Blade Warp

Stacy E. Apelt - Bladesmith

Stacy E. Apelt - Bladesmith

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There has been a lot of threads and posts about blades warping.

Some comments about warp;
WARP HAPPENS It isn't a plot against you, or bad steel, bad grinding, bad HT, etc......WARP HAPPENS.

Warp is an inherent problem with heating and working steel. You do your best to alleviate the problem, but have to expect it to happen....usually when you don't expect it.

Ways to avoid it;
Think inside the box...OK, inside the blade-
Whatever happens to the grains and crystals, or gnomes and faeries if you prefer, inside the blade will affect warp. Normalizing multiple times, grain refinement, proper forging technique, etc. all will help reduce warp.
Thing outside the box -
Everything around the blade may affect the warp. A cold counter with a warm blade set on it may make the blade warp. Uneven cooling in quench , or uneven heating in temper, etc. Stresses placed on the blade by the way it is held or suspended, etc.

Ways to not have to deal with warp -
There is one sure fire way to not have to deal with a warped blade.....try and straighten it after quench and before temper. This will break the blade in half, and end any need to straighten the warp.

Ways to deal with Warp-
Learning simple straightening practices will help you remove warp. Learning basic HT metallurgy will help you know when you can remove it. There are times when you can easily straighten warp, times when it will laugh at you for trying, and times when it will make you cry.
Start avoiding warp by getting the steel ready for hardening, This involves normalization, stress removal, and grain refinement.
The best time to avoid warp is at austenization temp. Before quenching the blade, pull it out and take a good look. Is it straight or twisted. If so, straighten it now, or that will be the set it takes in quench. Handle the blade gently when transferring from forge/oven to quench tank/plates. It is very rubbery at this point, and can be bent merely by moving it too fast. You have plenty of time to get it smoothly into the tank ( even of 1095).
After the quench, the blade has a period of time before the rubbery austenite starts to convert to brittle martensite. This is both a time when warp can be introduced as well as a time it can be removed. Once the blade has been in the quench long enough to drop below the pearlite nose, around 1000F, pull the blade and give it a good look-over. If there is anything that needs straightening, do it promptly and smoothly. Having all the straightening tools and materials in place is important. You don't have time to go get them. A simple slotted board clamped in a vise can straighten 99% of most warped blades in two seconds. Once the blade cools to where you feel it stiffening STOP STRAIGHTENING.
The next time you can try and adjust the warp is at full tempering temperature after at least one hour soaking time. The 400F or so blade can be gently nudged into shape. If it needs more, put it back in the oven for 15 minutes and repeat. You can do this for hours with no harm. A second temper cycle is needed, and may require additional straightening. Clamping jigs and such during temper can allow this to happen without needing removal or working on 400 degree blades by hand.

If you see a warp later on in post HT grinding and polishing, just reheat the blade to the tempering temperature and soak for an hour..then straighten.

Straightening at room temperature is only for the most minor warped steel, and only for the most steel nerved makers. A blade can break if pushed beyond its elastic limit. Where that limit is can't be known for sure.

Experience is the only teacher for dealing with warp. I normally say that you are best to learn by others experiences, and you can get some great info on warp from others.....but it is your own warp experiences that will teach you how to handle it, avoid it, and straighten it. Don't panic, and don't toss the blade saying, "I ruined three blades today, because they warped." It is a rare blade that can't be straightened.
Don't over think it either. I have heard people say that their new knife is ruined, because the blade tip is warped 1/16" to the left. That isn't ruined, and isn't even what most would call warp.


Lets hear from some others about how you
1) Deal with Warp.
2) Avoid Warp.
 
Stacy, thank you for this post, it puts together a bunch of tips in a way I haven't seen before.

bladsmth said:
Lets hear from some others about how you
1) Deal with Warp.
2) Avoid Warp.
Click to expand...

I think grinding the blade after heat treatment should be included.

I've been making some very thin kitchen knives (my stock starts at .060"). They are so thin they warp easily, and even a little warp is noticeable.

Based on advice, I've started to heat treat after profiling and before grinding bevels. With a plate quenching stainless, it has worked perfectly. I haven't had a warped or broken blade since.

Tait
 
I did two things on the last blades, following the advices of the experts around here:

1) A little stress relief, the blades, not me. ;) 1 hour at 1200F and let cool in the oven
2) After a count of about 7 in the Parks50, clamped the blades in my Al quenching plates.

Can't tell you which one did the trick of both but the blades came out straight as an arrow. This was with Damascus from Alabama Damascus.
 
Great information Stacy!

Here's how I deal with blade warp.

Buy straight stock - Precision ground costs more but is generally straighter. It also seems the more high end steels have more consistency. I also look into how the steel is cut, that really effects the outcome. Steels can also bend during shipping, etcetera but it doesn't hurt to try to get straight stock.

I never let my blades get hot while grinding. I feel this can distort the steel later and any heat buildup is avoided even before heat treat. Normalizing the steel is a great way to remove any stress before hardening.

Tip down, even quench. It is very important to quench straightly and evenly.

Grind post heat treat especially with thinner stock.

I'm still trying to learn how to hot straighten and will mostly stick to shimming during temper for now to help straighten my blades.

I try to stick with steels that distort less. Steel manufacturers give out this information and more such as how the steel grinds compared to others. It seems that the steels that need a faster quench distort more.

That's just my process and I have only heat treated about 200 blades. I feel I have barely seen the tip of the heat treat iceberg!
 
I've had some trouble with warpage when learning to make kitchen knives. Here's what I do right now to avoid or correct warp with forged 52100 carbon steel blades:

After forging, one normalization and two sub critical cycles up to about 1300, down to 900ish and quench. This way, I figure I've austenitized it once after forging, then the sub-critical cycles soften the steel for grinding.

I rough grind but leave about .040" or a little more at the edge. It may be 2" wide and .100 at the spine at this time. When grinding bevels, I hog in pretty good but keep an eye on the blade and try to apply the grinding heat evenly- grinding one whole bevel and then the other will warp it considerably. This seems worse with stainless, AEBL was kind of shocking how it pulled bent in grinding. Now I'll take a couple hard passes on one side, then the other, back and forth. Also, using a push stick can bend the blade in combination with the heat if you don't stay conscious of where it's at.

I flash normalize three times, checking for warp at critical. If I see it, I usually will tap it out on my wood block or the anvil.

Heat back up to critical, using a pipe muffle helps for even heat, and I'm usually having to pump the blade back and forth slightly to get the heel up to heat without overheating the tip. Get even color just a bit above critical.

Quench in P50, no interruption.

Pulling the blade out, I look for warp. Unless it's there pretty bad, I temper for two hours. Then, I take a large wood chipper blade and shim and clamp the blade to it however necessary. Another temper cycle. If it's fixed then, it's done- if not, another temper cycle with shims adjusted, one hour only.

If the warp has been corrected enough that finish grinding the edge from .040 down to .010 or so will fix it, I'm good to go.

If I have warp that's beating all of this, at some point I'll just straighten at critical and re-quench. No more cold straightening in my shop, it's too expensive to make a mistake. Plus, it seems to me as if cold straightening could harm the blade with stress.

Even heating and added thermal cycles, as well as a thick edge during HT have pretty much cleared it up for me. I MUCH prefer prevention to repair.

Added note: I've had a couple blades that frustrated me by just NOT correcting with shimmed tempering that I then tried water cooling while still clamped to the steel plate. In these cases, it seemed to lock in the shimmed shape better when taken out of the clamps. Normally I'd water cool after unclamping. Just something to try, your mileage may vary.
 
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Some blades do warp, especially long thin ones. I either straighten out of an interrupted quench and/or out of the temper with a round "U" shaped piece of mild steel in a vice... or just hand pressure with gloves out of the quench.

I haven't had any problems to speak of with warpage for a long time, and don't worry about it much. I've only lost about one blade per 1000 due to warpage, on an average.

With a little time and experience it really shouldn't be much of a problem.
 
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… if you do everything else right, warping shouldn’t be a problem. ;)
 
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Hi guys, just seeing this thread since I was searching different ways to get rid of warp. I am working on a .090 1074 12" blade slicer for roasts and such, and of course was having a wee bit of trouble with blade warp. After oh 10 Temper cycles with all manner of shimming I am straight and ready to go. I have done a few (10 :p ) blades this way, and have gotten fairly comfortable with it, even if my first try is unsuccessful due to lack of experience I just shim more and more until results are achieved. That being said, on to my my question(s)

My question has to do with the interrupted quench straightening method. Stacy Mentions the interrupted quench method. I understand how and why this works, but I am trying to get a better understanding of the timing involved. I have tried this method, and ended up with a blade that straightened but did not get as hard as it should have (could have been my imagination, since I do not have proper testing equipment.) and a blade that snapped right in half.

bladsmth said:
Once the blade has been in the quench long enough to drop below the pearlite nose, around 1000F, pull the blade and give it a good look-over.
Click to expand...

I know with all the variables, long enough will not be a set exact number, but about how long would be long enough, with some of the following factors?

1. Medium Oil Quenched Steel:
a) .250 thick - .040" at the edge.
b) .090 thick - .030" at the edge.

2. Fast Oil Quenched Steel:
a) .250 thick - .040" at the edge.
b) .090 thick - .030" at the edge.

3. Water Quenched Steel:
a) .250 thick - .040" at the edge.
b) .090 thick - .030" at the edge.
 
I like Tai's post!

That said, the best "jig" for straightening blades that I have seen and used came from Wally Hayes' Katana Tac video. Just made and used another one today and it works great.
 
Seems about 50% of what I order ends up being warped unless its 1/4 inch thick, which really sucks if you're purely stock removal. Ill keep all of this in mind in case I notice I didn't manage to eyeball it all the way straight before grinding.
 
Avoid, I send out for heat treat. Saves time and money for me to let a professional do what they do best, and for me to just grind.
 
Once the steel has cooled below 900-1000F in the quenchant, it is super-cooled austenite. That is a very soft structure, and can be hand bent easily. When the blade reaches 400F, it starts to convert to martensite. This is a hard and brittle structure. You can feel the steel stiffening up, which tells you to QUIT STRAIGHTENING.... no "one more twist" or "just a tad more"....unless you like broken blades.

The hardness should be about the same if it was left in the quench tank or pulled out below 1000F hand straightened. If you took it out of the quenchant too soon, it could be a little softer, but that is unlikely unless you pull it out after only a second or two. I use 5-8 seconds in the oil as a rule of thumb.
 
Now reading all of these techniques is great though I'm that guy that likes to know a sure thing before trying it. I have a warp at my tang/spine transition. Sounds like a simple fix except it's a scimitar where the warp is dead center of the rise in the offset handle. I've used 3 pins in my vise to straighten a blade but not where the most shock happens in a knife. Should I try something else due to the curved profile or give it a go?
 
I learned my lesson about warp recently when I had a long (14" plus") knife take a bow. It was very slight but enough that when I put to the flat platen to finish the flats it went great on one side and the top roller hit the grind line and ruined it on the other. Now I put each heat treated blade on my granite slab after heat treat and try to rock it on both sides before finishing. Larry
 
HDCutlery said:
Now reading all of these techniques is great though I'm that guy that likes to know a sure thing before trying it. I have a warp at my tang/spine transition. Sounds like a simple fix except it's a scimitar where the warp is dead center of the rise in the offset handle. I've used 3 pins in my vise to straighten a blade but not where the most shock happens in a knife. Should I try something else due to the curved profile or give it a go?
Click to expand...

Without a picture, it is a bit hard to say how to deal with it. First thing I would do is soften the tang by heating it with a torch. Heat the last half until it turns barely red. Watch the colors flow down toward the blade and cool off with a wet rag as the darker color just gets onto the ricasso. Then place in the temper oven and temper at 400-450F ( depending on original hardness) for 30-40 minutes, and straighten while hot. You should be able to remove the warp that way.

That said, warp at or in the tang isn't usually a big issue ... as it gets covered by the handle.
 
I appreciate the softer approach and will give it a shot. Pics will be up as soon as its done in the exchange area
 
Great explanations, should go in the stickies!!

I add just a consideration. Straightening in the supercooled austenite state is very useful, but you have to watch for your carbon in solution, if it is less than 0,7% your Ms will happen hotter than 400 °F, and the autotempering effect will be too much intense. For those steels (hypoeutectoids or underaustenitized hypereutectoids) it is better completing the quench and straighten in the temper.
 
That is why I recommend placing the blade back in the oil until completely cooled after a quick straightening. Once it stiffens, quit all straightening and place back in the oil.
 
A bit risky imho, with hypoeutectoids you get early precipitations+autotempering even with a full uninterrupted quench, due to the high temperature of Ms (around 500 °F); interrupting there robs the steel precious hardness, but if one makes very long blades meant to be abused it could be done as you said without many concerns.
For hypereutectoids it is a great technique for straightening major warps and you could even let the steel cool in air to room temperature since the autotempering effect will happen at the right temperature and the transformation will be without unneded harshness from the liquid cooling.

Anyway i must confess i'm still not too confortable with interrupted quench straightening; i have done it many times for trials, and it works, but the minor warps i prefer to take care during tempering.
I don't feel well the progressive "stiffening" and that gives me thrills ;)
 
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