly consist of few nodular graphite, pearlite and carbides. While the heat-treated samples contain graphite nodules, carbides of varying sizes, ausferrite and retained austenite. As the antimony content increases, corrosion resistance of the carbidic austempered ductile iron increase until critical antimony content is attained. With increase in antimony content, the amount of nodular graphite decreases, pearlite and carbides present increase. As antimony content increases, the acicular ferrite becomes finer, corrosion resistance increased. At austempering temperature of 300 o C, CADI sample with antimony content of 0.192 wt.% Sb has the best corrosion resistance in neutral environment at austempering time of 1hour with the least corrosion rate of 6.0234E-07 mmpy. While at austempering temperature of 325 o C, the best corrosion resistance was obtained with the CADI having 0.096 wt. % antimony content held for austempering time of 2hours (least corrosion rate of 6.6108E-4 mmpy). From the corrosion results obtained, it was observed that the electrochemical behaviour of these CADIs in neutral environment was influenced by antimony content, austempering temperature, and austempering time.
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