Heating up knives a la "Seraphim Falls"

[GrReaper]

So I was watching the movie Seraphim Falls the other day. Like many westerns, it has a scene where a guy has to heat up his knife on the fire to cauterize a gunshot wound. Obviously that was a movie and it was all made up. But if you really had to do this, doesn't this ruin the temper of the knife? Will it be unusable afterwards, or can it be re-tempered properly and be good as new?

 

[NJBillK]

So I was watching the movie Seraphim Falls the other day. Like many westerns, it has a scene where a guy has to heat up his knife on the fire to cauterize a gunshot wound. Obviously that was a movie and it was all made up. But if you really had to do this, doesn't this ruin the temper of the knife? Will it be unusable afterwards, or can it be re-tempered properly and be good as new?

The only need to "re-temper" a blade would be to lower the hardness from the austinitizing step of a heat treat process. That would be the heating to required temperature, soaking (holding) at said temperature if needed, then quenching the knife. If this is done correctly, you will have a hardened blade, which will be very brittle. Tempering would be done to reduce the hardness and improve the toughness.

The heat treatment process can be finicky or straight forward. Likely, given the time frame in which the movie was staged, and the environment it was filmed in, a basic carbon steel would be likely used in the making of the knife. So heat treatment should be fairly easy, even for those times.

Due to re-heat treat process, you will likely have some decarb (burning out of some of the carbon on the outer skin), which would need to be removed, or at the very least, sharpened through.

This all being said, if you alter the temper, you can only soften the blade, making it tougher, but holding worse of an edge. To "fix" the damage done to cauterize a wound, you would have to completely redo the heat treat process.

 

[GrReaper]

This all being said, if you alter the temper, you can only soften the blade, making it tougher, but holding worse of an edge. To "fix" the damage done to cauterize a wound, you would have to completely redo the heat treat process.

Not being a knifemaker, I think I mixed up the terms temper and heat treat. But thanks, that's what I wanted to know.

 

[Man with no name]

That was a great film. Watched it several times on cable the past few months. That Bowie played a prominent role to the survival of Pierce Brosnan's character. Still wondering about Madame Louise selling her elixer in the desert scene. If I didn't know any better I think she was the Devil in disguise.

 

[I'mSoSharp]

So I was watching the movie Seraphim falls.
But if you really had to do this, doesn't this ruin the temper of the knife? Will it be unusable afterwards, or can it be re-tempered properly and be good as new?

Cool film .

Difficult to say how hot the blade got in the film.......... I'd guess it would have softened it but it would still be useable, I imagine in times gone by many knives were quite soft.

Basically (many variations) a blade is quenched to make it fully hard, then tempered at a temperature +/- 400f to soften it to make it less brittle & useable.

Any temperature over the original tempering temperature will soften it more, any redness is way over the tempering temperature & will make it fully soft.

It can be re-tempered but only after being hardened again, that means quenched again from (generally a bit over) cherry red then baked at around +/- 400f to temper.

Obviously this means a bare blade with no handle unless it's made from asbestos

 

[NJBillK]

Any temperature over the original tempering temperature will soften it more, any redness is way over the tempering temperature & will make it fully soft.

This isn't entirely accurate, as any significant increase in temperature (say over 200°-250°) would be enough to damage the temper.

How temper works is the temperature affects the amount of hardness drawn from the blade, but the variable that changes is the time it takes. Higher temperature = a faster drop, but also can increase grain growth if the temperaturegets too high.

It can be re-tempered but only after being hardened again, that means quenched again from (generally a bit over) cherry red then baked at around +/- 400f to temper.

The part that I am addressing here is the "cherry red" part. It is Very hard to tell temperature by color, and different alloys need different temperatures (and a soak at said temp). Some alloys can be brought to a a shade or two over "cherry red" then quenched, but some need to be brought much higher (dull orange) and held within a stainless pouch.
"Cherry red" is around 1400° and 1084 (a simple carbon steel) is to be heated to around 1500°, which is a shade or two lighter. Just hitting 1400° won't bring you over the Curie point, and wouldn't be non-magnetic yet, which is still too low to harden even a 1084 blade.

A better judge for 1084 would be to use Kosher salt as it melts at 1475°. Once the salt melts, go a shade brighter and quench.

After that, your tempering should be at least two cycles (one hour per) at 400°-425°, but three cycles is preferred. Temp variance above would alter the finished "as tempered" hardness.

 

[Wowbagger]

This isn't entirely accurate, as any significant increase in temperature (say over 200°-250°) would be enough to damage the temper.

Totally this ^
Depends a little on the alloy though . . . if you think about it High Speed steel can take some high temp . . . some of them retain hardness at red heat and won't anneal once hardened.
That's splitting hairs though.

Fortunately the guys here have, by comity , figured out the best way to sanitize your knife : lick the blade and then wipe it under your arm. Done. Germ free and ready for anything.

 

[herisson]

I always thought he ruined the temper of the knife and probably damaged the handle. The knife may still be usable afterwards but this kind of operation definitely destroys the original knife. If the steel isn't overly soft, it may have "air hardened" = too hard / brittle or just lost his hardness. Now, he did what he had to do. The integrity of the knife wasn't his first concern at the time. On this subject, in "All the pretty horses", the main character (Matt Damon) disassembles his revolver and heats the barrel in the fire to burn out a wound. I certainly wouldn't shoot that gun afterwards (and he doesn't, if I remember well).

 

[GrReaper]

That was a great film. Watched it several times on cable the past few months. That Bowie played a prominent role to the survival of Pierce Brosnan's character. Still wondering about Madame Louise selling her elixer in the desert scene. If I didn't know any better I think she was the Devil in disguise.

Yes she was the Devil, trying to get them to kill each other.

 

[LastRodeo]

If I’m in a position where I have to cauterize a gunshot wound, whether my knife may lose it temper is the least of my problems.

 

[dirc]

I don't think it would really have to wreck a blade's heat treatment... you could cauterize a wound with 100+ C, (or 212+ F)... putting a blade in a fire for a few minutes should do it, and I doubt it would go to a much higher temperature. It really would most likely act as a low temperature temper, I expect. So maybe a tiny reduction in hardness?

I don't have first hand experience with testing this, does anyone?