Basic Metallurgy

As the saying goes, it’s always best to start from the beginning. A metal, when melted and cast, transforms from a liquid into a solid. It forms crystals and looses its fluidity. Those crystals fuse into a cohesive, ordered structure, while the metal continues to release heat. The crystals also contract, making the structure shrink in size. This is a very quick process, obvious to the eye.

However, it’s the transformations in the metals that we don’t see, which become the game changers. Some metal alloys used, when they cool, fall out of solution at different rates, and segregate along the grain boundaries. You may not realize, but this migration of differing metals happens for a longer period at temperatures well below the freezing point. Secondary microstructures—known as phases—can form, causing a dramatic change in the hardness of the metal, depending on the elemental metals from which they were formed.

Examples of these fluctuations in metal hardness are:

  • The copper in yellow gold often forms a secondary phase when it solidifies.  But the relative hardness differences between the alloyed gold and copper are not that dramatic so we do not perceive a hardness difference. 
  • In nickel-based white gold, on the other hand, the differences are quite dramatic.  As the metal solidifies and cools, it changes from a single- phase to a double-phase structure.  The longer the nickel is allowed to form the second phase, the harder the microstructures. The result is a profound change in the hardness of the entire metal.  As the relative hardness increases so does its brittleness.

Timing and Temperature Matter

Because timing is critical when breaking out certain metal castings, this is when understand what is going on in the metal applies to breakout.  In yellow gold alloys, the formation of a copper phase has little obvious impact on the finished casting, so time is not a critical factor for breakout. 

This is not the case in the nickel-based white gold alloys.  In this case, critical temperature considerations have to be accounted for during breakout.  It’s necessary to rapidly cool the metal while it’s above 450° C (842° F) in order to prevent the formation of the secondary phase.  This temperature can be higher or lower depending on the mix and ratio of alloys.

Though the investment material has thermal properties advantageous to the casting process, they don’t help immediately after the piece solidifies.  While the investment provides a thermal blanket around the casting and a mold for the formation of the piece, the average jewelry investment transfers heat at a rate of 400° F. per millimeter per minute.  This allows the interior cavity of the investment mold to remain hot enough for the cavity to fill before the metal solidifies.  However, after that happens, it also prevents the metal from cooling quickly if there’s a need to prevent it from forming a secondary phase.  At this point, the only way to allow rapid cooling to take place is to break the majority of the investment from the casting.

Grain Growth

A major concern among manufacturers is grain size in the cast pieces. However, grain growth in the casting process is entirely different than it is in annealing.  The amount of grain growth is controlled in large measure by the amount of energy stored in the metal. In a cast piece, this energy is in the form of heat.  When the metal cools and solidifies it allows the grain structure to form. However, unlike a fabricated metal, the grain does not grow significantly beyond the initial formation, due to the lack of additional stored energy.

So, what does this mean to the caster? Simply that they don’t have to be concerned with grain growth occurring in a casting as it cools.  Also, there is little difference in the relative grain size between a casting that has cooled in the investment, and one broken out of the investment while still hot.  This means the metals that don’t form a strong double phase structure can be broken out after the investment has cooled.  Yellow gold and palladium white gold castings, which don’t contain nickel, can cool before breakout.

Next up in this article series on casting breakout: Part 2 – The Investment. It focuses on wet and dry shattering, disposal, and other important considerations when moving into the final phase of casting – the reveal.