How to fix a damaged Heat Treat/Temper?

[Cyber_Demigod]

I don't currently have an example knife that I am working on to show you guys, but I have a question about how you would go about fixing a damaged heat treat/temper on a knife.

I am talking about blades that have been heated, people intentionally heat their knives often. I saw a young boy (about 10) heating his knife with a lighter once before sharpening, he said his reasoning was it made the steel softer and easier to sharpen. His knife had chips along the edge from this. I was a kid at the time too of course, but my father told me that child was ruining his knife.

There is a more common instance of knife heating however, with it I usually see lighter burns on one side of the knife, in a single concentrated spot.
This is from what is called "Knife hits" (I think, don't quote me on that) which is when drugs of a sort - usually Marijuana or something, are placed on a flat knife, and the underside of it is heated just below the drugs, a straw is used to inhale the vapors of the burning substance - sort of like how people use foil to do drugs, but with a knife instead....
It seems stupid to me, worse than using a knife as a screwdriver, prying tool, etc.... really makes me cringe when I see a knife with evidence of this. Although it is usually a kitchen knife or a knife with a large blade.
Knives get heated for many reasons other than drugs or lack of knowledge of course. Like falling into a fire or dropping it into a fire accidentally. So it is possible I will run into another knife that has been heated to a level that damaged it, in need of repair or replacement.

The reason I am talking about these types of damage is I am interested in how they are repaired, I was unable to help my friend some time ago and would like to be better equipped with the knowledge to answer a person with advice or repair.
The friend I mentioned had acquired a Spyderco Citadel which had intentional burns on it. He bought it from somebody on the cheap because of this, and it seemed to him like a deal he should not pass up. Do not remember the price, it was below 50 dollars though but it was a couple years back. He asked me to help him fix it up, as he was not comfortable working on anything automatic or spring assisted. I was able to clean it up and get the rust off of it, but I had no idea how to help him with the very likely damaged blade from frequent and high temperature burns applied to it at long times. I told him that the blade needed to be fixed by a professional, and that I was certainly not one such professional - and that I was unsure of Spyderco warranty or repair service policy on that knife.

Also, has anybody else seen this and fixed it or seen it fixed?
Is there any way to fix a blade with a damaged heat treat/temper, or are they a lost cause? I am curious so that in the future I can either fix knives like that my self after I learn how to do so, or send them to a person who can.
I have no experience with heat treating a knife and my knowledge is limited, so I figured I would ask here, and that the "Maintenance" part would qualify here.
Just trying to improve my knife repairing abilities a little, or at least learn something new.

Also, how hot does a knife have to get before a heat treat is ruined?
What is a safe level to heat a knife to?
I ask because I have heard that Frog Lube works better if the blade is heated before application. I would probably do that with a heat gun, but do not want to damage it so have not attempted it yet, would like to know the ideal temperature to do this at if anybody knows?

 

[eKretz]

If the knife is heated to a level that ruins the temper there's no cure but to have it heat treated over again. On most knives, 400° F or above will damage the temper. Certain harder knives, 350°F or less can do it.

 

[Cyber_Demigod]

Is there any way to test if the heat applied damaged it, without breaking it?

I could use some recommendations to good heat treating services - I know of a few in my area but nobody has told me if they are any good for this specific purpose.

I figured that was the only thing that could possibly repair it but I don't like to assume, because if I assumed I would just be one of those people who has no real idea what they are talking about, but say stuff as if they do. Which is why I tell people that , you know, hey I am making an educated guess, not a professional. Unless I am experienced and educated in the matter that is.

 

[bucketstove]

I doubt the damage goes too deep,
maybe if he empties an entire cigarette lighter fast enough ( 3 grams butane)
so I imagine solution is just keep sharpening to go past the damaged apex

 

[eKretz]

Watch for discoloration. If much more than a tiny bit is colored light brown, blue or purple it's going to require more than a bit of sharpening to put right. You'd be surprised how easy it is to trash a knife with something as dinky as a lighter with enough time - guys vaporizing their drugs repeatedly can easily destroy heat treat on a part of a blade.

Testing it would be difficult without a hardness tester. You could try running something like a pine stick uniformly down the edge and see if one area rolls before the rest of the edge as a sort of shadetree test I suppose.

I don't know of any heat treat places that specialize in knives, only industrial heat treaters, so I'm no help there.

 

[Bill DeShivs]

You would have to know the exact type of steel before you can heat treat.

 

[Bo T]

The modern alloy steels will require a complex and concise heat treatment. You would have to disassemble the knife first. Just given that, unless you are using it to help educate yourself. It probably isn't worth it. The temperature reached from heating the blade with a lighter will certainly soften the temper of many steels - Note Bill Deshivs post. The effect, as also noted, will not limit the blades functionality to any great extent. There is a lot of technical information on the net (always check multiple sources) regarding temper vs hardness for different steels at different temperatures. You can check the hardness with a file.