Differential tempering, laminated steel and cracking?

Q

Quy4n8

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Nov 27, 2002
Messages
659
If one has three blades with the edge or core made from the same material and heat treated the same, why doesn’t the edge of the laminated and dif. tempered blade crack when bent beyond its yeild point? Or does it. Do the two non-homogeneous blades return to straight after the stress has been applied? I have often wondered about this, but have not found and answer nor do I comprehend the physics behind the phenomenon. If you have an answer or can point me in a direction, I would be most grateful.

Richard
 
When a piece of steel is bent, the strain that it induced through a given reflection is proportional to the thickness.

So for example, if you took a piece of 1095 1/4" thick, and one 1/8", with the same heat treatment, you would find the 1/8" one bent *MUCH* further before breaking.

This isn't because it is more ductile, or tougher, it is simply that the steel itself has to stretch less to achieve the curvature, if you want to look at the math, calculate the difference between the inner and outer radius and you will see it is greater the thicker the steel, and thus the strain it greater.

On laminates, the fully hard blade is only 1/16" thick, or even thinner, and thus even when full hard comes until little strain when flexed. Take a hacksaw blade, full hard and see how far it bends before it breaks. This isn't because M2 is tough, it never comes near the yield point because of the thinness.

Similar issue with soft spined blades.

of course what you lose with such blades is overall strength, the blades as a whole get *much* easier to bend, how much depends on how much of the blade is left full hard. Some japanese blades get trivial to bend as only a thin strip is left hard and the body can be annealed steel, I have seen some blades be so weak you can slap them off a lg and they will bend around it and stay bent.

-Cliff
 
That makes sense Cliff. The thinner the hardened part of the blade, the less strain you induce on the outside radius for a given angle of bend. From the end of you post, if I understand it correctly, these blades also won't return to their original shape as well as a homogeneous blade will, when both are bent to the same angle. Is that true? If this is the case, it doesn't seem like there is a need for a laminated or differentially tempered blade, unless one expects to place a great deal of lateral stress on the blade.

Richard
 
Yes, they stay bent really easily. Of course there is a *LOT* of variance in the behavior. Some of the better ones have spring hardened spines which give the blade a lot of flexibility with only a little loss in strength. They will retain a lot of spring, the Swamp Rat blades for example. The ones to watch out for are the ones left dead soft - these bend trivially, like you can hit them with a piece of wood and watch a 1/4" blade bend around a 2x4" .

To be clear, many of the blades made in that manner (bend really easily) are made that way intentionally. The Japanese guys look at lateral flexing really odd, they simply don't built their tools to withstand that use. Just like their saws can crack trivially if you lean on them the wrong way, but a western saw is near impossible to break. Just understand their limitations and work within them - or choose another tool.

-Cliff
 
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