Making Ductile Cast Iron

There is no 'metals and alloys' thread, hence I post it here.

I have seen videos on making Ductile iron pipes, which is made from an alloy of cast iron which is more ductile. I have heard that adding magnesium to cast iron and performing a heat treatment makes it more ductile. Or is that too simpistic ?

 

There's an alloy that you inoculate the molten iron with right before pouring. It's Nickle-Mag 3, there are other of the same type but this is the one I've read about. It causes the carbon in the melt to be spherical instead of flat plates. You only have about 10 minutes to pour the iron or the Mg will burn off and the iron returns to brittle.
Here is some papers a caster sent me years ago. He worked in an Iron foundry and we traded some info. It wouldn't let me load the paper so I pasted the whole thing.

For making the ductile iron at home there are a number of ways you could do it. Let me think on that and suggest some. We use two processes at Cast-Fab. Most of the iron is made using the tundish process. In this process we have special ladles which are covered, they are like a vessel of sorts. The alloy is dropped into the opening in the top, and contains magnesium ferrosilicon, an inoculant, and cover steel, and the hot metal is tapped into the hole. We fill it very quickly, about 100 pounds per second. Most treatments are 5,000 to 8,000. In the small foundry we use another converter called the Sigmat process. The vessel is a flow through type where the iron is poured in one end and travels horizontally to flow over the magnesium ferrosilicon and out the other end to be captured in the pouring basin. We discontinued another process several years ago called the open ladle treatment, and at other foundries I have used the Gazal process and the sandwich process. The open ladle treatment uses a Nickel-Magnesium alloy. It is pretty simple – you make your base iron, then after skimming the ladle and setting it on the floor, the Nickel magnesium alloy is dropped (actually slid in over the side) into the ladle where it sinks to the bottom. As it dissolves the magnesium is released. There are two alloys made by INCO for that which I have seen used: Alloy #3LC and Alloy #4. 3LC is almost pure nickel and is highly efficient. About 80 to 90% of the magnesium is recovered. #4 is only about 57% Nickel, with the rest iron (except for the magnesium). It is claimed to be by INCO as efficient but in practice it is not. We still use #4 alloy as trim additions. Both alloys contain about 5% Magnesium. Therefore since ductile iron runs say 0.035% to 0.045% Magnesium, to treat 30 pounds you would need only about a quarter pound. After treating you would need to inoculate which you could do with the ferrosilicon and do an in stream inoculation. It could be put into the sprue.

http://www.millerandco.com/products/inco/incomag_alloy3lc.html http://www.millerandco.com/products/inco/incomag_alloy4.html

I’m assuming you don’t want to go the route of the second vessel (tundish or Sigmat) which would be awkward for a smaller batch size. I’m also going to see what I can find out about the in mold process for you. We used it in a test at the first foundry I worked at in 1976 but I don’t have any experience with it. The sandwich method also involves a second ladle. I will check to see if I have any Incomag #3LC. I know I have Incomag #4. If you want to try that method I will send you a piglet, they are about 2-1/2 pounds. You would have to cut it into the smaller pieces. For PPE you would want the obvious equipment for handling the molten metal, plus the magnesium reaction is quite bright, like flashbulbs going off, for seconds to minutes. You would need tinted glasses. There is some magnesium smoke that comes off. I would suggest doing it in either an open area or have a good ventilation. The base iron needs to have a low sulfur content. Can you check your base iron chemistry with a test heat? The magnesium may addition can be adjusted to handle some sulfur levels that are high as magnesium and sulfur readily combine, but then you will have a little more slag and dross.

NICKEL MAG 3

Nickel Mag 3 is a nickel alloy containing 5 to 6.5 percent magnesium. It was developed for the foundry industry as an additive for the production of ductile iron. A highly efficient magnesium transfer is archived with this alloy, resulting in magnesium recoveries of 80 to 90 percent.

This foundry additive alloy has been used in the commercial production of ductile iron since its introduction more than fifty years ago.

The product has a density of 7.7 g/cc compared to 6.9 g/cc for liquid cast iron. Nickel Mag 3 can thus be dropped into the melt in ladles or furnaces. The relatively quiet reaction which follows is structure-controlled and depends on proper balance of the alloy composition.

The capability of adding Nickel Mag 3 directly into the melt makes it possible to treat at the most advantageous point in the process cycle. Small trim additives can also be made to furnaces or ladles to bring subcritical magnesium residuals into range and restore full spheroidization.

The usual relationship of improved magnesium treatment efficiency with decreasing temperature also applies to Nickel Mag 3.

Nickel Mag 3 is used as a deoxidizer and desulfurizer, and to control the morphology of sulfide inclusions in the steel industry.

Nickel Magnesium CarbonIron Silicon Manganese

Balance5.0-6.5%0.015% max.0.05% max.0.05% max.0.08% max.

Forms : Half rounds

Approx. 3″ Long x 2″ to 3″ Wide x 1-1/2″ High

Weight approx. 1 –1/2 lbs. to 2 –1/2 lbs. per piece

 

We added a premix in the gating system. We had a thin metal disc that would hold the mold metal weight at the top of the mold for 3 seconds allowing the metal to homogenize before melting the disc out and pouring the casting. We did this for CGI but if we used too much or the wrong chemistry in iron before hand we would get ductile. CGI is just a lower concentration of nodularity. If it is between 55 to 70 percent nodularity it is considered CGI but anything over that is ductile. CGI has some really good benefits for making cast iron skillets. You will bend them before you can break them and if you drop them on the floor you better watch out because they will bounce back up without much loss in velocity. We made them for about six to nine months as a trial and the issue we had was not that it didn't work well, they wanted to go thinner than any casting has gone before with CGI and found out that everything has its limits. We could not pour the big super thin jobs they wanted to make without causing a heap of other issues in the process. In general it is easy to do with the right additives and the right cast iron composition for normal cast iron castings. The one thing you really had to watch was the Sulphur getting too high and killing the mg. addition. I wanted to try other forms of getting the additive into solution but they had already admitted defeat and moved on. If you smell one of our early 2000 era skillets and it has a hint of Sulphur smell to it than you may have one. Drop it onto concrete and watch it bounce. If it breaks than you didnt have one and now need to replace the one you just broke.

 

I recall reading that calcium carbide was used to reduce sulfur in cast iron melts.

 

You are correct sir. When I asked them about using calcium carbide as a way to reduce sulfur they cautioned against using it as it creates a lot of slag in the furnace and autopour. We switched to a low sulfur pig iron in our melt but the sulphur still affected the nodularity at times. I would think it would be easier to maintain a consistent level with the calcium carbide addition but due to our process we did not want to reline the autopour units every six months as opposed to every two years that we do it now. I'm sure there were ways to get it down that we did not really explore but mainly because we were trying to hit CGI and not ductile and we did not need but 70% nodularity or somewhere close. That is the great thing about not working in an automotive industry...we can get away with making engineering changes whenever we want without all the red tape. For the home foundry enthusiast and hobbyist I would say calcium carbide would be the easiest way to control sulphur levels that I know.

 

So perhaps put a bit of magnesium ribbon in the mold, say a piece in each of the runners, to give a "tougher" iron? Say, sorta-half-baked ductile? Perhaps some filings dusted in the mold?

 

Well...honestly I think it could be done, but we never did the experiments I had wanted to try that were similar in process. Part of the issue with doing it that way was not getting all of the melt converted to CGI or ductile. The mg. needs a pool to homogenize in or the mg. will just be picked up by the initial melt and carried to the end of the casting. Making a large runner that can carry the majority of the melt weight where the mg. can be converted and replaced by non converted iron as the casting fills seems very doable to me. Want to try it? Try making a thin casting that you can easily break...add some mg. to a runner that is big enough to carry the majority of the melt weight. Might want to vent the runner due to the reaction inside the runner. After cleaning, see if you can bend them before they break. If so then you are good and then can improve on the method from there.

 

No place at this time to run iron. (So as to try this out) I'm needing to get an oil-burning setup to go first!

 

Me neither...yet. I plan on playing around with it once I get up and running. Im seriously considering making a bigger furnace for iron and just using the one I just made for aluminum and brass and bronze.

 

Is it possible to make ductile iron with just silicon carbide additive?, a friend visited a small local foundry and tells me that's what they told him they were using...it's entirely possible he misheard them.

 

Ive never heard of it being done that way but if it helps change the nodularity then I guess it is possible. I will ask our metallurgist what he thinks.

 

The silicon carbide adds both carbon and silicon. It's a useful trick if one cannot find ferrosilicon - as silicon carbide is found in lapidary shops for rock-tumbling.

 

I've only heard of this second hand, I've obtained ferrosilicon from the same place and I asked my friend to enquire about ductile iron additives, they told him they had lots of it: silicon carbide.

Edit, I later realized this foundry is mostly using steel scrap in an induction furnace to make their iron.

 

I spoke to our guy and he said they add silicon carbide in most ductile but that you would still need mg. to get nodularity. We add it here and do not have nodularity in our grey iron. You can use some rare earth minerals instead of mg. to get nodularity but it is much more expensive. We tried them here once for a short while and found that it was difficult to transition between regular grey iron and rare earth ductile iron so we went back to mg.

 

Thanks for confirming that Billy.

 

Why not adding Mg ribbon (or turnings) to the iron just before pouring ?

 

Go to the 1:09 minute mark to see that reaction:

 

I think that vigorous reaction is why nickel-magnesium alloy is preferred. (?)

 

That's much more volatile than the in mold reaction...or maybe just on a larger scale?? Seemed excessive compared to the ladle size he had so maybe he had a long way to carry the iron and didnt want fade....cool video though! My camera just blurs it all out and you cant see anything.

 

Is the nickel-magnesium an actual alloy, or is it a highly-compressed mixture of powdered nickel and magnesium? Say, two grams of powdered magnesium to fifty of powdered nickel, mix well, then dump the mix into a receiver and give it time in a hydraulic press?