I forged a blade, hardened it and tempered it for two 1hr cycles @ 400 deg in my wife's toaster oven. After tempering I noticed that it had a slight warp in it and tried to straighten it. You guessed it, pop! 

  So I believe that 400 is not hot enough for this type of steel, or the temperature setting is off. I think I will do some experimenting with the pieces and try some higher temps to see if I can figure out what temp will work for this steel. The steel is from a large carbide tipped saw mill blade.

  Also how do you guys straighten a slightly warped blade after hardening?

I generally reheat, straighten; reheat, normalize; reheat, quench then directly into the tempering.

No, it's not the temperature you drew it to, it's trying to straighten it after you hardened it. Did it warp drawing the temper? If not you maybe should look closer after you harden next time.

Frosty The Lucky.

Thanks Thomas, I thought of that after it broke.

Frosty, I am not sure, it was a very slight warp, I did not see it until I started to finish grind it. I probably could have removed it in the grind.

When you pull your blade out of the quench, if you are quick and easy on your pressure you should be able to straighten it as long as it is up above 450 F or so. I have also had success using clamps along the warp during temper when setting it on a true flat before starting.

I've had some great success with clamping it to a straight edge, like apiece of angle iron, while tempering.  I don't get warps often (Proper normalizing before heat treat will help a lot) but when they do show up, they have always straightened out after two 1-hour tempering cycles.  Just make sure the straight edge you use is truly straight, and make sure your clamps don't have any plastic pieces that will melt in the oven.

11 minutes ago, HEAP of JEEP said:

I've had some great success with clamping it to a straight edge, like apiece of angle iron, while tempering.  

So if the blade warps during hardening, you clamp it flat to the straight edge with all-metal clamps and temper as usual? How much warp can you eliminate this way?

7 minutes ago, JHCC said:

So if the blade warps during hardening, you clamp it flat to the straight edge with all-metal clamps and temper as usual? How much warp can you eliminate this way?

I'd say a fair amount, but impossible to give you a number.  When I do get a warp, I don't measure, but have more of a naturally occurring scale... like, "Eh, that's not horrible"... all the way up to "Holy ______... how the ______ did that happen?!?!?!"  I've been using this process for the last 18 months or so, and its taken care of the handful of blades that I've had warp in that time... but I don't recall any that were all that bad, so hard for me to say.

The recommendation I've seen is first temper just temper, clamp for second temper and do subsequent tempering runs for straightening if required. There is also the window of opportunity right after the quench to straighten before it drops below 400, but that really only comes into play for blatantly obvious warps.

Thanks for the info to all.

The warp was so slight that I didn't see it until I started to clean it up, so I could have straightened it by clamping it to an angle iron and doing another tempering cycle or two. Like I said, a good lesson.

1 hour ago, HEAP of JEEP said:

I'd say a fair amount, but impossible to give you a number.    I've been using this process for the last 18 months or so, and its taken care of the handful of blades that I've had warp in that time... but I don't recall any that were all that bad, so hard for me to say.

Good to know. I've got a blade with some twist to it that I might try this on, especially since I recently came into possession of some hefty angle iron.