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Heat treating with foil: What am I doing wrong?

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DMS

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#1
Not sure if this is the right place, but I didn't see a "heat treat" section anywhere, and I figure it's at least related to tooling and tool making.

Here is the deal: I am trying to heat treat some D2 tool steel (this is a knife blade for a small slip joint knife). I am getting OK results, but I think it could be better. I am using some small SS foil envelopes to prevent decarburization. This works pretty well. The problem is, when I open up the oven after the parts have reached temp, the envelope looks like a small, incandescent balloon. That stuff is tough, even when orange hot, and my attempts to simply squash it flat have not been that successful.

My concern is I'm not getting a fast enough quench because the envelope has formed a little insulating bubble. I am using a double fold and crimp on the edge, so should be air tight (apparently it is).

Am I doing something wrong? Am I worried about nothing?

-DMS
 

Tony Wells

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#2
Since you can't really evacuate all the oxygen from inside the envelope, the old timer's trick was to put a small piece of cigarette paper inside with the part so that it burned, and took care of the oxygen. Not sure if that will help with what you are seeing, but won't hurt.
 

DMS

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Since you can't really evacuate all the oxygen from inside the envelope, the old timer's trick was to put a small piece of cigarette paper inside with the part so that it burned, and took care of the oxygen. Not sure if that will help with what you are seeing, but won't hurt.
Thanks for the reply Tony. I have been throwing a small strip of regular printer paper in the envelope. Seems to work well, the last blade I pulled out had only a very light oxide coating, very cool looking, kind of a rainbow sheen.

I am more wondering if I should be worried about quench rate because of my little stainless steel balloon, the de-carb issue seems to be OK.

-DMS
 

Sandro

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#4
I don't think you need to worry about it much. D2 is an air hardening steel so will likely cool quickly enough to give you a hard edge. I've heat treated large blocks that have taken hours to cool and they've still been around 60-62 Rc. BTW, how are you quenching it? Air hardening steels are supposed to be left to cool in still are at room temperature, though in one tool shop I worked at we would put the part on a cooling table. It had a fan below that blew air up onto the part. You'd lay the part on the table, turn the fan on and keep rolling the piece around until all the colour was gone from it, then let it sit until cool. I strongly recommend no quenching it in oil or especially water since D2 can easily crack because of the high chromium content.

Hope this helps.

Sandro Di Filippo
 

DMS

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I don't think you need to worry about it much. D2 is an air hardening steel so will likely cool quickly enough to give you a hard edge. I've heat treated large blocks that have taken hours to cool and they've still been around 60-62 Rc. BTW, how are you quenching it? Air hardening steels are supposed to be left to cool in still are at room temperature, though in one tool shop I worked at we would put the part on a cooling table. It had a fan below that blew air up onto the part. You'd lay the part on the table, turn the fan on and keep rolling the piece around until all the colour was gone from it, then let it sit until cool. I strongly recommend no quenching it in oil or especially water since D2 can easily crack because of the high chromium content.

Hope this helps.

Sandro Di Filippo
Thanks Sandro, that's what I was looking for.

I am using an air quench. I open the kiln, grab the packet, pull it out, and drop it on a fire brick next to the kiln. I have also been trying to flatten the packet. Trouble is, by the time it makes it to the brick it is only a dull cherry, and the foil has become something akin to adamantium.

The piece I am working on is very small (2" x 0.6" x 0.0625") so it should cool very quickly, but I was not sure how fast you needed to cool to make sure you got good hardness.
 

randyjaco

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#6
I wouldn't worry about the bubble. It won't make much difference in an air quench and with a water or oil quench, the bubble will disappear pretty quickly. (if you have an air tight seal)

Randy
 

KBeitz

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#7
I don't quench the first time with foil. I reheat with no foil for the quench .
 
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