Effect of niobium in medium alloyed ductile cast irons

The present work studies the effect of niobium addition on the mechanical properties and microstructure of an as-cast 4.3%Ni alloyed ductile iron. The experimental iron was made from high purity raw materials in a laboratory induction furnace. Eight castings were produced with niobium content in the range 0-0.8%. Silicon and carbon content were fixed at 2.4 and 3.1% respectively for all the castings. A complete microstructural characterisation was undertaken for all the as-cast alloys. Niobium was observed to be directly related to the formation of polygonal niobium carbides of the type NbC. Such carbides increased in size and amount as niobium content increased in the alloys. The amounts of pearlite and ferrite phases were not affected by the niobium content added in the present study. Neither nodule count nor nodularity was affected by such niobium addition. Mechanical properties such as hardness, tensile strength, yield strength and ductility were measured for the alloy; a small strengthening of the alloys was observed as the niobium amount increased. Such strengthening is explained in terms of NbC formation during solidification. (C) 2003 W. S. Maney Son Ltd.