... That is, when copper is worked small crystals form. How does
the formation of small crystals "cause" the previously soft metal to
become stiff and no longer pliable? What is the physical
explanation?
It doesn’t. Work hardening deforms crystals, and stresses the
crystal boundaries. The stress on the crystal boundaries accounts for
more of the hardening effect than does the actual deformation of the
crystals themselves. With pure copper, the degree to which a single
crystal can be deformed is truely remarkable… It’s the crystal
boundaries which eventually start to fail. The material transforms to
small crystals with increased boundary area, and again undeformed, but
smaller crystals, when you anneal the metal. This increase in the
area of the crystal boundaries when the crystal size (usually
referred to as grain size) is decreased accounts also for the fact
that worked and annealed metal becomes better able to be worked
without cracking than is metal with large crystals (such as castings,
etc).
I raise this question because we are all familiar with quenching
copper and it staying soft. In contrast if we quench iron it
becomes brittle. Why the difference, even though both are metals
have cubic systems?
Pure iron does not become hard. It’s iron with sufficient carbon
incorporated in it which does this, due to different types of crystal
structure that can form with high or low temperature phases of the
alloy. Quenching the metal from higher temperatures “traps” the high
temperature structure, which by chance, happens to be hard and
brittle.
And what exactly are the ferous metals? Are they Iron, Ruthenium
and Osmium. It is my understanding, and I could be wrong, but if one
quenches gold, or at least its alloys, it also becomes brittle.
Ferrous metals are those containing iron. Ruthenium and Osmium are
two of the platinum group metals. Platinum group metals share a few
characteristics with iron (they are adjacent in the periodic table of
the elements), but iron is not a platinum group metal, nor are the
platinum group metals considered ferrous.
While you can crack gold alloys by quenching from too high a
temperature, due simply to thermal shock, non of the gold alloys I’ve
ever seen will harden upon quenching. Many of those which include
sufficient copper, however, can be age hardened by a sufficiently slow
cooling, or better, a heat treat cycle at below the annealing
temperature. This is the opposite of the treatment processes used
with steels, where that “below annealing” heat treat, or tempering
process, is used to reduce excess hardness to the desired level,
rather than increasing it.
That is what I have read also. But it raises a question. What if
we heated a piece of copper very slowly for a long period of time,
just before it reaches a dull red colour versus what if we could
very rapidly heat the copper to dull red for an extremely short
period of time. Would both sheets be equally annealed? And what
happens when the copper is heated to bright orange red or close to
its melting point? Can the sheet be "over annealed"?
With pure copper, no heat treatment will increase the hardness. Only
annealing is possible. Slow heating vs. rapid heating will make
little difference, other than perhaps the amount of oxidation that
takes place. However, any heating, especially to temps considerably
higher than needed for annealing, allows the atomic mobility that
allows deformed crystals to recrystalize/reform into smaller
undeformed crystals, to proceed even more. At elevated temperatures,
in addition to just recrystalizing, the then reformed small crystals
start to grow together. The recrystalization becomes a process where
some grains grow in size, as smaller adjacent grains become
incorporated with their neighbors. The result is again large
crystals. While this metal then is still fully annealed, the increase
in grain size will reduce the malleability and ductility of the
resulting metals, since the amount of deformation then possible
before crystal boundaries start to rupture is reduced. So extended
high temperatures are usually to be avoided. In some alloys, such as
golds or sterling silver, the effect can be quite pronounced.
Over-annealed sterling silver may be soft, but simple bending of a
sheet that’s been overannealed can cause an obvious “orange peel”
texture for show up on the sheet, as the crystals deform differently
from the crystal boundaries.
So how does the metallurgist who makes the sterling know that
there is a perfect mix and how does he or she know how slowly or
quickly to freeze it? How do they maintain quality assurance?
quality assurance is maintained by testing/observation of the
finished product. Normally, the faster one can solidify the ingot, the
more uniform the ingot will be. The processes used can also have a
big effect. The modern continuous cast methods of actually extruding
a continuous ingot from a hole in the bottom of an induction melt
crucible, for example, produces a more uniform ingot structure than
does a more traditional poured ingot. Among the concerns is that
poured ingots freeze from the outside in. This does, as you’ve
mentioned, lead to ingot structures which are not always the same from
the outside to the inside. Usually, in rolling and annealing, over
multiple passes through both processes, the grain refinement that
takes place can allow the finished sheet to have a pretty uniform end
structure. But it’s not totally ensured. Thus the need for a refiner
to take care with the processes used to produce the finest possible
end product.
One final question. Does it matter whether we know this stuff or
not? I believe it does, not from a techincal perspective from the
point of view of respect for the materials with which I am working.
I don't "make" the piece. My tools, the metal and I together make
the piece. It is my belief that by understanding what we are working
with we transform the activity from craft to art.
I pretty much agree. The more you understand your materials, the
better your control over them, and the greater your ability to make
them do the things you wish. These are aspects of the craft of
metalwork, not the making of art. But to be able, as an artist, to
have the greatest freedom of expression, and the widest possible
ability to create in whatever means/forms you wish, the better you can
expand your skills and understanding of your craft, then the better
able you will be to express yourself as an artist.
Peter Rowe