Why Does Work Hardening Occur??

Mr. Cashen or Mete,

To what effect does the cubic structure of the material affect work hardening? It's been a few months since I took welding metallurgy, but I seem to recall that different materials respond differently depending on whether they're FCC (face-centered cubic) or BCC (body-centered cubic). As I recall (I really should go back to the texts) the cubic structure of steel changes in relation to temperature. How does all this relate to workhardening, annealing, and bladesmithing in general?
 
That will take a bit of research to get those answers ! The most common crystal structures in metals are cubic [FCC and BCC] and hexagonal but there are others. Some of the metals have phase changes.Alloying elements can make a big difference.In the 300 series of stainless steel ,301 work hardens the fastest and 305 the slowest with a considerable difference between the two.It's all about dislocations ,how they are created and how they move through the metal.
 
Several metals will "squeal" when bent in the hardened state. The "tearing" of the structure creates minute vibrations as they cascade. The sound usually precedes breakage.

Silver and similar non-ferrous metals,indeed, harden by breaking up the planes into smaller and harder to move pieces. Annealing allows the pieces to re-align and become more malleable again. Since there is no change in the structure (as the BCC to FCC in steel) the metal only has to be heated to a point where the metal re-aligns. Then it can be quenched to lock the structure into the smoother slip planes. That is why most non-ferrous metals soften on quenching and ferrous metals harden on quenching

Just to add to this topic:
There is a stone called Moldavite. It is a greenish natural volcanic glass found originally in Moldavia. It is similar to "Apache Tears", and was formed by rapid air cooling in droplets ejected in eruptions. In faceting or grinding Moldavite the extreme internal stresses can be released and the stone can "cry" ,usually just before exploding. I have personally had one do this while faceting a large stone. It is weird. The stones are often annealed in an oven prior to cutting to avoid this. Yes, stones are annealed,too.
Stacy

BTW, Kevin: Back in the 50's, I was involved in setting up a demonstration of a nuclear chain reaction using 100 mousetraps and 100 ping pong balls.
 
Moldavites are tectites. They form from molten Loess material (glass) that is melted by impact events-meteorites. The Moldavites were formed from ejecta from the Ries Impact crater in Germany. Other tectites are well known from other impact events

Apache tears are obsidian, which is glass formed from very slow moving Rhyolitic volcanc lavas the freeze in place instead of fully crystallizing. As the volcanic glass weathers, it forms clays that sometimes have remnant spherules of glass in the matrix, which are called Apache tears.

Occasionally droplets of obsidian wiil be enclosed in Rhyolitic ash-Pumice and will weather out as "tears.

The same droplets can be welded and flattened to form "Fiame" structures in densely welded rhyolite tuff.
 
The discription of twinning reminded me of someone flexing a saw blade (carpenter's hand saw) and genated some very cool sounds. Is this the same principle?

Kevin-I like your analogies. I teach my kids about chemistry from stuff we cook in the kitchen.

Ric
 
Twinning is associated with permanent deformation. If there was no deformation to the saw, then I doubt twinning was involved. Those saws can resonate quite well though.
 
Steve, Thanks for the fill in. I was oversimplifying the source ( I should have said "impacts", not eruptions).
Stacy
 
question that i did not see thrown in yet.

i know "Edge packing" is supposedly a myth by many but would "edge packing" at below forging temperature work as a work hardening. i guess it becomes pointless since you will HT later but i guess its an interesting idea seeing as depending how it is done it could work harden but then would HT affect it differently?

-matt
 
Edge packing was originally used for wrought iron blades, you work hardened the edge since they could not be heat treated. As Kevin pointed out, as soon as you heat it up to Ac1 all those dislocations are realigned and it makes no difference in that respect. However, it can still hurt the steel by making microcracks in the steel and making the knife easier to break. Also edge packing is not "supposedly" a myth, it is one, you can not pack atoms closer than they want to go, unless you utilize a black hole or something.
 
ok ya thats what i thought, i said supposedly because there are still the few things that every once in a while say it might do something.

i assume it could help if for some reason there was any micro voids that it could help fill but like you said it can do damage as well.

-matt
 
In general you could consider annealed steel dislocation free and void free !! When you hammer it you create dislocations and voids !! Therefore as you hammer steel it actually increases in volume !!
 
One part of the "packing" fantasy that may work is the strain induced by subcritical deformation.

By the way a definition for cold working could very well be deformation that exceeds the rate of, or occurs below, recrystalization. So once again we have carefully used terminology that takes advantage, either intentionally or carlessly, of the ignorance of the public. The smiths that embrace the myth should at least be willing to call it by its actual name - cold working.

But on the topic of strain. The imparted strain will allow for increased nucleation of new austenite grains upon reheating and thus a possiblity of finer grains. However one must never forget how exponentially more powerful heat is than our meager hammers in affecting the steel. One quick heat and quick cool can do so much more for grain refinement, and if you were to not get complete and consistent deformation you invite uneven grain coarsening, and how many of us can say that they managed to nail every single grain with the exact same ammount of force? Heat has no problem getting to every single grain with equal effect, that is why I harp on making your normalizing heats as even as possible, if you don't you may as well be edge packing.;)
 
In a simplistic sense we could call work hardening brittle strain. It is caused by the accumulation of nonrecoverable strain at a higher rate than the lattice can stretch (ductile or plastic strain). All crystalline material can be annealed (recrystallized) by the application of either temperature or pressure. We call the process of controlled cooling that maintains the recrystallized state annealing, but in physical chemistry, the recrystalliztion itself is the annealing.

When we quench any material, it is brought to a metastable state because it is cooled too fast to properly recrystallize. Quenched material will eventually rearrange the lattice to the stable state. In steel and glass this just takes too long for the human time frame to notice. Glass weathers to clay and I assume steel would revert to a pearlite type if it didn't oxidize too fast. Glass does this over what we call gelogic time spans.
 
hmm so a question on that, can you anneal steel by pressure, if so how would yo do it?

-matt
 
SR_matt said:
hmm so a question on that, can you anneal steel by pressure, if so how would yo do it?

-matt
Click to expand...

Theoretically anything with a crystal lattice will recrystallize under pressure. Temperature is much more efficient. Both are thermodynamic factors.
 
hmmm at first i thought its the same as how diamonds are formed then i realized that is the opposite result. is it a lack of pressure that will anneal or more pressure?

-matt
 
Matt I have to admit to oversimplifying above. Thermal annealing will occur below the critical temp. The ferrite lattice will anneal. The Austenite transition is a phase change where the lattice takes a more stable form at that temperature. This will be followed by annealing of the Austenite lattice until the grains all become the same shape and size. This is called a crystalloblastic texture. Continued heating will cause the grains to get larger, especially at higher temperatures as the process continues. I would assume that driven to the ultimate conclusion, the whole mass would become one grain.

The quench freezes the structure, preventing it from resuming the stable low temperature form-Pearlite.

Diamond is the high pressure phase of Carbon (cubic) that forms from Graphite at several megabars of pressure at a hundred to two hundred miles below the surface of the Earth Recrystallization of minerals at high pressures is by ductile deformation that first shortens or stretches the lattice until it shears and the same kind of crystalline textures form as in the thermal annealing, but a slightly different process. I am not sure what the depths would have to be to bring the diamond to the point where it would deform plastically.

You can observe ductile deformation by stretching silly putty. You will know when the ductile failure occurs. You can also see the silly putty deform in a brittle manner-hit it with a hammer. Crystalline materials will all deform in the same manners under different rates and amounts of stress applied.
 
ok makes sense,

i just got a copy of the complete metal-smith pro edition and have just started to read it cover to cover, only on page 9 so far but it is very interesting and simplified a lot of the talk here that was just slightly over my head. one thing that surprised me was that you can heat harden some metals by heating for a certain time and a certain temp.

-matt
 
Great thread!

It proves once again that you really need to keep exposing yourself to information or else it won't stay with you!
 
SR matt, I assume you are talking about precipitation hardening [PH ]! That is a major type of hardening for metals ,especially non-ferrous metals. There are PH stainless steels that use this mechanism too.
 
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