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PostPosted: Mon Jul 06, 2009 8:06 pm 
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Since a couple of questions about heat treating have popped up lately, I thought I would take an old post of mine and add to it in its own thread.

Begin old data:

Heat treating a blade is fairly simple if you work with simple steels.

The properties of the steel (hardness, edge holding, flexibility) derive from the internal stress of the steel, which in turn derives from the shape and the size of the steel crystals or grain. The crystals are manipulated through heat. These are not magic elves and rainbow unicorn crystals. They have all scientific-y names like cementite, perlite, austenite, martensite.

By heating it past the point that the steel loses its magnetic properties (and a bit highter), you are changing the crystals to a certain configuration. Quenching the blade freezes the crystals into that configuration (for the most part). You heat it and quench, making it hard but also brittle because of the internal stress. Depending on the steel, you will use a different quench medium to cool it at faster or slower rates. Some steel will actually harden in air. Others require a faster quench than straight water, so brine or brine plus surfactant (a.k.a. Superquench) is used. Sometimes if an alloy is cooled too quickly, the blade will have so much internal stress that it will crack in the quench or soon afterwards, leading to the dreaded "PING!" Most blade steels will be either quenched in water or oil. Leaf spring is typically an oil quench, but you'll need to experiment with a test piece.

After hardening, if left in the fully hardened state, the blade will be so hard yet brittle that simply dropping on the ground or even maybe leaving it sitting on a bench overnight will crack the blade. You need to take some of the internal stress out by drawing temper, heating it up several hundred degrees. The steel will be softer, but won't shatter if you've done it right.

Blades are a balance between holding their edge and being tough enough to withstand the stress of use. Putting it very simply, the harder the blade is the better it holds an edge, but the more likely it will snap under stress. The softer it is, the tougher it will be but the quicker it will dull. So the good thing to do (and one of the factors where a custom knife excels over most production knives) is to differentially heat treat, getting a softer spine for toughness and a harder edge for edge retention - in the same knife. There will always be compromises.

There are a number of ways to get a hard edge/softer spine combo. One is to edge-harden, commonly done by quenching the edge and leaving the spine out of the quenchant, then drawing temper. This can also be done by clay quenching, coating the blade with a clay mixture, leaving a thicker coating on the spine than the edge. The edge will cool more quickly than the spine. This is the technique typically used on Japanese blades, and is usually how you get a hamon on the blade (or you can laser-etch it if you are a Chicom factory selling flea market stainless sword sets for $20). Another way is to differentially temper, where you quench the entire blade, but then draw temper on the spine more than the edge. You can do it with a torch, like Madmatt said, or use a heat reservoir. I've used a pair of tongs with thick jaws that I heated to red, then clamped the spine of the blade and watched the temper colors run. Thirdly, you can do the hard edge/soft spine by forge welding a harder alloyed steel for the edge into a softer alloy steel for the sides and spine. San mai is the typical construction for this, where a high-carbon core is sandwiched between two low-carbon sides, so that when a blade is forged out, the core becomes the cutting edge. The lower carbon steel will not harden as much anyways.

So with, for instance, a small-to-medium general use blade made of piece of leaf spring that I've worked with before and know that it is an oil quench, I will pre-heat my quench oil to about 130 degrees Fahrenheit. The pre-heating makes the oil less viscous, so that it flows around the blade more quickly and evenly, actually pulling the heat out more quickly than cold oil. I use vegetable oil. It works well and has nothing toxic about it. I heat my blade past non-magnetic, then quench with the edge down. I start by plunging most of the blade in, then bringing the spine out of the oil after a few seconds. The heat retained in the spine will draw the temper as it is exposed to air, while the edge immersed in the oil keeps the edge from drawing temper. I plunge the blade back into the oil again, then pull the spine out again after a few seconds. I do this several times, making sure that there is no sudden line where the temper changes, reducing the chances of stress fractures. The edge stays in the oil the whole time.

After the blade has cooled most of the way off, which takes longer in oil than you might think, I pull it out, clean the blade off with a grease rag, then go ahead and cool it in water to the point that I can handle it without tongs. I clean the blade off quickly with soap and water, then use sandpaper to clean the baked-on oil and scale from the blade. Then I will pop it into my kitchen oven at 350 degrees for an hour. For increased performance, I harden three times and draw temper three times, letting it cool completely between cycles.

There are more sophisticated methods, but they are all variations on what I've said here. This is simple, workable for most people, and can produce a better heat treatment than most factory knives if done properly.

Begin new data:

To soften steel completely, making it easier to file or drill, you anneal it. This is done by heating it to non-magnetic and then cooling it slowly. This can be done simply by burying the heated steel in an insulating material such as wood ash or vermiculite. Leave it overnight and it will be ready to work the next day. After you're done working, harden and temper as above.

To reduce the amount of stress and refine the grain structure, it is a good idea to normalize the blade occasionally. Normalizing is the process of heating the blade to non-magnetic and then letting it cool in the air until it is cool enough to handle. This makes the blade tougher and less prone to breaking. If you are in a long forging session, you might normalize several times. This also gives you a chance to rest and knock back a cold Dublin Dr. Pepper or other beverage of choice. If it's a small blade that doesn't take long to forge, normalize after you have finished forging to shape and getting everything straight, as your last heat with the blade. Some people normalize again after doing the rough grinding on the forged blade and before final finish and/or heat treatment.

Begin newer data:

Just a random piece of steel isn't likely to have enough carbon to harden into a good knife. The rule of thumb when using salvaged steel for a tool is that if in its previous life it cut, resited impact, or flexed, it probably is hardenable. So things like files, jackhammer bits, car springs, plow disks, car axles, pry bars, etc. The next and more important rule of thumb is to always test a piece before putting a lot of work into it. Heat a blade-thick section of the steel above non-magnetic, then quench in oil. Check it with a file. If it skates across without biting, it hardened well. If it bites in, it may need a faster quench. Heat it again, then quench in water. If the file still bites in, there is no point in trying to make a tool out of it.

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Last edited by Storm Crow on Thu Nov 08, 2012 12:21 am, edited 3 times in total.

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PostPosted: Mon Jul 06, 2009 9:48 pm 
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Very nice! So, I just shaped my first blade today. I started with an old metal file and a very basic knowledge of what I was doing.

To anneal it, I put it on top of some coals from a July 4th barbecue, then piled a bunch of wood on top to get the fire going really well. I let it burn down and picked it up the next day.

It was a little bent, so I hammered it flat, and ground/cut it into the shape I wanted with a hacksaw, hand file and angle grinder. I will throw some pics up later.

What would you do next? Begin the hardening cycle? Or do I need to normalize it?

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PostPosted: Tue Jul 07, 2009 12:29 am 
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If you got the blade hot unevenly while grinding with the angle grinder, you may want to normalize. It should help reduce the chance of warping when you quench. Otherwise, go ahead and harden and temper it.

What kind of a file was it? In my experience, files marked "India" or "Pakistan" are poor quality and often case hardened (the carbon does not go through the whole file, just the outside layer). So if you used one of those, it might not harden well.

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PostPosted: Tue Jul 07, 2009 2:40 am 
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Would you be willing to share more about your "tin can forge". Making a coal forge is impossible here due to high fire season.


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PostPosted: Tue Jul 07, 2009 11:38 am 
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StormCrow's coffee can forge: http://www.zombiehunters.org/forum/view ... 39&t=46806

The file I used is really old, all I know is it is US made. I think I will normalize it because although I did try and keep putting water on it and not hold the angle grinder on there for a long time, it still got pretty hot.

Some other links I have been using for making a simple knife with a simple set-up (definitely check out the GreenPete page):

Basic How-To from Woods Monkey
Mini Forge How-To from Woods Monkey
Awesome Videos from GreenPete

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PostPosted: Tue Jul 07, 2009 1:31 pm 
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Veritas wrote:
StormCrow's coffee can forge: http://www.zombiehunters.org/forum/view ... 39&t=46806

The file I used is really old, all I know is it is US made. I think I will normalize it because although I did try and keep putting water on it and not hold the angle grinder on there for a long time, it still got pretty hot.

Some other links I have been using for making a simple knife with a simple set-up (definitely check out the GreenPete page):

Basic How-To from Woods Monkey
Mini Forge How-To from Woods Monkey
Awesome Videos from GreenPete


Thanks, perhaps Now I can get some heat treating done!


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PostPosted: Tue Jul 07, 2009 6:43 pm 
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Thanks, Veritas! Good links, too.

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PostPosted: Wed Jul 08, 2009 9:33 pm 
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hey, stormcrow, you arent the same stormcrow from the old virtual Blacksmiths Junkyard are you?

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PostPosted: Wed Jul 08, 2009 11:49 pm 
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Fire - Yes. Yes I am. :D Been a while since I have checked in there. Been too busy and been researching many other topics. Obviosuly I'm still doing blacksmithing, though.

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PostPosted: Thu Jul 09, 2009 4:19 pm 
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Awesome, need to get off my ass and do some knife making!!!

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PostPosted: Thu Jul 23, 2009 4:11 am 
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sadly, last time I checked the junkyard was shut down, and winikof had died......

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PostPosted: Thu Jul 23, 2009 9:08 am 
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Yes, but the Virtual Junkyard lives on under a different name run by different people but with the same look and many of the same old names still around.

www.forgemagic.com

Check it out!

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PostPosted: Sat Nov 20, 2010 12:49 pm 
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This is a great thread, and with elkhill's new masterpiece getting people interested in starting to make their own knives, I think it deserves to be resurrected.

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PostPosted: Thu Jan 27, 2011 12:07 am 
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http://www.hossom.com/jonesy/

The old link isn't working, so here are two other places with the same tutorial:

http://www.knife-making-supplies.net/ch ... orial.html

http://knifedogs.com/showthread.php?317 ... -by-Jonesy

I saw this years ago and have been trying to find it off and on to post it here. Someone on another forum linked to it today, so here y'all go.

The only thing I would take exception to would be to please use veggie oil instead of used motor oil to harden the blade, particularly if tempering afterwards in your kitchen oven like he's showing.

This is a lot of work, but I can pretty much guarantee that if you do it right (shaping as well as heat treatment), this will outperform anything you can buy for under $100.

And, quite frankly, I'd rather see a lot more folks posting things like this that they've made than asking, "I have a $20 budget and want a great knife. What can I get?" or any of the blue phone cord ninja nonsense.

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Last edited by Storm Crow on Wed Jul 25, 2012 10:12 pm, edited 1 time in total.

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PostPosted: Thu Jan 27, 2011 12:41 am 
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Wow, that's awesome, and it looks like something I could actually DO. Neat!

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PostPosted: Thu Jan 27, 2011 1:35 am 
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nice! Beats the hell out of the 30 cent hacksaw knife...

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PostPosted: Thu Jan 27, 2011 4:50 am 
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One thing I'd add is that when you get to the quenching stage, you're going to be tempted to swirl the blade around in the oil. Don't do it; you'll just end up warping it.

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PostPosted: Thu Jan 27, 2011 4:59 am 
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I like it! Now to find a piece of steel...

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PostPosted: Thu Jan 27, 2011 6:23 am 
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My son Ben and I have been working on making a knife from an old file, and it turned out great (we think!)...

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as we were shaping and grinding the blade from a file

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the finished product

I have some more steel, and we have plans to keep on cranking out homemade knives...they're not perfect, but we had a great time making the first one, and we'll treasure the blades forever!

Jamie - nfa

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PostPosted: Thu Jan 27, 2011 10:02 am 
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Interesting... :roll:

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PostPosted: Thu Jan 27, 2011 10:07 am 
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JoeTosco wrote:
Interesting... :roll:

why the rolly eyes... ?

OT:

ABSOLUTELY FANTASTIC :)

Looks like a great project. I will have to get going on this asap! I want!


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PostPosted: Thu Jan 27, 2011 12:36 pm 
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Very nice!


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PostPosted: Thu Jan 27, 2011 1:29 pm 
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Jeriah wrote:
Wow, that's awesome, and it looks like something I could actually DO. Neat!



Hey Jeriah if you decide to do this let me know, I'd love to participate and I have some tools that would make the job easier.

mystic_1

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PostPosted: Thu Jan 27, 2011 1:37 pm 
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ZS newb knife build contest??? :lol:

Great stuff Storm Crow!

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