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Hey guys,

I'm working on a little 5160 blade, got it heat treated. I was testing it by cutting into a bit of brass rod, the lower 3/4 of the edge is holding up great, but the part towards the tip is taking some dents. Naturally I'm assuming that the tip got hotter than the rest of it, since the soft part starts right where the distal taper makes the blade pretty thin. I took care to heat the blade evenly, but who knows, I'm not very experienced in heat treating.

Question is, could overheating potentially make the edge soft? I would've thought the opposite, that it would be too hard and brittle, if anything. 

Thanks, 

 

Alex

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Dunno if it's necessary, but my HT is as follows:

 

Thermal cycle 3 times- just above nonmagnetic, right to NM, and just below, cool to black each time

 

Heat using the "dragons breath" at the mouth of the forge, to just above NM, edge quench in canola oil, which was approximately 130 F.

 

Temper, 2 one-hour cycles at 400F.

 

I noticed a bend after tempering, so I clamped and shimmed the blade with a slight counterbend, and did 2 more short cycles at 400. Probably 20 min each, and quenched from tempering heat in water. Took care of the bend nicely.

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My guess is that "just above non-magnetic" was not hot enough to harden the thin part.  5160 will sort of harden kind of okay from 1475 (nonmagnetic is 1425-sh), but it really prefers to get to 1550 or so.  If the tip was already falling from 1500 or so before it hit the oil it may not have had enough heat in it to fully harden, i.e. the crystal structure was not fully transformed and you didn't get martensite.  Or what little martensite there was isn't enough.  

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39 minutes ago, Alan Longmire said:

My guess is that "just above non-magnetic" was not hot enough to harden the thin part.  5160 will sort of harden kind of okay from 1475 (nonmagnetic is 1425-sh), but it really prefers to get to 1550 or so.  If the tip was already falling from 1500 or so before it hit the oil it may not have had enough heat in it to fully harden, i.e. the crystal structure was not fully transformed and you didn't get martensite.  Or what little martensite there was isn't enough.  

Ahh so since the tip cools faster, it was too cool by the time it hit the quench. That makes sense, thank you.

I'll try bumping up the temp a bit, and getting it into the oil faster next time.

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I'm betting Alan guessed the problem, but I'll just add that the tip is also easiest to over heat (over temper) when grinding post heat treat.  Not an issue if not using power tools.  

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