Guy Thomas Posted January 19, 2007 Share Posted January 19, 2007 This came up on another forum under a discussion of differentially heat treated blades (the whole bend rather than break topic). It was put forth that a new heat treatment might fix any damage done to a blade that was bent to 90 degrees and then bent back straight. Unfortunately I'm a bit ignorant of the exact nature of the mechanical damage done to a metal when it is bent or over-flexed (are the grain boundaries stressed/fractured by the storing and releasing of energy from the bending?). My instinct is to doubt that simply re-heat treating such a blade would "repair" any damage. Just exactly what damage if any is done to a steel blade that has been bent 90 degrees and then straightened. (Obviously if you keep bending it the blade will break but is the steel damaged by that first bend?) Anyway seemed like an interesting topic. Guy Thomas Link to comment Share on other sites More sharing options...
mete Posted January 19, 2007 Share Posted January 19, 2007 Any serious damage such as cracks will certainly NOT be corrected by re-HT. Any other changes will be corrected. If permanently bent at 90* I would anneal, straighten, normalize and re-HT. In permanent bending you will distort the grains and work harden the steel. Link to comment Share on other sites More sharing options...
Guy Thomas Posted January 19, 2007 Author Share Posted January 19, 2007 A heat treatment wouldn't fix cracks, which is why old leaf springs aren't reliable as blade stock. So a single permanent bend on a blade heat treated to do such a thing (pearlite back/tempered martensite edge) wouldn't necessarily physically damage the blade, that is cause cracks (and for this example lets say that the martensite edge did not crack either)? If you continued to bend it back and forth it would obviously work harden and break in two. So how does this physical damage begin? At the granular level with microscopic fractures along the grain boundaries? I think I need to read up some more on how physical deformation affects grain boundaries! Guy Thomas Link to comment Share on other sites More sharing options...
David D. Posted January 19, 2007 Share Posted January 19, 2007 A heat treatment wouldn't fix cracks, which is why old leaf springs aren't reliable as blade stock. Are you serious!?? I thought old leaf spring made great blade steel!??? Crap.......... I just started forging a new sword out of leafspring... might not be good..... I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend" J.R.R. Tolkien www.CedarloreForge.com Link to comment Share on other sites More sharing options...
Howard Clark Posted January 20, 2007 Share Posted January 20, 2007 Just check it for cracks David, don't panic. I made a great many blades from old leaf springs. Most of them do not have any cracks in them. I don't really understand the initial premise very well that started this thread. If the blade was bent severely, there may have been some damage done to it at the loaction of the bend, but re-heat treating it will cause a great many more problems that it will fix. First there will be material loss from scale, and neccesary reshaping. Second is the sori issue. That can be a bloody nightmare when re-doing a differentially hardened blade, with all sorts of possibilities, most all of them bad. It is a pointless issue, IMO. Link to comment Share on other sites More sharing options...
Guy Thomas Posted January 20, 2007 Author Share Posted January 20, 2007 Hmmm...perhaps I should try and explain better. It's not unusual for things to be crystal clear in my brain and then discover I've failed horribly to make it so for others! Good example, I was thinking 'old cracked leaf springs' shouldn't be used but all that I typed was 'old leaf springs'. I think it is perfectly fine to use old springs for blades, sorry about that. The idea is that a blade with a differential heat treatment, whether by edge quenching, using a torch to only austenitize the edge, using a clay coating, etc. will result in a blade that will bend rather than break when a person uses it in a way that it shouldn't be used. The premise is that people will inevitably do this and that it would be better to have a tough blade that bends rather than snaps. If it snaps, you have a useless tool. If it bends well the blade can then be straightened and you can go on your merry way with a knife that is still useable. This type of heat treatment is often tested by bending a test blade to 90 degrees which it should easily survive though the edge may or may not crack. So my thoughts are, just what has happened to the microstructure of that blade? Did the initial bend to 90 degrees cause damage? Can it be straightened and be as good as new? If it visibly cracked well obviously it couldn't, but something was done during that initial bend which if continued (by further bending) would result in breaking the blade. What I'm getting from what Mete said is that the initial bend would result in grain distortion which would be erased by a new heat treatment as long as cracking didn't occur. Really I should have just asked a simple question like "What happens to the micro structure of steel when it is stressed by bending beyond the point that it flexes back to true and can that 'damage' be fixed by a subsequent heat treatment as long as cracking has not occured?". Guy Thomas Link to comment Share on other sites More sharing options...
Ty Murch Posted January 20, 2007 Share Posted January 20, 2007 David, Depends on what angle you are coming from. If you want to make blades from leaf springs, do it. Do what you want to do. Make them honestly and with love. If it just so happens to break in use, replace it. All that matters is that you are following what you truly want to do and making them with honest love. . Link to comment Share on other sites More sharing options...
Walkers Edge Posted January 20, 2007 Share Posted January 20, 2007 If permanently bent at 90* I would anneal, straighten, normalize and re-HT. In permanent bending you will distort the grains and work harden the steel. Hi Guy, It seems to me that what Mete said is a fair assumption of what might be best to do. Upon bending hardened steel up to 90 deg. 2 things happen to the steel. First the outer edge of the steel becomes stretched while the inner edge becomes compressed. The grains of the steel that are stretched have lost some of their molecular cohesion. The week stretched grains loose their temper from friction stress. These are not good bonds (molecularly) any more. The inner compressed side of the blade causes the metal to harden and become more brittle. If you bend it back without annealing the blade it will most likely break even if you can't visually see this happen. Because straighteneing the blade will cause the compressed grains to separate instead of stretching as softer metal would. My suggestion is to not straighten it but to anneal it then straighten it normalize it and then reharden. I really can't say if this will work as I have never had that problem. But I think if any thing will that should. Dan Link to comment Share on other sites More sharing options...
Don Hanson Posted January 20, 2007 Share Posted January 20, 2007 Guy, I read through the other thread on the subject of bending a blade 90 degs. Personally, I disagree with "a bent blade is a good thing", I would rather have a blade that would flex (come back to straight) and not bend. The only time a blade should be flexed/bent 90 degs is for a personal test or the ABS test and that blade, if bent should not be straightened and sold. A blade put to normal use will never see anything like the 90 flex/bend test. I find it very interesting that a blade with a hard edge can be bent (not flexed) back and forth many times before the edge cracks, it makes me wonder if the edge was brought to it's full cutting potential. Don Hanson lll My Webpage Link to comment Share on other sites More sharing options...
Guy Thomas Posted January 21, 2007 Author Share Posted January 21, 2007 Thanks Don, I wish I had a bit more aptitude for being as concise as you made your post. Howard is correct, it really is a pointless issue and in my convoluted way all I was trying to find out was just what happens to the micro structure of steel when it is subjected to mechanical stress such as a permanent severe bend. In my mind a permanently bent blade is as much a ruined blade as a broken blade. Guy Thomas Link to comment Share on other sites More sharing options...
Howard Clark Posted January 21, 2007 Share Posted January 21, 2007 What happens, exactly, to the microstructure of the bent portion depends to some degree on what it was to beging with. If it is a blade of 1050, for instance like the leate Bob Engnath made swords, the unhardened portion is a a pretty soft mixture of ferrite and pearlite, and if it is one of my 1086 blades, it is all pearlite, with some speroidal vanadium carbides in it and they behave differently in the bending, and in the straightening. "Edge quenched" blades of 5160 will be different yet, as will blades that were through hardened and then differentially tempered. I'm with Don Hanson on this one, I'd rather my stuff not bend either, but in the venue I am working, the hamon is sometimes almost as important as the shape of the sword. So I make the best sword I can and still have a hamon. In general though, if it went 90 degrees and took a serious set, and you straightened it out, there will be damage to the structure that is more serious than if it had flexed back on its own instead of taking a set. There are all sorts of "in between" things possible too. Kinked and straightened is bad though, no matter how you look at it. Link to comment Share on other sites More sharing options...
Don Hanson Posted January 22, 2007 Share Posted January 22, 2007 Just to be clear. Although I perfer a blade that flexes and doesn't bend, my blades with hamon don't spring back 100% after a 90 deg flex BUT the beauty and addiction of the hamon is worth this small annoyance. (I don't see my customers bending mine or any blades to 90 degs) I did play around with doing a high edge quench, quenching up to within a 1/4'' to 3/8'' of the spine, with the right tapper and grind, these blades would spring back to straight but no hamon, so I moved on Don Hanson lll My Webpage Link to comment Share on other sites More sharing options...
Karl B. Andersen Posted January 22, 2007 Share Posted January 22, 2007 Hi Guy, " lost some of their molecular cohesion. The week stretched grains loose their temper from friction stress. These are not good bonds (molecularly) any more. If I'm not mistaken, we don't have to worry about "molecular" anything in steel - there are no molecules in steel! There are only atoms. Link to comment Share on other sites More sharing options...
Don Hanson Posted January 22, 2007 Share Posted January 22, 2007 Thanks Don, I wish I had a bit more aptitude for being as concise as you made your post. Howard is correct, it really is a pointless issue and in my convoluted way all I was trying to find out was just what happens to the micro structure of steel when it is subjected to mechanical stress such as a permanent severe bend. In my mind a permanently bent blade is as much a ruined blade as a broken blade. Guy, I think the reason my post appeared so coincise is because I had read the other thread and have given this some thought lately. Don Hanson lll My Webpage Link to comment Share on other sites More sharing options...
Al Massey Posted January 23, 2007 Share Posted January 23, 2007 Guy, I think the reason my post appeared so coincise is because I had read the other thread and have given this some thought lately. I've seen something truly scary- a Wilkinson broadsword blade flexed 90 degrees and taking no set whatsoever, with the edge hard enough to bite into (unhardened) medium carbon steel without dulling or chipping. Link to comment Share on other sites More sharing options...
Guy Thomas Posted January 23, 2007 Author Share Posted January 23, 2007 I've seen something truly scary- a Wilkinson broadsword blade flexed 90 degrees and taking no set whatsoever, with the edge hard enough to bite into (unhardened) medium carbon steel without dulling or chipping. That's an awesome heat treatment! How thin was the blade? Guy Thomas Link to comment Share on other sites More sharing options...
Don Hanson Posted January 23, 2007 Share Posted January 23, 2007 (edited) I've seen something truly scary- a Wilkinson broadsword blade flexed 90 degrees and taking no set whatsoever, with the edge hard enough to bite into (unhardened) medium carbon steel without dulling or chipping. Easily done with a long thin blade fully hardened (no hamon) But a cool thing to watch Edited January 23, 2007 by Don Hanson Don Hanson lll My Webpage Link to comment Share on other sites More sharing options...
Walkers Edge Posted January 24, 2007 Share Posted January 24, 2007 If I'm not mistaken, we don't have to worry about "molecular" anything in steel - there are no molecules in steel! There are only atoms. Good catch kba I saw that as soon as I hit the send button. You are right of course, I did mean to say "Atomic structure" thanks for the "keen" eye. Dan Link to comment Share on other sites More sharing options...
kb0fhp Posted January 24, 2007 Share Posted January 24, 2007 (edited) What about carbides? Fe3C is certainly "molecular", with covalent bonding. There are all the other carbides too which also have covalent bonding, and then there are inclusions, such as MnS, which exhibits both covalent and ionic bonding. We should also include the oxides, such as Al2O3, FeO, Fe3O4, Fe2O3, etc.... Just thought I would stir things up..... Edited January 24, 2007 by kb0fhp D. Scott MacKenzie, PhD Heat Treating (Aluminum and Steel) Quenching (Water, Polymer, Oil, Salt and Mar-Tempering) Link to comment Share on other sites More sharing options...
daryl meier Posted January 24, 2007 Share Posted January 24, 2007 What about carbides? Fe3C is certainly "molecular", with covalent bonding. There are all the other carbides too which also have covalent bonding, and then there are inclusions, such as MnS, which exhibits both covalent and ionic bonding. We should also include the oxides, such as Al2O3, FeO, Fe3O4, Fe2O3, etc.... Just thought I would stir things up..... Thanks Scott. I have wondered about the bonds in CPH? ( looks so organic), whereas the atoms in ferrite and austenite seem to have a classic approach/avoidance conflict. Link to comment Share on other sites More sharing options...
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