Cost of metal

From _Metallurgy Funamentals_ by Brandt and Warner, 1999:

"In its initial state, pig iron is a rather useless material with essentially no product value. It is hard, brittle, and not very strong."

Blast furnace makes pig iron, BOF and EAF make steel from pig iron, EIF and cupola furnaces make cast iron from pig iron.

We could go a couple steps along and start calling "iron ore" "milling machine base", but I don't think it would be proper...

Regards,

Robin

I'll have to drop out of this because I'm really, really, busy. But I'd be careful about making this too complicated. There are types of pig iron that are made for steel making, for gray-iron casting ("foundry" grade pigs) and for feedstock in the alloying of ductile iron. Those slight refinements are modern variations on traditional pig iron, which was nothing more than iron made from ore in a blast furnace, or, in the very early days, in the same cupola furnaces that melted iron pigs and ore together for casting.

But pigs are the stuff of which gray-iron castings have been made for a couple of centuries. In fact, they've often been made in the same cupola melts. When the products were cast and the iron was still running, the remainder would be run off into pig molds.

Hasta luego. I have a weekend of work ahead.

Ed Huntress

(snip)

The problem of slag defects in gray vs. ductile is that you are creating additional slag in the process of making ductile, above and beyond what slag is produced during melting. The magnesium, calcium and rare earths in the treating alloy react with oxygen during treatment and make a mess of oxides. This requires a more agressive approach in designing gating to prevent the slag from entering the mold cavity. One common method we use is to place ceramic filters in the gating system to trap slag.

I can assure you that no customers are tolerant of slag in their castings, gray or ductile! Slag is basically a ceramic, one can imagine the results of hitting a slag inclusion during machining operations!

We use a washed & dried 1010 punchings, nodular pig and ductile returns for our ductile and a mix of 1055/1075 punching and OTM (rail spikes and plates), foundry grade pig and returns in our gray iron. We do use a very high quality steel in our charge, it is expensive, but the cost is justified in consistent chemistry and mechanical values.

(snip)

We tap our ductile base iron out at about 2800' F to 2700' F. . Final pour temps can range between 2620' F. for thin-wall castings to 2380 for very heavy, chunky castings. You can lose a lot of heat in the treatment process.

Mike Malone

I would certainly hope so. ;)

So there's no time to skim them off the top, or do they tend to be entrained?

Bah, I would be... but that's because I do very little machining. Yet. LOL.

BTW, have you seen my bed casting?

of curiosity, how much would it have cost to do it in say low tensile gray iron? It's 15 pounds of aluminum so about 45 pounds iron, 3' long, 3" square...

I imagine you don't use much of the punchings/stampings, eh? Too much and you lose the necessary carbon...

Yep, I'm familiar with how fast heat is lost at yellow-white heat! Gosh, it really goes that cold? I thought iron didn't start melting until

2500s...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

So if silica is left out of the cupola, and you just melt mild steel, you'll get nodular carbon as a result? Interesting... Is it hard like white CI or reasonably ductile as the condition would imply? Extra silicon allows flakes to form, then?

There's something about "effective carbon" that I need to remember more on......

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

It can be cost effective to refine steel anywhere. Exchange rates and trade balances often determine where the lower cost material can be purchased from.

In theory "1 pound per ton" of pure Magnesium would do it but I don't know of any commerical practice where the recovery is so good that 1 pound per ton of pure magnesium is used. And, it's rwally not that simple anyway.

There are several methods used to add Magnesium to the melt. In general terms it's a good idea to have the sulfur below 0.020 or 0.025% first, then add the magensium according to whatever method your foundry desires. In the beginning of ductile iron production International Nickel developed the process with Nickel-Magnesium master alloy, starting with No. 1, then No.2,

3, and so forth. No. 3 alloy and No. 4 alloy are still used a lot. No. 3 is about 57% Nickel, about 5% Magnesium and the balance iron. According to the way the foundry adds the Nickel (pour the iron over the master alloy already placed in the bottom of the ladle, drop the master alloy into the top of the ladle, plunge the master alloy into the ladle bath with a tool, etc.), and the base chemistry and treating temperature determines how much to use. The foundry will often calculate the recovery rate of the Magnesium. Normally ductile iron will contain 0.030 to 0.060 % Magnesium, but the recovery rate for the Magnesium may be only 40 to 70% of the amount added. The Nickel and Iron are just carriers for the magnesium but the Nickel will also cause the ferrite (a microstructure component) to harden and raise tensile and yield strength. International Nickel owned the patent rights for this process from the late forties until the early sixties.

Another common alloy used to make ductile today is Magnesium Ferrosilicon alloy. In this case a Ferrosilicon that is perhaps 50% silicon and 50% iron has magnesium added to it, and usually rare earth alloys and aluminum and/or calcium. The Magnesium content of these master alloys can be between 3 and

9% but a 5% alloy is a common grade. The rare earths (sometimes called 'mischmetal' can be Lanthium, Praseodymium and Neodymium. Cerium is also present but technically not a rare earth due to it's position in the atomic table, it is generally considered to be part of this group of metals. Magnesium Ferrosilicon generally can contain about 0.4 to 3.0% rare earth. There are many grades and combinations - these are general ranges.

Mike already covered this pretty well, I would only add that with ductile iron the magnesium on the exposed layer of molten iron is constantly oxidizing. It is sort of like if you took a saucepan, put milk in it and heated it up. A thin skin would form on the top of the milk. Then if you were ready to pour the mil you might want to "skim" that layer. Now imagine that as soon as the layer were removed another one immediately formed, so you never really get "clean" metal you are always having to deal with the light layer of magnesium oxide drosses that are present on the exposed metal suface. As you pour from a ladle these skins beform entrained into the liquid metal and can form thin layers or leaves within the casting. The foundries have to use several techniques to keep the metal clean.

Gray iron also forms slag but after you remove it from the ladle's exposed metal surface it will not re-form nearly so quickly.

We use Ajax medium frequency furnaces where I work, (2) 10-ton and (1) 15 ton. Scrap is very specific also as for what we want, especially for the ductile iron.

Yes, pure nickel is currently in the range of $8 to $10 per pound, if you are making a "Ni-resist" casting then that adds a consdierable cost to it. For example if a foundry were making a drain sewer grate, like what is in the curbs on a city street, and the casting cost $90 and weighed 100 pounds in gray iron, the exact same casting in a 20% Nickel grade of "ni-resist" then it might cost comething like $300.

Yes, all that and doing the process in a repeatable manner, same temperatures, same sequences, etc.

Yes I was specifically refering to the magnesium sulfide but also that as magnesium sulfide forms there is then less "free magnesium" to act upon the graphite in the melt to form the spheroidal shape of the graphite; the result could be a mixture of spheroidal graphite and flake graphite (as is present in gray iron).

We poured an 86,000 pound pour weight casting a few years ago, yet with only (2) 10-ton furnaces and (1) 15 ton furnace (i.e. 70,000 pounds total melt capacity). This involved tight scheduling of the furnaces, tapping and reladling the big ladles, and covering the some of the big ladles to reduce the heat loss from radiation. In the foundry radiation loss is the one that causes the iron to cool the fastest followed by conductive losses. After that 86,000 pound casting was poured, some software we used forecasted that it would take 5 to 6 hours in the mold to solidify due to the section sizes involved. In order to control the cooling down process and avoid casting strains, and changes to the metal structure we allowed that casting to stay in the mold for 10 days before shakeout. We had thermocouples buried in the mold to monitor the shakeout temperature. Even after that time the casting was still above 1000 degrees, in areas, when shaken out.

Mark

If you go back to the way iron was produced perhaps 200+ years ago you could generically say that the molten iron going into a sand mold to make a shape final product would be "cast iron" and the same iron poured into an open mold for later remelting would be "pig iron". Even today the excess metal in foundries that is not poured into a mold is said to be "pigged".

If you are talking chemistry only then you could simplify it like that but iron foundries are unsuccessful if they do not understand inoculation and graphite shape control. The essential part of gray iron metal, the part that makes it unique, has less to do with chemistry and more to do with the shape of the graphite within the casting.

Nobody would bother to control the shape of the graphite to make pig iron because it's going to be remelted again anyway.

That's the basics. For malleable iron you reduce the carbon and silicon content, sometimes in an air furnace but an induction furnace works fine too, but the carbon and silicon is reduced according to the materials charged into the furnace. The castings are poured with the deliberate attempt to have them freeze "white". Meaning a fractured casting will have a shiny appearance. White iron is very brittle.

The carbon is present but it is supersaturated in the metal. Later "annealing" causes the carbon to precipitate and once a local precipitate forms then additional carbon atoms migrate to it until these clusters of carbon are relatively large (under a microscope). The clusters are "generally" rounded and spheroidal - that's a 3D description - but more would appear like a huge cluster of grapes. When the process is complete then the metal, the ferrite, has lost most or all of it's carbon and it is relatively soft. This metal is now bendable, formable, etc.

In ductile iron you cause the process of the carbon combining into the tightly concentrated areas (spheroids). Instead of a cluster of grapes it would be more like a huge beach ball with the carbon atoms inside.

May not be the best analogy but I'm describing things that can be seen only under powerful microscopes.

Mark

Amen!

Unfortunately this process takes days, no? If it were stopped early, you'd have an...erm, malleable white CI? Now there's a contradiction...

Beach ball in comparison?

So what, mechanically, is the difference between ductile and malleable irons? Is malleable iron not ductile, and vice versa? ;o)

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

I didn't think so. :^)

- So magnesium recovery (resultant percentage of Mg in the pot after it's mixed) is worse when sulfur (and oxygen I would suppose, but that's kept out due to carbon, right? How about hydrogen, that gas loves me when I'm melting bronze...) is high? What of the MgS, does that come to the surface and burn off as MgO and SO2 gas?

Hm, okay...

- What's the resulting difference? Dropping or plunging wouldn't mix quite as quickly as pouring over it I imagine, but the goal is a homogenous alloy so it has to end up even one way or another.

So it's not a wise idea to simply stir in magnesium? Aside from its tendancy to burn of course. I take it the nickel reduces melting point, in addition to strengthening the alloy a bit?

Aluminum is a deoxidizer, right? I've heard of killed steel before...

So if I were making ductile iron myself, I could grab a few striker flints (mischmetal flint encased in an aluminum threaded holder which attaches to the striker arm) and a bit of magnesium?

- Actually, cerium is right after lanthanum. Maybe you're thinking of cesium?

Okay.

Oh, like aluminum. Does it have the same protective ability or is it too thin/porous?

Yeah, when I do a pop sprue to a casting (as I did just yesterday with a

5x13x2" box with 1/4" walls), I find a few inches around and including the sprue the metal surface is darker, from entrained slag sticking to the mold.

Hmm, that must be from the silicon, since the carbon would oxidize rather than the iron (or alternately, would reduce the iron oxide)?

So snaking the gate between cope and drag plus a ceramic filter is the order for quality ductile iron?

Yeah, I've seen it up to 45% Ni... ouch!

But then, manhole covers and grates can be pretty crappy. (I would hope)

I was reading a thing about ductile iron a few days ago and I forget what the term is but there's a middle ground where the flakes are kinda rounded, not spherical, not actual flakes. And I also forget what it said causes it. But an insufficient magnesium content sounds about right to me.

Ouch, that must've been an interesting day.

Yup, even at bronze temperatures I notice how big a difference a firebrick covering the last square inch of heat escape can make...

Yeah, stuff that big can do some interesting things...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

There have been some attempts to speed up the process. In one malleable iron foundry where I worked, the parts would be placed in a type of coarse sand in a large iron tubs. Iron lids would be placed on these tubs and the joint sealed with clay or some other tpye of material. The tubs would be loaded into the annealing oven and when the oven was fired off, (natural gas) it would literally take days to heat up, be held at temp, and slowly cooled. As I recall the cycle was 1 week. The same foundry also had car bottom furnaces using electric heating elements and a gas generator system to provide atmosphere control. I think I remember that cycle as being 30 hours. There was a critical point in the cooling down where the temp had to be maintained or transitioned at a slow rate for proper annealing. If you sped up the process during that phase, instead of "ferritic" malleable iron you would get "pearlitic" malleable iron. Ferritic malleable would be about

50,000 tensile, 32,500 yield and 10% elongation. The pearlitic grades have higher tensile and yield but lower elongation, for example 70,000 tensile, 50,000 yield and 5% elongation.

One thing about malleable is the importance of getting the metal to solidify white. If the casting design is very thick, like 2+ inches, it is difficult to get the casting to freeze "white" and there will be areas with "mottle" which is free graphite in the structure. The desire to to make the carbon stay into solution until the heat treat is done to cause it to occur in a planned way.

LOL I thought that might be a poor comparison. What I was trying to illustrate was that the carbon atoms coming together in the malleable iron happens while the iron is in the solid state (after casting) whereas in the ductile iron the carbon atoms are put together in the liquid state, before casting.

Malleable iron is ductile in behavior but ductile iron is not "strictly speaking" malleable in behavior.

One thing in malleable iron's favor is that it can be shaped by bending, or in a coining die, after being cast. This is not easily done in ductile iron. The main difference in the iron from a designer's standpoint is that ductile iron is strain RATE sensitive.

If you took a bar shaped piece of ductile iron and put it in a vise, you could take a hammer and lightly tap it until it bends. You could bend it but you would have to do it slowly. If you tried to bend it rapidly the iron would tend to crack.

Malleable iron on the other hand can be deformed relatively rapidly and cracking will not occur.

The main difference for that is that ductile iron has higher levels of silicon in it and the silicon atoms are spaced within the iron atoms in a manner that "locks up", if you will, the iron atoms so that they cannot move as easily. This is a simplification of what is doing on at the atomic level.

Malleable iron was invented, or discovered first and was the standard material when you needed a cast iron that could be formed or would bend before breaking. When Millis invented (or discovered) ductile iron in the late forties, it took a while for designers to know how to exploit the differences between the materials. Some malleable iron is made today but many parts formerly made in malleable iron are now made in ductile iron, according to whichever is the most overall economic design.

Also of course with malleable iron there was a limit to the size of the casting you could make, since you needed an oven to heat treat it. With ductile iron there is really little size limit; I think the largest one produced to date was a 230 ton press frame casting.

Mark

Mag recovery is worse with high sulfur content of the base iron. Also oxygen in the base iron would readily combone with the magnesium too. There is a "hierarchy" that the elements follow, and the ones which will combine most readily with the magnesium being consumed first, then the next most, etc. until all that is left is the free magnesium or mag that has not combined with any other element used to form the graphite which is left. If you took a chemical analysis of a piece of the iron though the magnesium content result alone would not tell you the answer to how much is in the form of magnesium oxide, magnesium sulfides, etc. There is expensive test equipment that will do this but it's not often done in production foundries.

As for the oxygen, it's a good idea to use silicon carbide in your melting practice (induction furnace) to de-oxidize the melt. Much of the oxide comes from rust on the steel or pig iron.

How about hydrogen, that gas loves me when I'm

No, the hydrogen is not particularly high and not likely to cause casting defects. rogen in iron is not anywhere near the problem it is in aluminum melting, or in bronze melting.

What of the MgS, does that come to the surface

No the MgS is actually fairly stable when it is formed and stays in the metal or when it floats to the top is skimmed off with the slag. MgO is very white and smoky, when ductile iron is treated the MgO comes off as white smoke that from the heat of the ladle tends to rise up. If there's cooler steelwork, like the bale of the ladle above the molten bath, it will tend to "distill" onto the colder metal as a white powdery mass. It can be chipped or broken off.

The main differences used are strictly the foundry's choice according to what equipment and process they design. In general though No. 3 alloy is not as dense as No 4 alloy (which is 95% Nickel and 5% Magnesium). You can add No. 3 to the top of a ladle but it won't sink to the bottom as easily as No. 4 so there is more flaring off of magnesium. But, No. 4 is 80% more expensive per pound than No. 3 so you have to trade off price vs. performance.

No, the magnesium can't be stirred in, because the boiling point is 1994 deg F. It would immediately vaporize and "burn". Actually the Nickel acts as a delayer, if you will, to allow the lump to sink into the iron and gradually allow the iron to take on the magnesium. As soon as it is melted in though the magnesium content will start to drop because it is constantly fuming away at the top of the molten bath of metal.

Yes, that is the function. Calcium is in treating alloys so the reaction is "quieter" because Calcium is a very strong deoxidizer.

Better to buy Magnesium Ferrosilicon. It would be less expensive.

Yeah I was not clear on that. Cerium is next to Lanthanum and I should have written that Cerium is not all that "rare". Often people will talk in terms of Cerium only in their Mag alloy or else "rare earths" and be excluding cerium.

No, there's no protective ability in the magnesium for the iron casting. Probalby because it's too thin.

Well, there are a few rules, things like using non-pressurized gating (i.e. choke at the sprue base), position the gates so that there is not a large metal drop within the mold itself, perhaps no more than 4" max, and a ceramic filter is a great plus.

"ni-resist"

Well, interesting night actually. We melt at night because electricity is cheaper. It took about 4 hours to collect all the needed metal and get the ladles into position. Once the pouring started we poured off in about 140 seconds. That was about 5 AM. I have pictures somewhere.

We actually made two of them, for Erie Press Company. Second time around was less exciting.

Mark

Not something I'm inclined to do myself, unless someone is feeling generous and wants to donate a 500 gallon propane tank to the cause... ;)

So pearlite is to ferrite what say, as-cast aluminum is to heat treated aluminum... well, mechanically. Still probably a horrible comparison though.. :^)

Yeah, I would've figured...

Not if your point was that malleable iron tends to form clumps of small graphite nodules as opposed to large blobs. If it doens't, then no it wasn't a good comparison. ;) I'm familiar with photomicrographs and their scale...

Hmm, so ductile would be a bad choice for say, an engine crank? Or is that still fine so long as the stress is below the yield strength (and fatigue limit, if you want a really long running crank)?

Hmm, interesting.

So malleable is a lower melting and easier cast form of mild steel, to a horribly simplified extent?

So it's kind of chemically work-hardened? ;)

I imagine ductile takes the cake for a large percentage, discounting shock loaded applications as you mentioned above..?

Nice...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

- Oxygen isn't removed by the carbon or silicon? Or is it that something like CO2 is soluble enough in iron to co-exist?

That makes sense. Reminds me of the story behind the first Bessemer converter... it was going smoothly for a few minutes, then literally all hell broke loose as the silicon gave way to the carbon...

So if I were doing this in a crucible furnace, I'd want to use an SiC crucible? ;) (Seriously, would that dissolve too much or what?)

Ok, that's one small thing off the list. ;)

Hmm.. so either ceramic filters in the mold, or no dice and you simply can't use a high-sulfur feedstock?

Very interesting, so it's like zinc in bronze? I would've figured a crust or skin given your description. But then, (...doublechecks...) wow, Mg has an even lower liquid range than zinc! No wonder. So then yeah, it is pretty much like zinc, eh?

So how do you get the Mg in the nickel? I recall nickel melts around

2500°F. Very carefully I guess...

Oh, despite its atomic number it's not rare enough to be considered as such?

Okay...

Neato.

Experience always calms things, eh? Reminds me of the first time I poured from my reverb. furnace...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Snip

Ferrite is basicly a solid solution of iron and a very small amount of carbon. It is soft and rather weak. Pearlite is a structure composed of alternating layers of ferrite and iron carbide. The iron carbide layers give gray iron its strength. The ratio of pearlite to ferrite in irons give a fair indication of tensile strength. For example, a Class 20 ( 20,000 psi tensile gray iron will have an approximate ratio of 10% pearlite to 90% ferrite, while a Class 40 gray iron will be 100% pearlitic.

I have seen many crankshafts cast in ductile, typically 80-55-06. Typically high-performance cranks are made of steel forgings!

My understanding of the role of silicon is it provide an inter-atomic glue, somewhat similar to the role tin plays in bronze, but to a much smaller degree.

Tim, you're correct in this assessment; most malleable castings have been converted over to ductile for several reasons, mainly that most malleable foundries are no longer in operation due to the dramatic rise in energy costs. Malleable is probably the most energy-intensive commonly used material, due to the lengthly HT cycles. Ductile is a much more cost-effective material.Typically, I purpose an annealed 60-40-18 to engineers when they are looking for a replacement material, but I have refused to quote several malleable-to-ductile conversions due to the shock-loading factor.

Mike

About on par with aluminum in terms of strength, but breaks due to stress risers from the graphite flakes, right?

Okay.

What would you get if you threw like 5 to 10% Si into the pot, assuming it dissolves, and assuming it doesn't form an intermetallic compound..?

I see. So what again of shock loading, a higher rate of force (dF/dt?) reduces yield strength, eh? So if you apply a very fast force, it turns to glass and falls apart? That doesn't sound right.. Good reason to use a forging in high-performance engines in any case...

In any case, thanks Mike and Mark, this thread has been very educational.

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @