Re-HT'ing An Already HT'd Blade

DonL

Well-Known Member
I have this 1084 blade that is crooked. I've tried and tried to straighten it with a variety of methods. While it is better than what it was, it still has a slight bend in the handle just behind the blade. I'm afraid to bend it much than what I'm already am. I'd hate to break this blade!

I put it back in a 400 degree tempering oven between attempts at straightening it, but it still wants to retain the bend, just like a car spring!

My thoughts are to take it back to my HT oven and heat it up and straighten it on an anvil, then start the HT process again. I'm just not sure if re-HT'ing will harm it?
 
I would normalize the blade a couple times. During the first cycle I would straighten it then give it one more cycle. Once I was done that I would heat treat it again. Hope it works out for you, warped blades suck
 
I have this 1084 blade that is crooked. I've tried and tried to straighten it with a variety of methods. While it is better than what it was, it still has a slight bend in the handle just behind the blade. I'm afraid to bend it much than what I'm already am. I'd hate to break this blade!

I put it back in a 400 degree tempering oven between attempts at straightening it, but it still wants to retain the bend, just like a car spring!

My thoughts are to take it back to my HT oven and heat it up and straighten it on an anvil, then start the HT process again. I'm just not sure if re-HT'ing will harm it?

There are about a dozen things that can go wrong in hardening and only one that can go right. So although you should be able to do it again it is a pretty big step jsut to straighten a light bend in the tang area. What are the methods that you have exhausted until now? I will suggest two more things- 1. Pull the blade straight by clamping it to a bar and then temper it this way, 2. put the back of the tang in a vice, heat the area you want to move until it is blue while protecting the edge of the blade with wet ceramic wool or a wet cotton cloth and then push it straight while the area is at temp. If none of these will work (repeating them isn't a problem so long as the blade itself doesn't exceed the target temp of your initial tempering) then I would do a simple stress relieve, i.e. heat to 1200F and air cool, and then re-heat treat using an interrupted quench so that you can keep it straight the second time around.
 
I have a piece of angle iron and two small steel C clamps to do just what Kevin said to do. The reason that I use angle iron is that it will resist flexing better than a flat bar of steel. I clamp the blade to the angle iron, maybe with a penny under it at where the knife is bent to counter bend it and then do another tempering cycle. I also like to do at least one tempering cycle on the knife blade after the quench before I try any straightening unless the blade is fresh out of the quenchant and while it is still real hot.

One other thing. I try not to quench my tangs. This can make them less strong but tougher. It also allows for easier cold bending in the tang. You can try cold straightening by clamping the work in a vice with one rod on one side of the blade at the peak of the bend and two rods on the other side of the blade to supply the push as you tighten the vice. I've use this method successfully but I've also broken blades giving the the handle of the vice just one more quarter turn. It works much better on soft steel. The method Kevin is talking about is much less nerve racking and less likely to snap a knife that has been hardened.

Doug
 
Thanks for the info guys.

I tried using 3 bolts in my vise. I turned it as much as I dared to. Still warped. I put it in a drill press vise with washers on both sides of the bend and one on the other side (just like the 3 bolt method but in a drill press vise with washers). Still bent. I then tried clamping it to my drill press table. First time I put washers on each end and clamped the bend down. Second time I put washers under the bend and clamped the ends. After each try, I put it back in the oven at 400 degrees and heated it before trying again. I also tried clamping it flat to the drill press table and heating the tang with a propane plumbers torch. I didn't clean off the metal to see the color change, so I'll take Kevin's suggestion and do that. If that doesn't work, I'll clamp it flat and put it in the oven for a while.

I'll let you know how it turns out.
 
You may need to "over correct" the bend before you put it in your tempering oven. This may go without saying, but make sure that you are giving it plenty of time to reach 400 degrees after placing it in your oven. I usually let it soak for a minimum of 30 to 45 minutes, if not more. Fortunately, I usually have about a million other projects to work on while I'm waiting for my blade to cook. :)
 
Okay, well I finally "fixed" it. I had an old grinding jig I made out of angle iron a while back. It holds a knife by tightening a flat plate against the knife and angle iron. So I decided to use this. To make a long story short, it still had a slight bend in it.

This knife is my interpretation of a Sharp Finger knife. The bend always occurred in the narrow section of the handle just behind the blade. I normally don't worry about hardening the handle. But on this one, since it was so short, I decided to harden the entire knife. So I decided to re-HT it. During the last normalizing cycle, I sit it on an anvil and straightened it. Or so I thought! The problem is, I do all my HT after dark so I can see the blade color. I do this even though I use a magnet and Tempilstiks. It looked straight but after HT it and taking it in the Garage, it was still bent!

So the next afternoon, I waited until nearly dark and straightened it again. Only this time I had enough light to make sure plus I decided to only HT the blade and not worry about the handle. I got the bends and twist out and proceeded to HT it. It came out straight this time and made it through all temper cycles!
 
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