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[Norman Castle]

I have a length of K & S 8mm brass tubing which is as drawn i.e. quite hard which I'd like to soften so that I can stretch the bore a few thou at one end.  What's the best way for me to anneal this stuff?

[Mark T]

Personally I would heat it until cherry red and then just let it cool back down naturally. This has always worked for me with both copper and brass

[ballastanksian]

Amen.

[Umi_Ryuzuki]

Well now you guys have me messed up again... 
I thought it was the other way 'round.


http://chestofbooks.com/crafts/machinery/Shop-Practice-V1/Hardening-And-Annealing-Brass.html#.VkKMNWdxWhM


And even the artisans can't agree...
http://www.artmetal.com/blog/tantoinette/2008/11/heat_harden_brass


 

Copper alliance UK

http://www.copperalliance.org.uk/copper-and-its-alloys/alloys/brasses


PDF location Brass processing, publication 117
http://www.copperalliance.org.uk/resource-library/pub-117---the-brasses

Annealing
When cold worked brass is progressively heated, the first effect,
at about 250ºC, is for the internal stresses to be relieved. This
prevents stress corrosion cracking subsequently occurring and
also minimises the amount of distortion which may occur during
machining. This low temperature heat treatment, which should be
applied for 1⁄2 to 1 hour, is known as ‘stress-relief annealing’ and
has little, if any, measurable effect on the mechanical properties
of the material. The improved strength due to the cold working is
therefore retained.
As the temperature is increased further, a rather more
fundamental change occurs at about 400ºC and above and the
material starts to ‘anneal’ or soften with time at temperature.


The strengthening effect of the cold working is progressively lost, until
at about 500ºC the alloy is in the fully annealed condition.
Restoration of the cold worked properties can then only be
achieved by further cold work.


Due to the volatility of the zinc at the surface of the brass, it is
not easy to anneal in a batch furnace with a ‘bright’ finish solely by
the use of a controlled furnace atmosphere, although strip is now
commonly continuously annealed during production.
When designing components which will be exposed to temperatures
of 400ºC or above during manufacture (e.g. pipework with brazed or welded flanges),
strength calculations must be based on the properties of the

material in the annealed condition. Although cold worked material
may be specified initially, it will be locally annealed during
fabrication or joining operations that involve heating.
Annealing (full)
In order to fully soften most brasses, heat to 500-550ºC for 1⁄2 to
1 hour at temperature, then either air cool or, especially for alpha
alloys, ensure that excessive grain growth is prevented by a
quench or rapid furnace cool. ‘Flash’ annealing can be carried out
at higher temperatures for considerably shorter times, but care is
needed to avoid excessive grain growth.
The use of a protective atmosphere reduces oxidation. Normally
this can be prepared from cracked or partly burnt ammonia to
give an atmosphere high in nitrogen and water vapour. Since zinc
is volatile, care needs to be taken to avoid overheating.

[Norman Castle]

Thank you, lady and gentlemen.  I had a feeling it was "one of those" questions ... 

[ballastanksian]

The latter part of the passage is the best and most informative bit for most work with copper/brass etc.

[Brian60]

We did this a few weeks back, a redux of that was heat to cherry red up to four times depending on how soft you wish to go. I found once was enough for me with K&S tubing, albeit I was using 3/4/5mm sizes, 2mm just split as soon as it started to get warm. Don't heat and then quench in water to try and get the hardness back, that doesn't work either.