Gray cast machinable iron consists of flake or spheroidal graphite in a ferritic, pearlitic, austenitic or martensitic matrix. This variety of microstructural combinations is used in a multitude of applications. Typical examples are vehicle brake systems, combustion engine blocks and machinery equipment.
The particular microstructural design of gray iron and thus its characteristic properties is controlled by the alloying concept and by the casting as well as heat-treating process. The standard alloying elements in gray cast iron besides of carbon are silicon and manganese. Other elements like nickel, molybdenum, vanadium and magnesium are added to influence the microstructure.
Although the research work on the effect of niobium in cast iron is rather limited, a few statements on the effect of niobium can be safely made: Nb additions up to 0.3% tend to improve the mechanical properties of gray iron resulting from a reduction in the cell size and correspondingly blunt graphite flake size, Nb decreases the tendency to produce chill carbides due to an inoculating effect and the increase in cell count, Nb is a mild pearlite stabilizer and refiner, and Nb alloyed in larger amounts forms primary MC-type carbides of high hardness improving the wear resistance of cast iron.
Based on these effects specific applications of niobium-alloyed cast iron have been developed leading to appreciable technical and economical benefits. Work rolls for steel mills are often niobium-alloyed giving the advantage of reduced wear and thus longer endurance. Alloying of niobium to brake discs and drums allows to increase the graphite content at a given strength level and thus to optimize the heat conduction and damping behavior. The strength increasing effect of niobium can also be of advantage for the production of thin walled engine blocks. Additionally niobium helps to avoid the formation of chill structures which are difficult to machine.
The present paper discusses the effects of niobium in cast iron in more detail and also disseminates practicable alloying techniques in the foundry.