Spheroid nodules

StUl another ore preparation is the nodulizing process where the ore is heated in a rotary kiln to incipient fusion. The tumbling action in the kiln causes the phosphate ore to cohere and form spheroidal agglomerates. Combustion of carbon monoxide from the furnaces is used along with supplemental fuel to supply heat to 1300—1500°C. A boring bar is used near the kiln discharge to aid in breaking up the fused ore. The material is then cooled, cmshed, and screened to the appropriate size for furnace feed. 

The role of the nodulizing elements can, therefore, be speculated as two-fold. They cleanse the melt of elements such as oxygen or sulfur which prohibit spheroidal graphite growth. The... 

The modern foundry process for producing nodular iron can be oversimplified by describing it as the treatment of a base iron (3% to 4% carbon, 1% to 2% silicon) having low (0,005% to 0.05%) sulfur levels and containing little (<0,05%) phosphorus. The treatment is carried out by means of the introduction of the appropriate nodulizer into this base iron. Inadequate addition of nodulizer results in incomplete spheroidization. Excessive concentrations of nodulizers promote the formation of unwanted iron carbides. The nodulizing elements include the rare earths, magnesium, yttrium and calcium. The latter two elements find little or no use today because of economical and technical problems. 

It should be recalled that the final step in the nodular iron treatment process is termed "post inoculation." The purpose of this procedure is to aid in the elimination of iron carbides and promote enhanced nucleation and proper growth of graphite spheroids. This is accomplished by the introduction of the element silicon (usually a ferrosilicon alloy) along with calcium and maybe some magnesium or rare earth. It has been demonstrated that the benefits of rare earth additions are not affected as a function of the time in the process that they are added (23). For example, the elimination of iron carbides by use of the rare earths is possible if the rare earths are introduced along with the primary nodulizer or with the post inocu-lant. In passing, it should be remarked that both the primary nodulizers and ferrosilicon inoculants contain about 1% calcium. 

Micronodules from the C-CF.Z. occur mainly within siliceous ooze. They consist of 10 A manganate with traces of quartz and sometimes phillipsite. They have Mn/Fe ratios of 4.7 andNi+Cu contents of 1.7%. Again, the Mn/Fe ratios are somewhat higher than for the associated nodules but the Ni+Cu contents somewhat lower. The micronodules are dominantly spheroidal or have rod-like stractures. Under the SEM, the surfaces of the micronodules consist of plates. 

In SG iron, the graphite is present as spheroids or nodules. Fig. 13.7(c), which are induced by adding magnesium before casting. This gives values of mechanical properties higher than with most other cast irons. 

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