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hey guys, I have a few knives I'm going to HT tonight, I was planning on using my forge to heat and then edge quench in some motor oil.

I guess my question is, in what situation would you not want to edge quench? I can't figure out why you would ever need the tang and spine hardened, so why not just edge quench every time?

Also, isn't there less chance of warping if you only submerge the edge?

Thanks,

 

Alex

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Posted (edited)

Hi Alex and welcome!

 

First of all, I would advise you not to use motor oil as a quenchant. It can emit toxic fumes and the cooling rate is too slow for most steels. Canola oil is cheap and widely available and will do fine for a wide range of steels. 

 

Second, an edge quench may be fine, except when your blade needs flex or just may flex. Depending on the proportion of hardened steel, an edge quenched blade will stay bent at various degrees if you bend it enough, while a fully hardened blade will completely spring back. 

Edited by Joël Mercier
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11 hours ago, Joël Mercier said:

Hi Alex and welcome!

 

First of all, I would advise you not to use motor oil as a quenchant. It can emit toxic fumes and the cooling rate is too slow for most steels. Canola oil is cheap and widely available and will do fine for a wide range of steels. 

 

Second, an edge quench may be fine, except when your blade needs flex or just may flex. Depending of the proportion of hardened steel, an edge quenched blade will stay bent at various degrees if you bend it enough, while a fully hardened blade will completely spring back. 

Ahh makes sense. I knew there was a reason! So maybe a filet knife or some such.

I have veggie oil as well, would that be suitable?

Thanks,

 

Alex

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Yes. Pre heat it to around 130°f before quenching. Warmer oil is more fluid so it'll increase its cooling speed. 

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I used old motor oil for my tooling, did my first quench in canola oil. So different, or so it seemed. Firstly, it doesn't stink, secondly it doesn't flash as much in my opinion, thirdly, it leaves a nice black finish!

 

 

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The biggest problem with old motor oil is it's not consistent, and there's no way of guessing how fast or slow it will be without a lot of testing.  It absorbs water, which speeds it up, it stinks, it may be carcinogenic, etc.  Real quench oils are graded by speed in seconds*,  with slow oils for steels like O-1 rated as a 15-second oil, medium oil for 5160 is an 11-second oil, and fast oil for 1095 is a 7-second oil.  Used motor oil can be anywhere on the spectrum.  Hot canola is a 9 to 10 second oil, while cold canola is more like a 13-second oil. This makes it useful for pretty much all oil-hardening steels.  The only downside is it can get rancid, and mice love to drown in it.  You won't know until you quench, and then you'll be lucky to make it outside before you hurl from the smell.  So, cover it tight when not in use!

 

*this has nothing to do with how long you hold the blade in, it's from a test involving a 1" nickel ball and the curie point. If you're nerdy as I am and want to know exactly where it came from, look up "G.M. Quenchometer test."

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i try to edge quench every knife i make unless i know it will get flexed a lot in use. if a blade ever gets bent you can bend it back.

sometimes the blade will bend tip down a bit if you edge quench.

 

there is edge quenching with the whole blade hot and edge quenching with only the edge hot, they are a little different, you can go either way. 

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I never do an edge quench. It just doesn't seem like a good idea to me. Tempered martensite is very strong and flexible. I am one of those guys who through quenches everything, including the tang. Every knife then gets the tang area drawn back to blue with a torch all the way into the ricasso area. 

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I do believe that the ABS journeyman's test would require a differentially tempered knife. The last part of the testing is to put the blade tip down in a vice, three inches from the tip. The blade is then bent 90 degrees. The edge is allowed to crack, but not the whole blade. This indicates that the spine and majority of the blade body is tempered to a much lower level of hardness, probably to around 580 or light blue color. If it was completely drawn down, the knife would most likely bend during the 2x4 chop. Instead, the edge is hard, tempered at around 375, while the spine and body are springy allowing it to flex.

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AS an ABS JS I can speak to the ABS testing.  The test is 4 parts, a rope cut (Speed and sharpness) a 2x4 chop, X 2, with no visible damage ( HT , mostly) shave hair (edge retention)  and the 90 degree bend (HT and geometry).  Personally, I prefer an edge quench, followed by 3, 4 hour temper sessions.  That is probably overkill, but every time I shorted the temper, I broke a blade.  It would be worth testing the process, but finding the time to do it right eludes me.

That said, I have seen blades make the bend test (and that really is the test, anyone with a bit of skill can build a blade that will do  the first three tests) that were full quenched.  One was torch drawn with an OA torch with the edge of the blade suspended in water.  It was drawn to sliver/blue 10 times.  A group of us tried for nearly an hour (failed and gave up and had beers) to break that blade, bending it 90 one way, then straight, then 90 the other way.  I believe that Nick Wheeler's JS test blade was also full quenched and tempered, but I don't know the process.

One of my shop tested blades (not the one I build for the test) bent 90, straight, and 90 the other way 15 times before failing.  That one was 1084, I had a number of failures in 5160 and stopped making them in that steel.

 

If a blade bent during the chop test, I don't think that would fail you, so long as you got through the test.  The MS's prefer a blade that bends like a spring, rather than taking a kink, but that won't fail you either.

 

I use a stop plate in my quench, so that I can get a nice 3/8 to 1/2 strip of hard steel.  You don't need a point for any of the tests, so a square end chopper is what I made, it's easy to HT and easy to get the right distal taper.

 

Geoff

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Awesome, thanks for the great responses y'all. This has become my number one source for all of the many questions I have while bladesmithing. 

I normalized/edge quenched a few blades last night, tempered at 400 for two 1hr cycles, they all came out nice and straight and hard.

 

I'm doubting myself a little with the file test, I don't have experience with it but I'm just comparing the edge to the soft spine and tang, and there's definitely a big difference.

Is hardening sort of a black and white process, where it's either hard or not, or can it be more of a spectrum from hard to soft? Just wondering if I maybe have half-hardened any of them.

Thanks,

 

Alex

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You can get a mixed structure, but the file biting after hardening is usually due to surface decarburization.  After a couple of strokes the file should skate.

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i know of a master bladesmith who differently hardened his test blade but doesn't make or sell a blade treated as such but hes also way obsessive about metallurgy :rolleyes:

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Yeah.  The test blade, as Geoff noted, is meant to do one very specific job, and is not meant to be the "ultimate" heat treatment for every knife.  And our boy you mention is interested in the ultimate heat treatment. B)

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And he no longer recommends JS applicants edge quench the test blade.

 

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