Limit above which graphite degeneration may occur. The effect of trace/interfering elements, in particular titanium, lead and bismuth, is generally enhanced as the wall thickness increases. Moreover, their effect is cumulative, i.e. low concentrations of several interfering elements which would be without effect if only one element were present have a detrimental effect on nodule formation. The interfering elements dissolved in the melt can only reach the graphite/melt phase boundary by diffusion which is a time-dependent process. If interfering element levels increase or the cooling rate decreases, the interfering elements will have enough time to diffuse to the boundary surface, and the diffusion paths will be reduced at higher interfering element levels, disrupting graphite cristallization.
Fig. 1 shows the influence of various trace elements on the propensity for graphite degeneration. Below the curves, graphite is present in nodular form, and above the curves, other graphite shapes prevail. The example of titanium illustrates that the detrimental effect of trace elements is intensified if several of them are present simultaneously when the interference threshold concentration is exceeded. Estimating the permissible trace element concentration (interference threshold) is therefore difficult.