Annealing gray iron castings

[GreenTwin]

I am moving this to the casting section, so it will be more easily found.

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[GreenTwin]

I have consulted "The Book" (that is what I call it), page 179 (index attached).
The Holy Grail of castironism, so to speak.
Be back shortly.
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Edit:
Annealing generally reduces the grade level of gray iron one level, ie: if starting with Class 40 gray iron, it will be slightly less strong Class 30 gray iron after the annealing process. Not a problem for model engine castings in my opinion.

There are three typical gray iron annealing processes:
1. Low-temperature (ferritizing) annealing.
2. Medium-temperature (full) annealing.
3. High-temperature (graphitizing) annealing.

Low temperature annealing is between 1,300 F and 1,400 F, at 1 hour per inch of section.
Medium temperature annealing is between 1,450 F and 1,650 F.
High-temperature annealing is between 1,650 F and 1,759 F.

Once a piece of cast iron has been heated, it should be cooled slowly.
Slow cooling is also important for stress relief.

Apparently the medium and high temperature annealing are used when the iron is of higher alloy content, and does not respond to the low-temperature annealing process.
I would suggest starting with the low-temperature annealing process, and try that method first, to see if it improves machinability significantly.

If you don't have a kiln with electronic temperature controls, then I guess you could try the charcoal method that JasonB mentions.

I have heard that if you use a torch of some type to heat a gray iron casting, then it should be heated somewhere between "medium red" and "red", per the attached cast iron color chart (not an exact method, but would probably be accurate enough).
The torch method (depending on exactly what type of torch is used, and how concentrated the flame is) is a good method for annealing just a small area of an overall casting, such as the bosses you were trying to drill.
I guess if I were annealing, I would just anneal the entire piece, just in case there were other hard spots somewhere else in the casting.

And as I mentioned, use slow cooling regardless of which method is used.
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[ChazzC]

I wasn’t aware of annealing Cast Iron castings, but I have heard that they should be aged (preferably by storing them outside so they get thermal cycling).

 

[GreenTwin]

The cast iron book I mention above notes that by slowly cooling an annealed casting, you can also get stress relief, and so that is another way to do that, in addition to putting iron castings outdoors for aging.
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[IC-man]

The cast iron book I mention above notes that by slowly cooling an annealed casting, you can also get stress relief, and so that is another way to do that, in addition to putting iron castings outdoors for aging.
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I seem to remember Rolls Royce used to leave thier engine block out in a field for years before machining.

 

[GreenTwin]

I seem to remember Rolls Royce used to leave thier engine block out in a field for years before machining.

Someone told me Packard in the US did the same thing, and he said he saw very little wear on Packard engine parts, even when the mileage was high.

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[Oldiron]

What causes aging in the back lot to make them better? Is it the daily relatively small heat cycle day to night or is there some change simply due to age? I have some castings that must be great by now!

 

[Henry K]

In the late 1960s I was working in a small machine shop. One machinist, during WW2, worked for Curtiss Wright making B29 engines. He told me CW had their castings rough machined and then stored them outdoors for about a year before final machining. This was in New Jersey so metal stored in the summer sun could reach probably 120 degrees or more - HOT to the touch. Then, in the winter, the temperature could go to about zero degrees and they would be under snow for days, or weeks, at a time. A huge amount of money must have been tied up in this "inventory" procedure. No way the US Government would allow this dollar amount of inventory to be tied up (the US was paying the bills) unless it was proven that this was absolutely necessary. It must work somehow.

 

[GreenTwin]

There is the cost of the energy required to anneal castings, but in many of the commercial foundry videos I have seen, large annealing ovens are an integral part of the process.
With just-in-time manufacturing processes, the annealing and stress relieve immediately after casting the parts would be a must.
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[ChazzC]

There is the cost of the energy required to anneal castings, but in many of the commercial foundry videos I have seen, large annealing ovens are an integral part of the process.
With just-in-time manufacturing processes, the annealing and stress relieve immediately after casting the parts would be a must.
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Faster and cost-effective doesn’t necessarily mean better.

 

[BobsModels]

I have been annealing gray cast iron for many years. In the past I used a commercial place since castings were large ie 1 1/2 live steam locomotive castings. I am now building smaller stuff ie hit miss, steam engines, hot air etc. I had a small Paragon kiln 6 x 6 x 6 since the late '80s, and about 12 years ago I built a heat controller. Parts came from Omega and my cost was about $200, see controller photo. Their controller came with software that made it convenient to set up heat steps, see photo of a trial run. The difference between that run and what I do now is my soak is 1 1/2 hr that way I do not need to change that step for different thickness of iron. My unit is small so it takes some time ie about 2 hours to get to temp. I have one program to take it to the full anneal stage ie 815C. A second program, if I have some castings that I do not know their origin I go to 900C. I load up the oven turn on the controller and the next morning castings all cool and ready for action. Sometime I get a crust on the surface and I knock it off with a sand blaster.
When I have done this with a casting I rarely have had a hard spot remain.

 

[GreenTwin]

Bob-
That is a really nice annealing setup !

I purchased the materials for a controller for my kiln, but have not set the controller up yet.
I tempered some aluminum parts for my green engine.
I did not have to anneal my green twin flywheel, since it machined easily without annealing.
I found out the hard way that for aluminum castings, if the temperature is not controlled carefully, you can get too hot and start melting castings.

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[GreenTwin]

For comparision of Bob's annealing temperatures with the annealing temperatures I found in the iron book, 815 C is 1,499 F, and 900 C is 1,652 F.
So that matches the "Medium temperature annealing" from the iron book, which is between 1,450 F and 1,650 F.

My pour temperature for gray iron is somewhere between 2,400 F and 2,600 F (estimated, not measured).

Aluminum tempering is a multi-step process, and requires a water quench.
You do not want to quench iron castings unless you are trying to make a very hard casting.

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[Jasonb]

The graph is a good one for annealing if you want strength as it converts the hard carbides to perlite and graphite, if continued down to 240deg C @ 100deg/hr will give a nicer to machine iron rather than a strong one. Leaving it too cool in the oven as you do overnight may have a similar affect to that final stage down from around 540 down to 240.

Things like flywheels don't often suffer with hardspots as there is usually a good mass of material and no small features sticking out. The exceptions I have found are very slender spokes and any bosses such as found on hit & miss engines for pulleys as these cool faster than the main body of the casting.

 

[Charles Lamont]

Faster and cost-effective doesn’t necessarily mean better.

While outdoor seasoning will gradually relax internal stresses, it will not soften chilled spots. Annealing will.