How hot can a blade get before the heat treat is ruined?

[stonproject]

My friend has a camping knife hes been using as a fire poker for years, and it got me thinking, how to can a blade get before the heat treat is affected/altered, or ruined?

Will different steels and alloys have the heat treat get ruined at different temperatures? also does the specific heat treat itself come into play here?

I havent found an exact answer using the search function, so bear with me if this is redundant.

Thanks guys.

 

[RX-79G]

It depends. Some steels are tempered at 350F, so going above that will change the temper, softening the blade somewhat.

Others are tempered closer to 1000F.

 

[stonproject]

Well, which steels are tempered at 350? which are tempered close to 1000? which are in between?

When you say itll make the blade somewhat softer, how long does the blade have to be hot to actually affect the heat treat or is it instantanious?

what happens if you make the spine softer but do not alter the cutting edge itself? will there be a noticable change in use?

 

[chiral.grolim]

Different steels are tempered at different temperatures. Tempering brings a specific area of the blade to a specific temperature and holds it there for a set period of time. The changes that occur through tempering require both temperature and time to occur, how much time depends on how much material needs to be affected - larger/thicker amounts of material take longer to reach temperature than thinner/smaller sections. For very thin sections like the very apex, the affect can occur quite quickly when temperature is reached. Average campfire temps can reach ~1500 F, easily enough to soften the edge, but how far into the edge depends on the thickness of the material and the amount of time it is in the fire. Poking a knife in and out of the fire will heat the blade but shouldn't affect the temper of more than the very edge. The edge may soften slightly such that it deforms a little more easily when cutting hard material - you may not notice a problem if you never cut anything that exceeds the threshold. Grinding away the softened edge will present stronger material at the apex. It's his knife, he can use it how he wants, especially if he never experiences a problem as a result of that use.