Heat treating in stainless foil issues

N

Nicolas Joseph

Joined
Jun 2, 2018
Messages
16
Hi everybody,
I have done a couple heat treats now with my knives wrapped in stainless foil and I have had a recurring issue.
When I put the knife wrapped in foil in the forge or my friends heat treating oven, the foil always seems to blow up like a balloon. I have watched YouTube videos of people heat treating in foil, and I have never seen anyone else have this issue. In fact, often it looks like the foil actually vacuums onto the knife.
My issue with the foil blowing up is that I am worried that when I quench in oil, the knife will not cool down fast enough as it is not directly in contact with the oil, there is that layer of air in between the knife and foil preventing this.
I have no way of definitively seeing if this is or is not affecting my heat treats either, I do not have any sort of hardness tester. I have tried to see if a file skates over it's surface but I am unsure of how to tell properly if it is skating or not.
I have tried a couple of things to fix this as explained underneath, but nothing has worked yet.
Things done to try fix this issue:
1. I have made sure my foil is airtight
2. I have squeezed out as much air as possible before sealing the envelope of foil
3. I have tried placing a small piece of paper in the foil to burn up extra air/oxygen
4. I have coated my knife in corn starch to burn up extra air/oxygen

Any help would be greatly appreciated, thanks guys!
 
SS foil is normally used with quench plates. What alloy are you HT'ing? What temperature? It's normal to see a SS foil pack balloon up with heat, just shows the packet is sealed and as the air inside heats up, it expands.
 
Ken H> said:
SS foil is normally used with quench plates. What alloy are you HT'ing? What temperature? It's normal to see a SS foil pack balloon up with heat, just shows the packet is sealed and as the air inside heats up, it expands.
Click to expand...
I have used the foil on 01 tool steel and SAE5160 in a homemade forge so I do not know the exact temperature, I was just experimenting with it.
Then the other day I used it on some Bohler N690, and I heated it up to 1913F/1045C and both times it ballooned up.
Is this ballooning an issue?
 
Stop putting paper and cornstarch in the foil with the blade.

Problem solved.

Edit: Yikes, Don't use foil for oil hardening steels. Too slow.
 
To reiterate the other's posts:
Use HT foil for air hardening steels (high alloy tool steels and stainless steels). These have long soak times and higher oven temperatures and thus need to have oxygen excluded. These steels are either cooled by hanging in the air, or more commonly, between thick aluminum quench plates.

Oil quench steels should be bare or the quench will not work. A coating of one of the anti-scale compounds ( clay wash, ATP, Brownell's, Turco, etc.) is fine, but never use foil.
 
I don't have my own oven, so may be well off or behind the times, but I remember seeing instructions by big name makers to roll the foil tight, get as much air out as possible, then put a small hole, like 1/64 to 1/32 in the foil to allow remaining air to escape and prevent the foil being pressed too hard against the steel when cooled.
That would certainly deal with balloon effect.

I am interested in the idea of "burning the oxygen" out of the trapped air in an effort to reduce the volume of gas. Would be nice to hear from a chemist or similar about the volumetric effect of taking gaseous O2 and solid material of complex chain hydrocarbons, and heating to get CO2 and possibly CO.
 
C_Claycomb said:
I don't have my own oven, so may be well off or behind the times, but I remember seeing instructions by big name makers to roll the foil tight, get as much air out as possible, then put a small hole, like 1/64 to 1/32 in the foil to allow remaining air to escape and prevent the foil being pressed too hard against the steel when cooled.
That would certainly deal with balloon effect.

I am interested in the idea of "burning the oxygen" out of the trapped air in an effort to reduce the volume of gas. Would be nice to hear from a chemist or similar about the volumetric effect of taking gaseous O2 and solid material of complex chain hydrocarbons, and heating to get CO2 and possibly CO.
Click to expand...
Nah, no hole, that defeats the purpose of the foil, just wrap it and use clean double seams on all three corners.

The problem is over thinking it.
 
C_Claycomb said:
I don't have my own oven, so may be well off or behind the times, but I remember seeing instructions by big name makers to roll the foil tight, get as much air out as possible, then put a small hole, like 1/64 to 1/32 in the foil to allow remaining air to escape and prevent the foil being pressed too hard against the steel when cooled.
That would certainly deal with balloon effect.

I am interested in the idea of "burning the oxygen" out of the trapped air in an effort to reduce the volume of gas. Would be nice to hear from a chemist or similar about the volumetric effect of taking gaseous O2 and solid material of complex chain hydrocarbons, and heating to get CO2 and possibly CO.
Click to expand...
The simple equations of combustion say that you'll get a greater volume of gas after combustion of the oxygen in the foil with the primary products being CO2 and H2O. The only simple way to get the oxygen out of the gas phase without creating another gaseous byproduct (within the parameters of this situation) would be having it adsorb and/or react with a surface. The creation of oxides on the surface of steel is a perfect example of this (and exactly what we're trying to avoid)!
 
Just to reiterate what Stacy and few others have said, you don't use foil for an oil quenching steel. Anti scale compound like ATP 641 works very very well. But I also have had the balloon thing happen a few times when heat treating stainless steel blades and wondered, "what the heck?" I had a habit of spraying the blade down with WD40 and casually wiping off the excess with a paper towel. Every time I did that, I would get the balloon, but luckily I wasn't getting any scale or decarb, just the usual oxidation rainbow. Now I no longer hose the blades down. As a matter of fact, I now de-grease them with acetone and then alcohol. No more ballooning.
 
Ballooning is caused by the trapped air in the envelop heating up. I have never had any issue with it. Reduce the trapped air and eliminate the ballooning. If you are a nerd, use PV=nRT to determine how much increase the air volume will be. P will be more or less constant, since the packet can expand. V is the change we want to know about. n is the number of moles of air, which is also a constant. and T is the other variable. if the temperature rises from 100F to 1500F, the increase is 15 fold (1500%). The volume will also increase the same amount. If there is one cubic centimeter of air trapped, it will increase to 15 cc. If there is 5cc air, it will become 75cc.
The point is, if there is very little excess space in the packet for air, the expansion is also small.

Liquids in the packet, be it water or oil, are far worse, as the volume of gas created when they vaporize creates a lot of gas to then expand by heat. Just a few drops of water or oil could make as much as half a liter of gas at 1500F. You want the blade clean and dry.

Forget paper and spraying with WD-40. The tiny amount of oxygen in the small amount of air is insignificant as far as creating decarb to any degree that matters. One cc of air contains 2.53X10^19 of molecules., but air is only 21% oxygen - which means that a cc of air contains a mere .5X10^19 molecules of oxygen. I'm not going to bother calculating how many carbon atoms are in a steel knife blade, but compared to the small amount of oxygen molecules in the packet, it is not reduced by any calculable amount due to decarb.

Here are the basics:
Make the packet just large enough to hold the blade. Double fold and press the seams flat along the sides and one end. Use a terrycloth towel to squeegee down the packet and make it fit snug against the blade ... fold and crimp the remaining end.
 
Stacy E. Apelt - Bladesmith said:
Ballooning is caused by the trapped air in the envelop heating up. I have never had any issue with it. Reduce the trapped air and eliminate the ballooning. If you are a nerd, use PV=nRT to determine how much increase the air volume will be. P will be more or less constant, since the packet can expand. V is the change we want to know about. n is the number of moles of air, which is also a constant. and T is the other variable. if the temperature rises from 100F to 1500F, the increase is 15 fold (1500%). The volume will also increase the same amount. If there is one cubic centimeter of air trapped, it will increase to 15 cc. If there is 5cc air, it will become 75cc.
The point is, if there is very little excess space in the packet for air, the expansion is also small.

Liquids in the packet, be it water or oil, are far worse, as the volume of gas created when they vaporize creates a lot of gas to then expand by heat. Just a few drops of water or oil could make as much as half a liter of gas at 1500F. You want the blade clean and dry.

Forget paper and spraying with WD-40. The tiny amount of oxygen in the small amount of air is insignificant as far as creating decarb to any degree that matters. One cc of air contains 2.53X10^19 of molecules., but air is only 21% oxygen - which means that a cc of air contains a mere .5X10^19 molecules of oxygen. I'm not going to bother calculating how many carbon atoms are in a steel knife blade, but compared to the small amount of oxygen molecules in the packet, it is not reduced by any calculable amount due to decarb.

Here are the basics:
Make the packet just large enough to hold the blade. Double fold and press the seams flat along the sides and one end. Use a terrycloth towel to squeegee down the packet and make it fit snug against the blade ... fold and crimp the remaining end.
Click to expand...
I completely agree on all counts, except for one. If you're going to use PV=nRT in that way then you must use a temperature in kelvin which has an absolute zero and not a relative zero. Using your example, 100-1500 ˚F is approximately 300-1100 K which would give a little under a factor of four increase in volume. All that said the point you were making is still perfectly valid.
 
Stacy E. Apelt - Bladesmith said:
To reiterate the other's posts:
Use HT foil for air hardening steels (high alloy tool steels and stainless steels). These have long soak times and higher oven temperatures and thus need to have oxygen excluded. These steels are either cooled by hanging in the air, or more commonly, between thick aluminum quench plates.

Oil quench steels should be bare or the quench will not work. A coating of one of the anti-scale compounds ( clay wash, ATP, Brownell's, Turco, etc.) is fine, but never use foil
Click to expand...
I have a problem now after reading this information.
The steel N690, as far as I understand, can be oil hardened, or plate quench hardened.
I have access to oil hardening, but not to plate quench hardening.
I do not have access to anti-scale compounds as I live in South Africa and cannot find a place to buy them.
I have heard that at the high temperatures needed for N690 (+-1913F/1045C), you need some type of anti-scale method, such as foil or the anti scale compounds.

This gives me 2 options:
1. I do not use any anti-scale compounds and get large amounts of corrosion on my blade
2. I oil quench the N690 with foil and risk a bad heat treatment.

Does anyone have any solutions?
 
Do you know the specifics of plate quench hardening? It’s just appropriately sized aluminum plates clamped together with the blade in the middle.

Maybe you can contact Des Horn for guidance or advice, I think he’s in Cape Town.
 
When N690 or other high alloy steels are quenched in oil, you have to remove the blade from the foil packet. That isn't a big deal. Take it out of the oven with tongs holding the tip end of the packet and cut the other end off the packet with heavy scissors. Pull the blade out by the tang with pick-up tongs and quench in the oil bath. You have lots of time to do this, so don't get crazy trying to rush it into the oil.
As for plate quenching, you could even plate quench between large steel plates. The quicker cooling is only part of why plates are used. It is preventing warp that they do best.

A caution to those who aren't experienced with HT foil. It will cut you like a razor when cutting and folding it. Wear gloves! After HT, the foil is stiffer and even worse. Despite what you see online occasionally, foil packets can not be reused after HT.
 
Hey everyone. My heat treat oven just came today. I've made a few stainless blades and didn't use foil. I plan to use foil in the HT oven. My question is when to remove the foil? Right after plate air cooling, after cryo or tempering? I'm assuming after plate and air cooling. Thanks
 
Normally, remove it after plate quenching.

A good trick for getting as much air as possible out of the packet is to close the side seams and tap them tight. Close one end and tap it tight. Slip the blade into this envelope and roll it with a low-nap 4" roller from the paint department of a hardware store. 1/4" nap is OK, 1/8" is even better. If you are creative, you can make one yourself easily.
After rolling it flat, fold and tap tight the open end.
 
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