Steel Toughness Issues

M

me2

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Oct 11, 2003
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Doing some tinkering this weekend on some steel blades, a question came to mind. Are high hardness steels as easily shattered as they are made out to be? I remember reading about a concrete floor drop test that some high hardness O1 failed. I tried it on a 1095 knife I made in the garage and didnt have any trouble, even though I aimed to get the knife as hard as possible. A friend using the same heat treating techniques on a piece from the same bar got a tested hardness of HRc 65-66. I didnt have access or I would have tested my knife as well. I also tried the drop test on a knife made from a file that tested out to HRc 64 after the teeth were ground off. Again, no problem on this one either. This weekend I took a ball peen hammer and a 3/16" diameter brass rod to a piece of M2 in an attempt to punch a hole in it. I would have used a steel punch, but didnt have one in that size. I cracked the paint on the surface of the steel, but didnt crack or even scratch the steel itself after 5-6 solid blows. I just wonder if the issue is not made out to be worse than it really is.
 
But that said nothing about impact. He is just showing the stress-strain curve.

I think geometry is a big factor, and I think it will depend on how you hit it, how it drops. But I know that I have lost a tip on a knife in 5160 just by stabbing it slightly of center (read clumsily) into a thick branch. I also know that Mike Steward has had A2 blades that were hardened for test purposes higher than they usually run their blades, break by dropping onto the shop floor.
 
I thought it would help emphasize the importance of geometry.

A drop test seems a little random. The knives would have to be built the same, especially tip dimensions and weight. They need to not tend to rotate, need to be dropped from the same height, gotta make sure it isn't landing the tip into a big rock in the aggregate, stuff like that.
 
I've done a little bit of concrete floor testing with some 63 Rc AEB-L and dropped from a couple different angles as well as point straight down and never had any problems. For a pure cutter this is plenty of toughness for me. However, there are many with Japanese kitchen knives that are 63+ Rc that break their tips, the tips are very thin, and the White and Super Blue steels are significantly more brittle than 1095, M2, AEB-L, etc. Here's an example of a pretty large chip in an extremely hard M2 blade with a very thin hollow grind: http://www.cutleryscience.com/forum/read.php?7,92
 
Larrin said:
I've done a little bit of concrete floor testing with some 63 Rc AEB-L and dropped from a couple different angles as well as point straight down and never had any problems. For a pure cutter this is plenty of toughness for me. However, there are many with Japanese kitchen knives that are 63+ Rc that break their tips, the tips are very thin, and the White and Super Blue steels are significantly more brittle than 1095, M2, AEB-L, etc. Here's an example of a pretty large chip in an extremely hard M2 blade with a very thin hollow grind: http://www.cutleryscience.com/forum/read.php?7,92
Click to expand...

Oh no, my idiocy is out there for the masses to see! Actually, I really had no clue what steel that knife was made of when I used it, but that was a righteous chip I took out of it. With that thin hollow grind sharpened flat to the stone it was inevitable that once I made an idiot twisting cut the knife would chip bad, no matter what steel it was (though a softer and tougher steel may have just bent bad). What surprised me was how my DMT XX Coarse blew through that M2 like it was AUS 4 when I sharpened it, but I was using massive pressure and that stone is like a belt sander. After I got to good steel and finer grit sharpening I started seeing the signs of the high hardness of the steel, as it would microchip on ceramics but cut cleanly on waterstones, as I have had happen with Super Blue and ZDP before.

Mike
 
Ok, nevermind about the M2. It took a few good whacks, but when it finally broke, it cut loose into about 6 pieces. I did have to move up to a steel punch though. I couldnt break it using the brass rod. Oh well, back to the drawing board.
 
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