Why do knife blades only bend once?

M

Michel Droz

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May 4, 2015
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I am into second-hand pocketknives. Alot of times I buy nice knives because they are interesting models and I can get them for cheap, especialy after showing the vendor the bent blade. I put them on my anvil and tap around a bit with a brass-headed hammer. Sometimes I can get it straightened perfectly. But quite often the blade breaks and after regrinding it I have an OK knife that is 3/4 of an inch shorter or it ends up in the " for parts box ". Does anybody have any good tips for me so I don´t loose to many of mine. PS: Why do I get so many pocketknives with a bent blade because some previous owner used it to pry open a can of paint or it has a half-moon ding on the blade because it was used as a screwdriver when it has a 2 screwdrivers, a caplifter, and an awl on it? Some real fools out there.....
 
Fatigue or lack of ductility. Ever bent a coat hanger back and forth a bunch of times until it snapped? Same principle. The harder steel just takes a lot less bending to do it because it's way less ductile. The harder a steel is or the greater the degree of bend, the less likely this cold straightening will work.

If you want to guarantee success at straightening the bent blades as well as that is possible, anneal the blades, bend them straight and then reharden them. Don't forget to temper.

And yes. Many people are idiots.
 
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What you are seeing when a blade is permanently bent is plastic deformation. Note that in order to cause plastic deformation, some of the bonds holding the atoms in the blade together are permanently broken (see below). So when you straighten the blade, the damage is still there. I believe what eKretz suggested is a way to actually repair the damage done to the steel.

Elastic/Plastic Deformation
When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves the stretching of the bonds between atoms, but the bonds hold and atoms do not slip past each other.

Plastic Deformation
When the stress is sufficient to permanently deform the metal, it is called plastic deformation. Plastic deformation involves the breaking - not just the stretching - of a limited number of atomic bonds. While the force needed to break the bonds of all the atoms in a knife blade all at once is very great, a much smaller load will cause the atoms in the steel to permanently slip past one another and cause permanent, plastic deformation.
rummels
 
Their answers are of course correct.
The short answer is harder means more brittle and knife blades are hardened.
 
Michel Droz said:
I am into second-hand pocketknives. Alot of times I buy nice knives because they are interesting models and I can get them for cheap, especialy after showing the vendor the bent blade. I put them on my anvil and tap around a bit with a brass-headed hammer. Sometimes I can get it straightened perfectly. But quite often the blade breaks and after regrinding it I have an OK knife that is 3/4 of an inch shorter or it ends up in the " for parts box ". Does anybody have any good tips for me so I don´t loose to many of mine. PS: Why do I get so many pocketknives with a bent blade because some previous owner used it to pry open a can of paint or it has a half-moon ding on the blade because it was used as a screwdriver when it has a 2 screwdrivers, a caplifter, and an awl on it? Some real fools out there.....
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Thanks for all the feedback and tips, but they are not too useful to me because I only collect pocketknives. It would not be worth the effort of taking them apart, annealing, straightening the blade, tempering, polishing, rivets, filing, and so on. I will keep on with my cold hammering process and take the risk of breaking the blade. Have a nice day, Michel Droz.
 
M Michel Droz : have you tried the 3 pieces of wood in a vise trick ?
That is what i use to straighten certain bent knife blades.
The process is much more gentle compared to hammering, which reduces the chances of breaking a blade considerably.
 
kwackster said:
M Michel Droz : have you tried the 3 pieces of wood in a vise trick ?
That is what i use to straighten certain bent knife blades.
The process is much more gentle compared to hammering, which reduces the chances of breaking a blade considerably.
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Hello! What a coincidence! I spent most of my free time today trying to set up just that kind of " device " on my vise. I foünd it in Wayne Goddard´s $50 Knife Shop book. But now it is 8:30 PM here in Switzerland, and I really need a couple of cold beers, so trying it out can wait untill tomorrow. Have a nice day, Michel Droz.
 
A little tape wont hurt the process but save your blood pressure.

Trying to align 3 rods, a blade, and tighten the vise is a challenge
 
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