Straightening thin kitchen knives after HT and final grinding

nickandersonart

nickandersonart

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I just made a kitchen knife (Gyuto) from a steel in the W1 family (spec SK4) and decided to go quite thin on this one. After hardening (1500F into Houghton's K and temper at 400F) I took it down to about 1.8mm (.070) at the spine above the heel.

As I was doing the post-HT grinding the blade began to warp repeatedly on each side...in fact I continuously "countered" the previous warping by flipping to the other side and grinding until it warped a bit in the other direction.

In the end, using this technique I was able to get it almost straight but it now has just the slightest bit of warp to it that I couldn't get fully out. My question is, what's the best way to fix this?

I've previously seen folks recommend bead blasting one side of the blade, or shimming/clamping and tempering again. I don't have a bead blaster and I'm a bit worried that the fine edge will overheat quickly if I try to temper again...thanks for any advice :)
 
Are you sure you weren't bending it because you were pressing too hard while grinding such a thin blade? Is it through hardened?


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I wouldn't have been pressing hard enough to bend it, and as to hardening, yes hardened all the way through except for around the spine which had clay on it for hamon
 
That's odd...I'm stumped! Maybe someone with more experience will shed some light on this. You could also use a 3 pin bending jig in your vice and carefully straighten it during tempering.


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That is true and definitely can work, but considering the thickness of the steel (thin) and the high carbon content I'm afraid to try that method first as I've cracked blades with that method in the past. The method had to be done while the steel is hot and a thin blade like this just cools so quickly, the window for being able to bend would probably just be a few seconds. I'll have to resort to it if there isn't a safer method!
 
Thinner blades can still bend after HT, and note that this isn't bending because of flexing, it's from grinding.

Ive been very careful to make sure that the blade doesn't overheat enough to get into the tempering ranges. It could still be, however, warpage due to some of the heat caused by grinding?
 
I just had a conversation about this with Brad at Peters. Ive been having fits with a certain stubborn 250mm W2 blade. He told me to properly straighten it with tempering and clamping will sacrifice a few RC points. So that Isnt really an option if you are going for a high RC. He suggested I can use a torch to carefully heat a little on the spine and try and bend the warp a bit in between an opened vise. I still have not tried this yet but It sounds like it could work with some practice and being careful not to heat the edge at all.
 
Why would an additional tempering cycle overheat your edge? What does it matter how thin the edge is if you control the temperature of your oven?
 
Because of the temperature swings within the oven -- ideally with proper pre-heating and added thermal mass the oven shouldn't fluctuate too drastically but an edge that is .0005 thick is going to match that heat almost instantly. I guess this is where stacy's "box" method with thick steel plate above and below the blade itself could be invaluable.

Timos, yea I'd also like to avoid any additional loss in hardness. I've tried using a butane torch a little bit already but no dice...maybe I should heat a larger area and see how that goes. I'm guessing that the best approach would be to heat the general area, then just flex it with gloved hands against a flat surface? The 3-point vise system scares me a bit for thin blades
 
It was because you would need to go to a higher temperature than desired in order for the steel to take a set. In my experience I have not been able to get any W2 blades to straighten If they warp during finish grinding. At least not with a 350 deg. temper.
 
Good suggestion and one I may use if I can't do it in-house, though I'd really like to find a solution with available tools that I have since this is something that will happen on most thinner blades.
 
nickandersonart said:
Good suggestion and one I may use if I can't do it in-house, though I'd really like to find a solution with available tools that I have since this is something that will happen on most thinner blades.
Click to expand...

Luckily I have one at work. The first time I tried it I went into it knowing it wasn't going to work and shockingly it worked perfect. It was easy and quick. I broke a few before trying this and it amazes me that if all the research I've done I haven't ran across it. Not saying it isn't out there in the world of the internet just that I hadn't seen it.
 
Clamp the bottom half of the blade including the tip between plates, steel or aluminum and sping temper the spine. If it is clamped flat when you are heating it, it should stay flat once cool. The plates will keep the heat from traveling to the edge or tip.

I use a similar method for straightening the tangs/handles on my full tang tomahawks.

And for tempering thin stuff in the oven, you can put the knife, at least the edge, in wet sand while tempering for even heating.
 
kuraki said:
Why would an additional tempering cycle overheat your edge? What does it matter how thin the edge is if you control the temperature of your oven?
Click to expand...
Usually it takes at least 400F to correct a warp. If you're aim is a very high RC number, then 400F is going to take few points away from you. Usually 400F puts me in the 63HRC range, low carbon alloy steels. If I want 64-65, any straightening will need to be done Ms-Mf.

I have successfully straightened a blade at 375F, done with a water quench AS it was still clamped up.
 
Cool method there BenR.T. I'll do that as my backup method if the tempering method is unsuccessful. And good call with the sand! Only thing - does it need to be wet? Seems like the thermal mass of dry sand would already be enough...unless you want to keep the blade edge from reaching the tempering temp entirely. Hmm [emoji848]

Samuraistuart, I actually tempered at 400 the first two times so I think I'll be at the same RC if I do it again at 400...Your suggestion sounds promising, a 3rd temper at that exact temperature with shim and clamping and then water quench the clamped rig. Main thing is just to not overheat that thin edge so I may also try putting it in a tray of sand.

As an aside, on future kitchen knives I may start tempering a bit lower as well maybe in the 375 range...seems like a handful of people are using that temp for kitchen knives with good results.
 
And Weo thanks for the link...I had intended to watch that video previously and never got around to it!

So it seems that there are quite a range of techniques people are using to straighten after hardening...

-Lightly peening concave side of blade with small-faced peen hammer
-Using Brass hammer on wooden stump
-Tempering while shimmed and clamped (with optional quench of entire rig in water)
-Bead blasting concave side
-Clamping straight and spring tempering spine
-Heating and using straightening jig handles (I say yikes)
 
Nick , thanks for starting this thread! I will have to try that peening method, it sounds too good to be true.
 
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