Foil Wrap Amazement -

R

rlinger

Joined
Mar 29, 2002
Messages
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This has not before happened for me. I just got through air quenching a full tang blade and the foil packet lost all noticable air pressure very soon after lowering into the wind tunnel. I mean the blade looked like it was vacuum packet in the foil. I could even see the plunge lines !!

I do not know why this happened except that it is obvious the air within the packet was thin and hot. There are two things I did differently on this packet: I only folded the edges twice instead of my usual three folds AND I left the packet extra long to keep the hard folds away from the blade end and tip (warp stuff, you know). A inadvertant pin hole does not make sense because that would allow cool air in, so it must have been air tight in the cool air but I wonder if the 'only two folds' allowed some of the air out during time in the oven.

There is no noticable decarb or scaling. After cutting the packet, pulling the foil away from the blade was like pulling good tape off. It was stuck like tape. Oven temperature during HT was 1965 F.. Soak was 25 minutes.

Anyone else have this happen?? I just knew the blade would come out twisted and warped but it did not (thank God) and it sure seemed to quench good.

Roger
 
Roger are you using the same foil as before?
or some from new stock?
there is a SS foil that is used for lower temperture.
 
Rodger, I've had it happen a couple of times. My sense was that my oven got up around or above 2000 degrees at some point. It also happened the times when I wiped the foil with acetone.

My guess has been that it got hot enough and clean enough to start to weld to the blade. I always double fold so I can't see how that plays in with my experience in this area.
 
Andrew, thanks for that experience. I do doubt my oven went above 2000 since I have used this foil at 2050 and at that it degraded. This foil remains strong after cooling and the target temp. was 1965 F.. I did not and do not wipe my foil down with any liquid so that wasn't it however, I did wipe the blade down with alcohol but thought I had let it evaporate before wrapping in the packet. I do put a small piece of brown paper bag in the packet (about 1 to 1 1/2 sq. inch of paper).

It was truely amazing especially because the partial vacuum was such that you can not only see the entire outline of the blade on the foil but also the foil was sucked into the tang drill holes !! You can actually feel the dimples in the foil where the tang holes where (both sides of the foil packet).

RL
 
the only thing I can think of Roger is
it may have lost internal air thought a pin hole
and as it cooled the suction collapsed the hole like a valve
and then the foil collapsed more than normal?
if you can't reproduce the same thing it has to be a fluke of some kind..:confused:
 
Roger,

Do you usually put something "sacrificial" in the envelope to burn off the oxygen, and did you do it this time? I've had the same thing happen before, never did have an explanation...
 
Rodger, I had the exact same thing you describe happening. My foil was pulled right around the blades including outlining the pivot hole and the profile. The foil was completely intact and cut the same as any other packet with the expection of being stuck to the blades. I use a small piece of paper tucked in one corner and wiped the blades with acetone just to get light oil off. The blades came out as clean as any I've seen including those I've sent out. They were hard and took a good edge.

What you describe as a "vacuum" seemed to me to be more of an issue of the foil sticking itself to the blade. I wonder if an oxide from the foil formed and welded itself to the blade at a temp lower then the welding point specified for the foil. Just a thought.
 
Andrew, I believe I actually stood there and watched the foil loose pressure and suck up around the blade as I was air flow quenching it. I want to believe in Dan's idea of a pin hole but would that not simply allow outside pressure in, thereby preventing this to happen??

Fitzo, in practice I almost always place a small amount of combustable in the packet. I use brown paper bag as my material and usually try to keep it to about 1 sq. inch or slightly greater in size. It is true, I had also done that for this one.

As it turned out I ended up HT'ing this again. The story is this: It Rockwelled 61 after snap tempering @ 350 F.. That I though great and a little higher than I expected after a snap temper. However, deep cryo only brought it up to 62.5 - 63. That is short by one point. After a single temper of 550 F. / 2 hours it Rockwelled 58. It should have been 59 - 60. As an after thought, did I ever mention what the steel is?? It is RWL-34.

RL
 
Originally posted by rlinger
I want to believe in Dan's idea of a pin hole but would that not simply allow outside pressure in, thereby preventing this to happen??

RL
Click to expand...

Roger what I said/meant was

bear with me I'm not good with words.

It may have lost the internal air
pressure from the paper burning and heat caused pressure
expanding the pack faster than it could be released
through this alleged hole or gap
hence still blowing it up and
though a pin hole or gap some of that pressure got released
and as it cooled the suction or contraction of air in cooling
may have sealed that hole or gap sealing it
like a one way valve
and then causing the foil to collapsed more than normal
actually causing a vacuum in inside the pack,
try less paper,
it don't take much fire to burn up that amount of air.
and some guys believe it's not necessary anyway.
because I believe you'll get some fire inside with just
impurities on he fold and or blade burning out the o2 anyway
it's the seal that keep anymore O2 from getting at it that is working for you.
 
With the moon being almost full the gravatational pull could explain what has happened. Mr. Wizzard

I am full of crap this morning.

I have had the foil weld itself itself to the blades at that temp. That was a long time ago and can't remember what I had done but do remember I had a bunch of blades in the oven at that time, thinking maybe like 17.
 
Roger,
I have had this happen. A pin hole did it a few times.
The other times is was the folds. I fold three times also.
Less than three it seems to leak. Also certain stainless steel damscus will give you grief if you use paper in the pouch.
 
It was a dry day.

I only folded that one twice, which is unique for me. So far I have to lean toward that or the pin hole theory, or the moon phase.
 
Roger,
I don't want to throw anymore confusion into it, but, of the thousands and thousands of blades I've heat treated, I've never folded the edges but twice.

I've not had the problem you mentioned to that extent but sometimes on very dry, cold days I notice the packets do get snugger. Btw, I use a cigarette butt in each packet.

My heat treating ovens point North/South too ;)
 
Kit, it was cold and dry. I've never tried the cigarette butt yet. I sure have plenty of those.

Maybe it did get a tiny pin hole in it.

RL
 
Hey Kit,
I think the cig butt is the the trick on yours!
North South is a good thing if you using a live sheep to quench the blade!~:D
I was having a small decarb area on stainless damscus. The bag was sealed. I did a little experiment. I quit putting paper or anything in the bag. The problem went away! The spot didnt look like decarb . It was bright white looking.
 
Ok I am going on pure theory here as my fiscal situation does not allow for much practical experience. Thus my sig; I put time into learning the "book knowledge" from your posts and other sources on the internet.

Please bear with me for a while. I will try to explain this in a different, humorous way that I think we can all relate to.

I've Heard of Shrunken Heads, but Shrunken Pizza!?:
Once I was microwaving a piece of leftover pizza.
When put away, the piece had been tightly wrapped in traditional clear seal wrap stuff, often falsely called by the trademarked "saran wrap". Being lazy at the moment I did not unwrap it. I had not been keeping an eye on it as it heated up, and I probably set it for too long also. A few minutes after the timer ended, I went to retrieve my pizza. Not only was the pizza piece now shrink wrapped, it had actually been shrunken!
Keep in mind that a microwave works by making water molecules move faster bumping into other molecules more, which increases the temperature.

The Technical Explanation:
Keep in mind that a microwave works by making water molecules move faster bumping into other molecules more, which increases the temperature. What had happened is that live steam from was able to escape the heat softened wrapping. Regular, room temperature air would not have the energy to escape or enter the tight wrap. Eventually enough steam was escaping for the pressure inside the wrap to be the same as that outside, while at the same time being much less dense than the outside air because of its high temperature (this is the principle that makes hot air balloons work). Then the microwave turned off. No more thermal energy was being produced inside the pizza piece, producing steam. The superheated steam inside rapidly cooled, reverting to water vapor with a much lower volume and pressure, easily resealing the now highly elastic seal wrap. As the steam, which filled both the still soft wrap and the internal spaces of the softened pizza, continued to contract, the force of the partial vacuum squeezed everything together.

What Does Pizza Have to do with this!?:
The only differences between the wrapped pizza and the foil wrapped blade are the wrap materials, and the heated gasses involved and the wrapped object. These differences can be ignored: The blade is solid metal, rather shrinkable spongy pizza; the hot air and live steam behave in the same fashion as heated gasses; the foil, while not as elastic as the plastic wrap, still is malleable enough to conform to the blade.

Putting it All Together:
Despite the triple foil wrap, there is not a perfectly airtight seal. Short of welding a hermetic seal or some other excessive, brutish, and unnecessary method, you are not likely to get a perfect seal. During heating the hot air leaks its way out through the crimps just like with the pizza above. Then at the quench, as the air inside the foil wrap cools and contracts, air outside is unable to force its way in past the sealing fast enough, producing the vacuum. The vacuum forces also help seal the crimps further. The conformation of the foil that you see is also happening on a microscopic scale, with virtually no gasses to separate the foil and steel surface. This is what is causing the glue like stickiness. Remember that part of what makes glue work is that it fills in and (more relevant in this case) grips surface variations, one of the reasons you should scuff-sand smooth surfaces before gluing.

The Important Thing to Remember:
When this foil vacuum wrapping does not happen it means that the tightest possible airtight seal was not formed and maintained during heating and quenching.

Please tell me if I am making sense, or am I completely insane?

Edited for granmar and spelng :D
 
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