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Warping Damascus


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I'm supposed to know these things, but obviously I don't have it down yet.

 

My current 8" chef knife started warping in my final set of normalizations?!? Any insights welcome (even those I've heard or read a dozen times).

 

Blade history: Billet of 1095/15N20 I forged - several hundred layers (theoretically 700+ but I know I burned off some and ground off some layers between cutting & stacking for each round).

 

Forged pretty much to shape & thickness - bevel & distal taper. Mainly using drawing dies on a press - some flat dies on power hammer and some hammer & anvil work.

 

3 normalizations starting pretty high (since the billet had been forge welded it seemed reasonable to start high) 1650f then 1600f then 1575f. Then an anneal in pre-heated vermiculite - from 1475f.

 

Then ground to final shape. Straight & true.

 

3 more normalizations all at 1575f +- 7f for 5 minute soak - air cooled in still air to where I could hold the blade in bare hand. At each normalization I noted a slightly increased warp toward the back of the blade (to the right looking down the blade).

 

Heat to 1475f - soak 5 min - quench in Parks 50. And the general warp that developed during normalizations was slightly enhanced - plus a couple of small wobbles at the thin blade edge.

 

Temper 3 times for 90 min at 375f. Straightening a little between each temper. Got most of the warp out. Will head to the shop this afternoon to get the last bit out of the blade edge (heating to 375f).

 

It's probably silly to ask for advice when nobody was watching me to see where I might have *caused* the warpage - but here I go: Any advice?

Edited by Michael Kemp

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Michael Kemp

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Only a very recent bladesmith (3y) but did you have it completely vertical in the forge/heat treat oven? If it was leaning slightly to the right, at that temperature it could have started to bend?.

 

James

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James - the blade sat vertically in the forge - and I'm of the opinion that the heat in the forge was even... but that could be an issue.

 

It did not warp in the initial set of 3 normalizations and an anneal. Of course it was slightly thicker steel at that point. At any rate it was straight when I started grinding (and when I finished grinding).

 

Maybe it had something to do with my grinding techniques - but I thought I worked on front and back about equally. Hmmm.

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Michael Kemp

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Hmmm?

I cannot say that I know the answer to this one.
Something that I have found is that the cooling of the steel after normalisation can have an effect on later tendency to warp. Do you have a draft in your forge that affects one side more than the other? Did you place the blade on a surface as it cooled? You do say that the blade was cooled in still air? Inside a tube? Was it held closer to one side in the tube? (This might be a silly thought...)
Can you think of any thing that might have made for different gradients in cooling of the blade, since this might result in warping later on.

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Peter - good queries.

 

I *believe* that the forge temp is even - but that may be the culprit. I'll be building a new forge with better temp control at austenitizing temps - so in a month or two we'll see if that's the culprit.

 

For cooling in "still air" I have a rectangular chamber of firebricks with one end open - the floor is those soft refractory bricks - and I have a couple of fragments of kiln shelving that barely contact the blade to keep it vertical. Hmm.

 

As for thinking of "anything that might have made" heat gradients - I'm at wits end (not that I'm all that long in the wit to start with) hence posting this thread. This isn't the 1st blade I've had issues with - I just paid more attention this time to try to spot where the problem occurred.

 

It's curious to me that the 1st round of normalizings and an anneal did not bring out a warp - but the 2nd round of normalizings and quenching did. As close as I could see the forge and "air cool" were consistent between 1st round and 2nd round. And the warp started to appear in the 2nd round of normalizings, even before the quench.

 

Maybe I should just get the next forge built and see how it goes. As I say - at wits end - grasping at straws. The blade is *almost* straightened out - just a slight wow left in the edge about 1/3 up from the heel. It's soaking in the oven again for the next straightening round. I have to say I'm impressed with how springy the edge is. I'll be curious to see what the Rockwell is.

Edited by Michael Kemp

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Michael Kemp

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thin blades warp... that is one reason I have gone to heat treating before grinding, and doing a interrupted quench as they can warp even then.

 

my normalizing schedule for that mix is 1650 1550 1450 and forge to shape using reducing temp heats. over normalizing can affect hardenablity.

MP

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Matthew - you may be right about thin blades - they've sure given me fits.

 

As for extra thermal cycling impacting hardenability, I believe it also improves toughness - so kind of a trade-off. I've been getting around 57 Rc on my Damascus kitchen knives which I consider appropriate. Hrisoulas noted in one of his books that Damascus tests low in Rc - and heck - my Wusthof knives test the same.

 

But yah, the only real "cure" may be to harden a profiled but un-beveled blank before grinding and sacrifice more Damascus to the grinding gods.

 

I'll try flipping sides more during grinding (and an improved forge) first... but I may just be delaying the inevitable.

Edited by Michael Kemp

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Michael Kemp

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Michael,

If I read your process correctly, you did three (3) normalizing heats after grinding (?), but before heat treatment, and this is when your warping started to show up.

Personally, I think three normalization heats after rough grind is over-kill. The residual stresses caused by grinding will probably be relieved with a single normalization heat or sub-critical anneal. If you are not doing a differential quench for a temper line, there is probably no reason to normalize or anneal between rough grind and HT (providing you properly normalize/anneal before rough grind).

Recently I saw a post in another forum by Ed Caffrey about his experience with 1095 in Damascus warping dramatically during HT. Another thing to consider is the thickness of the blade when quenching. I think Matthew hit it right and leave the final beveling until after HT is complete. What I typically do with kitchen or chef knives is to rough grind to parallel before HT and put the bevels in after tempering. The cross section is so thin, even at parallel, that warping is difficult to control. You might want to assemble some sort of quench plate device for these knives to help limit the warping.

 

Edit: If you normalize in your forge, make sure you have the blade in a tube or other heavy container to ensure the blade heats evenly.

Edited by Joshua States

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I've been having a side conversation with John Emmerling about this - and he also grinds after heat treat. And when he hardens he quenches in oil for 8 seconds, then clamps between aluminum plates.

 

So the warping issue isn't just me... I just can't treat Damascus like it was 5160.

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Michael Kemp

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57 is a bit low for a chef in my exuberance, mine are 60RC or so and that seems to be the general standard among high end chef knife makers so going as high as 61-62RC.

I have never had any issues with my pattern weld getting less hard than the equivalent materials.I think your lower hardness is more due to the normalizing. when steel has been over normalized the carbon can have a hard time coming into solution resulting in lower hardness, in some steels it can result in a lot of fine iron carbide or graphite with in the matrix. you might gain some toughness but I would rather temper back to gain that toughness.

MP

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Good points Joshua - and if Ed Caffrey warns about 1095 Damascus warping - well then.

Then again, Tim Hancock has been using it for his Damascus for years and doesn't have a problem.

Ask two knifemakers a question........

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“So I'm lightin' out for the territory, ahead of the scared and the weak and the mean spirited, because Aunt Sally is fixin’ to adopt me and civilize me, and I can't stand it. I've been there before.”

The only bad experience is the one from which you learn nothing.  

 

Josh

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https://www.youtube.com/channel/UCdJMFMqnbLYqv965xd64vYg

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The approach I take to undo warping is clamping the blade straight to a thick bar and temper it again. This has never failed me in un-warping a blade :-)

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Thanks Niels - I've always taken it out of tempering and used a 3 point jig in a vice... I *have* to remember to try your way next time... and despite all efforts, I'm confident there will be a "next time." ;-]

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Michael Kemp

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  • 2 weeks later...

Update: I've tossed the chef knife in the "oops" drawer. I was down sick for a few days - so this was about a week ago. I had it straightened - but while cleaning up the bevels I let part of the blade get hot. Not hot enough to color - just hot enough to warn me ?200f? - and the warp returned to that section of the blade.

 

I'm making up a couple of new Damascus billets - but will only be making small knives until I can get some hands-on mentoring. David Lisch is 4 hours away so I'm watching for him to post his next Damascus class - I believe he's in the process of moving. John Emmerling is busy for a month or two and then I might be able to work with him (3 or 4 hours away). The Bells (Dragonfly Forge) work with cable - which is a different beast. Murray Carter is out of my price range these days.

 

I'm particularly curious how folks who do layered Damascus with integral bolsters do their heat treat. Somehow I doubt if they harden a slab the full bolster thickness and then grind all the way down to blade thickness?!? - so I doubt if they plate quench.

 

Damn shame too about the chef knife - I loved the glimpse I got of the Damascus pattern.

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Michael Kemp

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What direction is your quench tank pointing. It could be that your tank might not be pointed magnetic north. The reason blades warp is because when you heat the blade the metal loses its magnetism. When you quench it it regains its magnetism and pulls at magnetic north creating a warp. Its a simple thing that many people get wrong or don't know, but it means the difference between a good blade and a bad one.

Andrew Karow

 

Of the four elements, air, earth, water, and fire man stole only one from the gods. Fire. And with it, man forged his will upon the world."-anonymous

 

The Armorers accomplishing the knights, With busy hammers closing rivets up, Give dreadful note of preparation." -Shakespeare

 

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Andrew,

 

I hate to tell you this, but I'll probably be kinder than the next guy in line. The whole magnetic north thing is an old wives tale that has been perpetuated on the internet, and more recently, on TV. It is total hogwash.

-Brian

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It sounds like it COULD be accurate. Buf if you give it a little more thought there are other possibilities. I would think gravity has more effect than magnetic north especially with the mass larger than that of a needle. Having read some of Mr Caffreys posts on the ABS website i've come to suspect he's basing his information on a bad batch of steel. All things being equal, more times than not the warpage is coming from different variables in your heat treat. Was the blade heated EVENly on both sides, is the blade geometrically symmetrical and so on. Most warps for a beginner are one of these two things I would bet. If one side cools faster than the other side due to being hotter, uneven, etc it will constrict faster and create the warp. Theoretical rant finished. I haven't experimented with 1095 in damascus yet but I have used it quite a bit lately for chef knives. I noticed a lot of curves due to my lack of normalizing my steel before heat treating to reset the granular structure.

 

With Mr. Caffrey's skill level I can confidently contest that he has all that technique dialed in and he just got a batch of lower quality "specification loosened" steel from a supplier not named Aldo :D

 

but i dont know nothing im just a country boy from the sticks :P

Edited by Gabriel James
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Heya all - entertaining reading. While I *am* using a forge rather than a Paragon or Evenheat - the blade was normalized set vertically on it's spine on a bridge across the forge above the burner. I use a pyrometer mounted near the center top of the forge. The kiln shelving bridge and blade support were at forge temp. Visually speaking left and right side of the forge were the same color. I soaked the blade for 5 minutes on each normalization once the blade visually reached the forge interior color.

 

That said the blade is friggin' thin (0.062" spine at the point that the warp reappeared - see above).

 

And the initial warp appeared in the 2nd set of normalizations - after grinding to shape... and got a little more pronounced during the quench. So I wouldn't say that the quench caused the warp.

 

As noted above I had the warp straightened out during tempering - but it reappeared while finish sanding. I just *had* to see the pattern - so I did a quick etch & bluing to see the Damascus pattern. And I wanted to take the opportunity to see how the grain looked - so I broke it at the start of the warp.

 

What the heck - here's a snap of (1) the blade profile (2) closeup of the pattern (3) closeup of the broken edge - to the naked eye the grain looks a little rough but not "bad" to me.

 

201605Profile.jpg

 

Here's a closeup of the pattern:

 

201605DamascusPattern.jpg

 

And here's the broken end - approx. 0.062" at the spine - you can see the etched/blued layers on the sides of the break. To the naked eye there is a texture to the surface of the break but I can't resolve individual grains. If this closeup photo can be trusted I'm guessing the grain size is around 0.002" to 0.003" - which is larger than I'd like.

 

What is your definition of "fine grain"? (in microns or thousandths, please)

 

201605BrokenEdge.jpg

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Michael Kemp

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"What is your definition of "fine grain"? (in microns or thousandths, please)"

 

Practically invisible and impossible to measure. Like the pieces on the left, not on the right.

Grain pics (2).JPG

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“So I'm lightin' out for the territory, ahead of the scared and the weak and the mean spirited, because Aunt Sally is fixin’ to adopt me and civilize me, and I can't stand it. I've been there before.”

The only bad experience is the one from which you learn nothing.  

 

Josh

http://www.dosgatosdesignsllc.com/#!

https://www.youtube.com/channel/UCdJMFMqnbLYqv965xd64vYg

J.States Bladesmith | Facebook

https://www.facebook.com/dos.gatos.71

https://www.etsy.com/shop/JStatesBladesmith

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OK then - my grain is somewhere between your two examples. As mentioned at the top of this thread, after the forge welding I assumed I had huge grain size - and for the first set of 3 normalizations started pretty high: 1650f then 1600f then 1575f. Then an anneal at 1475f and into in pre-heated vermiculite. Then ground to final shape. Then 3 more normalizations all at 1575f +- 7f for 5 minute soak - air cooled in still air to where I could hold the blade in bare hand. This is the grain size I got.

 

Any helpful hints are appreciated!!!

 

201605ChefGrainSize.jpg

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Michael Kemp

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