Heat Treating for the 90 degree bend test

theWeatherman

Well-Known Member
So I know there is a couple different ways that would allow you to Heat Treat the blade to allow for the 90 Degree bend test for the JS test.

However, I know that you can use a torch and edge quench and that would work but what about other methods to differentially Heat Treat that would allow the blade to pass?

I am guessing that using clay wouldn't allow enough change in hardness from the edge to the spine. So what about drawing back the spine? You could use a torch on a fully hardened blade and draw back the spine by sticking the edge in water or sand and then heating the spine to a blue/grey which would temper it and making it springy.

What would you need to look for to be able to draw back the spine or differentially harden the blade so it will pass the 90 degree test?

-Brian-
 
My advice is to never harden the spine (on a "test" blade). For a lot of years I've supervised many JS and MS tests. Those who fully harden, and then try to "soft back draw" the test blade(s), have about a 50% pass rate. My advice is to follow the information I've laid out here: http://www.caffreyknives.net/journeymanarticle.html

The short answer is to "edge quench" the blade. Remember....your creating a blade to pass a given set of circumstances.....and you want the best chances for success that you can give yourself.
 
My advice is to never harden the spine (on a "test" blade). For a lot of years I've supervised many JS and MS tests. Those who fully harden, and then try to "soft back draw" the test blade(s), have about a 50% pass rate. My advice is to follow the information I've laid out here: http://www.caffreyknives.net/journeymanarticle.html

The short answer is to "edge quench" the blade. Remember....your creating a blade to pass a given set of circumstances.....and you want the best chances for success that you can give yourself.

Thanks Ed. I have seen your page. Seeing that and reading it a couple times, as well as seeing some other people talking about the ABS and testing has got me thinking about some of the different ways to do it. I was wondering if it could be done another way, other than edge quenching? Of the 50% that passed half of them did something correct.... I just ask so I can understand some of the more detailed and intricate parts of heat treating. There are a lot of different ways to do things. So does that mean that edge quenching is the only way to make a good knife, if it passes the JS test? OR is it done that way to pass the test?
 
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Ed, another good question for you, since I have been thinking about it for a bit would be. Why is the success rate only 50% when doing drawn back spines?
 
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It's not drawing back the tang, it's drawing back the spine. It was explained to me that it's the blade, not the handle that has to bend 90°. The edge of the blade can break but the spine has to stay whole. Check the regs to see how far up the blade the edge is allowed to crack. Also don't stamp your blade with your makers mark. Etch it. I was doing a 90° bend test on a blade that I was testing to destruction. It was a blade from 52100 that I had heat treated to give me a mixture of lower bainite and martensite and I had the blade just short of 90° when it snapped. The thing is it didn't break where the blade was bowed the most. I broke right up by the handle where I had stamped it with an identifying letter so I wouldn't get it mixed up with another test blade that I marquenched and tempered. That one bent right where the handle joined the blade and I had to beat the crap out of it with a 4 lb maul to get the blade to break. Both had real smooth grain to them, though. I was happy about that.

Just an FYI. When you check the ABS regulations go to their site and then call them up. There are some links out there that will take you directly to some regs but they are evidently old standards and are not up to date. Even before the changes that happened about a year and a half ago they were missing a lot of detail. It's even possible that those links were never the official standards.

Doug
 
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Doug, your right. My first post said spine and then that above post said tangs for some reason. I fixed it. Thats for the info.
 
I'll give you the explanation that Ed gave me. The knife for the performance test is a test of the knifesmith not the knife. It is to show that the maker has a certain set of skills that makes it possible to produce a knife with a given set of characteristics. This is not a standard that knives should be made to nor do you ever have to make a knife like that again unless it suits your purposes.

As for why an edge quench instead of a soft draw goes, with a edge quench you will have pearlite in the spine. That is less strong and more ductile than tempered martensite, which would what you would have if you did a soft draw on the spine. Once you create martensite in steel it will be there until the steel is austinized again. Regardless of how hot you temper at, some martensite will remain. If you temper the martensite hot enough you may release enough carbon from it to restore enough toughness and ductility to allow a 90° bend without the spine breaking. You also can't just heat the outside of the spine. It has to be all the way through or you just have a lower carbon martensite on the outside of the spine and the core of the spine has not changed sufficiently to bent 90° without breaking. The most sure way to achieve the 90° bend is to edge quench or choose a shallow hardening steel that will not harden in the spine

If you choose the shallow hardening steel route to achieve a pearletic steel spine, however, you will run into some more problems. The spine will have to be thick enough and the grain fine enough that the the steel won't harden in the spine. You also could lose some wear resistance if you don't choose your steel well and fail the edge retention test. You could increase your chances by clay coating a higher carbon simple steel but the simplest way to go is choose something like 5160 and just simply edge quench. Steel selection and knowing how to heat treat that alloy is a big part of the test. If you can achieve it with another alloy and a different heat treating method, fine, what Ed laid out is only his suggestion based on what he has seen work.

Doug
 
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I’ve wondered whether or not there might be more to the edge quench than what’s been stated and if traditionally it might actually have been more about edge quality and geometry than the whole 90 degree bend thing.

Heat treating a triangular or wedge shaped cross section is much different than heat treating a rectangular cross section, much more complicated and problematic because of the “temperature gradient“. So much so, that it is often actually considered a design flaw which should be avoided or at the very least dealt with. The problem is that there are different heating and cooling rates, differences in expansion and contraction etc., between the edge and the spine. In essence the whole edge is a stress riser.

Perhaps the edge quench is one way of dealing with this problem. Maybe it allows for higher quench speeds without as much possibility of cracking and minimal stock reduction post heat treat.

It talks about design influences on heat treating in this article.

http://books.google.com/books?id=SX... part geometry on heat treating steel&f=false

I'll start a new topic for this:
http://knifedogs.com/showthread.php?29184-Edge-Quenching
 
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I have to admit that I have some problems with the ABS standards, but they're more about how what's not in them than what is. I think that edge quality and geometry are addressed with the rope and 2X4 cut. Do I think that the 90° bend test is realistic? No, I don't. I think that I could make a knife that would pass that part of the test, and probably the others too, and be a rather inferior knife by going for a totally bainitic blade. It would be far too easy to bend but it might not even crack in the edge. Edge geometry might be a little bit of a trick to give the edge more support, but it might be doable. That said, the ABS had to establish some standards and they wanted one that dealt with blade durability. Other than change to degree of the bend, which is a matter of judgement, I can't think of anything different.

Doug
 
Just a note about the mechanics of bending a blade. You can make the very edge as hard as you want, as long as it's in the center (not a chisel grind). Take a sheepsfoot profile and harden the edge and bend it. Then take a cutoff wheel and cut down the edge, say 1/8" or less up the blade. The edge will spring back straight. A certain distance up the blade from the edge is where the bending starts. The trick is how high up the blade needs to be hardened. I've bent small blades 1mm thick that were 64+ HRc spine to edge and they just go back to straight, and did it as many times as I care to try. There is a certain thickness above which such a blade will just snap. Stay below that and it doesn't really matter how many times you bend it. The problem is a knife that thin just isn't heavy enough to do the rest of the tests.
 
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