Effects of Micro-alloyed Nb on the Microstructure and Mechanical Properties of Nodular Cast Iron

초록

Microstructure of the Nb microalloyed nodular cast iroon was examined by optical microscope and scanning electron microscope, and quantitative analysis of the microstructure was performed with an image analyser. The nodularity was found to change little with the microalloyed Nb content. Graphite nodule counts increased markedly with an increase of Nb content, whereas mean diameter of the graphite nodule decreased. Also, fraction of the ferrite in the matrix increased with the addition of Nb, and eutectic lamellar spacing decreased with addition of Nb up to 0.03wt.%. Mechanical tests revealed that tensile and yield strength were slightly increased, but impact toughness and fracture elongation increased markedly with an addition of Nb. Such an improvement in impact toughness and fracture elongation for Nb-microalloyed nodular cast iron is considered to be mainly attributed to the correlative effects of the microstructural changes such as an increase of the ferrite areal fraction, an increase of the nodule counts, and a decrease of the eutectic lamellar spacing.