Given the recent posts about the precipitation hardening of silver
alloys and some of the confusion which seems to have appeared over
what exactly precipitation hardening is I thought I would try to
answer some of the questions raised.
Let me declare my interest right at the start. I am a qualified
metallurgist who has worked in the silver manufacturing industry for
over 20 years. For the last 18 months I have been employed by
Argentium International Limited as their Quality Assurance Manager.
So first of all what is meant by precipitation hardening, what is it?
If we consider an everyday analogy to assist in our understanding of
what happens within the silver alloy; think of sugar being dissolved
in hot tea. At any given temperature there is only so much sugar that
can be dissolved into the tea. When no more sugar can be dissolved at
that temperature we have what is called a saturated solution. If the
temperature of the tea is increased then more sugar can be dissolved
into the tea until the saturation point is again reached. If you then
allow the temperature of the tea to fall, the tea becomes
super-saturated and the sugar is precipitated from the tea as small
crystals.
The same situation exists with metallic solid solutions. The
solubility of one metal in another metal increases as the temperature
increases. In the case of silver-copper alloys, molten silver and
copper are completely soluble in each other in all proportions. When
solidified, silver alloys having a copper content in the range from
about 2% through 27%, contain, two discrete constituents or phases
that can be seen when examined under a microscope. One is nearly 100%
silver; the other is a silver-copper eutectic* (71.9% silver; 28.1%
copper) which has a melting point of 780degC (1435 F ).
So if we go back to our analogy of tea and sugar, the silver rich
phase is the tea and the sugar is the silver-copper eutectic. All
silver-copper sterling silver alloys contain a mixture of these two
phases. The exact proportion of the phases depends on the cooling
rate (for investment castings) or the working and annealing schedule
that the piece has received for sheet and wire products. All we are
aiming to achieve with the precipitation hardening heat treatments
is to first take all the silver-copper eutectic phase into solution
in the silver phase. Then quickly cool the alloy to create a
supersaturated solution; finally to use a low temperature heat
treatment to encourage the growth of the silver-copper eutectic
precipitate, to harden the alloy.
The heat treatment schedule to achieve precipitation hardening in
sterling silver is well documented (1). It consists of the following
stages:
-
Heat the alloy to 745-760degC (1375-1400 F). This takes all the
silver-copper eutectic into solution in the silver - rich phase.
-
Hold at temperature for 15 minutes. This allows the piece to be
heated to temperature throughout its cross-section.
-
Quench rapidly into cold water. This retains the silver-copper
eutectic phase in the silver-rich phase as a supersaturated
solution. The alloy is now in a softened condition.
-
To re-harden the alloy it is now heated to 280-300degC (536 - 572
F) for 30-60 minutes and then allowed to air cooled. The exact time
to achieve the maximum hardness is dependent on the cross-sectional
thickness of the piece being treated.
If we now look at how the hardness can change at each stage of this
process (these figures are from numerous databases, publications and
personal experience):
-
Sterling silver, as-cast investment pieces and annealed sheet and
wire (as supplied from the manufacturer) has a typical hardness of
65-75VPN (DPN).
-
Heating to 745-760degC and then quenching produces a soft
malleable condition in the alloy. This has a typical hardness of
55-60VPN (DPN).
-
After precipitation hardening it is possible to achieve
hardnesses of up to 120-140VPN (DPN).
There are two aspects involved in precipitation hardening to
consider, the first is that at 745-760degC, the solution treatment
temperature (red heat when torch annealing) the alloy is very soft
and malleable. Many pieces will distort at this temperature when
picked up and quenched. The second is that when heat treating at
280-300degC to increase the hardness, if the ideal heating time is
exceeded, it is possible that the alloy will then start softening
again. This is due to the silver grain size enlarging and coarsening.
If only the first heat treatment is carried out at 745-760degC then
you have a very soft piece. With time it is possible that some of the
super-saturated silver-copper eutectic may precipitate out of the
silver-rich phase and harden the piece. As this hardening effect
occurs at room temperature over a long time period this process is
also sometimes referred to as age hardening. This is a very slow
reaction and is the reason for the second stage of the precipitation
hardening heat treatment at the lower temperature (280-300degC). It
allows the hardness to be increased in a controlled way, in a
reasonable time.
If however only the second heat treatment, at 280-300degC, is
carried out then there is only a slight increase in hardness. This is
due to the grain coarsening effect described earlier and also the
precipitation of any silver-copper eutectic already retained in the
silver rich phase. This is not, however, a significant increase in
hardness, typically only being a 10VPN (DPN) increase on the original
hardness value.
One other fact to remember is that one of the reasons that this
process did not find a major commercial application was due to the
deoxidation of silver alloys with phosphorous. It is well known that
molten silver alloys readily absorb oxygen. To prevent this and also
to remove oxygen from the molten silver alloy, graphite crucibles and
charcoal melt covers are used. In addition, commercial manufacturers
use a deoxidant to assist in scavenging the oxygen from the molten
silver. Often phosphorous is used and any residual phosphorous can
create a low melting point constituent in a silver-copper alloy,
which might result in hot shortness. This may cause cracking when
standard sterling silver alloys are quenched from high temperatures.
It must also be remembered heating sterling silver at high
temperatures for long periods will produce deep firescale unless
protected. This is normally done with an inert gas, nitrogen or
argon.
That deals with the traditional silver-copper alloys and I hope
explains the theory of precipitation hardening. It is not my
intention to discuss the hardening of Argentium[tm] silvers as this
is detailed elsewhere on Orchid by others. I would just make the
point that Argentium is an alloy with three constituents, silver,
copper and germanium. As a consequence it has a different structure
compared to traditional sterling silver alloys and hardens in a
different way.
*Note: In a two-metal alloy system, the “eutectic” is a specific
ratio of the two metals that exhibits the lowest melting point.
- Butts and Coxe, Silver, Economics, Metallurgy and Use. (Chapter
18: Alloying Behaviour of Silver and its Principal Binary Alloys.)
Charles Allenden.
Quality Assurance Manager