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Author Topic: Anneal  (Read 1600 times)
hardlec
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« on: July 24, 2010, 03:47:26 pm »

I hope this topic will be made sticky

How do people anneal brass, copper, and nickle silver for various projects.
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« Reply #1 on: July 24, 2010, 08:25:36 pm »

Copper, red hot dump in water...
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xenomagnet
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« Reply #2 on: July 24, 2010, 08:32:03 pm »

Essentially heat the metal massively then cooling it.

The cooling process depends on the metal:

Brass- immerse it in water
Silver and copper- cool slowly by just leaving it to cool natuarally
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« Reply #3 on: July 24, 2010, 08:37:24 pm »

Essentially heat the metal massively then cooling it.

The cooling process depends on the metal:

Brass- immerse it in water
Silver and copper- cool slowly by just leaving it to cool natuarally
for copper, cooling time makes no difference. If you are set up to work it hot, go ahead. If  not, cool it in water...
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xenomagnet
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« Reply #4 on: July 24, 2010, 08:45:25 pm »

Essentially heat the metal massively then cooling it.

The cooling process depends on the metal:

Brass- immerse it in water
Silver and copper- cool slowly by just leaving it to cool natuarally
for copper, cooling time makes no difference. If you are set up to work it hot, go ahead. If  not, cool it in water...

Test both methods and see what results you get. Nice to see there are so many excellent minds here.
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Narsil
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« Reply #5 on: July 24, 2010, 09:05:12 pm »

It's worth pointing out that heat treatment for ferrous and non-ferrous metals is completely different and aluminium can be different again.

Ferrous metals:

Hardening: heat to critical temperature  (around 800degC depending on the alloy) and quench. There is a critical window of time in which the metal must be cooled below a threshold temperature in order to harden successfully, the length of the time window depends on the type of steel in question and can vary considerably. In practice the quench speed in controlled by using different quench media. Water gives a very fast quench, oil somewhat slower and some steel can be quenched in a stream of air. For common types of tool steel we're talking about a temperature drop of several hundred degrees in under a second. For oil quenched steels it will go from red hot to merely warm in less than 10 seconds with the biggest drop happening in a much shorter period of time.

In terms of metallurgy, at the critical temperature the crystal structure of the steel changes, if it is cooled fast enough that new structure can be 'locked' into the steel, albeit in a modified from.

Accurate control of temperature is vital to successful hardening, not hot enough and the steel won;t fully harden, too hot and very large crystals will form, compromising its mechanical properties.

Steels will also work harden, and this is indeed the only way to harden steels with very low carbon content. Work hardening is often designed into sheet metal processes such as pressing, folding and raising and can also be generated by shot blasting eh to pre-stress the roots of gear teeth to mitigate fatigue.

Quenching is an exceptionally stressful process for the steel and careful design of the part and precess is required to reduce the risk of distortion or cracking.

Tempering:

Fully hardened steel is hard and brittle, too brittle for most uses, therefore a tempering stage is used to relieve the stresses caused by the rapid change of structure on quenching and to reduce hardness and increase toughness. The balance between hardness and toughness can be accurately controlled by tempering at different temperatures. For most low alloy steels the tempering range is around 180 (hard) to 300 degC (tough;spring temper).

Normalising:

Normalising is used to relive internal stresses and refine the grain structure after heavy machining or hot work. The work is heated to critical and allowed to cool in air. The process may be repeated several times for maximum benefit. Normalising should be carried out immediately prior to hardening.

Annealing:

Annealing is usually carried out on steels prior to and during cold working, especially during heavy and prolonged cold work where excessive work hardening could cause cracks. It may also be used prior to machining operations to reduce the load on tooling and improve machinability, especially if the work is to be hardened subsequently. The process is to heat to critical temperature and cool slowly over several hours, either in a furnace, slowly ramping the temperature down or by packing the hot part in insulating material.

Exceptions:

Some exotic and high alloy content steels, particularly stainless and high speed steels have markedly different heat treating processes which may require controlled atmospheres and sophisticated temperature control over long and complex heating cycles.

There are also alternative hardening processes for conventional steels which involve quenching into a much hotter quench medium than normal such as molten salt or lead. Which produces a completely different structure which is exceptionally tough and does not require tempering.

Some heat treatments also incorporate and extreme low temperature phase post-quench to promote full hardening.

Non-ferrous metals:

Hardening

Most copper alloys etc cannot be hardened by heat treatment and are exclusively work hardened, either as part of the forming process or by polishing or shot blasting etc.

Annealing

Heat to a critical temperature and allow to cool, in most cases rate of cooling is not critical and it is common but not necessary to quench for convenience, although this may introduce residual stresses into the work.

Aluminium

There are many many different aluminium alloys (pure aluminium is rarely used except as a chemical reagent) which have widely varying properties.

One process particular to aluminium and related alloys is precipitation hardening. This is a process which occurs naturally over time (referred to as 'ageing')as copper atoms diffuse throughout the metal structure effectively locking it together but it can be greatly accelerated by storing at controlled temperatures.

Certain grades of aluminium, particularly those used for sheet metalwork behave in much the same way as the generic non-ferrous alloys described above. It can, however be tricky to anneal, since it melts before there are any visible indications of temperature. A trick to get round this is to coat the metal with black soot from pure acetylene flame and then heat with a torch, when the soot burns off it's at about the right temperature.

Another peculiarity of aluminium is that if it has been heavily worked the residual stresses can shift over time as it ages, causing it to distort.

« Last Edit: July 24, 2010, 09:12:30 pm by Narsil » Logged







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« Reply #6 on: July 25, 2010, 03:34:57 pm »

Not much to say after Narsil's overview, but I will note that aluminum (and some plastics) can be purchased in a "strain-relieved" condition, which minimizes its tendency to distort after machining. If you find that the nifty part you made from billet is developing an interesting twist, that may be the problem. The process of rolling and compressing the metal at the manufacturer causes strain to be unevenly distributed through the cross-section of the stock. This can be mostly eliminated by a "soaking" cycle at a controlled temperature, which allows some amount of equilibration.
Aluminum alloys are frankly just kind of weird. Machinery's Handbook provides a nice overview. It's good to know what you are getting into if you step outside the comfortable familiarity of the 6061 family.
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hardlec
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« Reply #7 on: July 26, 2010, 01:47:24 pm »

Having worked with silver and nickle silver, which have different characteristics:

Silver can be purchased in Hard, Half Hard and Soft condition.  Sometimes there is a state called "dead soft."  Silver can be heated and left to cool in the air to soften/anneal. 
Silver, especially Sterling (which is what most Jewelers use:  an alloy of silver and copper) work hardens quickly and to do intricate work it is often necessary to anneal a project several times.  This is why I keep a ceramic block (It is almost "Lacey" in structure, and I keep it on a firebrick) near at hand.  I would anneal my work often. 
Nickle Silver was a lot more forgiving.  It would not harden as fast and took less heat to anneal. 

In both cases I treated the metal with a commercial compound that prevented, or at least reduced, fire scale. 

One thing I am grateful for:  There is a great deal of difference in the experience of a "smith" and a jeweler.  Many valuable lessons on both sides.

When I annealed silver I did not quench. 
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hardlec
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« Reply #8 on: July 26, 2010, 01:52:36 pm »

One thing important to the annealing process is getting the metal to the right temperature.

With the exception of Aluminium, which stays "dull" until it melts, or nearly so,  the color of a metal is a way of estimating temperature.

A:  Does anyone have a good graph that they have used and trust?

B:  Is this more fiction than fact?  My own experience is somewhat limited.  I would heat silver to "bright red" (dull red, cherry red, bright red, orange, I never went hotter unless I was brazing)

Wonderful stuff, folks.
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jringling
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« Reply #9 on: July 26, 2010, 02:10:24 pm »

Can silver (sterling or higher) be hammered while hot, or should it be allowed to cool slowly?
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hardlec
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« Reply #10 on: July 26, 2010, 02:25:28 pm »

Hammering while hot will not harden the work as fast and it will make each blow more effective, but silver will shed its heat pretty fast.  It's not like hammering hot steel, where you can get several blows between returning the piece to the forge.

Of course, when I work metal I think a ring is a big piece.  I have to position the piece very carefully and I use a punch rather than a direct strike more often than not, so, by the time I can make a blow, the metal has cooled.

When I used to put a "hammer finish" on a sheet, I worked the metal hot.
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jringling
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« Reply #11 on: July 26, 2010, 02:53:09 pm »

So far, the only silver work has been melting some random "scrap" into a puddle and hammering it flat into a sheet. I then used the sheet in the same way I use brass: ironing on a mask and etching. I quenched the puddle in the ladle I was using and hammered it cold. If I ever revisit this type of work, I am thinking I can get a thinner, more uniform sheet if I hammer the slug while it is hot.

I am in no way a jeweler, but the silver was cool to work with...
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Narsil
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« Reply #12 on: July 26, 2010, 08:57:20 pm »

One thing important to the annealing process is getting the metal to the right temperature.

With the exception of Aluminium, which stays "dull" until it melts, or nearly so,  the color of a metal is a way of estimating temperature.

A:  Does anyone have a good graph that they have used and trust?

B:  Is this more fiction than fact?  My own experience is somewhat limited.  I would heat silver to "bright red" (dull red, cherry red, bright red, orange, I never went hotter unless I was brazing)

Wonderful stuff, folks.

Usually the best way to refine heat treating processes sis by experimentation. Both ferrous and non-ferrous metals come in a large variety of different alloys which can have significantly different requirements. Probably the best source of information is the original manufacturer, who should be able to provide the basic information required .

The other issue is that, unless you have fairly sophisticated kit then you are indeed going to end up eye balling it and its difficult to be that specific about colours, especially since lighting conditions will have a significant effect on how you perceive the colour of hot metal. Forges are usually situated in a fairly dark space for exactly this reason.

A dull red glow seems to work pretty well for sterling silver and copper, brass can be a bit more problematic as some alloys will be very close to melting by the time they get any colour at all.

Steels require rather more precision and unless you have a very experienced eye colour on its own won't be accurate enough. frotunately steels will become non-magnetic around critical temperature. This may not be a precise indication for some alloys but its as close as you;re going to get without fairly serious kit.

 
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Mr. Boltneck
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« Reply #13 on: July 27, 2010, 05:07:52 am »

Once you get past color descriptions and rules of thumb, you are more or less left with two choices. One is to buy a non-contact IR thermometer (http://www.instrumart.com/ProductList.aspx?CategoryID=4110&Landing=1&Phone=5&gclid=CLTLyOHoiqMCFQ9qgwodvDHocg for examples) or an older-style optical pyrometer. The other is to use an accurate temperature-controlled kiln, and wait until the temperature reaches equilibrium. This leaves most of us content to use rules of thumb and descriptions like "dull red," albeit I am given to understand that some Japanese schools of sword-making have descriptions which are both more poetic and more exact.
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