Annealing cast iron bar stock

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HDB
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Annealing cast iron bar stock

Post by HDB » Mon Aug 01, 2016 11:29 am

Hello all,

I recently built a siphon nozzle type oil/diesel burner + kaowool furnace.

Works like charm, even melts steel, and sintering the kaowool. Other topic.

So I melted and poured say 2 kg of cast iron from an old garden bench into a (cold) steel pipe with a bottom welded onto it. Objective: create cast iron bar stock which is easy to machine.

Problem: the cast iron is friggin hard, but like HRC 60 hard. I tried to get it above the curie temperature and slowly cooled it in a bucked of sand, but still hard.

I break carbide tips by the bushel trying to machine this stuff, and it takes an eternity.

Any advice on how to prevent this or how to solve it?

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10KPete
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Re: Annealing cast iron bar stock

Post by 10KPete » Mon Aug 01, 2016 1:26 pm

The fault is your iron mold. That's how foundries purposely 'chill' the cast iron to make it hard. Sand molds don't cool the iron quickly and don't create the problem. Even then, you don't want to knock the casting out of the mold until it's cooled
quite a bit.

Pete
Just tryin'

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steamin10
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Re: Annealing cast iron bar stock

Post by steamin10 » Mon Aug 01, 2016 2:28 pm

Yes indeed, you have 'white iron' or chilled iron that has little carbon lattice. No amount of reheating will induce carbon into this , and it may be that you cooked the carbon out of the iron to begin with. This is one of many problems with casting, as physical properties can be manipulated during the cast, either by purpose or by lack of understanding. In founding, adding lumps of shaped metal into the sand provides 'chills' that will lter the grain structure of the cast iron, making for ready made hard surfacing. Mill rolls are one example, as cheap cast iron rolls have taken over for tool steel rolls being very much cheaper. Holding any heat is usually detrimental to its chemistry, Slow cooling in sand is the common process for cast iron, as mentioned the sand allows for some insulation factor getting white iron less chance to form. Mold design is important too, as thin sections have a tendency toward white iron, and the stresses induced by unequal cooling i the reason for post cast annealing, to allow those stresses to abate and lessen the crack tendencies at design joints and curves.

There is alot of basic knowledge out there about the do's and dont's of casting, I encourage you to read some old founders books back into the 1930's where I think hand founding was at its peak. Many facts can be gleaned from such a good read.
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Re: Annealing cast iron bar stock

Post by Harold_V » Mon Aug 01, 2016 3:37 pm

Melting cast iron in a crucible is, and has always been, a crap shoot, for reasons already mentioned. The prolonged heating period tends to burn away the required free carbon, resulting in an alloy too low in carbon to yield gray iron. Pouring to a mold that chills is yet another of the issues, as that pretty much guarantees hard iron. Prolonged heating at the prescribed temperature may result in malleable iron, but the heating cycle is VERY long, and likely not worth the effort. 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.

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.

Harold
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HDB
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Re: Annealing cast iron bar stock

Post by HDB » Wed Aug 03, 2016 4:38 am

Thanks for the interesting replies. I'm gonna reply in depth to each reply when I have more time.

I'm going to do some tests (cut a thin piece and try to harden /temper/anneal this, spark test, break it for grain structure,...

If it's due to carbide forming, maybe I'll have to reheat it much more...
If I'm decarburazing then I'm heading to carbon steel. But it takes only say 30 minutes from lighting the furnace to pouring, running rich and I have a graphite crucible, so doesn't this seem unlikely?

Time, your experiences and more experimenting will tell.

Metallurgy is so complex and addictive.

HDB
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Re: Annealing cast iron bar stock

Post by HDB » Thu Aug 04, 2016 9:20 am

10KPete wrote:The fault is your iron mold. That's how foundries purposely 'chill' the cast iron to make it hard. Sand molds don't cool the iron quickly and don't create the problem. Even then, you don't want to knock the casting out of the mold until it's cooled
quite a bit.

Pete
Okay, so that would mean that the core of my cast is still somewhat softer. I'm going to verify that.

The `chill`, is this due to martensite forming or carbide forming (i.e. cementite (FeC), WC,...)?

HDB
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Re: Annealing cast iron bar stock

Post by HDB » Thu Aug 04, 2016 9:28 am

Yes indeed, you have 'white iron' or chilled iron that has little carbon lattice. No amount of reheating will induce carbon into this , and it may be that you cooked the carbon out of the iron to begin with.


So a spark test should give me a good indication of the carbon content, right? Are there other methods to verify?
There is alot of basic knowledge out there about the do's and dont's of casting, I encourage you to read some old founders books back into the 1930's where I think hand founding was at its peak. Many facts can be gleaned from such a good read.
Absolutely. I think I've seen most available stuff on YouTube which covers casting and read everything The Machinery's Handbook has to offer about casting. Yet, recognizing (~converting) the literatury in real stuff is confusing sometimes. That's where forums like these come into play.

HDB
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Re: Annealing cast iron bar stock

Post by HDB » Thu Aug 04, 2016 9:46 am

Melting cast iron in a crucible is, and has always been, a crap shoot, for reasons already mentioned.
I understand. But for now it's all that I have. I do believe that, under good conditions (running rich, not to long heating/overheating,...) good results are still possible and repeatable.

The prolonged heating period tends to burn away the required free carbon, resulting in an alloy too low in carbon to yield gray iron. Pouring to a mold that chills is yet another of the issues, as that pretty much guarantees hard iron.
That must have been my primary mistake, to cold a mold. What's a good preheating temp for a steel mold?
Prolonged heating at the prescribed temperature may result in malleable iron, but the heating cycle is VERY long, and likely not worth the effort.
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.
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.
Can you provide a link? I get very few Google results. I want to know what Glomag consists of.
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.
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?

Thanks everyone for the interesting replies.

HDB
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Re: Annealing cast iron bar stock

Post by HDB » Thu Aug 04, 2016 11:50 am

BTW,

I just cut the bar in 2, and the center is soft. So definately chilled on the outsides. Did spark test and compared it to the cast iron I used for the melt, identical sparks. Not a lot of sparks though.

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Re: Annealing cast iron bar stock

Post by Harold_V » Fri Aug 05, 2016 4:22 am

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.

Harold
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BAdams
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Re: Annealing cast iron bar stock

Post by BAdams » Fri Aug 05, 2016 7:39 am

Here is a link to Globe Specialty Metal's products spec sheets:

http://www.glbsm.com/Customer-Products.aspx

GLOMAG is right at the top of the page.



Brook

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Re: Annealing cast iron bar stock

Post by Harold_V » Fri Aug 05, 2016 2:43 pm

Thanks, Brook. You saved me some uncomfortable digging!
Interestingly, I saw no offer for bags of the product, which is what I have. The home foundry would be hard pressed to consider a drum of the stuff.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

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