Wendell Bryenton,

Bob I got one from Niroc because I was having such a problem with warped thinner forged blades. I have watched a few videos on how it works, but a "how to hammer with some test examples" might be a video we should collaborate on. I know a few things:

1) I have been told by a few Mastersmith that hammering treated steel is never a good thing.

- That is countered by MS Murray Carter who is professionally Japanese trained. I have no idea, however, how much knowledge that goes into understanding the forging methods of cold steel and understanding the science that goes into it. Maybe that is why other MS have said don't do it?

2) In the videos everyone seems to show the technique starting with the "U" shaped warp facing upright. This is obviously antithetical to previous idea of blade straightening with the "U" shape warp facing downward, in order to push the high center down. 

3) What seems to be the best advice thus far is taking a series of very thin metal shims and a completely flat metal plate or anvil and testing where the air gaps are. After you have located the air gaps, mark each side of the bottom of the parabola or "u" shape and then hammer in the middle, (hopefully the center), of that U shape. Now  I have seen people hammering randomly, or people hammering in a pattern moving toward the point from the Ricasso, and people hammering from the spine toward the bevel. Maybe this is just innate understanding to other folks, but I found it hard to understand and some of my knives have warped in other directions. 

Is it better to just anneal, restraighten the blade, and re-quench? How much time/value do we save by using the carbide hammer, and does that outweigh the risks of potential stress fractures by hammering cold and treated steel?

These are like black magic, the should not work as well as they do. especially on fussy stuff like stainless. they are one of the best tools I have added to my tool box or tricks in the last several years. 

I always to an additional temper after, but more than one guy I trust has told me it isn't needed.. it can't hurt though. 

These are not "bending" they rather they stretch the surface of the inside of the curve pulling it straight. that is why you work the inside of the curve.. what i have found it a lot of light taps is best. You're not trying to "forge it" straight just stretch the surface a small amount. I made mine and I think it weights around 4-5 OZ and I used a 1/2 carbide ball. i really just let eh hammer do the work. light taps just bouncing the hammer off the work.  you really NEED to make sure the opposing side is supported on the anvil.  

That said most of the time I flex things straight in the temper , either with shims or in a three point jig ( i have low temp salts that make this way easier) . Where the hammers really shine is when an edge is rippled or warped and the spine is straight. The fact that you can Isolate areas to straighten is so awesome. 

when to anneal and straighten VS temper or hammer straighten, is really depended on a number of factors. how thing the blade is how its warped and most importantly WHY it warped. 

ALL warps are caused by one of three things uneven structure/lattice structure (IE bad normalizing), (also areas of decarb can cause change in structure) uneven heating. or uneven cooling. In the case of the first Normalizing and re heatreating can solve the issu. in the case of the latter two we need to examine why. If the uneven heating or cooling is due to crosection then re heat treating is pointless. was one side hotter because the burner was hitting it? did one side cool faster because the blade went in the quench at an angle. all super common causes. 

When doing swords they will bend under their own weight when hardening,  the long edges are all but impossible to keep perfectly even and perfectly symmetrical.  Swords always warp (to the point when I get one that doesn't i freak out!!) I start by interrupting The quench at around 600 degs. then clamping between boards, that will generally get the worst of it.  after the first temper if they are still not straight Ill either flex at tempering temp (if it is a gentle curve or bow) or use a 3 point (if it is a kink or short bend.)  I'll straighten , put it back in temper. straighten back in temper etc . once I pull it out the oven and its still straight Ill let it cool and start grinding. sometimes grinding can induce a warp in that case I'll use the low temp salts to heat and straighten the warp. 
 doing an few swords will make doing a smaller blade seem easy. 

MP