Cast Iron Quality in my Foundry (From Brake Rotors)

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Xyius

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Hello everyone,

First time poster here. This is a weird question, but I am really curious as to what is going on. I hope this is the appropriate place to post. I don't know who else to ask!

I have a home foundry, from which I do mostly iron castings (at least right now). Recently I melted down some old brake rotors I had in the scrap bin. I cut them up into pieces and just for fun, I wanted to measure the density of the rotors and compare them to my other scrap iron from which I had already melted down and formed into ingots.

The density of cast iron is about 0.28 lbs / in^3. And this is what my iron ingot measured when I weighed it and measured the volume. However the rotor piece only measured 0.24 lbs /in^3. I checked a few times too!

When I melted them down, they seemed to have a LOT of slag. So I figured they were just cheap rotors and were probably alloyed with something else for what ever reason. But then I took an ingot that I made purely FROM the rotors and measured the density again and STILL got 0.24 lbs/in^3! I thought for sure it would go to to 0.28 since I assumed all the impurities would come out as slag.

Does anyone know what is going on here? What the heck is in these rotors?
 
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Except for a high school class way back when, I know little about foundry so I can't answer your question. However, what does the "^3" mean?
 
For the price they sell those Chinese rotors, I can't imagine it being anything to exotic. If you can imagine what ever the cheapest alternative is, that's probably it. Mike
 
Thanks for the replies. Yes ^3 means raised to the third power, in other words, cubic inch.

I don't think it was steel, steels density is pretty much the same as iron and it is much harder to melt than iron. I was reading today that the cheap rotors are made from the "bottom of the barrel" scrap iron, so you never know what is in it. I guess I just wanted something along the lines of "oh! it is probably XYZ!" But I think this is as good as I am going to get. Mystery scrap iron lol.
 
I can't speak for brake rotors, but I have some general knowledge of cast iron and ingredients that goes into the mix to make cast iron. When I was involved with real cast iron made at Alamo Iron Works in San Antonio, Texas back in the 1980's, we were told that heat consisted of pig iron and A-36 structural scrap. Nothing else was added to the heat. It was melted in a electric furnace. It yielded a fine grade of Class 40 gray iron. (To this day, I think the ground under the Alamo Stadium had a little to do with it too!) Now, brake rotors, are basically cast iron but with things like silicon and maybe nitrogen added and I can't think of the other ingredients, to make it more of an nodular iron. This makes it more heat resistance as well as wear resistance also.
I'll go find my iron casting handbook and review it a little and report back. BTW- True ASTM A-48 Class 40 gray iron or even class 20 or 30 iron, there is no control on the chemistry. So foundries will add such things as nickel, copper, and stuff like this to get the strength up there to meet tensile requirements.

EDIT. My Iron Casting handbook says that brake rotors are made from "pearlitic" gray iron. I remember that quite well from the cast iron we used to use. Ken
 
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Xyius said:
I have a home foundry, from which I do mostly iron castings (at least right now).
Click to expand...

Hi Xyius,

Sorry I have no answer for you, just a couple questions. What's your heat source? Using propane and a homemade burner I can currently reach temperatures for aluminum. I have some plans for a waste oil burner that's supposed to reach cast-iron temps. Is yours home made or something purchased?

Thanks,
-brino
 
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4gsr said:
I can't speak for brake rotors, but I have some general knowledge of cast iron and ingredients that goes into the mix to make cast iron. When I was involved with real cast iron made at Alamo Iron Works in San Antonio, Texas back in the 1980's, we were told that heat consisted of pig iron and A-36 structural scrap. Nothing else was added to the heat. It was melted in a electric furnace. It yielded a fine grade of Class 40 gray iron. (To this day, I think the ground under the Alamo Stadium had a little to do with it too!) Now, brake rotors, are basically cast iron but with things like silicon and maybe nitrogen added and I can't think of the other ingredients, to make it more of an nodular iron. This makes it more heat resistance as well as wear resistance also.
I'll go find my iron casting handbook and review it a little and report back. BTW- True ASTM A-48 Class 40 gray iron or even class 20 or 30 iron, there is no control on the chemistry. So foundries will add such things as nickel, copper, and stuff like this to get the strength up there to meet tensile requirements.

EDIT. My Iron Casting handbook says that brake rotors are made from "pearlitic" gray iron. I remember that quite well from the cast iron we used to use. Ken
Click to expand...

Thanks so much! This is exactly what I was looking for!

RJSakowski said:
The Engineering Toolbox gives a density range for cast iron from 6800 - 7800 kg/m3 which is .24 - .28 lb./in3
Click to expand...

Oh really? Huh that makes me feel better then!

brino said:
Hi Xyius,

Sorry I have no answer for you, just a couple questions. What's your heat source? Using propane and a homemade burner I can currently reach temperatures for aluminum. I have some plans for a waist oil burner that's supposed to reach cast-iron temps. Is yours home made or something purchased?
Click to expand...

My furnace burns waste oil. I actually have a YouTube channel where I am documenting building a steam engine from scratch. Making the castings, machining them etc if you are interested. One of my videos explains how my furnace works (although I HAVE made some upgrades to it since making that video, mostly in the burner assembly, the one in the video is a bit crude). The channel can be reached here: (Sorry for the shameless plug, I just figured you might be interested)

https://www.youtube.com/channel/UCbpe1Qd_ZKzcViwfxzFgeiA

Thanks all for the replies!
 
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