anneal exhaust valve

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Lornie McCullough

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Is it possible to anneal an automotive exhaust valve so that it can be machined and used for another purpose?

I was melting aluminum yesterday, and when done, I placed an exhaust valve into the hot charcoals, watched until it got red-hot, then closed up the furnace and let it cool until this morning.

I expected to find that I could machine it, however a file still glides off it. Did I accidentally case-harden it by leaving it in the charcoal to cool, or was I simply on a fools mission?

Lornie
 
Charcoal would certainly impart carbon to the valve, but slow slow cooling should not have hardened it. I would suspect your exhaust valve wasn't steel as we know it.
 
You may have case hardened it, but my guess is that you didn't get it hot enough.

You asked a great question so I had to look up the properties. It's probably 21-4N steel, very high temperature steel. Very high chromium and manganese content, about 21% and 9%. This is some tough stuff. It looks like you would have to heat it to around 1500 to 1600 F to anneal it, but the annealing temperature is not specified anywhere that I could find. It may not be possible to anneal it. I wonder how it would work as a HHS tool bit? Someone needs to give that a try just to see what happens.


http://www.azom.com/article.aspx?ArticleID=7648
 
Just use a carbide cutter, cuts like any stainless, witch is to say not well, screech, screech, screech.
Keep the cuter always moving, if it dwells it will harden that spot, then you'll have to sharpen and try to take that spot off by cutting deeper than the hard spot and move sideways to it. Not always doable depending on what is being made.

Grinding is a good way to work with it.
 
When I was younger, My dad did make various tools etc from the old engine valves. Although some of those old valves also had sodium in them so they were also dangerous to grind if you did not know it. But they are hard so they work good for some things, but not for others.

KVT
 
Many valves are multi piece welded assemblies. The heads are stellite type material (cobalt, chrome,tungsten and who knows what) spin welded to the stem. Running a magnet along the stem will tell you if this is the case. The magnet will lose attraction as it gets near the valve head. That is the point where it is friction spin welded. Many diesels used sodium filled valve stems for more efficient cooling transferring heat to the valve guide. Sodium has a tendency to explode if exposed to the moisture in the atmosphere, so the valve were supposed to be buried when disposing.
 
That was a great question, and I'm learning something from this thread.

I would think that 1600 degrees is too low of a temp to anneal that. I'm guessing the anneal temp would need to be somewhere in the 2500 degree f range, or more.
 
Thank you everyone for your help and advice. I suspect I did not get it hot enough.

I will admit that my use for the exhaust valve was quite pedestrian: I wanted to use it as dial and stem for a diy thread-dial for my SB9 lathe. I didn't want to ruin my number-stamps trying to label the dial without annealing it first.

Jim, I think as the crow flies, we are practically neighbors. Again, thank you everyone for your help.

Lornie
 
Something to keep in mind when selecting steels for repurposing is their original intent.
An exhaust valve is designed to run at elevated temps, and it is designed to cool down slowly in insulated areas.
Valves are often faced with monel or stellite on the sealing faces and pretty much no heat treating in the home shop is going to deal with that.

Cheers Phil
 
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