A little "bent"

[JohnHV]

A question about straightening blades after heat treating...

After quench, I will sometimes get a subtle bend in the blade.  I usually curse a little and then reheat to strtaighten, then bring up to quenching temp and try again.  Sometimes it works and sometimes I just curse some more.  Especially see this with thinner stock (1/8").

IT could be that my grinds are not quite perfect and the quench imparts a bend or maybe something else?

I read in W. Goddard's book ($50 knife shop) that he uses three metal dowels in a vise to straighten his blades, but I could not find mention of when he does this in the process,  before quenching?  after tempering? etc.

So my question is how do you keep your blades from bending during the quench, and if they do, what do you do about it?

[Lin Rhea]

This a good question. I appproach this in two ways or even three.
 First I do a few things to try to prevent warpage from happening.
      Thermocycle the blade to remove stress and  reduce grain size. A blade under stress will warp more often than one not under stress, all other things equal.
       Turn off fan and close the windows, etc to prevent air movement on the blade.
        Position the quench tank as close as possible so I dont have to carry the blade far, especially swinging it in the air.
        Heat the blade evenly from both sides.

After all of that it will sometimes warp. What I do to correct it depends on some things. If it's pretty bad, I'll heat it up and straighten it and re-quench. If it's just a small amount, I might can grind it out during the finish grind. If it's a long blade and edge quenched I will use my straightening jig. It's silmilar to what Mr Goddard uses.
   
I have found that some blade shapes and cross sections will often warp in particular ways unique to itself. I may leave extra material in some places to change the mass and "fool" the quench. Afterward, just slowly grind the final shape. Lin

[JohnHV]

Thanks Lin!  That makes a lot of sense.

I am doing three normalizing cycles at decreasing temperatures for each cycle and my quench tank is within arms reach of the forge, but I haven't always controlled exposure to the wind which can be whipping around pretty good!

I will also experiment more with the edge quench technique and I am going to start using thicker material (1/4") for forging which will hopefully give me more "wiggle room" during the finish grind.  

I tend to forge too close to final cross-section and then have no material left to fix things during the grind  

[kbaknife]

I'm gonna tell you guys a secret.
I have seen this dilemma struggled with by even the old timers and long time professional knife makers as well.
Just remember where you heard it.
Like Lin mentioned above, some blade shapes or geometries seem to warp a little more than others.
Keep in mind that martensite has a different SIZE than other grain structures.
If your blade is not ground PERFECTLY from side to side, and you have a little variation in physical mass of steel, you will be creating MORE martensite on one side of the blade than the other, and this imbalance of martensite SIZE/AMOUNT will push the blade to the weak side.
That's in the situation where mass was the  problem, but consider un-even heating where one side is hotter than the other, and the sides create un-even martensite at a different rate or in a different amount if one side was so hot you got grain growth, and poor martensite creation or one side so cool that it didn't create martensite at all!
Anyway, there could be a zillion reasons for warpage, but I've found uneven grinding the most frequent reason due to the imbalance of martensite from side to side.  
As a result of a dagger class with Kevin Cashen this winter, I am now clamping my blades to a surface plate right at the ricasso. I then take a height gauge and scribe a line at 1/2 the ricasso thickness around the cutting edge and spine directly in the center of the blade and grind to that.
This ENSURES that the blade is ground right down the middle.
My warpage has virtually vanished even on 10 inch bowies.
Regardless, I right now have a warp on a blade I quenched last night and am fixing it as I type this.?
I take my blades so close to finish that I have not enough material to grind the warp out.
So try this next time.
Do your first temper. We need to make the blade a little less fragile.
Remember that edge quenched blades almost NEVER warp due to the minimal martensite creation, but fully quenched blades are far more prone to this malady.
Like the one I did last night.
So, I take a 1 1/4" inch wide piece of bar stock and a small steel shim and a C-clamp.
Lay the blade on the bar with the warp TO the bar.
Lay the shim under the blade out toward the tip so that it lifts the blade UP off the bar at the tip. C-clamp the blade, at the apex of the warp, to the flat bar so that you can visually see the tip go to the OTHER side of straight! So, basically, you are forcing the blade warp to the opposite side of where it was approximately the same distance away from straight.
Give it its second temper, only this time let it completely cool down in the oven after you shut it off.
Remove the C-clamp and you may find that it is perfectly straight.
This way there is no fooling around with extreme temperatures on a fully hardened blade doing who-knows-what to the steel structure.
I've seen some pretty goofy solutions, but every one of them threatens the integrity of what you just worked so hard to make.
This method hurts NOTHING! And works almost every time.
To prove that it works, I have on occasion, clamped the blade TOO FAR the other way, and put the warp on the other side of straight after it cooled.
Even though some steels like to be tempered at 400-450, even if you don't get it on the second temper, you can clamp and temper a third time at around 350, still fix the warp, but not effect hardness to any appreciable degree.
It's worth a shot.
Give it a try, because if you've done everything right, your blade needs to get at least two tempers anyway.

[Lin Rhea]

Karl,
     Good idea and it's a little more safe than just sticking it in a straightening jig. Even when I straighten a blade with my jig, I then do another tempering draw on the blade.

      I watched Jim Crowell straighten a blade with a torch and some water. He lightly heated the outside of the warp then wet, with a saturated rag, the same area to allow the steel to contract alternating heating and cooling. He always kept a bucket of water just under the blade so he could dip the edge not letting it get too hot. It was a dance but he was able to get the blade to pull straight on it's own.
     
         I straightened a shotgun barrel that way many years ago. Lin

[JohnHV]

Thank you Karl!

I was reading some of Mr. Cashen's posts last night and read about tweaking the blade during an interrupted quench with gloved hands.  I know I don't have the guts or the skill to do that!  But the use of a clamp and shim between temper cycles is something that even I can certainly do!

So, my game plan for my next blade is:
1) start with 1/4" stock and forge roughly to shape but take it easy on the bevel (ie. stop hammering away no matter how much fun I'm having!)
2) figure out how to do a center scribe line and grind to it EVENLY. I need a lot more practice in grinding!  Again don't grind too far, leave some of the material for a finish grind.
3) Do my normal heat treat and try an edge quench.  
4) If see no warp, thank Vulcan or whatever other spirits inhabit the forge.  If small warp, temper for one cycle, use the clamp and shim to correct during the second cycle, check again when fully cool and if necessary do it again for a third cycle.
5) if all goes well, repeat process on another 1000 blades and then call myself an experienced beginner.

BTW. Karl and Lin thanks for all your support on this forum.  I will be at the Blade Show next month and will make a point of stopping by your tables to thank you in person.

[kbaknife]

Now, you don't use the bar and clamp BETWEEN temper cycles, you use it DURING the second cycle!
Regardless of anything else that is done, you need to do two temper cycles, so why not use the heat during the second cycle to straighten the blade?
The one I did this morning was 100% successful in getting my blade dead straight and I did not need to subject any portion of the blade to extreme temps or stresses and risk altering any steel structures.
And while it was tempering, I went out and  forged another knife.
Those other "methods" of straightening knives are last resorts as far as I'm concerned.

[JohnHV]

Thanks again Karl!
I think I got it...  
Set up the clamp and shim to correct the blade to a little past straight and put the whole shooting match in the oven for the second temper cycle.

I do in fact do at least two cycles anyway!  (Sometimes 3 if the wife doesn't need her oven    )

[kbaknife]

You got it. Clamp it the same amount that it is bent to the other side of straight.
Do the temper cycle.
Then let it cool in the oven.
Take advantage of that second temper cycle's heat.
During the first temper cycle, you are converting retained austenite to martensite, that's why you need to do a second cycle, because you now have NEW, UNTEMPERED martensite which needs to be tempered.
At around 350-450 degrees, there's enough migration of alloy occurring that you can fix any bend problems that you have.  
Take advantage of that heat and you won't risk screwing up anything you've accomplished at this point.

[JohnHV]

One last question.

Do you do a longer second temper cycle to allow for all that "extra" material (Clamp, bar, shim plus the blade) to uniformly heat up to 350-450degrees?  I would think it would take longer for the clamped blade to get to the desired temperature with all that stuff around it.

[Lin Rhea]

John,
       The tradgang guys attending Blade are going to have a group photo done. We'll know more closer to Blade.

Karl, you are so much smarter than you look.    
                          Lin

[JohnHV]

Lin,

It would be great to meet some of the other members of this forum.  I will be there on Saturday with my sons in tow.  My 12yo won't let me go to the Blade show without him!

[kbaknife]

That got a laugh out of me - thanks, Lin.
It was a tough day.

John, good question.
Bad answer - not really.
I just do both tempering cycles for two hours.
But to be certain, and extra fifteen minutes would be a good idea.
I think I'll start doing that now that you've brought it up.
Thanks.

[Scott Roush]

great thread...

[Lin Rhea]

Actually, I allow an extra 15 minutes for the oven to come up to temp most of the time.

I have a blade in the oven right now and it's clamped to a bar of steel. It's pretty good, but needs to move just a goat's hair to be perfect. We'll see. Lin

[Steve Nuckels]

Very interesting information!!!!!

Thanks for the question and the anwsers!

Steve
--------
Potomac forge
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IN GOD WE TRUST

[kbaknife]

Originally posted by Lin Rhea:
Actually, I allow an extra 15 minutes for the oven to come up to temp most of the time.

I have a blade in the oven right now and it's clamped to a bar of steel. It's pretty good, but needs to move just a goat's hair to be perfect. We'll see. Lin

Lin, I think he was asking if I allow more time with the blade clamped to the bar than when it's just a regular temper. So the blade will still get a full temp soak after the extra mass of the bar comes up to heat.
I let my oven sit at temp for a full 1/2 hour before I put the blades in.
When clamped to a bar for straightening purposes, I should probably add another 10-15 minutes.
I think that was a legitimate idea.

[JohnHV]

Yes Karl, that was my question exactly.  I have a nice digital read-out on my oven so I wait until the temp reads my target before putting in the blade and starting the timer.  When using the clamp method I will add 15min to the soak time as you suggest.  Can't hurt!

I learn so much from these threads, thanks for the replies everyone!

[Lin Rhea]

Yes, I understood. My comment was incidental.
Sorry for the cornfusion. Good discussion. Lin

[prarieboy]

Thanks for all the info and questions!