EFFECTS OF HEAT TREATMENT AND ALLOYING ELEMENTS ON CHARACTERISTICS OF AUSTEMPERED DUCTILE IRON

Abstract

The effect of three variables on ductile iron has been investigated in this study. The first variable was the effect of austempering time on ductile iron. The second variable was the effect of austenitizing temperature and the third major variable was the effect of alloying additions on ductile iron. The alloying elements selected for this purpose were copper, nickel, a combination of copper and nickel and lanthanum. The initial study was conducted on unalloyed ductile iron castings. The effect of austempering time was examined by varying austempering time in the range of 30 minutes to 90 minutes, while keeping austenitization temperature and austempering temperature constant. It was found that with the increase of austempering time, the tensile strength increased significantly. However, at 90 minutes the tensile strength decreased. The optimum temperature was found to be 60 minutes. The second variable was the effect of austenitization temperature on ductile iron. Based on the result of the first experiment, the austempering was carried out for 90 minutes. The austempering temperatures were kept at 270 oC and 370 oC. The austenitization temperature was varied from 850 oC to 925 oC. The study revealed that tensile strength increased at 900 oC but it decreased at 925oC. The third major variable involving the effect of alloying additions on ductile iron, was studied by adding copper with three different values i.e. 0.5 wt. %, 1.0 wt. % and 1.5 wt. %. The fourth melt was without the addition of copper. It was found that with the increase of copper the tensile strength continued to increase up to 1.5 wt. %. The second alloying addition was nickel. One melt was made without nickel while the remaining three melts were made with the addition of 1.0 wt. %, 2.0 wt. % and 3.0% x nickel. The tensile strength increased correspondingly with the increase in the addition of nickel to 3.0 wt. %. The effect of a combination of copper and nickel on ductile iron was also examined. The effect of the last alloying element which was studied was lanthanum. Four melts were made for this study. The first melt was without the addition of lanthanum while the remaining three had 0.006 wt.%, 0.02 wt.% and 0.03 wt.% lanthanum. The results indicated that the tensile strength increased with the increase of lanthanum content with and without austempering. Furthermore, the highest nodule count was obtained with 0.03 wt.% lanthanum while the nodularity remained almost unchanged. Thus, it was observed that the addition of alloying elements results in an increase of tensile strength. The optimum austempering time was 90 minutes and the optimum austenitizing temperature was found to be 900 oC.