Precipitation Hardening - Argentium vs Traditional Sterling silver

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. So in the case of a simple silver-copper alloy (e.g. traditional sterling silver), 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 780C (1435F).

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 traditional sterling silver is well documented (see Butts and Coxe, Silver, Economics, Metallurgy and Use. Chapter 18: Alloying Behaviour of Silver and its Principal Binary Alloys).

It consists of the following stages:

1. Heat the alloy to 745-760C (1375-1400F). This takes all the silver-copper eutectic into solution in the silver–rich phase.

2. Hold at temperature for 15 minutes. This allows the piece to be heated to temperature throughout its cross-section.

3. 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.

4. To re-harden the alloy it is now heated to 280-300C (536–572F) 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):

1. Sterling silver, as-cast investment pieces and annealed sheet and wire (as supplied from the manufacturer) has a typical hardness of 65-75VPN (DPN).

2. Heating to 745-760ºC and then quenching produces a soft malleable condition in the alloy. This has a typical hardness of 55-60VPN (DPN).

3. After precipitation hardening it is possible to achieve a hardness of up to 120-140VPN (DPN).

There are two aspects involved in precipitation hardening to consider, the first is that at 745-760C, 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 280-300C 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.

After the first heat treatment is carried out at 745-760C 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 (280-300C). It allows the hardness to be increased in a controlled way, in a reasonable time.

If however only the second heat treatment, at 280-300C, is carried out in traditional sterling silver alloys 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.

It must be remembered heating traditional sterling silver at high temperatures for long periods will produce deep firescale unless protected. This is normally carried out with an inert gas (nitrogen or argon).

That deals with the traditional silver-copper alloys and I hope explains the general theory of precipitation hardening.

For Argentium silver alloys it needs to be remembered that Argentium silvers are alloys with three constituents, silver, copper and germanium. As a consequence these alloys have a different structure when compared to traditional sterling silver alloys and have a different hardening response when heat treated.

With Argentium silver alloys it is possible to get a significant hardening response by carrying out the lower temperature (280-300C) only. This is well documented and there are numerous video clips on You Tube which show how this can be carried out.
(See www.youtube.com/watch?v=YJDkXyvNkJM for one of my personal favourites).

You can also carry out the more traditional two stage heat treatment process with Argentium silver alloys. However care must be taken at the higher solution treatment temperature that the alloys do not sag when heated or crack when quenched.

For Argentium 935 alloys a solution treatment temperature of 650C is recommended, also for Argentium 960 alloys a solution treatment temperature of 650C obtains an optimal response. These pieces were only quenched after there initial 'red' heat had disappeared, in accordance with good annealing practice for Argentium silver alloys.

So, happy hardening!

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