Cold Forging

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mete

mete

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Since Stacy thinks we should have a discussion about cold forging , I'll start it.
Cold forging has been around for a long time .The bronze age blades and later iron age blades were often cold forged on the edges to thin and work harden the blade since they couldn't be hardened by HT.
Cold forging distorts the grains ,flattening and elongating them .But what happens inside those grains ??
If you start out with annealed metal the lattice is one of uniform regular positioning of the atoms - all lined up in neat rows . When you start hammering you start to create disorder with atoms no longer neatly lined up . The disorder produces dislocations and vacancies ,specific types of disorder.
Lets take a look at vacancies .If you create vacancies where do the atoms go ? They don't disappear but they move .Then the grain you started with must then get LARGER as the total number of atoms is the same but the density is reduced [ atoms per CC ]. NO edge packing !! NO compacting the grains.
The creation of dislocations and vacancies greatly strain the lattice ,strengthening it .BTW the formation of martensite creates large numbers of dislocations !!!
There is a limit to how much cold working you can do without breaking the metal. Take a paper clip and bend it back and forth .As you do you are creating ,by work hardening, dislocations and finally you you will break the wire.
When you work harden you are adding energy .The energy levels and dislocations create many points where new grains can form when you harden the steel . Thus cold work + hardening results in a finer grained structure.
 
I don't have a horse in this race, either way, but can I ask if you have any sources?
 
I'll explain it the way I do for folks who ask me. Not everyone may agree, but based on metallurgy, this is how I see it.

A blade that has been cycled in forging should have a fine grain if all was done right. Lath martensite in steel can be considered as a long 3-D stack of blocks ( grains) - all neatly arranged and stacked in layers. If we start hammering on the steel/stack, we create dislocations - places where the joints between the blocks/grains get moved, and don't line up anymore. If we move it more, the blocks/grains get turned on their edges, making spaces called voids. These empty spaces make the volume greater, and the density less, not tighter and smaller as the edge packing and cold forging folks claim. The example I use is to take a box and fill it half full with neatly stacked blocks. Draw a line around the top of the blocks on the inside of the box. This is thermally cycled fine grain steel. Now, shake the box up good , and look at the height of the blocks. The blocks/grains are the same size as they were, but the level in the box is much higher than it was, due to dislocations and voids. No amount of pushing or pounding will make it go back to the same level. You will have to re-arrange the blocks neatly again to get it back in order and tightly packed again. This is what normalization does - rearrange the grains/blocks back to a neat "normal" order. If you hammer the blade after normalization, or after any thermal cycling event, you will create chaos, not order.

As you hammer the blade, and the dislocations and voids are created, the steel seems to get harder ( thus someone thinking it got denser). This is because the grains are disarrayed in such a way that they get jammed up and don't want to move easily. If we continue to hammer, the grains on edge may start wedging themselves between the neatly aligned grains. This creates bigger voids, and weaker bonds. Enough of this and a rift forms such that the bonds/boundaries between the grains can not hold on anymore - and a crack forms. So, a little cold forging may do little harm (and certainly no good), but a lot of cold forging may actually weaken the blade. The traditional cold forging advocates always explain that they are very gentle and use a small hammer, watching temperatures, etc., and that is how they avoid this. The actual fact is that by being judicious,they make either no change in the grain structure, or minimal bad change, and the effects are going to be wiped out in HT anyway.

As to that HT statement - Regardless of the disarrangement done by cold forging the blade in the final shaping steps, a proper HT including:
1) thermal cycling ( which is what a multiple quench really is)
2) the lowest austenitization temperature that will fully dissolve the structures
3) a sufficient quench
- will always lead to a fully martensitic and fine grain blade.
Any previous procedure that did not make catastrophic dislocations ( cracks) .... prayers, facing true north, rubbing it on your gal's boobs for luck, or waving a dead chicken over the blade at midnight, had no effect .... it was the proper metallurgical procedures used in the HT that made the blade come out well.

So, one would ask, "Why do some well known traditional smiths and/or master Japanese smiths claim to do this?" It is because of tradition being the source of their reasoning. That is not necessarily a bad thing, but can lead to mis-interpreting the results. The reasoning system may go like this - Cold hammering the bevels has been done for 1000 years, and the entire blade making procedure has been honed to the point where very good blades are made. The smiths don't know the science, or reason why it works, but they know the results are good. Someone surmises that the cold hammering works because the molecules are packed tighter together. They then claim that because the blades are hard and of good quality, ergo, the cold forging reasoning must be right. They offer the good quality blades as "proof" of their theory. If someone was to argue against that reasoning, the answer will always be, " We have been using this procedure for 1000 years and the results are always the same." What is failed to be seen, is that there may be no connection between the cold hammering and the final blade results. They could have said that the reason the procedure worked was feng shui, or the that the Forging Gods were pleased by the incense and prayers they offered before the blades were made. ( actually, that was ,and still is in some cases, the reasoning used). The smith would say, yes, the blade was good, so the Gods smiled on it. If there was a problem with a blade, they would assume that the Gods were unhappy with them - not because something went wrong in the materials or HT. As the world has become more educated and metallurgical science has become more extensively understood, things like facing true north and the dead chicken have become humorous anecdotes, but some old traditions are clung to for a long time.

So, to sum it up, there is a metallurgical reason for some things and a traditional reason for others. They sometimes agree with each other ( like thermal cycling/multiple quenching the blade for fine grain, or repeated folding to even out carbon distribution), and sometimes disagree ( like cold forging making the steel denser, or an overnight stay in the home freezer making a blade a lot harder). Both sides will claim they can prove it, and neither is likely to change their mind. As things get more modern, the new generation of smiths has more modern metallurgical training as well as traditional training, and new and better processes are developed. This will replace some old traditions, but many explanations/traditions will die only when the folks who do and teach them die.
The Japanese smith's reverence for the master who taught him will make the newer smith reticent to speak against his master's traditional explanations, so he may just keep quiet and make good knives. If asked why his knives are good, he may well, out of respect for his teacher, state what he was taught ( not what he knows). It may take many generations for these ideas to finally drift into folk lore and be replaced with metallurgically sound explanations and procedures. The dead chicken already has passed into the realm of foolish superstition, the prayers and incense are mostly gone, and some day cold forging will be ,too.
 
somber said:
I don't have a horse in this race, either way, but can I ask if you have any sources?
Click to expand...

A for sources, most any physics book will show that solid mater doesn't become more dense with hammering.
Any metallurgical textbook or HT guide will explain thermal cycling and grain refinement, with photographs and charts to show the reasoning.
 
I'd say that sounds pretty darn accurate.

I have limited knowledge in Metallurgy, so there's nothing I can say that can really add to this topic. :D
 
Thank you Mete and Stacy:thumbup:... I was wondering how in the heck I was going to start up a strain hardening/cold forging thread with my limited metallurgical vocabulary. I could have easily made a fool of myself by making a lame statement like "knifes ain't snowballz, fellas"... oh boy, looks like I just did.:o

The history and tradition of Japanese bladesmithing is quite intoxicating and for some, addictive... but as with any drug, you run the risk of altering your perception of reality.;)
 
When I started out as a metallurgist there were many theories about things such a dislocations. Sometimes there was substantial info to indicate is was more than just theory.But today we can sometimes actually see what's going on. 1000x was the limit of a microscope magnification. Now it's about 1,000,000 X .Recently i've seen a video of dislocations moving ! WOW ! They will slow down or stop when the come upon another dislocation or precipitate particle.
As you continue bending the paper clip you are creating more dislocations .They interfere with each other more and more so the paper clip becomes harder to bend.

http://info.lu.farmingdale.edu/depts/met/met205/imperfections.html

Here's a simple picture of what you have .In very cold worked or martensitic structure it's multiplied greatly and would look like a box of worms !
 
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http://www.feine-klingen.de/PDFs/verhoeven.pdf

While not particularly focused on or referencing edge packing myths, Verhoeven does discuss the strain on grain lattice as Mete discusses, and the subsequent likelihood for what errors can occur if done on hardened steel, or can happen if HTed without going through normalizing to first relieve those internal stresses.
 
This discussion was spawned from a thread linking a Murray Carter video. Great video, BTW. There are a few seconds in it, where he explains that coldworking the blade refines the internal structures. We took it to mean "edge packing", which we now know is a false premise.

For the record, Murray is a dedicated smith and still one of my favorites. His knives certainly speak for themselves and nobody has said otherwise. There is a danger of strain induced dislocations forming cracks but for someone who has done it as many times as Murray... I don't think it is the case. If anything, the only critique is that he is doing the step for nothing because the moment he heats it to critical, all that cold forging is erased. It would be like someone carving a statue out of ice, melting it down, refreezing it and expecting the statue to show up again. It just doesn't happen.

We are only speaking of one short step in his process. It takes nothing away from his blades... but at the same time, I do not believe it adds anything. I certainly do not wish to disrespect the man.
 
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bladsmth said:
.... prayers, facing true north, rubbing it on your gal's boobs for luck, or waving a dead chicken over the blade at midnight, had no effect ....
Click to expand...

Heck I don't even forge but I think I'm gonna have to try this for the next few (dozen) blades I make. :D
 
Given that there seems to be a back-story to this thread, of which I know nothing at all, I doubt that this will be seen as a helpful contribution but I have to put it out there anyway. I have never heard about Japanese smiths doing such a thing, and have no particular opinion.
All the metallurgy put up here is sound. The explanations given are clear and precise.
But, here comes the but, piano wire and the highest tensile steel wire rope are cold drawn to size in my understanding because equivalent properties can’t be achieved by quench and temper procedures. If I recall rightly, some wire rope is quench and tempered but has inferior properties in yield, ultimate and toughness to the fully cold drawn stuff.
Certainly some of this can be attributed to the introduction of beneficial residual stresses, and possibly the avoidance of some of the negative results of time at temperature but it suggests that there should be some room to consider/investigate whether this could translate to larger sections.
The reason that I have seen put forwards by industry for not pursuing strain hardening in sections larger than wire, when you are pushing the steel property envelope in any event, was simply that the forces required became prohibitive. Dies/rollers become an issue when you can’t rely on a temperature difference to hold their properties at a higher level than those of your work piece. Clearly at the same time if you are pushing up towards the limit you will run out of available strain and the margin of error becomes very, very small just when you are applying maximum forces. Industrially this is just asking for large amounts of reject product in a process less precisely controlled and uniform than wire drawing.
I don’t know that there is a benefit to be had here in cold forging blade sized or shaped objects but it doesn’t seem way out of line to think that there could be. I can’t see that any of the very fine explanations given above about what actually happens when forging/heating preclude that possibility. In light of the industrial experience it would seem at the least feasible.
 
There are lots of considerations in making wire. It's heat treated in the larger size then cold drawn to final size. 'Music wire' was usually made of 1095 .When used especially as springs surface conditions are important as surface defects are likely to cause failures. Drawing large diameters may require large machines but it's possible .Some of the procedures are very old .I had a good tour of the old Worchester Wire Works in MA There they had machines that were perhaps 100 years old !! Each steel and each product has different criteria.
 
Music/piano wire is a different thing from making a blade and then heat treating it. The effects of drawing do not increase the density, but instead elongates the structures. This creates elongated dislocations. Because this type wire is hyper-eutectoid, the softer ferrite is elongated and the hard cementite is left alone ( more or less). Done with the right equipment it produces hard and thin wire. In this case, this is the desirable outcome. Stretched along the axis of these dislocations, as in a music instrument, the wire is very strong. Bent across these dislocations, the wire is very brittle.

In forging a knife, the only reason the dislocations would be desirable is if the only hardening was to be from the cold forging. In that case you would have a very soft knife that had a slightly hardened edge ( Rc low 40's?).
If you HT the blade, all effects of the cold forging are erased, and the structure is changed from dislocated pearlite to lath martensite. You end up with a blade in the upper Rc50's to low Rc60's.

For the record, I wasn't saying that any of those methods of getting a better blade ( some were for humorous effect) are used any more by most people, and certainly not by Murray Carter. I was saying that none of those methods would have any beneficial effect on the blade, and the cold forging will be erased in HT anyway. The main claim that was being countered is the one stated that it increases the density of the steel. This is not only physically impossible, it is actually the opposite of what is happening.
 
You won’t find any argument from me that work hardening does anything but decrease the density of metal. I guess that like I said above there does seem to be a back story to why this thread was started which involves the usual hoodoo voodoo, but of which I am completely ignorant and happy to remain that way.

I took the humour in the vein intended and I think it is a fair assessment of the way the world works and the people in it. I never imagined that no treatment was ever applied to the steel, but from an appropriately annealed state it is then drawn without external heat application and not subsequently heated again. This seems to be at odds with the suggestions which prompted this thread and the proposal that subsequent heat treatment would erase such effects is of course correct. Scale effects are understood, but blade edges are reasonably proportional in scale to many wires.

However, it has been something I have idly mused over from time to time and I suspect there may be a danger here of throwing out the baby with the bath water. Plain carbon piano wire, or music wire, has to be able to be formed around a mandrel the diameter of the wire to be acceptable and is commonly wound into springs of pretty small diameter which I would like to see any fully hard cutlery steel do. You can buy it up to 60Rc. Having used the stuff for taut wire alignment of casings it has strikes me as anything but fragile.

Hopefully this will link through to an interesting paper exploring the properties of cold drawn plain 0.7 and 0.8 C carbon steel wire:
http://ens-cachan.academia.edu/Sylv...t_I_metallography_and_finite_element_analyses

Note that the yield strength implied is 2770MPa or 400ksi. That is not to be sniffed at. While pure tensile failure of the hardest wire occurs after only 1% to 2% total permanent elongation, the actual failure mode is entirely ductile and this apparent contradiction is then explained by examining the residual stress resulting from the wire drawing process leading to intense localisation of the plastic strain such that once one spot shows a tiny plastic deformation, instead of benefiting from further strain hardening at a faster rate than the cross sectional area reduction, the reverse happens and the failure happens involving a very, very small volume of the material. Clearly this effect is process dependent and could potentially be manipulated to advantage in a different forming process. The manufacturer didn’t stop drawing the wire with only a 1% margin of error before failure and the material itself is apparently not showing any sign of strain exhaustion.

Equally quenching operations can induce alarming thermal shock to steel that leaves residual stresses locked in the final piece that are frequently just overlooked and set up an eventual and mysterious failure at modest loads.

Anyway, I am done, I don’t have anything much else to contribute on the subject. I have no actual directly relevant results or testing, it just seems to me that there could be some interesting results from a deeper examination of cold strain hardening plain carbon steels in the blade arena.
 
Now Stacy, just a cotton picking minute. Rubbing your girlfriends (or your wifes, but not both)
(at least not at the same time) boobs is always a good thing.:D

Bill
 
Personally, I couldn't care whether or not a bladesmith quenches his blades in horse piss, ultimately it is the performance of the knife that matters. Here's what Murray says in regards to this technique:

A common technique used by Japanese bladesmiths but seldom exploited by western cutlers is the process of cold forging. In simple terms cold forging is the mechanical reduction in the steel's grain size through force when the steel is below normal forging temperatures. Cold forging makes the steel denser. In its extreme, cold forging is done with the steel at room temperature. Amongst metallurgists, the value of cold forging is controversial, because the theory doesn't match up with reality.

Metallurgists argue that while cold forging does work-harden steel, the effects of cold forging are nullified when the steel is later heated to the temperature required for quenching. Further, they argue, cold forging will induce micro fractures in the steel that will eventually cause a hardened blade to fail under duress. I have conducted research that supports the attributes of cold forging in blades, and all highly respected Japanese bladesmiths incorporate the process. Japanese bladesmiths unanimously agree that cold forging enhances cutting performance in blades. In the final analysis, it is difficult to argue with the results.

Here are some thoughts and considerations. Carbon/mild steel laminates seem to really benefit from cold forging and do not generally show evidence of micro-cracking. While I have seen homogeneous blade fracture from too much cold forging, in 16,000 blades, rarely have I seen a laminate blade fail as a result of cold forging. I suspect that the softer outer laminations absort excess force and energy, thus protecting the inner carbon steel from over stress. On the other hand, I have purposely destroyed a laminated blade by cold forging it until the blade fractured. Therefore, the amount of cold forging is a factor that must be understood.

...the blades that I have experimentally destroyed required this procedure to be repeated many more times before the blades would crack. As a general rule, Yasuki White steel can withstand more cold forging than Blue Steel before failure... Again, in order for the student smith to appreciate the process and value of cold forging it must be experienced first hand.
Click to expand...

Bladesmithing with Murray Carter, by Murray Carter, ABS Mastersmith pg. 53-54
 
somber said:
Personally, I couldn't care whether or not a bladesmith quenches his blades in horse piss, ultimately it is the performance of the knife that matters.
Click to expand...
I wouldn't care about the piss either, unless the maker claims that the piss does something it can't physically do.

Amongst metallurgists, the value of cold forging is controversial, because the theory doesn't match up with reality.
Click to expand...
If we can't present claims based in reality, we should not make them at all.

If the maker just left it at "Part of my process is cold forging, I get good results from it." There would be no issue. But Murray takes a leap into the realm of metallurgy when he says things like "mechanical reduction of the steel's grain size" and "makes the steel denser". The metallurgy that Murray attempts to back his claims with, is the very same science that completely disproves them.
 
Anyway, I am done, I don’t have anything much else to contribute on the subject. I have no actual directly relevant results or testing, it just seems to me that there could be some interesting results from a deeper examination of cold strain hardening plain carbon steels in the blade arena.

I am having a little bit of trouble understanding the process. If I extrapolate to knives I come up with the following; cold drawing the edge of the blade could result in a higher tensile strength, greater ductility, and an HRc up to 60 for a eutectoid steel????? That is as long as you don't reheat the blade above a certain temperature where the effects would be negated??

So pertainent to OPs discussion. Cold forging (cold drawing) correctly could give an excellent edge as long as a heat treatment does not follow???

When a eutectoid (simple steel) blade is quenched with the clay coating on the back is the curve of the blade partially a result of the pearlite back stretching the edge? During quench is the edge under compression or under tension or does anybody know?
 
There was a thread somewhere about curvature and clay coating and I refused to comment as there are far too many variables to set up any formula .One swordmaker tried an experiment by quenching in oil .The blade curved the opposite way !! LOL
 
I would have to look for that thread, but the short answer is that the curvature formed in yake-ire is formed mainly by the martensite expanding. When the edge converts to marteniste it bend up, because martensite takes up more space than the austenite it is converting from.

Here is the text of a previous post I made:

A VERY simplified explanation is that the different structures have different volume ( density).
As the edge cools down past the pearlite nose, it contracts, bending the blade down. The soft austenite spine under the clay yields easily to this stress.
Next the super-cooled austenite edge converts to martensite, which takes up more space than the austenite did, so the edge lengthens...and the blade curves upward dramatically and suddenly.
Then the spine becomes pearlite, and it cools, contracting. This final stress can literally tear the blade apart. The edge is now fully converted brittle martensite, and the spine is tough pearlite...in the tug of war, the edge looses easily.
The steel needs to be the shallowest hardening alloy possible. As little Mn as possible, and none of the normal other toughening elements, like Cr....and no carbide formers but the iron.
 
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