Is there a sttructural enginer who wants to talk theory?

Ed Fowler

Ed Fowler

Joined
Jul 21, 2001
Messages
2,869
I have been exploring the frontiers of tension and compression in differentially hardened blades. If you are interested please email me.
 
From a metallurgist's viewpoint I don't think there's very much to gain with playing with tensile and compressive stresses as far as blades go.[that's how the curve is created in a katana] In other applications they can make a very substantial difference in performance.
 
Mete,

When you say "not very much to gain", is there something to be gained or not? Seems to me that when someone is reaching for the stars any gain, no matter how small, can potentially be substantial.
 
At one time I was working with tool steel draw dies [donut shaped] and the steel was shallow hardening. We quenched it by shooting brine through the bore , leaving the outside to cool slowly.The compressive stresses caused by the differential hardening were tremendous and we never saw a broken die ! At one point they changed the steel making it much higher hardenability.The effect was no longer there and we then started to see broken dies .So there differential hardening had a great effect.If we look at a katana ,the fact that the blade bends means that some of the stresses have been relieved.As some of you have found stresses can also be relieved by breaking as in quench cracks!! Whatever stresses are left , what do they do for you ??Do they make the blade harder or tougher ? If you quenched in a fixture so it wouldn't curve the stresses would be there but I still wouldn't know what they do for you.
 
Mete: You have just described the dynamics I seek to explore further. When a blade is only partially differentially quenched, a martensite cone forms in the center, the transition zones as seen on the outside of an etched blade reflect what happens on the inside and the structural laminations that form from the cone to the outside of the blade. When done right from forging to finished blade these can develop a blade that is much stronger and tougher than a simple uniformly hardened blade. I believe they act much like the old horn bows that were laminated with sinue on the outside of the stress.
Again I refer only to 52100 steel forged carefully.
 
British longbows were made from yew and cut carefully so that sapwood[strong in tension] was on the outside and heartwood [strong in compression] on the inside.Some oriental bows were laminated IIRC, and of course we would laminate today for the same effect. But that's not residual stress .[I think the longbows were bent by steaming ?]Another composite , reinforced concrete , is the same though we at times fudge that by stretching the steel as the concrete cures...Is your differentially hardened blade tougher because you have a pearlite spine or because of some residual stresses remaining ?
 
I believe tough is developed through the entire process from 5 1/2 inch bar to finished blade, it is also influenced by the temper. The strength is the issue I wish to explore. At the finish of our last seminar I placed a test blade in the vice on my welding table. No one was able to flex the blade with their bare hands. All they could do was slide a 400+ pound table. It took a piece of pipe on the tang to flex her to 90 then 180 4 times before the edge cracked. She could have gone more 180's, but we had to shorten the heat treat cycles to fit the time schedule of the class.

I believe the strength comes from not only the geometry of the blade, but the influences I mentioned before. How to evaluate them is very complex to me at this time. One day I hope to isolate them and evaluate each influence, the only problem is that it takes time and costs money in the long run.

I was hoping to find some shortcuts.
 
I can see how it would be benificial in the example that mete gave about a doughnut shaped peice being quenched from the center as the outside would cool slower putting the inside under great pressure. When you differentially quench a blade the exact opposite happens. The edge is placed under tension. The closest I have ever heard of what I think you are looking for Ed, is in Japanese blade where they also harden the very spine to try to equalize the stresses and lessen the curavature.



You are trying through your multiple heat cycles to create a supe fine grain structure that is really shallow hardening, right? The shallow hardening is what gives you that hard edge and outer layer and leaves the core softer thereby opposing some of the tension that the edge would normally be under and allowing greater flexibility at high hardness.
Or maybe I just don't know what's going on :D
 
Jason: The edge and center of the blade is hard, the outside above the transition zones gets softer. A martensite cone (piramid with rounded top) forms in the center and runs to the outside of the blade and then down to the cutting edge the softer steel on the outside of the blade is soft.
If I knew how to draw on this outfit I could describe the phenonomon better - maybe.(?)
 
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