HDB wrote:That must have been my primary mistake, to cold a mold. What's a good preheating temp for a steel mold?
Unfortunately, I am unable to answer that question, although I'm of the opinion that pouring to a steel mold will always yield hard iron. The cooling rate is too rapid to not do so, and if you make the mold thin enough (lower the mass) to prevent chilling, I expect the mold would dissolve (happens below the melting temperature). Again, I could be wrong.
I visited a company in Utah engaged in making ductile iron pipe. It was cast in a spinning water cooled steel mold, in 20' lengths. Diameters from 4" up to 20" were cast, if memory serves. After the cast pipe was withdrawn from the mold, it was sent to an annealing oven. I expect that would be necessary for anything poured in a steel mold, water cooled, or not, as what is important is that the pour cool slow enough for full conversion of the carbon to occur before it has cooled enough to lock in hardness. Reading about the carbon cycle can be helpful in gaining an understanding.
Harold wrote:Prolonged heating at the prescribed temperature may result in malleable iron, but the heating cycle is VERY long, and likely not worth the effort.
HDB wrote:So you mean that heating the cast iron until it's completely decarburized is unlikely to happen? The iron I poured was not longer heated than 40 minutes, I believe it took less than 30 minutes.
I'm sorry. I didn't make myself clear. Prior to the creation of ductile iron, malleable iron was created from white iron. The process was to hold the castings
at the proper temperature for many hours (like 48), converting to malleable. That isn't really a practical idea for small volumes, and would be difficult for the home foundry. In its stead, one can simply pour ductile iron, although it, too, must not be chilled. Rapid cooling, due to carbon content, will always yield hardened castings.
Harold wrote:You'd be better served using the proper inoculants, shooting for gray or ductile iron. For ductile, one uses a product marketed in the US known as Glomag.
HDB wrote:Can you provide a link? I get very few Google results. I want to know what Glomag consists of.
in regards to a link, I will attempt to get to the two bags of Glomag and Ferrosilicon that I have. They won't provide a link, but I will be able to provide the name of the company, which may offer you the opportunity to learn more about the inoculants. These were acquired more than 20 years ago, long before it was fashionable to have a web presence. I will try to get this done in the next day or so, so please be patient. They are stored in one of my containers, at the rear. I know where they are, but they are not easy to access. Mean time, you might like to know that the inoculant that causes the formation of carbon spheres instead of carbon flakes is magnesium (the mag part of Glomag). The inoculation occurs in the ladle, and has a relatively short window of opportunity, although normally adequate. One has a few minutes to pour. When enough time elapses, there will be no conversion, and the heat must be low in sulfur, otherwise it fails to occur. In both instances, the charge yields gray iron instead of ductile iron.
The company casting the ductile iron pipe I spoke of was melting shredded cars in a 96" water cooled cupola, discharging to a large ladle in which they drizzled calcium carbide. The calcium carbide captured sulfur and was skimmed from the surface.
Harold wrote:If you begin your process with steel, there's no way you can end up with grey iron unless you obtain the required inoculant, which would be ferrosilicon. A little research on the subject will help you understand the relationship between silicon and carbon.
HDB wrote:I started off with cast iron (don't know what kind though, it were the legs of a garden bench). I don't understand how one upgrades the carbon content of steel so that it becomes cast iron by the addition of ferrosillicon. Ferrosilicon (FeSi) becomes SiO2 and Fe being quarts and pure iron. So the addition of FeSi pulls the O2 out of the mixture, which prevents the O2 from forming CO2, so less decarburization. But why would it upgrade the carbon content?
Iron has an affinity for carbon---so it is readily absorbed when it presents itself. It will absorb carbon when heated, even when in a less than liquid state (think of pack hardening). There's a relationship between silicon and carbon, one that dictates how much carbon will be absorbed. Unfortunately, I am not a metallurgist and can not address the chemistry involved. You should also understand that I do NOT have hands-on experience, although I've witnessed all of the things I've discussed.
If one operates a cupola, feeding only steel scrap, the output of the cupola will be gray iron, not steel. Fuel for a cupola is normally coke (carbon), so, considering the affinity to absorb carbon, it is absorbed as the steel melts, and then precipitates as the molten iron solidifies. Considering gray iron, the precipitated carbon occurs as flakes, while in ductile iron it manifests itself as spheres. They are thought to be less disruptive of the slip-plane of the resulting material, thus yielding a greater tensile strength, with far less brittleness. Ductile iron, annealed, machines much like gray iron, so there's not much benefit in fighting with gray, assuming you can keep the sulfur content low enough for proper conversion to ductile.