Flex in hardened blades.

[kbaknife]

I have found not only myself, but a number of close friends enjoying making fillet knives. And there have been a number of makers making more and more thin kitchen knives.
One aspect of both of these is flex.
And not just flex, but flex in hardened blades.
Oddly enough, I have found it a common misunderstanding that hardened blades don't flex and must be softened at the spine so as to "flex" more easily.
I digress to about 12 years ago around 2003 when I used to attend the Ashokan Seminar in New York state. The organizers of this event were master Smiths Tim Zowada and Kevin Cashen. They always did a demo where they took two knives of exact dimensions and subjected them to flexing/bending tests. The results were always the same. How a piece of steel flexes/bends is a matter of geometry - not hardness, or lack of.
They made a bending "thing" that clamped their blades of identical geometries and steel type, and then connected heavy weights and measured the amount of flex in both inches and weight.
Without fail - both blades bent exactly the same.
Up to a point.
What will happen, eventually, is that the unhardened blade will eventually take a "set" in that the steel structure will yield to the bend and the hardened blade - with an entirely different structure - will return to precisely straight.
Some folks confuse the torching of blade spines to flex. This is not the case.
When we over-temper a blade spine, we're simply making it so it won't crack, but it will take a set - big time.
This is done mostly in heavier dimensions like Bowie knives, etc.
For the most part, I would always prefer fully tempered martensite over unhardened pearlite. It's much tougher.
But in thinner blades, properly tempered, fully hardened is the way to go.
Think of your commercial fillet knives, like a Rapala.
Flex one if you have one.
It goes right back to straight.
It does that because it's hardened. If it had a softened spine, it would take a set.
So, if you have fillet knives or kitchen knives on your horizon, make sure you do fully hardened blades with good grain control, and temper them accordingly.
As an example, a friend of mine of Facebook just did an example of a fully hardened 52100 blade.
I hope this link works.
Keep in mind that if the spine was soft - that thing would look like a horseshoe right now.

   

[Lin Rhea]

Excellent point Karl.

I saw a guy chop a 2 X 4 and he make a mis-lick  slapping the blade on it's side. It took about a 10 degree set. You should have seen his face.

He, of course , had drawn the spine down dead soft "just like he was taught" in the class. The ABS class will teach you how to pass the performance test but in the real world will that be satisfactory performance? No. A dead soft spine will be good for one thing. Passing the test. I want mine to have spring to it. After all it's basically spring steel. We are making our blades into a spring with a harder, thinner edge for cutting.

I wonder exactly what the heat treat procedure is for truck and car springs? I bet they don't draw any of it back dead soft.

[gudspelr]

I always enjoy getting to read good stuff like this. Thank you both for taking the time to share.

Jeremy

[akaboomer]

I agree completely Karl. I took this picture testing a recent fillet knife. I had flexed it much farther, but it was not showing it well in the picture.  
   
Hope you don't mind me chiming in and sharing
Chris

[kbaknife]

Originally posted by Lin Rhea:

I wonder exactly what the heat treat procedure is for truck and car springs? I bet they don't draw any of it back dead soft.

I'm sure none of it is dead soft in auto use. It's always fully hardened. Then they get the spring action by tempering.
We might temper fully hardened 5160 at 350 or so, but for a spring we would temper about 500 or a little higher.
Also, a lot of leaf springs are heavier and thicker in the center and taper and get thinner toward the ends. As the load gets heavier, it transfers to thicker portion of the leaf spring. But, of course, they are fully hardened to avoid taking a set.

[kbaknife]

Originally posted by akaboomer:

Hope you don't mind me chiming in and sharing
Chris

Absolutely not. Great picture.