Heat Treating Knives

The purpose of heat treating is to bring steel to a hardened state. The correct hardness depends on the application of the steel being treated. Knives need to be hard enough to hold an sharp edge through continuous mechanical abrasion, yet be soft (flexible) enough to stand up to forceful use without breaking.

Equipment and tools:
- Visegrip pliers or tongs
- small forge or charcoal fire of sufficient heat (search for forge or smelting instructables)
- fire proof quench container with lid (I used an old cookie tin)
- quenching medium such as used motor oil
- BC fire extinguisher (the kind that puts out grease and oil fires)
- Heat resistant gloves and face shield.
- kitchen oven
- fireproof material for regulator block (I used aluminum tube)

Materials:
- one mild or high carbon steel knife blank (forged or stock removal)

The quenching method I decided to use for this knife was the “edge quench”. I learned this method from $50 Knife Shop by Wayne Goddard. Instead of dropping the heated knife into the quenching medium tip first, submerging the entire knife, the edge quench involves submerging one third to one half of the blade's width (cutting edge first) into the quenching medium. A regulator block is used to hold the blade at the correct depth. The quenching medium I used was old motor oil. After the blade has been quenched, its hardness is still not suitable for usage. In its hard and brittle state, the quenched blade will shatter like glass if dropped, it must be tempered before it is put to use. Tempering involves heating the blade to a non-critical temperature (350 – 450 F) to slightly soften the steel (I used a kitchen oven). A tempered blade will hold a sharp edge and still retain strength and flexibility.

Step 1 Surface and edge preparation

The heat treating method is the same for knives made from forging, or stock removal. Whatever method you used,the final bevel should be on the blade and the surface should be brought to the desired finish. This was one of my first forged blades and I found the hammer markings appealing, so I did not polish the blade to a mirror shine, but I used a file to cut the bevel and a rough stone (100 – 200 grit) to get it fairly smooth, and as even as possible. Remember, once it is hardened, it will be much more difficult to remove material via filing and sanding. The edge is the thinnest part of the blade, and therefore more prone to cracking during the quench. Use a strip of 150 grit sand paper to dull the edge and reduce the chance of cracks or warping in the edge.

Comments

[curvy77]

you say that in its quenched state it is still brittle. but with the blades i have made after i quenched them they were still very strong.

[Semper Fi]

Nice, thanks for the info. I needed it. Here's one i just finished

[makingcustomknives]

Handy looking blade Semper Fi. Details on the octagonal pins please?

[Semper Fi]

There small bolts from a pop up canopy that got destroyed in a storm. Just countersunk and ground flush.

[makingcustomknives]

Nice. Are the ends on the opposite side peened over? Did you heat treat it? The coloration on the bevel suggests it took some thermal abuse.

[Semper Fi]

Thanks, they are flush. no heat treating, thats why i was glad to see your instructable. I think its just a shadow from me taking the pic. I'm not sure what grade of steel this is, i got it at a rock quarry i worked at year ago. But i do know its really hard (Hard). lol it holds its shine and edge very well. Can't wait to see some more of your creations.

[teckner]

The professionality of your work is very inspiring.
I also saw your blog, and it says you created your instructables account only a few days ago.

Do you forge metal by profession, or just a hobby?

[makingcustomknives]

Thank you for visiting my blog! Although it is not making me money yet, the reward in the craft itself is enormous.

[the_burrito_master]

Great instrucatable, Do you mind if put a link to your instructable into my instructable of how to make a bowie?

[makingcustomknives]

Sure you can. I saw your "riveted sword of the monkey" instructable, looked like there was fair amount of forge work involved. I'm currently using the gas forged pictured above to draw out a pair of hunting knives from a file, trying to improve my blacksmithing skills.

[the_burrito_master]

Nice!

I don't take much pride in my riveted sword of the monkey any more, haha I've been working on a new katana. that's close enough to almost perfect. the only problem is is the the blade is too thin.

[Phil B]

I have never made a knife and doubt I will ever get around to making one. Your Instructable is very interesting and I appreciate all of the detail.

[makingcustomknives]

Thank you Phil B.