This thread got me thinking about my knives strength. So today I intentionally broke one of my knives for the first time. It was awesome and had the most beautiful fine tight crystalline structure in the edge half. The top part that wasn't in the quench had a layered look to the break almost like rope. It did bend way over and on the second bend back is when it broke. I was happy with the results being such a small knife (4 1/2" OA). I wonder though what the top half of the knifes grain should look like. Does anyone have any pictures of this?...I did not do any cutting tests with this one it was made just for breaking.

Danny: How soon are you getting your blade into your oven after the quench. This is a critical time frame.

Hello Danny. I know that you are frustrated. I looked at your heat treat regimen and noticed some things. Are you using the 1650 for quench temp? If so that is high for 1084, it should be around 1500. Also when I draw the temper on 1084 I use 425 degrees for two hours and two cycles. Drawing at 350 would probably give you a rockwell of 63-64, so the file may be right on. Although at that hardness the edge should not have rolled but chipped. Also by using a fast quench oil you may have gained some hardness as opposed to a slower oil. You said you took the edge down to .020 the first time, there is a possibility you got it to thin as the .020 line moved up the blade. Just some ideas.

By the way the round of tests you did sounds fine and the first run was pretty good for that blade.

Oh, when the tip broke, how was the grain structure?

Keep doing what you are doing.

Brion

Jared, right after the second quench, while the knife is still warm, I ensure the blade is straight then right into the preheated oven. Brion, I take the forge to 1650 to give myself a little more heat than critical to compensate for the temp loss between the forge and the oil. Basically, after my 3rd thermal cycle I bring the blade up checking it constantly On the magnet when I get close to my color, then right at critical I go in and Out of the forge a couple times quickly to go just above non-magnetic then pull it out and over to the oil which isn't to far from the forge. I would venture to guess the blade is closer to 1500/1550 by the time it hits the oil. I thought the grain looked good on the broken tip. After I do a little more sharpening and cutting I'll break it in the wider part of the blade to see it a little better. Thanks guys, Danny

Sounds good Danny. I usually take my before sharpening edge down to .020-.025 for hunters and .025-.030 for choppers.

Your methods and sequence sound fine. Just do some tests with different tempering temps and see which works best for your method. Like I said I use 425 for my temp on Aldo's 1084, and I do quench in Parks50. I usually leave it at room temp, which these days is around 90 degrees. Got to love Oklahoma summers. Keep on testing and let us know. I kind of figured with your thermal cycling you probably would have good grain.

Brion

Ok, to finish up here. Did one more temper then back to cutting. Didn't see much difference with the last temper at 325, I think my edge is already to thin, but I definitly found that the best cutting results for me were yielded when I had the edge right at .020. When I pulled that dimension up the blade the performance seemed to get worse. I will definitly do another blade with the .020 edge and test higher temps with my tempers. I went ahead and broke the blade too, this pic was taken with 10x magnification, maybe it makes sense. Any feedback on the grain would be appreciated. I definitly learned alot and I truly hope, even in some small way, this thread will benefit others as well. Thanks, Danny

Danny,

I do see a problem with the heat treat. You say "I take the forge to 1650 to give myself a little more heat than critical to compensate for the temp loss between the forge and the oil." Okay, that is not is correct. You do not want the blade to rise above the critical temperature. IT is not how hot the blade is right as it goes into the oil. It is how hot the blade got during that heat treat cycle. So, 1650 is just too hot for that steel and I would argue you are probably getting grain growth you do not need especially on the second or last quench. It is time and temperature and in this case the temperature is too high for this steel.

Here is another factor to consider. Harder does not necessarily equate to better edge holding. I would argue that a small blade (1084) at 60 RC gets better edge holding than the same blade at 65 RC. Abrasion resistance is what you are striving for and in certain mediums you cut through micro abrasions can be more frequent with a 65 than a 60 RC blade. Often we talk about a really fine sharp edge having a slight burr on the edge (too big a burr and it lies over and that affects sharpness). If the edge is too hard that burr chips off quickly. The result is dull. It is not chips you can necessarily see with your eyes, it is chips nonetheless.

Now this is all relative to the steel being used. Some steels will show the best edge holding at 64 and others maybe at 59/60. It all depends on alloys that inhabit that steel. My experience suggests that with 1084 you are too hard and I question your grain size. A smaller grain size will result in better strength and edge holding.

One last comment. I am not a proponent of testing the edge of a knife on aluminum cans or any other hard metal. If I want to cut metal the edge geometry will look more like a chisel than a knife blade. Even engraving tools have a sharp but chisel shape to them. I do not use a knife to cut aluminum or other metals. I use wire cutters or tin snips. Aluminum oxide is what they put on grinding belts and next to diamonds it is one of the hardest substances we have. Why would I try to test a blade on aluminum? Now the only value I see in aluminum cans for testing is whether the edge chips when I cut them from top to bottom in one slice using a knife big enough for that purpose. But that is not a test edge holding.

I hope that helps some.

This picture shows a burr curling off the hardened clip on a knife I recently finished.

The burr behaves in a particular way if you ever have opportunity to check it. In the above case, the burr was stiff but not brittle. That same behavior is appropriate for the blade edge on a different scale. The behavior of your blade is just not consistant with an edge that is hard enough and yet tough enough to be sevicable. You recognize that and that is a very good thing.

I agree with Dan. More is less when the "more" becomes too much. We want as hard an edge as possible till we start losing toughness. Testing behavior (in accoordance with the knife's intended use) is how we know where that tipping point is.

Hey Danny, here's my suggestion. Make a 10 in. blade... nothing fancy just a good chopper. I think you will get a better idea of the cutting potential with this size. To me the grain structure looked too big... ideally it should closely resemble a brake in glass. Try this for a thermal cycle process...just above critical, then cool--just below critical, cool--dull red, then cool. I like to thermal cycle after the grind, just before heat treat, to relieve stresses caused by grinding. Test after heat treat with a new file...it should skate on the cutting edge. After tempering test with the file again...it should just grab. Thin the edge to thirty thousandths and roll at about 20 degrees to the cutting edge. Sharpen and test. I like to test on seasoned antler. Give it a good chop, if the edge geometry/heat treat holds up ,it should handle any reasonable cutting task. Watch for edge distortion, chipping, rolling etc. You may have to play with the geometry to get a properly supported edge. After this move on to cutting free swinging rope...this will test the edge geometry. Chop some 2x4's, if the blade has performed well so far, you should be able to wear yourself out on 'em...don't avoid the knots! Good luck, Ben

An after thought. I'm not sure the five minute soak is necesary. I've been working with 1075 and no soak is necessary. Perhaps Lin Rhea could clarify this...I think he's been working with 1084. Another thing, in my opinion the closer to critical on the quench the better. I've been working with hamons and using a low critical quench for the pictoral effect of the hamon. This seems to result in a tougher blade, capable of thinner edge geometry. A low critical quench with 5160 seems to mirror those results. All the best, Ben