When "Good" Knives Break

Originally posted by Jeff Clark
Minuteman, you helped me decide what I'll pick as my next survival type knife. A knife that is optimised for toughness and has been tested seems like a natural choice.
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Ok, so let's take this discussion to the next level: based on Jeff Clark's conclusion here, what blades do we know for certain fit the criteria of (1) optimized for toughness (blade, tang, and handle) and (2) tested? Warrantees don't count towards toughness.:grumpy:

I'd say for starters the Swamp Rat Battle Rat is optimized for toughness (blade, tang, and handle) and they are individually penetrant tested.

Cliff recommends the HI khukuris which are known to be very tough (with the possible exception of the handle attachment in some instances) and are highly polished and then visually inspected by multiple individuals.

I keep going back to the Martindale Golok (which is very tough due to a simple slab handle and a soft steel blade). These are not individually inspected as far as I know, but due to the softness of the steel, the likelihood of failure is slim. You could probably fold one in half.

Tramontina machetes probably fit the description above, as well.

What else? Please include your rationale for the knife being "tested".
 
Most guys who forge blades make very tough and dependable knives. If you take a steel like 5160 or similar, spring back and hardened edge, it will take a massive effort to significantly damage it enough to effect the functionality. The main problem with knives breaking is the choice of steel or heat treatment which has radically different inherent abilities than what is required for the indended tasks.

For example I have used two of Ontarios machetes which NIB shattered on hardwoods, they came to pieces losing big chunks of the blade (quarter sized and larger). The Patrol Machete from Camillus also behaved similar, throught multiple samples. On the other hand many Martindale machetes did this without fracture, and if reprofiled so that they were too thin, they would just bend, damage which could be hammered out.

-Cliff
 
Originally posted by Cliff Stamp
For example I have used two of Ontarios machetes which NIB shattered on hardwoods, they came to pieces losing big chunks of the blade (quarter sized and larger). The Patrol Machete from Camillus also behaved similar, throught multiple samples.

-Cliff
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Cliff, based on the Wayne Goddard piece, from my personal experience, and from the discussion above, I'd bet money those knives you mention that broke had cracks in them, though it's probably too late to prove that now.

Wayne suggests that cracks most often occur at the time of the quench, and usually propogate out of a sharp corner at the tang or from somewhere on the edge itself, when it has been sent to heat treatment with too coarse of a grind (i.e., infinite tiny saw-toothed stress risers, all along the entire edge, just waiting to send a crack up into the blade when it goes into the oil).

Conversely, if these same knives went to heat treatment with polished surfaces and edges and good sized radii in all the corners, they may have came out just as strong as any other knife.

It is possible to ruin a blade in heat treatment by overhardening the material which would make it excessively brittle. However, I think this would be much easier to control (i.e., I should know the blade alloy and temperature and quench oil type and temperature). I guess they could also inadvertantly skip tempering steps, but I think the real production wild cards are surface flaws, sharp corners, and rough grinds.
 
This is possible, however I have heard horror stories about production heat treating methods, such as quench tanks not being cooled or even circulated which means the temperature changes from batch to batch. Tom Mayo has often described over filling ovens with poor circulation so that some blades are get too hot and others left cold. Both of these effects could produce blades too brittle and blades too soft. The point Goddard makes is a good one however, it doesn't hurt to check the blades, a decent magnifier is inexpensive.

-Cliff
 
Originally posted by Cliff Stamp
This is possible, however I have heard horror stories about production heat treating methods, such as quench tanks not being cooled or even circulated which means the temperature changes from batch to batch. Tom Mayo has often described over filling ovens with poor circulation so that some blades are get too hot and others left cold

-Cliff
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Well maybe I've been naive. If manufacturers are doing things like you describe above, then they really are screwing up the heat treatment processes.

There are always two processes for everything: (1) The one that was intended and (2) The one that was implemented. Number (1) is pretty straight forward for most alloys. Number (2) is always subject to murphy's law, etc.

However, the stuff you are talking about here is cost cutting negligence, not Murphy. No wonder they don't want to do much inspection.
 
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