Strip bending...

MTA · Strip bending...

I was looking at 'Victor' and remembered I said I'd wrap some brass strip around the chimney so it looked like a chimney cap. I decided to wrap the strip around the chimney, but it prooved a bit more difficult as the strip is quite hard to bend:

Length

Width

Height

I think it's made more difficult because of the small diameter of the chimney. So anyone got any ideas?

tmuir

As it is brass the strip will be 'Half Hard' Meaning it will try and resist bending and shaping.
Before you can bend it you will need to anneal it.

This involves heating it until it is a dull red and allowing it to cool to black before quenching (cooling it) in water.
Once you have done this it will be black and will need to be cleaned with either an acid pickle or fine W&D and then brasso it back to a shine.

You will find it will bend very easily then.
Just make sure you heat it on something heat proof.
If you dont have a blow torch I've heard a gas camp stove can do it as well as a gas top stove but the cooks in the house tend to get a bit upset by you doing that.

oldstuff

Not sure I grasp your particular dilema, but here's a few pointers.
1. Buy a thinner strip.
2. Use an object of larger diameter to roll it/shape it. You'll have more
leverage with a larger object and more surface to grip.
Then use increasingly smaller diameter objects until it's right.

IndianaRog

tmuir

Yes it is reversible.
As you work the brass/ copper it slowly goes hard again. Every time you bend, hammer, stretch the metal it goes a little harder hence when you are trying to make end caps you may have to anneal the item several times to complete the job.

If you mean once you have bent it to shape and its still soft can you harden it that I'm not sure off.

I also do silver smithing and I'm sure I read somewhere baking sterling silver in a kiln for a set period of time at a certain temperature will reharden it but not sure about pure copper or brass.
Will try and dig through my books tomorrow and find it.

Polishing the brass with a buffing machine will give a small amount of hardness to the outer layer as the final polish is actually a burnish.

IndianaRog

Thanks Tony,

If you find out any additional info on rehardening a brass strip made into a bracket, I'd appreciate it...otherwise I will just take my chances since I doubt I could bend it properly without annealing it first.

cheers,
Roger

tmuir

Doh didn't read your message properly.
To get it all hard again the best method would be to gently hammer the brass to reharden it or sit the brass under some fire bricks and only expose the bits you want to anneal and just heat those areas up.
It wont be perfect and you will get some creeping of the heat but you should still end up with parts of the bar still hard and where you have bent it will of partially hardened by the action of bending it

rough-shunter

the above proces is known as work hardening

Chris

Have you done it yet MTA? I am thinking of making a cap for Arthur.

MTA

MooseMan

Isn't it true that you can reverse the annealing by bringing it up to a dull red and then cooling it very quickly by dunking it in water? Seems to work for me....

Roly Williams

old_timer

I don't believe it's quite as simple as that Roly.

Steel has quite a complicated phase diagram and when steel cools at different rates from say austenite it may form, ferrite, cementite, pearlite, martensite, etc These are all different structures of steel that each have individual properties. Martensite, for example is formed by very rapid quenching, and therefore closely resembles the austenitic structure as it does not have time to form another structure. Martensite is very hard but brittle. However you can temper (heat treat) martensite and form bainite a different and more ductile structure, yet still quite hard. Obviously if you heat it enough to take it back into the austenitic phase, then you are back to stage 1

Work hardening on the other hand is created by 'pinning' of dislocations (or irregularities) within the crystal structure, whilst annealing causes the the crystal structure to 'relax' and hence become for ductile.

What I'm saying (in a very complicated manner) is that it is more or less unimportant what rate of cooling is involved when annealing brass as all you are doing is 'relaxing' the the crystal structure by heating it up. In Steel, the rate of cooling is important as different structures form depending on the cooling rate and each structure has it's own set of physical properties.

Sorry for the metallurgical lecture - guess what I studied when I was younger

SPOKESMAN

Any chance of a glossary of those terms?

Seriously!

Roly Williams

old_timer

An expert? Me? More of an x-spirt

x = an unknown quantity
spirt = a drip under pressure

Seriously, I'll try to explain the terms in an easy to follow glossary soon - must dash now.

mc_mc

I'm not an expert but I have a book on the subject:

SPOKESMAN

Oh no!! A level maths again . . . . .

Roly Williams

SPOKESMAN

old_timer

Ok, as promised, a quick lesson in metallurgy for those who have already had their appetite whetted by mc_mc’s post. Actually that post will probably be better than this attempt, but here we go…

… oh, and no mathematics in sight, but we may involve a few Greek letters just to keep the mathematicians interested

Let’s start with the basics (and apologies to any metallurgists reading, but I’m trying to keep things simple for the benefit of all):

Steel – an alloy of Iron (Fe) and Carbon (C). Ok so it may contain a few other elements but these are not really important here and will only complicate matters

Brass – an alloy of Copper (Cu) and Zinc (Zn)

First let’s consider the possible atomic lattice structure of materials. When materials crystallise they take up the smallest possible structure as this is the least energy configuration, but this will of course vary by the relative sizes of the different atoms.

Body centred cubic (known as bcc) is probably one of the simplest arrangements. Think of an atom at each corner of a theoretical cube and then one more in the centre, like so:

But then there is face centred cubic, (unsurprisingly known as fcc) where again the is an atom at each corner of the theoretical cube, but an extra atom then houses in the centre of each face of that cube, rather that it’s overall centre, like so:

Of course there are umpteen other possible lattice structures too, (e.g. hexagonal), but this should give you a basic idea.

Now lets look at the Phase diagram – also known as an equilibrium diagram is a graphical representation that shows the state, or phase, of a substance at any combination of temperature and pressure under equilibrium conditions (natural state). They can be multi-dimensional for all the different elements, but now you see why we only consider the 2 main elements that means we have a simpler, yet still complicated, 2 dimensional phase diagram

mc_mc already posted the Fe-C phase diagram for low concentrations of carbon, but here is a more complete version

You see that different states or phases occur not only at different carbon concentrations but also at different temperatures – remember that this is only under equilibrium conditions i.e. where the structure has time to form based purely on temperature and pressure.

Austenite – also known as gamma (γ) phase. A fcc structure that can dissolve up to 2.06% C at 1147 ºC.

Ferrite - also known as alpha (α) phase. Bcc iron dissolving a maximum of 0.025% C at 723ºC

Cementite – Iron carbide (Fe_3C); a hard brittle material with orthorhombic crystal structure

Pearlite – shown as P in the above diagram; a 2 phase lamellar structure of ferrite & cementite formed when allowed to cool slowly.

Martensite – this is a metastable structure that forms when rapidly cooling austenite. It is a bct (body-centred tetragonal) structure. Don’t ask me to draw that!

Bainite – A metastable aggregate of ferrite and cementite that is formed when rapidly cooling austenite below the crtical temperature of 723°C. The cooling rate to form bainite is higher than that required to form pearlite, but lower than that to form martensite, everything else being equal. Bainite has very fine particle size and is generally stronger and more ductile than pearlite.

Not convinced about a material having different properties depending on the temperature and pressure it was formed under? Think about carbon – it normally is found in the form of graphite which is very soft. Form it under the correct pressure and temperature and you produce diamond 0 the same chemical element, but with a very different appearance and hardness!

Ok, now lest look at the Cu-Zn (Brass) phase diagram

Don’t worry too much about the different phases that occur with different concentrations of Zn, what is important here is that there is no different phases that occur for a given concentration of Zn as it cools (except at higher Zn concentrations) You can also ignore the very tiny area representing the different phase (or structure) very close to the melting point as it is unlikely you will be able to heat and hold accurately to this temperature.

So this is where I got entangled in this discussion. What happens in steel may or may not be work hardening, but in brass it most probably is.

What is work hardening? Well consider a lattice structure again, but this time on a much bigger scale – it would not be unreasonable for an irregularity to form within such a structure – this is called a dislocation. There are again many types of dislocations, but below is an example of an edge dislocation

Line O-A would normally be a line of weakness in such a structure, allowing the material to deform like so to a much more stable condition

But when you work harden a material, you will induce multiple, cris-crossing dislocations which effectively pin each other and harden the material. By annealing the material, (heating it up) you give the material enough internal energy to effectively remove the dislocations and so take up the most stable (non-dislocated) state, although a few will still undoubtedly remain.

I think that more or less covers what I was saying in a simplistic overview. Again apologies to any metallurgists reading as simplification will by it’s nature will brings with it a few ‘short-cuts’. Alternatively blame it on the fact that I trained as a material scientist, not a metallurgist

Anyway, hope that helps generally; if you need more, let me know.

Roly Williams

tmuir

Wow oldstuff.

I was happy with 'You heat to read and let cool and brass is soft'
'You heat steel and depending on how hot and how quick you cool it the steel takes on different properties'

Good stuff