Trouble with warping blades

O

oklasam

Joined
Nov 13, 2013
Messages
5
Hey folks;

First I want to thank you all for the abundant information and inspiration I've gleaned from this website in the past...Really makes the workday drag on in the afternoon when I make the mistake of browsing the site during my lunch break.

I've run into a slight problem heat-treating blades recently that some of you might have some insight on...I'm working on several blades that are thinner than what I have typically made in the past (mostly larger camp knives/choppers), including a boning knife about 7" blade length. After grinding and hand-rubbing the forged blade to 150-grit finish, I proceed to my heat treatment; first stamping my initials lightly along the spine (which always curves the blade a bit) and correcting that curve, then normalizing. During the air-cooling normalizing steps, I always hold the blade edge-down or edge-up, never sideways, but have been experiencing major issues with more or less localized warping. I tried skipping the normalizing step and going right to the quench, and then massaged the blade straight again with some light hammering on my anvil face. But after it came out of the soak in my oven (375* for an hour), lo and behold, the warps had reappeared. I have repeated this whole sequence twice and hesitate to try again for fear of adding yet more scale I'll have to hand-rub out and reducing the carbon content of the blade below what's acceptable. I have left the microbevel unground (still just under 1/10 cm thick) so that I wouldn't get warpage from it being overly-thin. Does anyone have any ideas as to why this blade might be warping so much? The warp isn't localized to one spot of the blade, but seems to happen in different spots each time.

The steel is from a railroad anchor, but I had similar issues with 5160 on a large chopper in the past (though less pronounced, perhaps because of the thickness of that knife's spine). I’m quenching vertically in some oil I purchased from a knife site awhile back, after heating the oil (the brand escapes me at the moment, but I suspect the quenchant isn’t the issue, since I’m getting warping during normalizing before the blade ever contacts the quenchant).

My forge is a small round cavity carved inside two refractory firebricks wired together, with a MAPP torch firing in from the side at an angle. I am careful to heat primarily along the spine of the blade so as not to burn the edge, although the heat is still primarily coming from one side of the blade. However, I suspect almost any gas forge will have some directionality to the flame, relative to the blades. I recently moved to a new house and am working inside a garage (with open doors), rather than the covered porch I used to be on. I can’t figure out any other reason for this persistent problem…Perhaps I’m just not praying to the right gods!

Thanks in advance for your time and any insight you may have.

Sam Martin
 
There is a lot of possible reasons but one common cause is asymmetrical grinds, especially a spine that is canted along the top. Mystery steel could have spots of who knows what lurking beneath the surface and taking the heat differently. Feathering it in and out of your forge (even heating) is important for your thermocycle quality. Skipping those grain refining cycles will not only produce a lower quality heat treat but it can contribute to warpage in the quench. You do get a chance to straighten it directly after quenching, for a few seconds in the martensite start formation. After you pull it out of the oil and its still smoking off a little, wipe it with a rag and check it for straightness and bend it on the flat against your anvil by hand if it needs it. Go easy, it's really soft. Also don't hang it to cool with any breeze from the doors on it, that will warp it every time.

There's makers here a lot more qualified to help you out than me, although lately I've felt like the queen of the warped blades. Especially with integrals.

-HD
 
Thanks for the response, DesRosiers;

I will have to re-check to make sure the spine is perfectly straight; I did get the blade straight after the quench against my anvil, but then during its tempering soak in the oven it became warped again.

Sam
 
This has worked for me on slight warping: After quenching and leaving the blade in the quenchant for about 8 seconds, take the blade out. Use gloves as the blade will still be hot, and correct any warping. As the blade begins to cool it will become less malleable, and you should stop correcting the warping immediately as it will be brittle.

Have all of the equipment you need to straighten on hand before you quench your knife, as you won't have much time to correct the warp before it becomes brittle. For this reason, I would hesitate to hit it with a hammer. Try using a camp and shims to bend it back to shape.

You could also use a clamp and shims to bend it back straight during the temper. Temper it once before shimming though, as it will be brittle before temper, and you could crack the blade by bending it straight.
 
I just had another thought... How are you tempering? Toaster oven? Kitchen oven? Are you using an oven thermometer?

If you put your blade in your temper oven while it is heating up, your knife can become overheated. The coils on the oven can heat your blade above the ambient temperature of the oven itself.
 
Shane,

Thanks for your input too...I am tempering in the kitchen oven, and do have a thermometer in there, although the last couple of heats I've put in the knife while the oven was heating. I'll avoid that in the future, thanks!
 
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