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Black_Moons
02-06-2010, 01:26 AM
Hi, im wondering about hardening and how exactly it affects metal.
I know hardened steel will be just as 'rigid' as unhardened steel, or indeed other alloys. (Youngs modulus), this is the 'elastic' reigon of steel correct? where it returns to its origional shape?

So insted, it affects the point where elastic becomes plastic right? Ie does not return to its origional shape but insted flows into a new shape.

I know something hard is considered 'brittle', Is this because hardening also reduces shear strength? Or is it something to do with the loss of ability to asorb impacts/energy with plastic deformation?

barts
02-06-2010, 02:23 AM
Hi, im wondering about hardening and how exactly it affects metal.
I know hardened steel will be just as 'rigid' as unhardened steel, or indeed other alloys. (Youngs modulus), this is the 'elastic' reigon of steel correct? where it returns to its origional shape?


Young's modulus describes the ratio of stress/strain -> the "stiffness" of the material in the linear region. Steel (and other materials) have a linear relationship between force and displacement below the yield point.



So insted, it affects the point where elastic becomes plastic right? Ie does not return to its origional shape but insted flows into a new shape.


Yes. What happens is that you apply a force, and after releasing the material doesn't spring all the way back.


I know something hard is considered 'brittle', Is this because hardening also reduces shear strength? Or is it something to do with the loss of ability to asorb impacts/energy with plastic deformation?

Hardening doesn't reduce shear strength. It can reduce the amount of energy needed to break a piece for the reason you mentioned - a soft, ductile material will yield slowly, absorbing energy, while a hard, brittle material will fail suddenly at far lower displacements.

Imagine striking a file with a hammer - now substitute a similar size piece of hot rolled for the file, and repeat...

- Bart

Black_Moons
02-06-2010, 08:51 AM
Intresting! So this holds true for all types of hardening? (quench, age and work?)
Does anneling just make it 'less' hard just as if you did'nt harden it as much the first time around? (I assume then its done because hardening something part way in a controlable repeatable manner is much harder (impossable?) then anneling it in a controled repeatable manner)

strokersix
02-06-2010, 09:01 AM
I'm no metallurgist so take this for what it's worth:

Carbon steels go through a phase changes upon heating. The carbon atoms are in different positions in the iron crystal structure according to temperature. When cooling slowly (annealed) the atoms rearrange into the low temp structure. When cooled quickly (quenched) they are frozen in the higher temp structure. When quenched, the internal stresses within the crystal structure result in hardness and higher strength.

That's the way I visualize it anyway.

http://en.wikipedia.org/wiki/Steel

ikdor
02-06-2010, 09:52 AM
<delurking>
Ah there is something I've always wondered about.
If hardening doesn't affect the Youngs modulus and obviously not the weight, why does it ring at a higher tone when you strike a hardened piece of metal?
Isn't the resonance frequency determined only by geometric shape, elasticity and mass?

Igor

fciron
02-08-2010, 12:02 PM
I am a blacksmith, not a metallurgist, here is my plain english understanding of heat treating. It is useful for steel, not aluminum or bronze, and I am not addressing work-hardening which has different causes.

There a several different possible crystal structures for carbon steel that form at different temperatures and a couple of different descriptive terms that need clear definition.

Hardness is a lot like what it sounds like; how hard it is to dent or deform the metal. Toughness how much the steel will deform without breaking. Heat treating is a compromise between hardness and toughness. At maximum hardness, the steel is at it least tough (most brittle); think of a file. At maximum toughness (bends but doesn't break) it is at it's least hard; think of hot rolled steel.

Heat the steel to critical temperature and and quench in the appropriate medium, now it is in its hardest, but least tough, configuration. Don't drop it, it's brittle. In some high carbon or exotic steels the internal stresses can be so high that it should be tempered immediately to prevent cracking. (As stated above, the steel is trapped in a high-energy crystal state.)

Tempering? That is when the steel is heated to a lower temperature (usually below incandescent temps) to allow some of the high temperature crystals to shift to the lower energy form. This lowers the internal stresses in the part and makes it less hard and more tough. It is a necessary follow-up to hardening for almost any tool to be useful in practice.

Annealing is usually done before hardening and after working the steel to relieve work hardening and other stresses that could cause distortion or checking in the hardening process. (At least that's good practice for Blacksmiths who are dealing with a forged tool and planning to hit it with hammers;-) It is also used to put the material in its softest possible state for machining or bench-work.

Heat the material to critical state and cool slowly, for many tool steels as annoyingly slow as 50 deg. per hour. In practice in my shop, leave it in the furnace at shut down or toss in an insulated box. Cooling very slowly ensures that the steel is entirely in its low temperature (and softest) crystal state.

Clear as mud, right?

vincemulhollon
02-09-2010, 12:47 PM
Hi, im wondering about hardening and how exactly it affects metal.

I'm looking for a very specific book on that topic. I read it about 30 years ago, and it was an old book at that time.

Each chapter had the young apprentice briefly explaining what he learned from the previous chapter, then asking the old timer if that is all there is to it, at which point the old timer would peel another thin layer of the onion away from the mysterious art of steel hardening. Repeat at least half dozen times.

If you learned nothing else, you learned metallurgy is like an onion with about 20 layers of understanding, which in itself is interesting. It started off pretty simplistic, and ended up just underneath what I'd now call serious materials science / physical chemistry. No equations, just text. Vaguely read as if it were a magazine serial, collected into a book. Written in a conversational / Socratic style.

I am well aware that a modern metallurgical textbook would better serve me, or anyone else, I am purely interested in this for reminiscence, etc.

I don't remember the author or the title, but the book contents were very memorable!

strokersix
02-09-2010, 01:10 PM
If hardened steel is observed to ring with different tone when struck as compared to soft steel I propose this is caused by local yielding when struck.

The ring tone is probably made up of multiple frequencies. Local yielding damps (or does not impart) the higher frequency component, and therefore soft steel rings with a different tone than hard steel.

camdigger
02-09-2010, 01:25 PM
To fully understand steel behaviour on a molecular level, you need to understand the assorted and sundry crystalline structures and how they react to chemical composition, stresses, strains, and heat (time vs temperature). The exact mechanism of hardening is related to crystalline structure. FCC face entered cubic vs BCC body centered cubic crystals, IIRC. The crystal structure shifts above the transition temp, and gets trapped that way on quenching.

I strongly suspect that the difference in ring tone is due to the difference in crystal structure.

Toughness is closely related to crystal grain size. Bigger is tougher.

Failure mechanics are related to something called localized dislocations.

At least, this is what I remeber from the three or four physical chemistry and materials courses I took 25 years ago.:D

camdigger
02-09-2010, 05:44 PM
Hmmm, can't believe the amateur expert on all things metallurgical from BFN BC hasn't chimed in by now...?

boslab
02-09-2010, 09:00 PM
the difficulty you have is that the question is a little to broad, there are loads of ways to harden metals, some metals are not capable of being hardened at all, like lead they recrystalise themselves at room temp, the modified or distorted lattice recrystalises, how that happens is that metals have within thier structure dislocations, broadly half a plane of atoms stuck between a full plane during solidification, if you cold work a lump of steel as the metal flows the dislocations pile up like a traffic jam, thats when work hardening becomes apparent [the simplest form of hardening, hit **** out of it till it gets hard], to soften ie anneal a sample the sample Must have been cold worked, otherwise no dislocation pile ups, these pile ups are important as thats where theres a lot of energy to fuel the recrystalisation, where the pile up occurs is the point where a new crystal wil Nucliate, grow
thats roughly work hardening and the reverse, annealing.
things that are important for steel , the iron carbon equilibrium phase diagram,
that tells you what a specific iron carbon alloy should have in the way of crystal structure, [cementite is the hard bugger, Fe3c,]
to manipulate a steel we use a TTT diagram, time temperature transformation,
Heat to cherry red and quench is a bit rough for tool steel!
BTW heating to cherry [>720 deg C] transforms the steels structure by converting it to a predomanantly cementite, the carbon is absorbed by the grain/crystal , quenching traps it there end of story, faster the quench the more carbon gets trapped the more cementite the harder it is!
Problem, its glass hard, harder actually and mostly useless, enter Tempering or how to let some carbon out.
its only a rough guide and i havent gone into the mechanics and maths of it like the lever law and boring stuff, but its interesting...to metallurgists lol
regards
mark
[a little clue, Eutectics....if you look at a Eutectic micrograph of steel, looks EXACTLY like a micro of a lead/tin, or a copper zinc, in fact if only shown a black and white photo then a metallurgist will not be able to tell them apart!, ive tried it, just looks spikey]
the harder it gets so does the natural frequency, look up singing razor from lagoulle

Rustybolt
02-09-2010, 09:21 PM
Well Moons it depends on the alloy, but steel is what they call allotropic. Which means it can have more than one crystal arrangement depending on how hot it gets. At normal temps the crystals are locked much as bricks in a wall are locked by offsetting them. As its heated to a certain point the crystals align themselves more in rows. A good example of this is how the floors collapsed in the world trade center. Once the stell structure between the floors heated to the point where things became unlocked.