Straightening a Custom Made Knife after Heat Treating

My Knife 34 came out of the quench with a small warp.

Here is how I fixed it. I’ve used this several times since and it has worked fine every time.

I clamped it to a solid straight piece of metal.

I tempered it at 450 degrees for 2 hours. The warp was slightly diminished but still there.

I added a finish nail behind the knife to provide a slight over bend and tempered for another cycle of 450 degrees for two more hours.

This time it came out straight.

Another option when Straightening a Custom Made Knife after Heat Treating is to use a leaf spring with the curve sill in it. Heat the knife and spring in the oven to tempering temperature and place the knife against the concave side of the piece of spring. apply a small (1″) C clamp to the blade and spring at the place where it warped and tighten till the blade is straight (ware gloves as everything will be hot) temper for 1-2 hours, cool, check and if not straight, do it again this time going a little past the straight point when you clamp.

Straightening Immediately After Quench. Immediately after hardening, while the blade is still warm to the touch, it is remarkably flexible. After a while that goes away and you have a hard, brittle blade.

Being prepared ahead of time with a couple lengths of angle iron in the vise, you can squeeze the knife back to straight. Jason Knight has a video showing how he does it with two pieces of 2 x 4. You only have a short period of time after the quench, so be ready ahead of time.

I’ve also used a leaf spring. The natural curve will sometime help with the over bend. Also keep in mind that additional tempering cycles when Straightening a Custom Made Knife after Heat Treating doesn’t have a whole lot of affect on the hardness.

Knife 32 – Mini Cleaver

Knife 32 – Mini Cleaver is a small chopper or mini cleaver.

  • Made from 3/32” 1095 steel
  • Laminated Rosewood and maple scales with G10 spacers
  • Blade is 4”
  • Overall length is 8 ¾”

This mini cleaver or herb chopper was a trial and error kind of learning exercise.

The tang was a little too narrow, but I figured that would be acceptable for a herb chopper. As it turned out I was right

This was made from a left-over piece of 1095. The shape of the piece of steel inspired the knife. I also wanted to try grinding after heat treating before I did it on a full-size cleaver like Knife 30 – Cleaver and Knife 35 – Serbian Cleaver

I discovered with a good ceramic belt, and submerging in the water bucket every stroke, it didn’t take long at all. I keep the belt running about half speed. This worked out well and grinding after helps eliminate a potential for warping during heat treatment.

Knife 32 – Mini Cleaver

The other lesson learned on this knife has to do with carefully choosing the scales. There are some blemishes in the scales I didn’t notice before I put them on. It won’t hurt the performance of the knife, but if I wanted to sell the knife, it would be a harder sell and would definitely warrant a price reduction.

I now inspect the scales much more closely. Other than the minor imperfect in the scales, it came out well. The knife turned out to be an exceptional kitchen knife.

Because of the small blemish on the bottom of the handle, I thought about removing the handle and starting over, but it’s unnoticeable enough that I decided to offer it as it. I obviously had to point it out clearly in the description and priced it accordingly so someone got a great knife at a great price.

See more on this style of grinding on Knife 30 – Cleaver

Heat Treating for Beginners

Heat treatment for beginners.

One thing I discovered is heat treatment for beginners doesn’t have to be as complicated as it seems. Follow a few easy steps and you’ll be ok, but keep researching and learning. There is a lot to it when you get into different kinds of steel.

A lot of beginners, including me, wants to start out making stuff out of old metal we find. We have the mentality of the woodworking prototypes we make out of cheap softer wood. However steel is a different animal. 

I recommend you start with a known piece of steel. My research led me to 1080 steel. 1080 is well respected traditionally used steel for high carbon knives. It makes exceptional knives, and it’s a bit forgiving when it comes to heat treating. It fairly inexpensive as well.

You can then move on to old files and spring steel if you want. I’ve used this heat treatment for old files and it works well, assuming they are annealed for shaping.

You’ll obviously need something to heat your steel. Typically a forge is used unless you’ve been fortunate enough to have a heat treating oven. If you have a heat treating oven you can stop reading this. Otherwise, read on.


Heat-treating ovens

From my research into heat-treating ovens, there are many differing opinions of how important a heat-treating oven really is. It seems to boil down to what you plan to build. If you will be building knives with stainless, and want to do your own heat treating, the overwhelming consensus is a heat-treating oven is required.

However, if you will only be using high carbon steel for your knives, it isn’t quit as important, assuming you have the heat-treating and tempering process down using a forge. After all, if the appropriate hardness is obtained, it doesn’t matter how you get there.

Getting ready

First make sure you’re ready. The knife is fully shaped with the exception of the edge. You want to leave a little thickness on the cutting edge. It should not be sharp.

-All holes are drilled.

-Touch mark added.

Another option for beginners is to send the projects out to have them heat treated. I sent my first batch of plane iron blades out, so there is no problem with that. There are many bladesmiths and professional blacksmiths who offer this service.

But heat treating a knife made of 1080 steel isn’t that difficult. Here is my technique that works for me.

I use my propane forge, but any forge should do if it can heat to an even non magnetic heat. I’ve even heard of doing it with a camp fire, but I haven’t tried. I find the propane forge is easier to get a consistent and even heat, however if you have a little experience with a coal forge it can be used as well.

Heat the metal to non magnetic. I have a magnet stuck to the side of my forge frame. It’s easy to check. Once it’s non magnetic, heat for few seconds more and make sure the color is even throughout.

I then quench in Canola oil. I have the oil in a metal military bullet box. I quench only the blade (and on large knives only the edge for an inch or so) leaving the rest to have a little “give”.

I preheat the oil by heating a piece of scrap steel cherry red and quenching it in the oil. This heats the oil which slows the cooling process of the knife blade.

Move the blade through the oil slowly like your cutting a path in it. I keep it moving for about a minute. Avoid a side to side movement. Cooling one side of the knife faster than the other will cause it to warp.

I then let it cool slowly at room temperature. At this point you’ll want to be careful. The knife is brittle and will break if you drop it or thump it against something to hard.

Now you need to temper it. Your kitchen oven will work, but I bought a $20+/- toaster oven so I could do this in my shop. It keeps the smell out of the house. Canola oil doesn’t smell bad, but it just easier and I don’t have to schedule the oven time with my wife.

I clean the knife up. I will do a little more sanding to get it back to looking decent and get all scales and gunk off it. This is just a clean up to keep the heat more even.

I do 2 cycles at 425 degrees for 2 hours each. When I started I did 3 cycles, but after some testing I think the twice is just about right. At the end of two hours (I set the timer on my phone so I don’t forget) I just turn the oven off and let everything cool back to room temperature each cycle.

I’ve also never tried, but my research tells me leaving it to long doesn’t hurt, as long as the temperature never rises.

Now just finish beveling and sharpening the knife. That’s all there is to it.

For a deeper understanding here is additional recommended reading – Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel By John D. Verhoeven (2005)