Analysis of the Rationale and Accuracy of the use of Carbon Equivalent and Thermal Analysis in the Quality Control of Cast Iron

The motivation for this paper evolved from the question “when is a cast iron eutectic?” raised by a recent paper. The simple answer is when the carbon equivalent of the iron is 4.2%. This raises another question: How does one calculate the carbon equivalent (CE)? The paper summarizes the many different equations used for this purpose. It demonstrates that Mg and inoculation affect the calculation of CE and that the Si contribution in classic CE equations is inaccurate for all practical purposes. The use of isopleths is limited as they cannot consider the effect of elements such as Mg and oxygen, or that of the metal quality (nucleation potential). They also cannot illustrate the effect of small differences in silicon resulting from inoculation. Corrections for the contribution of Si and Mg are proposed. Yet, CE can be used to determine the eutectic carbon if enough data are available to find the intersection of the austenite and graphite liquidus. Despite its limitations, because of its repeatability for a given process, the foundry can rely on TA and CE for composition control.