Knife bending - feedback?

[OneStaple]

Hey all,

I'm trying to straighten a knife (240mm Old Hickory, which should be 1095 steel, if that matters). I've tried a few approaches and can't seem to get it straight. Every time, it just springs back to its original shape. I'm nervous to bend it further than I am and would appreciate feedback regarding whether I'm on the right path.

I've predominantly tried with a vise (yes, I know the included pictures are not in a vise), using three dowels. I have done this with the knife at room temperature and heated to 250F. I have done it multiple times, increasing pressure each time.

The first pictures below show approximately how far I have been bending it (and I have even gone a little further). Is this a "normal" amount and is it safe to go further? The last picture attempts to capture the curve of the blade.

(Pictures on my phone, so they will follow in the next post)

Thanks,
Tyler

 

[OneStaple]

 

[Matus]

Well, this is one of the properties of monosteel knives. If this approach has not worked, than you may be left with removing the handle, bring the blade to some 150 - 200 Celsius and do the same as you did above while the blade is hot. Should it work, than you will need to make a new handle. Whether that is worth the effort is up to you to decide.

 

[RDalman]

Yes upwards 200c (390f) should be the heat to do it at. But still it's really difficult and you almost have to break a few knives this way to learn the breaking point.

 

[OneStaple]

I'm planning to convert this to a wa handle anyway, so ruining the current handle is not a concern.

So, basically, heat it a lot hotter than I had been doing. Is there any concern that temperatures of 200C are starting to get into the tempering ranges for 1095 (at least for high hardness)? My understanding is that these knives are likely in the 59HRC range, which would correspond to a tempering temperature of about 260C.

Hmmm, I don't like the concept of breaking knives, but at least these are relatively cheap, which is the point. I got a few of these to play with as a learning experience with grinding/shaping/etc. before I jump into making my own knives from bar stock (don't have room for a forge, unfortunately).

Once I heat the knife, is the bend angle shown in the right ballpark? I certainly plan to try multiple times, gradually increasing the angle until I get a response.

Thanks!
Tyler

 

[RDalman]

The goal is to heat it to as close as it's tempering temperature as one can guesstimate, hotter-safer. 260 for 59 would be if they got it max hardness as quenched, and I wouldnt have bet on that, so safer to stay at 200.

Cant say based on pictured bend, depends on pin spacing too how much pressure gets applied in the bent area.

 

[milkbaby]

If you remove the wooden handle scales, you can do it in an oven. Clamp the knife with counterbend and heat to the desired temperature for like an hour. For example, the larger knife below came out warped after heat treat. I clamped it with counterbend during tempering in the oven. I use coins under the portion where I want to counterbend. Basically you can heat up to the temperature it was originally tempered at without much adverse effect. In the pic, the knife is just clamped to an old useless file, but it's better to use something stiffer like angle iron (I've just been too lazy to buy and cut some up).

Some people swear by the three point clamping while cold, but I feel the risk of breaking is much higher. You can do it in the oven multiple times if you find it's straightening but you didn't put enough counterbend at first.

Edited to add: Let your oven equilibrate to a steady temperature first to avoid a high temperature swing. I store my cast iron pans in the oven and use that as a thermal sink to help level off the temperature swings. Use a good oven thermometer to measure the temperature too instead of relying on the oven setting which is often wrong.

 

[Kippington]

Use a good oven thermometer to measure the temperature too instead of relying on the oven setting which is often wrong.

100% this. My home oven hits 200 degrees C when I set it to 170. This kind of thing is normal in domestic appliances.

 

[OneStaple]

Ok, some updates. I've tried a few things but am yet to be successful. Attempts (roughly in order, if I can remember). Oh, and Kippington and milkbaby - I had previously checked my oven for some temperature sensitive baking with a thermometer (and I verified while working with the knife). It is surprisingly very accurate. Certainly worth checking though.

1. Using some angle iron, used milkbaby's method in the oven at 410F. It only lasted about 20 minutes before it started to smell funny, as I hadn't removed the wood handle. The wood (hickory, I assume) turned fairly dark and some "gunk" oozed out between the handle and tang, so there must have been some glue in there. Knife was not any straighter.

2. Cut the pins on the handle and removed the handle. Tried the oven again, but again got a funny smell after 20 minutes. I think it was due to some paint on the angle iron, so I took it out. Again, no change.

3. Put the knife in the oven without anything to force a bend. Soaked at about 410F for 15 minutes, then ran (ok, walked briskly/carefully) the knife to the vise to use the three-pin method. I bent it fairly far, to the point that I was getting uncomfortable. Further than in the pictures at the beginning of the thread. Again, no change.

4. Back to bending in the oven, but I replaced the angle iron with paint with another piece of metal. This time, 410 for 60+ minutes (bent using a coin). I let it cool while still in the bent position. Again, no change to the bend of the knife. However, the full blade turned straw colored, as did the coin (American nickel). The color is in line with tempering color charts for about 400F that I see online. The tang on the handle did not change color though (I had previously sanded that area with 80-grit to remove rust/residue, if that matters).

Soooo, any further thoughts? Up the temperature? Try bending further with either method? Should I be concerned with the color change?

Thanks,
Tyler

 

[Kippington]

Bending further should work. For reference, I bend warps a lot more than milkbaby's picture above, albeit at an earlier stage of the process where the knife is thicker.
I put less pressure on it when it's still cold, and more as it comes up to temperature.

I've used enough pressure to break a small C-clamp in half:

Sometimes the knives break too. It's risky.

Don't go any higher in temps than you've been going, you'll change the hardness to much. It takes time, I sit it in the oven for an hour or two then turn the thing off and leave the knife in there as it cools, all while the blade is under tension.

 

[danielomalley]

I do a lot of knife straightening while sharpening knives. To be frank, I don't know what so many folks do to bend their knives, but I probably straighten 10-15 knives per day ... many of which are _very_ bent. It's true that san mai knives are inherently much easier to straighten (and to cause a bend), but monosteel knives can also generally be easily straightened.

Over the years, I've found that techniques that involve slow bending back are much more likely to cause a knife to crack. Instead, I have great luck with 'cold forging' the blade to straightness. I put the knife on my anvil and lightly tap it to true. The only downside to the approach is that you will sometimes be able to see very slight hammer blows on the surface if the knife (this will be visible afterwards about 1/2 of the time). I've tried a brass hammer to alleviate this, but oddly, the knife won't come to true with a brass hammer.

If you don't have an anvil and you want to send it to me, I'd be happy to straighten it for you. You can pm me.

Yours,

Daniel
Epicurean Edge

 

[milkbaby]

I agree with Kippington that there is always a risk of breaking when fixing warps by bending. That's just life.

Also, Daniel's method can work well too, though I personally never used on an already finished knife for the reason he mentioned about marking the surface with hammer marks. Here is a video that shows it:

 

[zetieum]

If you like the knife a lot, I would recommend sending it to a knife maker who will be able to redo the HT, make it it straight, refurbish it, and put you a new handle if you ask for it. I think, I know an excellent maker who will accept doing this in Europe. But since you are writing temperature in the so-widely-used °F, you may be located in USA.

 

[OneStaple]

Ok, another try. Again, at 410F (210C, just for you, zetieum!). In the oven for 2 hours with it on, then allowed to cool completely in the oven. I forced a larger bend than previously, with the bend increased at the 30 minute mark after it had warmed up a bit. Result: I think it's a little straighter, maybe, perhaps, I think, but certainly not quite there yet.

I might again tomorrow with more of a bend.

Over the years, I've found that techniques that involve slow bending back are much more likely to cause a knife to crack. Instead, I have great luck with 'cold forging' the blade to straightness. I put the knife on my anvil and lightly tap it to true. The only downside to the approach is that you will sometimes be able to see very slight hammer blows on the surface if the knife (this will be visible afterwards about 1/2 of the time). I've tried a brass hammer to alleviate this, but oddly, the knife won't come to true with a brass hammer.

If you don't have an anvil and you want to send it to me, I'd be happy to straighten it for you. You can pm me.

Daniel, that's a really nice offer, and I appreciate it. If this were a more expensive knife that I cared a lot about, I might take you up on it, but this knife is really meant to be a learning experience for me. So I plan on beating it into submission, even if it breaks. I don't have an anvil, but might try to rig something up and give it a try. I plan to thin/grind the blade a bit, so I'm not that worried about marks.

Also, Daniel's method can work well too, though I personally never used on an already finished knife for the reason he mentioned about marking the surface with hammer marks. Here is a video that shows it:

Good video, milkbaby. I can definitely see how it wouldn't be the method of choice for all straightenings. I think I'd now like to learn both by hammering and bending. Is there a special name for that hammer?

If you like the knife a lot, I would recommend sending it to a knife maker who will be able to redo the HT, make it it straight, refurbish it, and put you a new handle if you ask for it. I think, I know an excellent maker who will accept doing this in Europe. But since you are writing temperature in the so-widely-used °F, you may be located in USA.

Nope, definitely not a knife worth going to all that effort for. As mentioned above, it's meant to be a learning experience. And it's not an expensive knife.

Thanks!
Tyler

 

[JBroida]

I’m with Daniel on this… I straighten place every day, and I’ve never once needed to heat up a plate to straighten it. We straighten using sticks, hammering, and even chisel hammer is in some extreme cases. What you see in Japan is pretty much what you see me doing. This whole thing of heating up is really weird in me out to be honest.

 

[The Anti-Chrysler]

I’m with Daniel on this… I straighten place every day, and I’ve never once needed to heat up a plate to straighten it. We straighten using sticks, hammering, and even chisel hammer is in some extreme cases. What you see in Japan is pretty much what you see me doing. This whole thing of heating up is really weird in me out to be honest.

I was gonna ask him if he tried smacking it with a hammer.

 

[Kippington]

I’m with Daniel on this… I straighten place every day, and I’ve never once needed to heat up a plate to straighten it. We straighten using sticks, hammering, and even chisel hammer is in some extreme cases. What you see in Japan is pretty much what you see me doing. This whole thing of heating up is really weird in me out to be honest.

I'd agree with you when straightening knives with cladding/san-mai, but do you use the same methods on a high hardness honyaki or monosteel? For these I've tried everything you've mentioned so far (except the chisel hammer, I should give that a shot sometime) and each time the blade breaks just as it starts to bend... these methods all being a form of plastic deformation. The only way I can get around it is by either tempering higher to increase toughness (which I don't want to do) or by using creep deformation to do the straightening, as milkbaby, Dalman and myself have suggested: https://en.wikipedia.org/wiki/Creep_(deformation)

 

[JBroida]

yes to monosteel, no to honyaki. You can do some of this with honyaki, but there is always a chance of breaking (please keep in mind that my experience with this is limited mostly to Japanese blades). Also, the chisel hammer thing is quite tricky to learn how to do well... be prepared to break a few blades. And the hammering is very different from normal hammering in terms of how and where you strike to achieve results. The whole re-temperthing thing seems to defeat the purpose of the blade being made that way though, no?

 

[Kippington]

Well re-tempering at the same temperature shouldn't have much of an effect on hardness and toughness... but it's all theory. I've seen some videos of some wacky tempering methods done by the Japanese guys, so I really would have no idea how to follow the same exact tempering.

This one is quite amazing... Hardening and tempering at 2:35 into the video.

Those glowing hot tempering tongs at 3:08 are something I don't think I'd ever see outside of Japan haha!
Maybe I should do the same thing, I sometimes get hairline cracks in that exact same spot where he drew the extra temper...

 

[danielomalley]

I'm with @JBroida on this. I think the reheating is not a good idea is is pretty much unnecessary. Maybe chancing down the wrong rabbit hole? You don't want to change the heat treatment on a knife unless absolutely required (like it was in a fire).

I regularly straighten san mai, and monosteel knives using the hammer technique. I have done it to quite a number of honyaki blades as well, though it always makes me nervous (not because I think they have a much larger chance of cracking, but because he value makes cracking much worse). In most cases, there is no visible mark on the side of the knife (though this takes practice). In some cases, there will be slight visible hammer blows. When you are first learning the technique, it is pretty easy to break blades. Straightening 10-15 knives a day with about 1/2 being monosteel, I think the last one that broke was 4-5 years ago.

To make the technique work, you need a good, responsive anvil (just doing it on a thick piece of steel won't work) and a good, hard hammer (not a softer forging hammer). The blows are fairly soft, but with a bounce. If doing western knives, don't go too close to the bolster, since there is often heat weakness in this area from when they were manufactured.

Assuming you've got the right tools, it is just a bit of practice to understand where to hammer (and where not to!)

-daniel

 

[RDalman]

You can always heat to 150-160 c and be sure you will not change or lower the hardness. There's also no such thing as "heat weakness" but I can guesstimate you're referring to a poorly done welding process in that area. Or maybe temper embrittlement could be possible in some cases.

 

[danielomalley]

There's also no such thing as "heat weakness" but I can guesstimate you're referring to a poorly done welding process in that area. Or maybe temper embrittlement could be possible in some cases.

Quite a number of knives (such as Henckels), while appearing to be one piece of steel, are actually made from several ... eg. one piece of steel for the blade, one for the tang with a tig weld in between or a sintered bolster. Both techniques are perfectly fine if done perfectly. Both techniques are typically not done perfectly. Instead, the heat causes grain growth in the surrounding area, making that area more brittle and likely to crack when put under impact or bends (weakness caused by that area having been heated during the manufacturing process).

-daniel

 

[RDalman]

Yes, those issues from a weld can be dealt with/reversed/removed, production companies maybe don't though.

 

[OneStaple]

To make the technique work, you need a good, responsive anvil (just doing it on a thick piece of steel won't work) and a good, hard hammer (not a softer forging hammer). The blows are fairly soft, but with a bounce.

I find all of this very interesting, especially to see well-respected forum members with plenty of experience in both camps.

I'm up for learning/trying something new and giving the hammering approach a shot. As I said, the purpose of this knife is to learn. I definitely don't have the tools yet. What is the name of the hammer in the video that milkbaby posted? Is this the correct hammer to use, or are there others worth considering? Are they things I can find used on ebay and get at least decent quality (old is fine)?

A bigger question is what to do regarding an anvil. I don't have one, unfortunately, and getting one probably isn't in the cards at the moment. Are there any alternatives? A chunk of railroad? A chunk of cast iron? I suspect, given Daniel's comments, that these aren't adequate.

From some research, my understanding is that the hammer blows expand the metal a bit on the side being struck, thus forcing a bend in the other direction. Is this largely restricted to affecting the metal on/near the surface, or does it go relatively deep? For instance (for the future), if I am working on a knife that has only been rough ground or to which I want to add a distal taper, am I going to remove the metal expanded by the hammer and thus end up with a curve in the knife again?

Thanks,
Tyler

 

[Kippington]

The hammer in the video is a chisel hammer. Jon mentioned it as well, and it works by the method you described here:

From some research, my understanding is that the hammer blows expand the metal a bit on the side being struck, thus forcing a bend in the other direction.

A normal hammer blow works differently. It bends the knife in the opposite direction - like the center pin in a three-pin vise setup, and the coins in milkbaby's clamp picture

Daniel says you need a good anvil, but I have a good blacksmiths anvil and still cant get the hammer method to work. He also says "not a softer forging hammer", but I have no idea what hammers are harder than a forging hammer. Having seen cold blades crack after hitting them with a hammer on an anvil, I can't help but think that a softer option would be more successful, but Daniel seems to be convinced otherwise.

I have no doubt that he's telling the truth, but my own personal experiences are extremely different. For me, using a hammer on high-hardness steel always ends in a glass-like crack that trashes the knife. Doing it to a softer knife would be less of a problem, but the risks are still there.

I hit this one with a hammer, and it would've cracked all the way through if it were not for the differentially hardened spine. I checked after it broke and it wasn't any straighter.
I later stomped on it out of frustration, but couldn't get the thin strip of soft steel along the spine to crack.

On an semi-related note, it's fine to hit a honyaki above the hamon, just don't miss and hammer slightly below it... like I did here.
Unless you're like Jon and Daniel and have the magic touch!

 

[JBroida]

I have very little experience with western blade smithing, blade straightening, etc., so it always surprises me to hear about the methods employed. I’m used to Japanese methodologies, and that is really my main source of experience. It seems like many of the things that I have learned to do are not common, or even entirely unheard of in the world of western blade smithing. It’s a really fascinating subject for me.

 

[Kippington]

I have very little experience with western blade smithing, blade straightening, etc., so it always surprises me to hear about the methods employed. I’m used to Japanese methodologies, and that is really my main source of experience. It seems like many of the things that I have learned to do are not common, or even entirely unheard of in the world of western blade smithing. It’s a really fascinating subject for me.

I find it fascinating too. I mean, knowing what you do for a living, I'd say you comfortably have more expertise in the field than myself, but my own experiences don't seem to follow yours. One could argue that it's Japanese vs Western techniques, but it's all just a bar of steel at the end of the day so there should be many similarities. And there are, but this particular topic seems to be an exception.

I should take this opportunity to ask: Seeing you and Daniel have similar views, do you agree with what he says about a using a harder striking surface and hammer, almost the hardest tools you can find? Or would you go a softer route, something like the wooden stump style 'anvil' that are often used in Japanese knife-making.

 

[Dan P.]

I'd straighten a knife with a hammer and anvil too.
I press the concavity caused by the warp hard against the anvil and hit it , but it must be completely in contact with the anvil.
I've found that using soft things, wood stumps, mallets, etc. are more likely to cause cracks/breakages.
But... I'm a knife maker not a knife sharpener or repairer and at the stage where I'm straightening a warp the blade is unground, unhandled and does not have a huge amount of value, and they do break from time to time.
Talking about monosteel here.
For particularly stubborn warps, or a knife that was made by someone else/unknown hardness, I might draw the temper on the spine, but that is a faff, as is grinding out a warp, if you don't have the kit.

 

[comet_sharp]

Yes upwards 200c (390f) should be the heat to do it at. But still it's really difficult and you almost have to break a few knives this way to learn the breaking point.

So true, ive broke a few. Getting them to straighten under 375°F is hard is they are hardened properly. Id heat around 380/400 with a shim/jig or 3point and torch.
-Trey

 

[comet_sharp]

Sandblasting/or peening the inside of a warp or bow with a small hammer like a tack hammer is also a good way to straighten monosteel once you learn. By peening and stretching the steel on the warped side youll force the steel to lay back straight. Can leave peen marks that neeed to be ground out.
-Trey