> Forgings have the added advantage of variable grain direction which generally can be tailored to the stress patterns of a specific design.
This is a super underappreciated fact! It's often repeated that forging is just stronger, but just squishing steel does NOT make it stronger. Forging a part is so much more than just smashing it into a shape.
Steel cable is made of pretty ordinary steel which is stretched 100s of times its original length. That process alone makes it 2-4x stronger in that direction. You stretch steel and it gets stronger in that direction.
Do you see how complicated that optimization process becomes? The process steps are not just trying to take it to the final shape. Your piston rod needs to be strong lengthwise, so you actually want to start with a short fat ingot and stretch it out instead of one that is near-final size.
Think of making an I-beam. You could hammer out the middle, making it thinner. That would give you a bit of strength there but very little on the edges. If you instead pull the edges out, you create a long continuous stretch that will be very strong against bending. Where, how, and in what order you stretch makes all the difference. You may want to leave extra material and cut it off later, so that your grains are all oriented together instead of tapering to a point.
For any moderately complex part, this process is as complicated as modern engineering problems. With poor steel you genuinely need to understand how to foster and bring out those continuous lines or your corkscrew will unwind like playdough. Blacksmiths had a legitimately intellectual job back in the day!
This effect also applies to polymers! Perhaps even more so. Take a polyethylene bag (LDPE) and stretch the material in one direction. You might notice the material becomes thinner but also stronger. This is due to the polymer chains becoming aligned. Eventually you get "drawn fibers" where the molecular strands are aligned with the fibers for optimum tensile strength.
it varies a lot with polymers, and it's a different effect. steel is entirely crystalline; ldpe is mostly amorphous. a big part of what's happening in the strain hardening of ldpe, aside from making it uniaxially oriented, is that it's crystallizing; the crystalline domains become larger, greatly reducing the amorphous volume fraction. (there are also other ways of achieving this effect, such as annealing, which you will notice softens steel rather than hardening it.) ldpe's strength isn't determined by crystal dislocation density in the same way as steel's, and of course steel doesn't have polymer chains to align
HDPE is also what Nalgenes and the like are made of (or used to be anyway), and is very popular for storing chemicals as well. It is very nearly completely inert.
you cunninghammed me: nalgenes are polycarbonate, which is a lot less inert
all the polyethylenes are relatively inert, because polyethylenes are in some sense just heavy paraffins. paraffin is germanized latin for 'relatively inert'
just squishing steel does actually make it stronger, because it increases the number of dislocations in its crystal structure. smaller grains mean higher strength even without the variable grain direction. also, peening, which is not exactly the same as forging but is also just squishing steel, can give you higher strength for a third reason: areas with residual compressive stress can't initiate cracks until you overcome that stress, which increases strength. even more, though, it increases fatigue resistance
> Blacksmiths had a legitimately intellectual job back in the day!
ACOUP noted that blacksmiths might be assisted by unskilled laborers, strikers, who had the actual job of lifting the hammer and hitting the object with it.
> Unskilled feels unfair, it requires a fair bit of skill, and you're also learning how to forge while doing it.
Both of those points are untrue. Strikers are unskilled labor and in general are not learning how to forge. The smith shows where he wants the hammer to fall, and they let it fall there.
> Things were worse for the many strikers and other laborers who were essentially unskilled hired hands or even enslaved laborers (given their depiction in artwork, it seems likely many ancient strikers were slaves) of much lower status and who could not expect to be trained into blacksmiths themselves some day. While some strikers were probably apprentices in training, it is quite clear that not all of them were! These workers would also have been far less richly paid; indeed, the entire point of strikers was to have laborers who could be paid very little but still amplify the production ability of the blacksmith himself.
The reality is that, depending on circumstances, a highly skilled craftsman of yesteryear could have any number of obviously less-skilled assistants. Some would be "career track", some semi-skilled seasonal help, some minimally-skilled (whether due to youth, infrequent day labor, poor talent, or social status), and some in supporting type of skilled work - animal handling, cooking, bookkeeping, etc.
I tried to maintain a certain kind of optimistic humility, that almost anything which employs a person full-time is a problem-area that has fractal layers of complexity one don't have to know from the outside.
The only question is whether someone will pay you for doing the fancy skill/science tricks or not.
Not necessarily a culture war thing. People who aren't familiar with the subject might take "unskilled" in the plain-english sense, as a pejorative. And who can blame them? We're english speakers before we're technical speakers.
Anyone who has worked in a job classified 'unskilled' generally doesn't need a pejorative to feel unloved. It's the nature of the game.
Digging ditches generally sucks, same as I imagine being a striker, but most anyone can do it (until their body gives out, anyway, which in some cases is 'immediately').
One of the most interesting things I learned about blacksmithing (aside from all the metallurgy because I'm not a ME), was how much more valuable "the right hit" was than "the heavy hit."
Sure, it takes a ton of muscle, but you can quickly screw up a piece by repeatedly beating the hell out of it.
I'd really like to see some backing of these claims. I've seen "grain flow" claiming big gains for years in various enthusiast magazines (bike, motorcycles, cars, etc) as to why components are forged.
Then I started working in engineering, and I can't find any support for these claims. For sure when a steel bar is worked down to become wire for a steel rope, it cannot be pulled to an elongation of 100x increasing strength. A36 steel which is a basic structural steel has an elongation at break of 23% in a 2" gauge length [1]. In every rolling mill I've been in, there is a limited amount of reduction per pass through the mill, after which the metal needs to go for thermal treatment to be annealed to remove all the cold work. Every time you anneal the material, you completely resets the elongation (internal plastic strain) and strengthening due to work hardening. If they do too much reduction in one pass or at too low of a temperature, it cracks the material and makes it weaker.
For sheet metal, there is lore about the material being stronger in the rolling direction as that is the direction of grain flow. I have yet to find a source that can point to any large difference. In papers like this [2] there are claims of certain orientations of samples relative to rolling direction have different tensile properties, but when you look at the tensile charts, there is minimal difference. The yield strength in these charts isn't reported, but all three orientations look to yield at the same point. In this test the across the grain (90 degree to rolling direction) orientation had the highest tensile strength which is the opposite of the expectation of the forging "grain flow" promoters. But the magnitude of the difference isn't large, and is small relative to normal factors of safety in a reasonable design.
When designing automotive components, I've only ever seen forging methods selected for efficiency of production. If a part mostly fills the envelope of a bar or plate, it is cut from bar or plate in all cases. If there is a lot of void volume in the part, the calculation will be made to determine if the cost of developing forging tooling and development will get paid back in reduced material and machining cost. I have yet to see the dimensions of the part change with manufacturing method, which would be needed if the non-forged part was significantly weaker.
And finally, a lot of forged parts are subsequently heat treated. When heat treating steel all of the grains in the steel have to be destroyed and recrystallized. That is the mechanism by which heat treatment works. Depending on the exact process and part geometry, this process removes or reduces the grain flow in the finished parts.
Having said that, the claim of superiority of forging persists, and I'd love to see a technical reference that shows the magnitude of the change from someone who has plausibly actually tested the effect.
MIL-HDBK-5 [1] is a good publically-available source for strength allowables for several aerospace alloys, including multiple directions relative to the grain for some of them.
The first relevant example I found was on page 3-86, extruded 2024, 2.250 - 2.499 inch cross-section. For ultimate tensile strength, F_tu, the L (in the direction of extrusion) allowable is 57 ksi, while the LT (perpendicular to the direction of extrusion) allowable is 39 ksi. That's a 30% drop in strength.
Thanks for slogging through that one to find an example. I've come across that handbook before, and went looking through it in the alloys I normally work with. In the alloys I normally see, the L and LT are either identical, or with a couple digits in the least significant digit.
So it looks like the effect is very alloy dependent. I didn't see any of the steels having any notable directionality. Also Aluminum 6061 doesn't show any directionality either. Outside aerospace, I suspect that covers the majority of metal tonnage used.
Thanks for posting that as I was about to! I will note that MIL-HDBK-5 is no longer valid for actual aerospace design, as it has been superseded by Battelle Institute's MMPDS Handbook, which is locked behind a very very tall paywall. The MIL-HDBK is still all perfectly good data.
I think it is unfortunate the US government got rid of all their MIL and STD standards. You used to have a rich resource of technical specifications for materials, fasteners, fittings, etc for free access. Now they are pretty well all cancelled and have been moved to organizations like SAE and ASTM where they are hundreds of dollars per copy. For the metallurgical data, I'm curious if these successor organizations are actually generating any new data. Whenever I'm looking up references like that hand book, it appears they all summaries of investigations that happened back in the 1960's and earlier.
That’s the problem of capitalism : you need it but too much of it becomes a disease for the society abusing it…
Much of the western world is starting to feel the effects…
The best evidence for grain flow on a really atomic scale comes from what is called texture analysis in X-ray or electron-beam crystallography (or related techniques): you get a deviation in the distribution of Bragg peaks due to the fact that you have a non uniform distribution over the orientation of the unit cells within the crystallites in the bulk material. You can fit this in a spherical harmonic basis and quite accurately work out the excess or defect of the distribution, typically quantified in units of 'multiple of a random distribution' or mrd, again either in crystallographic axes or traditionally in three orthonormal axes – parallel to the surface of the workpiece ("rolling direction"), axially transverse to it, and normal to it. The phrase to search for is 'pole plot'. They're rotationally symmetric and an inverse projection over all space, and so usually only a quarter of a hemisphere is shown.
A very good example of the affect of annealing tungsten wire is here [1] – note that (a) there is a very clear orientation dependence that some difficult geometric transformations will undoubtedly show means that they are aligned in the wire drawing dimension; and (b) after annealing at 1600 ºC for an hour the preference is slightly reduced but still about 15 sigma away from random...
I'm not saying grain flow doesn't exist. My claim is that I can't find any support for the idea that grain flow results in superior strength characteristics in the grain direction.
Ahh, I see! This is a common problem with things that are "known" to be true -- often people don't rigourously test them.
This paper [1] has some good data in it:
"The experiments in this study were developed to verify the influence of the grain-flow
orientation on fatigue life and its impact on the anisotropic properties of a mechanical
component. To this end, steel specimens were made, and their fiber was oriented by
machining and hot forging. Subsequently, they were subjected to flexo-rotational fatigue
tests in a piece of specific equipment to determine their fatigue life."
(...)
They then describe three parts: A, properly forged, B, improperly forged, and C, machined.
(...)
"The results showed that specimens of configuration A achieved a much longer fatigue life than configurations B and C, actually doubling it. The results indicated a similar fatigue life behavior between configurations B and C. It is important to emphasize that this similar behavior between these two configurations is valid for this case analyzed (...)"
It's plenty real, and it matters for more than just strength. Cold rolled grain oriented electrical steel has better magnetic properties than non-oriented steel and is used in some applications where the field is in a straight line.
I very much agree with you and also would really like some clear evidence of that.
My opinion was formed from another area that is related : the pretense that “forged” knife are stronger, and hold their edges better, etc. I have seen some mostly nonsensical electronically microscope observations that didn’t prove anything except minor differences in “fiber” patterns that cannot be reliably be shown to be better in experimental protocols (provided you heat treat the metal in the same way and everything else being equal in particular the particular alloy).
I think this is one of those things that people keep repeating without any evidence because very is a large amount of marketing behind it as well as vested interests to sell more expensive supposedly superior “artisanal” stuff.
There are many examples of the likes, being mostly belief/lores repeated ad nauseam that becomes “true” just because everyone is saying it, yet with no hard evidence !
your comment is an extremely valuable contribution!
disclaimer: i don't have a relevant technical reference handy, and i'm far from an expert on the area, which is vast, and i recognize you know things i don't about it. still, i do spend a lot of time reading papers with metallurgical micrographs in them†, and i think i figured out the answer to your question many years ago, so i will explain my understanding
except for the part about grain orientation, anyway
> In every rolling mill I've been in, there is a limited amount of reduction per pass through the mill, after which the metal needs to go for thermal treatment to be annealed to remove all the cold work. Every time you anneal the material, you completely resets the elongation (internal plastic strain) and strengthening due to work hardening. If they do too much reduction in one pass or at too low of a temperature, it cracks the material and makes it weaker.
as i understand it, this is exactly right, but you say it as if it's contradictory. strain hardening increases the yield strength of metal (by making it yield). it can also change the tensile strength, but to a much smaller degree. when the metal can no longer handle stress by yielding, in particular by yielding in a way that produces further work hardening, so that the yield is distributed over the metal rather than being concentrated wherever it starts, it cracks. that's why strain hardening metal makes it more prone to cracking. in general, a given metal is more prone to cracking when you harden it, whether you harden it by cold forging, case hardening, or quenching. (peening is the exception; it inhibits crack initiation by a different method.)
the change in yield strength from cold working can be quite large, a factor of 4 or so. it doesn't change the ultimate tensile strength much (or at all in the case of your wire rope), but there are a lot of cases where what you care about is the yield strength, not the uts, because if the part yields by more than a tiny amount, it is out of tolerance and has therefore failed
(with respect to a36 steel, elongation at break, and wire rope, this is a minor detail, but it's possible to elongate it somewhat more through rolling than you can through wire-drawing. but you are certainly correct that you cannot elongate it 100×, and wire rope is mostly made by drawing, not by rolling.)
there are different kinds of heat treatment, but the most common kind for steel involves a phase transition to austenite and back, which does indeed destroy the entire grain structure of the steel, losing any potential advantage of forging, precisely as you say. i'd think this would also be mostly true for hot-forging, where steel is forged while still austenitic; the relevant grain structure for strength will be the one that the steel acquires when it leaves the austenite phase. there are other kinds of heat treatment (more commonly used with things like aluminum) that don't involve fully recrystallizing the metal, and i would expect some grain structure to survive those
probably none of that is telling you anything you don't already know, but perhaps it's a different way of thinking about the things you know that explains the apparent contradictions
as for which direction i would expect grain orientation to make things strongest in, i really have no idea at all
I'm not sure what you thought was contradictory within that quote. I thought it was reinforcing a single idea? I was pushing back on the idea that very high reduction ratios keep causing higher and higher strength. There is a pretty low limit to the amount of deformation you can make in steel and aluminum before you wreck the metal. You need to keep resetting the cold work via annealing to be able to keep forming the metal. Cold work is just done as the final pass with a very limited final dimensional reduction in rolling.
I'm not saying cold working doesn't happen, or doesn't affect strength. It certainly does. I'm pushing back on the idea that forging creates superior strength via grain flow. One of the sibling comments pointed out the MIL spec materials handbook[1] where he found some materials that do exhibit a strength dependency on grain direction. That is interesting.
That seems to be the exception rather than the rule. If you go to page 3-220 in that spec, they show 5052 Aluminum in varying degrees of cold work (H32, H34, H36, and H38), where higher degrees of cold work have higher ultimate and yield strengths, but the L vs LT directions are identical in many cases, or 1 different. That goes against the general idea that forging grain flow creates superior strength in general.
So basically what you say is agreeing with his observations ?!
If you cold forge there are some benefits (but the question would be what can actually be reliably be cold forged and be a useful object in our precise world ?).
If you hot forge and/or heat treat, most of the benefits are lost pretty fast, so it doesn’t make much difference.
As the OP seems to intuit there is probably not much real strength benefits to forging for useful objects in real use cases scenarios, the reason they are forged have to do with manufacturing processes more than anything else.
true, conventional welding or sintering would be a bad idea. but you can connect them together with brazing, laser-welding, explosive welding, ultrasonic welding, self-propagating high-temperature synthesis of an intermetallic like nickel aluminide, electrodeposition, lashing, or globs of glue
Doesn't annealing take hours, particularly at the lower range of temperatures? Perhaps the additive process can keep the metal hot for a much shorter time. Granted, this also means stresses from the manufacturing process will not be removed.
Most forms of metal AM require melting, which gives solidification microstructures. There are solid state (no melting) forms of metal AM though. Look up AFSD. MELD just got (part of) a billon dollar contract from the air force for it.
>"Can you forge metals in a highly controlled and directed magnetic field where you can orient the grain/alignment of atoms/fields in whatever direction you want. Further, if true, what happens when you make damascus from varying plated that have particular alignments/grains - and what are the features of this material?
> Can you forge metals in a highly controlled and directed magnetic field where you can orient the grain/alignment of atoms/fields in whatever direction you want.
This is how you make magnets. "Soft" ferromagnets have small, round grains that rotate to reinforce outside fields. "Hard" ferromagnets have permanent fields of their own and long grains that can't reorient.
Forging with a field has a very low impact on the material properties because of how weak a magnetic field is compared to the forces moving atoms- same reason steel loses its magnetic properties when it gets hot.
> what happens when you make damascus from varying plated that have particular alignments/grains
"Damascene" is the layered look most often made from acid etching sandwiched and forge-welded layers of different steels. Damascus is a single alloy for which the pattern is named.
Since in both cases the material is melted together, it's far too hot for any magnetic properties to have any impact.
This is a super underappreciated fact! It's often repeated that forging is just stronger, but just squishing steel does NOT make it stronger. Forging a part is so much more than just smashing it into a shape.
Steel cable is made of pretty ordinary steel which is stretched 100s of times its original length. That process alone makes it 2-4x stronger in that direction. You stretch steel and it gets stronger in that direction.
Do you see how complicated that optimization process becomes? The process steps are not just trying to take it to the final shape. Your piston rod needs to be strong lengthwise, so you actually want to start with a short fat ingot and stretch it out instead of one that is near-final size.
Think of making an I-beam. You could hammer out the middle, making it thinner. That would give you a bit of strength there but very little on the edges. If you instead pull the edges out, you create a long continuous stretch that will be very strong against bending. Where, how, and in what order you stretch makes all the difference. You may want to leave extra material and cut it off later, so that your grains are all oriented together instead of tapering to a point.
For any moderately complex part, this process is as complicated as modern engineering problems. With poor steel you genuinely need to understand how to foster and bring out those continuous lines or your corkscrew will unwind like playdough. Blacksmiths had a legitimately intellectual job back in the day!