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[Beginner's corner] Wire hardness


#1

I have a few questions about the hardness/softness of silver wire.
Is the only difference between the hardness of silver wire is
annealing/working?

Can soft wire become hard wire by working of tumbling?
Why doesn’t fine silver come in different hardness?
Can fine silver become hardened by tumbling as sterling will?

Michelle Teka and Zoe


#2
I have a few questions about the hardness/softness of silver wire.
Is the only difference between the hardness of silver wire is
annealing/working? Can soft wire become hard wire by working of
tumbling? 

Pretty much. Silver, if hardened, is softened again by annealing,
which allows distorted crystals to reform, thus relieving stress and
tension both in the crystals and in the boundaries between them.
Silver is generally hardened by work hardening, which requires not
just flexing, etc, but actual deformation of the metal, stretching,
flattening, or otherwise distorting the crystals, and stretching and
distorting also, the boundaries between the crystals (and yes, this
is an important aspect of annealing and hardening) In general,
sterling silver wire you purchase has been hardened by the operation
of drawing it down through draw dies or drawplates, reducing the
guage, lenghening the wire, and greatly compressing and stretching
the crystals. Annealed sterling silver can also be hardened to a
considerable degree, though not as much as by working, by a heat
treating process called age hardening or precipitation hardening. In
this a temperature below the annealing temperature is held, which
allows copper in the crystals to migrate to the grain boundaries, and
recrystalize there as copper crystals along those boundaries. What
that does is to make the grain boundaries much less able to distort
and stretch, so although the crystals themselves haven’t hardened,
the metal mass as a whole has, because the grain boundaries can’t
deform much.

For the record, tumbling is not a great way to harden silver if you
need a big improvement. Steel shot, like tiny hammers, does indeed
work harden the surface layer of the silver, as well as compressing
any porosity or flaws, and burnishing it. That can give a surface
that will take and hold a better polish. And with thin metal, the
surface hardening of that thin burnished skin layer, may indeed be
enough of a percentage of the overall thickness that the metal is
significantly hardened. But with anything a bit thicker than very
thin stock, the improvement in hardness is mostly at the surface, and
not an overall difference. In some cases, this may be enough, and if
one has no other options for making an item more durable, perhaps
it’s still a good method. Compared to butter soft totally annealed
metal, steel shot tumbled silver is significantly better. But
tumbling alone usually is not at all the equal to the hardening you
get with actual working of the metal by means that work the entire
thickness, such as drawing, rolling, hammering enough to distort the
shape, or in the case of wire, twisting it, which can harden it
without changing the dimensions.

Why doesn't fine silver come in different hardness? Can fine silver
become hardened by tumbling as sterling will? 

The copper in sterling silver is responsible for a good part of the
degree to which sterling silver can be hardened by working, and it
work hardens more quickly. Fine silver does indeed work harden, but
not as much, and you may need to work it more to get a desired level
of hardness. Tumbling does the same to fine silver as it does to
sterling, but as with other means of work hardening, it does it
less, so tumbled fine silver will have an increase in surface
hardness, which my give a noticable increase in overall hardness or
stiffness of wire, etc. But it will be less of an effect than you get
with sterling.

Peter


#3

In answer to your questions the simple answers are yes and yes.

Drawing the wire hardens it by making the crystals of the metal
smaller in at least one direction. this increases the number of
dislocations within the crystals which makes it harder to
plastically deform [stretch] Annealing causes the crystals to grow
and realign the internal lattice which will allow the metal to
stretch again. Silver is an alloy so the different components will
deform at a different rate which gives the wire/sheet etc its working
properties overall. Tumbling silver will cause peening to the surface
which will change the grain size of the surface crystals making the
skin of the metal harder. Shot polishing or sand blasting is common
for surface treatment of objects that cannot be polished in other
ways but need to be smoothed. It prevents fatigue failure by removing
focal points for stresses such as pits and bumps as well as hardening
the surface.

Pure silver will work harden but it will be less noticable as a pure
metal will allow more plastic deformation before it becomes brittle.

Nick