Forging and the physics of plasticity

[Tai Google]

Lately I've been hearing some of the guys from the "metallurgy camp" saying that it takes the same amount of force to bend or flex a soft piece of steel as a hard piece of steel of the same geometry, (or something to that effect). Or that,... heat treating and composition have nothing to do with the amount of force needed to bend or flex a piece of steel.

However,... bending is a type of "deformation", like twisting, upsetting or denting, etc. The harder the steel, the harder it is to deform. That's why we harden it! Hard means harder to deform. So, I don't think the original statement makes sense from the standpoint of physics.

[Tai Google]

The composition of the steel also effects the hardness or plasticity. Generally speaking the greater the alloy content the stiffer or harder the steel.

[Martin Brandt]

Forging an integral well like this one of Tai's is so far one of the hardest things I have done in smithing. I usually end up hitting the bolster area and damaging the nice transition to the blade, making it a much longer transition than I had intended, after I clean it all up. So one day when the thought came to me to make a big gaucho blade (12") (like Sfreddo and the other Argentine smiths) from some train coil spring, I pondered the doing of it for a while. I didn't want a long transition like I usually get. I wanted the blade to start about the same proportions out from the handle as on other knives. Started forging, and found the 1 3/8" 5160 rnd. was going to be the death of me, so saved it for a day at a friends shop with power hammers and helpers. Got the stock down to flatter with the power hammer, and then over edge of anvil, and with a top fuller and directing a helper deliver blows we got the blade edge drawn down and back towards the handle closer than I had ever been able to before. Also had to work the bolster up towards the back of the blade in stages as Tai had said. I used a large oak schwauker (club) to do this to avoid hammer marks on bolster. This must be done while very hot, don't wait til the later part of your heat. Also start early to move it up as you are flattening the blade out. As soon as you start getting a swelling above the bolster, take a fresh heat and club the bolster with powerful blows , with the blade resting on its back on the anvil. I also use this schwauker alot when forging bevels in as the straight blade wants to become curved when forging bevels in. By directing the fuller at a steep angle a bit towards the edge, but more towards the bolster we were able to really direct it where we wanted it, and by using the top fuller there were no "Oh ####!" blows accidentally hitting the bolster. I just don't yet have the hammer control to give repeated full force blows to within 1/4" of accuracy, if I ever will. But looking at my blades now, forged to shape within 1/8- 1/4" in profile as compared to several yrs ago when they were blobs somewhat knife shaped that i drew around on paper, then figured what knife would fit into that outline, I can see that practice makes perfect, and watching other masters working helps a lot to expand ones concept of what can be done with the plasticty of steel. I have learned much about upsetting blows and the power needed behing them to set up the metal for other more gentle shaping blows. How a well dressed cross pein works better at pulling the edge bevel out than the hammer face which spreads the metal in all directions. How the point can be drawn out of a squared bar end with a combination of upsetting blows, and drawing blows over the horn, or with the cross pein, or with a hammer like Tai's raising hammer. Forge something every day that you can to develop your hammer skills, and never pass up a chance to watch, or pick the brains of others to learn how to move this wonderful metal.

[Tai Google]

Couldn't "hardness" be defined as the ability to resist deformation?

Is ?strength? the ability to resist deformation, or the ability to tolerate deformation, or both,? or neither?

http://en.wikipedia.org/wiki/Stiffness
http://en.wikipedia.org/wiki/Plasticity_(physics)
http://en.wikipedia.org/wiki/Elasticity_%28physics%29
http://en.wikipedia.org/wiki/Deformation
http://en.wikipedia.org/wiki/Deformation#Metal_fatigue

[Thingmaker]

"Hardness" and "strength" are kissin' cousins. The metallurygiurigeristsistsis are right, it takes the same amount of oomph to bend 62 rockwell as it does 26 rockwell - all else being equal. But the 26 rockwell will stay bent! That's the "yield" thingie in the wikipedia article Tai sent us to.

I'll crawl back under my rock now.

[Tai Google]

It's the sliding "Y" thingy.

This may not be "metallurgically correct" but...

Intuitively...

If you take three pieces of steel with the exact same composition and geometry, one hardened, one hardened and tempered, and one annealed,... in the final analysis,... The hardened one will be the hardest to bend. The hardened and tempered one will be the most springy, and the annealed one will bend the easiest.

They are only equal up to the elastic limit of the annealed piece.

[Tai Google]

"Facts" about intuition:
http://en.wikipedia.org/wiki/Intuition_%28philosophy%29

http://www.angelfire.com/hi/TheSeer/intuition.html

http://www.awakening-intuition.com/

The Myth of Magical science:
http://www.dharma-haven.org/science/...al-science.htm

http://www.dharma-haven.org/science/...fic-method.htm

[Tai Google]

... intuition is more efficient than science! Ha Ha LOL

[prizzim]

There was a discussion about this a year ago on Fogg's forum, I just looked it up again.

The devil is in the difference between the terms "bend" and "flex".

Flex... metal gives way to the weight, but returns to original shape.

Bend... is deformation, where the modulus of elasticity has been exceeded, and is now deformed.

Two bars of steel, equal in composition and dimension, bolted down at one end with a weight hung on the other, will flex equally until the MoE is exceeded in the softer steel, which has a lower elastic limit than hardened steel (ie, you can bend a piece of mild steel in your hands, while an equal size piece of hardened and tempered O-1 will not be as easy, though both may flex before returning to normal).

If you're still in doubt, Tai, forge out a long blade (the longer the better for this experiment) or take a bar of factory steel that you can get a consistant heat treatment on, and bolt it down at one end and measure the flex with a weight; then heat treat it, and measure again... they should be equally flexed in both occasions.


Now, you wanna get trippy with it, the whole issue of "feel" in the different crystaline stages of steel will yield different vibrations, and affect the resonance of the material, but will not alter the elastic limit.


Of course, as you've already said, once the elastic limit is exceeded, yes "harder" means "harder to deform". So you, and your information sources, are both correct... as long as we're being specific about what you're describing. Again, it's the difference between Flex and Bend.

[BlueSkyJaunte]

Man lots of good info here but my head hurts now.

[Tai Google]

Right Chris, but it's a minor point.

The three bars of equal geometry, each with different heat treating will "flex" (bend and return to true when the load is removed) equally up to a point,... the limit of the elastic range of the annealed bar. After that point the heat treating will show very dramatic differences. What bladesmith works within the elastic limit of an annealed blade? The whole thing is silly, and has caused so much confusion. They have tried to take this simple point too far, and have focused in only on one side of it,... which really isn't that applicable to bladesmithing.

Sometimes I wonder what planet these guys come from...

[Tai Google]

... Of course when you "flex" a blade it never really returns to it's original state internally because of metal fatigue deformation mechanisms.

[Tai Google]

Originally Posted by BlueSkyJaunte

Man lots of good info here but my head hurts now.

Don't worry about it! Just go with your instict and intuition.

[Tai Google]

There's really no way to eliminate intuition from the bladesmithing equation,... without eliminating the bladesmith. However, we do use "facts" to feed our intuition.