The amount of Inoculant use per weight of Nodular Iron poured.

[MrKopow1]

Is there a rule of thumb for the amount of inoculant to use per weight of iron poured? I am dealing with a situation were I have seen carbides form in the nodular iron. The supplier is telling me that it is because they did not have enough inoculant. They have incrased the amount of inoculant by about 10%. However, experiements indicate that it is either still not enough. I have asked for this information from the vendor but they either do not know or are not willing to share it with me. If you have any information, please let me know.

Thank you.

 

[Grelling]

The effect of inoculation is fading with the time between the inoculation and the moment of pouring. If you have a device that adds the inoculant to the pouring jet of liquid iron, an addition of 0.2 % of inoculant should be sufficient *) to avoid carbides provided the analysis shows no Cr-level above 0.04% and absence of other carbide promotors. Also you should not melt with electric arc furnace which also promote carbide formation which should be considered by adapting your metallurgy. Your inoculation material should not have a CaSi content above 0.8%. High CaSi level can also promote carbides.

*) This recommendation bases on following:
Coreless induction furnace, Fe-Si-Mg 5% coarse, Tundish-Cover Treatment 1 t, transfer to pouring ladle 1 t, adding 0.3% of Fe-Si 75 while transfering, casting within 5 min. after the Mg-treatment. Tapping temperature 1500 °C, pouring temperature 1400°C. Ladle preheating.

I also understand that you follow the general rules stated in the book "Ductile Iron I" published by RTZ Iron & Titanium (former Sorelmetal), adapted to your shop conditions.

 

[MrKopow1]

Grelling:
Thank you for your response. Here is some additional information that might be helpful.

The vendor we are dealing with puts the inoculant in the gating of the molds. He does not inoculate the tundish ladle or the pouring ladle. Each tundish ladle holds four pouring ladles and each pouring ladle pours five molds. The temperature is controlled so that pouring occurs between 1400-1455oC.

 

[Grelling]

Generally, inoculation is really the best if applied as late as possible. If you add it to the gate, the first iron passing will not react because the inoculant must melt before it will act. That means, this makes only sense for larger castings where the uninoculated prime iron will lateron be mixed with inoculated material. If at all, I also strongly suggest to apply this technology only for castings with a very long run in order to adapt inoculaton product by quantity, grain size and grain distribution. Furthermore there is always a danger that some moulds will be poured with no inoculant or wrong size or wrong quantity. You will not be able to trace the castings being wrongly inoculated, but your customer will, and get a read head.
From the process you will also have to handle every casting as an individual heat and check it accordingly, and the price for the casting will not cover an individual examination of every casting.

In order to have a save, controled process with low reject rate I advise you to abandon in-mold inoculation and go to a double inoculation, first while transfering to the pouring ladle and second, into the pouring jet (I made good experience using the Foseco-Installation (MSE ?) Suitable inoculation products are offered by many producers. Also decide for a supplier of inoculant with constant production of inoculant, not just a "good" offer from the market.

 

[Mike450]

Inoculation of ductile iron has rules of thumb. However the occurrance of carbides in castings can be a complex problem because so many factors can potentially be linked to the carbides. Analysis of the iron for example, does not guarantee the quality of the casting, it is merely one measure, and the 'goodness' of the iron can be at different ends of the spectrum.
what can cause carbides?
low Si, high alloy element chemistry in the iron, low pouring temp (not in your case), high Mg and Ce, low S, poor inoculation, slow mold fill, slow iron processing, excess tap temperature, iron held too long either before or after Mg treatment, moisture, one particular impression on the pattern plate.
Inoculation is an absolutely critical step in the production of good ductile iron. It can be added to the furnace as a pre-conditioning agent, on transfer after Mg treatment to the pouring ladle, on transfer to the mould, and in the mould usually in the runner system or bottom of the downsprue.
An inoculant is a ferrosilicon alloy conatining 75%Si, and usually 1.2% Al, 0.8-1.5 %Ca, sometimes with 1% Ba.
This inoculant can be added at 0.2% into the iron stream into the mould or 0.2% in the runner or 0.4-0.8% by weight of alloy in the pouring ladle. Some combination of this may be needed.
If the moulds are manually poured then it is unlikely that a dispenser can be rigged to follow the path of the pouring ladle, and this type of stream inoculation is for fixed pour stations ( autopour furnaces).
You need to examine all aspects of the foundry process, and the repeatability, controls because the foundry (or you) need to figure out why the carbides are occurring instead of assuming the inoculation is the primary factor. To waht extent the carbides are occurring (1% or 20% of the castings and are they always in the same location on the casting or the casting from the same impression?) The design of the casting may need to be changed.