Detecting Graphite Degradation in Ductile Iron Production

Original Language: Dutch

The Challenge

Despite all the control methods available to modern foundries, it still regularly occurs that production errors cause a series of castings to have insufficient nodularity. In extreme cases, quality within a single series can vary from perfect nodularity to complete absence of graphite spherulites—with graphite precipitating instead as vermicular (compacted) graphite or even as flake graphite lamellae.

This problem typically arises from magnesium fading. When casting from a ladle with marginal magnesium content, the first pours produce perfectly nodular iron. But as pouring continues, magnesium content drops below the threshold needed for spheroidal graphite formation. The result is castings with progressively degraded graphite morphology that will fail to meet ductile iron specifications.

The Solution

Sound vibration analysis provides a non-destructive method to detect nodularity variations in castings. Because graphite morphology significantly affects the elastic modulus of iron, measuring resonant frequency reveals whether a casting contains spheroidal, vermicular, or flake graphite.

Castings with good nodularity exhibit higher elastic modulus and correspondingly higher resonant frequencies. As nodularity degrades toward flake graphite, modulus drops measurably. This relationship allows foundries to screen castings after solidification but before finishing operations.

Results

Implementing resonant frequency testing enables foundries to catch magnesium fading problems before defective castings proceed further in production. Suspect castings can be flagged for metallurgical examination, and the point at which fading began can be identified retrospectively. This information feeds back into process control, helping foundries optimize magnesium treatment and ladle practices to prevent future occurrences.