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Metal Hardening


#1
  The head of soldering takes out the hardness and anneals the
metal, leaving it in a soft condition. However if you take the
piece of jewelry and put it into an oven set at 360B0C for 60
minutes, th e hardness increases from 190 to 270 Vickers, even
harder than if you had hammered or rolled it!  

Alan …The quote above is from an earlier message from you
discussing gold alloying.This method of hardening is new to me
and I do have several questions.

1-I don’t understand the oven setting of 360BOC. What does BOC
stand for?

2-Does the technique apply to Sterling Silver too ? What would
be the temperature and time for this?

3-Is it necessary to flux the piece before heating or is the
temperature too low to be concerned about?

4- How critical is the temperature? or time?

Sol K


#2
   1-I don't understand the oven setting of 360BOC. What does
BOC stand for? 

It is the computer’s poor translation of the degree symbol which
I typed. It should be 360 degrees centigrade.

   2-Does the technique apply to Sterling Silver too ? What
would be the temperature and time for this? 

Yes, but it may depend on the exact composition of the alloy
(what the supplier puts in to make it cast better or tarnish
less, etc.) The info I have (and have not tested) is to take
sterling and put it at 750 degrees centig rade for 30 minutes,
followed by immediate quenching and subsequent heating to 300 for
60 minutes. (It melts at 893 degrees centigrade.)

  3-Is it necessary to flux the piece before heating or is the
temperature too low to be concerned about? 

Good question and I honestly do not know. It cannot nurt and my
guess is that heating silver that high would cause tarnish,
unless a coating of bor ic acid is applied to seal the surface off
from the atmosphere.

  4- How critical is the temperature? or time? 

Again I cannot say, but I think that if you are within 10
degrees it s hould not make a difference.

The gold was gleened from the World Gold Council’s
Aloy Data Sheets. The silver from Grimwade’s book, Introduction to
Precious Metals. I have not tried this, along with lots of other
things. The theory is wide ly accepted and in practice, it should
work. Please let me know if you try it and what the results are so
that I can tell others.

Alan


#3

This sounds like precipitation hardening, which occurs when a
second phase exsolves and forms tiny particles throughout the
rest of the alloy. Do gold and silver do this? I know lots of
structural alloys do, but they’re mostly aluminum. If this is
precipitation hardening, the temperature and time are probably
pretty critical.

Tas Raistrikia


#4
   This sounds like precipitation hardening, which occurs when
a second phase exsolves and forms tiny particles throughout the
rest of the alloy.  Do gold and silver do this?  I know lots
of structural alloys do, but they're mostly aluminum.  If this
is precipitation hardening, the temperature and time are
probably pretty critical. 

It IS precipitation or “age” hardening, based on copper’s poor
actual solubility in silver. The hardening allows copper in
quickly cooled sterling to migrate to grain boundaries as
seperate crystals. The result is a considerable increase in
hardness, caused by much reduced ability of the grain boundaries
to then deform.

Peter Rowe


#5
     Is it necessary to flux the piece before heating or is
the temperature too low to be concerned about? 

the initial heating is simply to assure a uniformly annealed and
soft piece with a uniform fairly large grain structure before
age hardening. With simple cast pieces, it’s of benefit. For
constructed pieces, unfortunatly, that temp will melt some
solders, and also give you some nasty fire scale if not
protected. Assuming the work was soldered together, you can
assume it to be annealed, though the grain structure may vary.
Skip the initial heating if you like, and you’ll still get decent
hardening, though perhaps not quite so hard or uniform, from the
second cooler age hardening step alone. As to protecting it, a
coating of pripps flux would be ideal. See the orchid archives
for my prior article on applying and making pripps flux.

Peter Rowe