The Effect of Different In-Stream Inoculants on the Solidification of Ductile Iron

Abstract: A new breed of inoculants has come on to the market offering greater improvements in shrinkage prevention with the promotion of late graphite growth and high nodularity count by using an engineered inoculant containing many more ingredients than the older inoculants. These ingredients can include specific rare earths as well as graphite and sulfur. This is a first look at how two of the main inoculants work to change solidification characteristics.

Recent changes in DI MeltLab program
With added experience in Ductile Iron final analysis, the MeltLab program has been revised to change the scaled Shrinkage numbers to reference numbers that range from 0 to 100 with 100 being the best, and zero being the worse. In addition, at the suggestion of Dr. Torbjorn Skaland, we have developed a method of measuring the energy production of the late graphite that is so very important to preventing shrinkage. We are calling it the expansion potential of the iron. This value seems to decline markedly with carbide formation, and appears to us to show the completeness of the carbon to graphite process. It may be interesting to look at this value with pearlitic irons.

As this was a quick experiment, no lab work was preformed to calibrate the nodule count with the MeltLab. The results may be slightly low. The nodularity results seem in line with the foundry experience of 90%, and the Lab's final Magnesium was running about 0.045% on an older spectrometer. The MeltLab reports effective magnesium which is the magnesium less the mag-sufides and mag-oxides. These final numbers seem in line, though the pouring furnace seems a bit high so the equations may need to be recalibrated for Fischer type of iron.

This is an example of new shrinkage and expansion values. This sample shows low expansion and moderate shrinkage potential, possibly due to hypereutectic arrest.

Pouring Furnace Iron
This iron is clearly a hyper-eutectic iron that shows the relative high magnesium content from a Fischer converter.

Further, the sample is highly shrinkage prone (89) but no carbides are showing up, and the graphite expansion is very good. The major problem with this iron is that it is Hyper Eutectic as shown by the two arrests. This drives out a lot of the necessary carbon in the graphitic liquidus and the iron looses about 10% of the ability to counteract shrink. Multiply a 10% loss in late graphite times a 10% volume of graphite, and the result is a 1% of volume shrinkage defect concentrated in the last iron to freeze.

First Inoculant trial
A measured amount of this inoculant is used to produce nodularity samples by placing the inoculant in the mold and filling it with iron. We used the same amount in a non-tellurium cup and produced the following results:

The inoculant sharply reduced the shrinkage potential and changed the solidification mode to eutectic. Where in the pouring furnace iron, the bulk of solidification occurred between 2040.0 and 2040.3, here the solidification is occurring between 2096.0 and 2108.0, a major change of over 60 degrees higher.

With the addition of sulfur in the inoculant, the magnesium is a little lower than the second inoculant trial. But the result is an excellent shrinkage value of 17, and a very good expansion value of 98 along with a significant increase in nodule count.

Second Inoculant Trial
This is a new product from a major manufacture. We are not sure of the exact chemical content, but it is reportedly lower in sulfur. Hence we have a slightly higher effective Magnesium content. The Lab was reporting an 0.045 total magnesium at this time. The same method of measuring was used on this cup as in the first inoculant trial, and the test was preformed 15 minutes later.

Here the bulk of solidification occurred between 2096.3 and 2109.8 degrees. This is extremely close to the previous trial of 2096.0 and 2108.0, with the difference in the higher number reflecting more effective magnesium.

The nodularity on the second sample is almost identical at 89 vs. 90. The nodule count is 50 points lower, the mag is higher due to lower sulfur in the inoculant. But the surprising result is that the shrinkage potential of this iron is significantly higher (40 vs. 17) even though the expansion value is almost the same (95 vs. 96).

Carbide example from MeltLab Archives

This is an example of an iron with very low graphite expansion that resulted in actual carbide formation. Due to the low magnesium, this could have been carbides in d or e-flake. Note how the rate of cooling rises at the end of the eutectic reaction. This iron was not from the same foundry, and was inoculated with only ferrro-silicon.

Conclusion
Thermal analysis does show the expected trends in effective magnesium, nodularity, and nodule count. While the new expansion number confirms the absence of carbides, the shrinkage number seems to show the most difference between the inoculants. By keeping the value of the shrinkage number instead of just showing Good/Bad, it is possible to quantify one inoculant from another. The Good/Bad evaluation is conveyed by color-coding the values Green, Yellow (brown here for readability on paper) and Red.

And Thanks
FIS cannot afford the kind of research done by major universities or large corporations. We are dependent on the generosity and help of the foundries we work with. We thank all those who have helped in this, though they remain confidential.