Foil wrap when heat treating?

[Weldingrod1]

The heat transfer between the foil, the co2 inside the foil, and the part is going to be poor. Too slow for oil or water quench parts. Ok for air quench.
So, yes, the foil will chill quickly, but not the part.
The box of carbon is a better tactic for oil/water quench IMHO

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[BladesIIB]

I believe that the s/s foil wrap is only useful for air-hardening steels. If an oil or water quench is necessary, the foil would interfere with the quench and the time it would take to remove it would allow the metal to cool below the critical temperature. I could be wrong, it happened once.

I don't do any O1 Carbon steel but people I know who do, don't wrap them when heat treating for those oil quench steels.

 

[3strucking]

A heat treat oven is high on my list of things I need. I see them on eBay but I’m concerned about getting a dud since they are never close enough to see it in person.

 

[jwmelvin]

William Bryson's book, Heat Treatment, Selection, and Application of Tool Steels, is an excellent reference for heat treating. Chapter 4 is "Surface Protection" and discusses stainless steel foil (tight, triple folded seams and a piece of paper to scavenge air). It points out that the technique is good for air-hardening steels and should not be used for oil-hardening steel (because of uneven quenching where the foil does not intimately contact the part). He recommends anti-scaling powder for oil quenching.

 

[jwmelvin]

One more: Larrin Thomas's book, Knife Engineering, is also excellent. It discusses decarburization and oxidation control, and notes that foil is good for air or aluminum-plate quenching (where the foil can remain during quench). Because of the difficulty quickly cutting the foil pouch away before oil quenching, he recommends antiscale coatings or an inert-atmosphere furnace.

Also keep in mind Dan Gelbart's approach of dripping methanol into his furnace to create a protective atmosphere. Seen at around 23.5 min here.

 

[dbb-the-bruce]

A heat treat oven is high on my list of things I need. I see them on eBay but I’m concerned about getting a dud since they are never close enough to see it in person.

I lot of the older mechanical ones are pretty easy to rebuild and upgrade - as long a the frame and bricks are intact and in reasonable shape, it's not hard to replace or upgrade the rest. Although I get you point about buying sight unseen - no way to know if the price is worth it.

I did a post about my my rebuild / upgrade here: Heat treat oven upgrade

 

[dbb-the-bruce]

methanol into his furnace to create a protective atmosphere. Seen at around 23.5 min here.

That sound intriguing - I look into it. I'll also checkout both sources you mention.

I'm also interested in in-oven silver solder techniques. I've tried it once or twice with little or no success. Keeping things in place, keeping the solder where you want it and reducing or eliminating scale and oxidization seemed unobtainable goals. Of course I really didn't know what I was doing.

I've gotten better using a torch for silver solder, but complicated stuff (like a 5 sided box) is still challenging and results in a lot of cleanup being required.

 

[pontiac428]

Huh. Even the authors of those books haven't put pencil to paper to prove that a piece of foil amounts to less than a speed bump on the temperature ramp during quench. Orders of magnitude matter. Even square-brained engineers know when to call a variable a "HUGAF" factor based on proportionality. As in, for a given context, a variable is HU-Gives-A-F in output significance. Compared to the heat in the system and the excess of quench capacity, the foil adds HUGAF worth of thermal impedance to the quench process.

If the thread title lands you in science class, then go to science class. Don't argue that the world is flat just because you've never seen for yourself that it's not.

 

[MrWhoopee]

Huh. Even the authors of those books haven't put pencil to paper to prove that a piece of foil amounts to less than a speed bump on the temperature ramp during quench. Orders of magnitude matter. Even square-brained engineers know when to call a variable a "HUGAF" factor based on proportionality. As in, for a given context, a variable is HU-Gives-A-F in output significance. Compared to the heat in the system and the excess of quench capacity, the foil adds HUGAF worth of thermal impedance to the quench process.

If the thread title lands you in science class, then go to science class. Don't argue that the world is flat just because you've never seen for yourself that it's not.

I don't think it's the foil itself. I think it's the foil to metal contact and any gases between the two that reduce the quench rate. General guidance for oil or water quenches specify agitation to prevent vapor bubbles from affecting the quench.

 

[dbb-the-bruce]

@pontiac428 - I agree with you regarding .002 of foil acting much if any of a thermal barrier. I think the real issue here is the foil functioning as a physical barrier the cooling quench (liquid or air). That is going to depend a lot on the geometry of the part. A thin-ish flat piece where the foil is in full contact with most if not all of the surface is not going to have a problem.

A part that has large or deep internal areas where the foil simply covers the opening are going to be a problem. Of course that shape of a part might be difficult to quench evenly without any foil at all. Most descriptions of oil quenching describe how to move a part around in a figure eight or some other way to provide constant new cool oil contact.

Not sure how to think about air quench on a thick part with an internal void where foil covers the opening to the internal space.

Regardless, it's not so much the thickness or the foil is physically limiting the quench.