Stress Cracking and Prong Failure

Few things can be as upsetting to a jeweler as having a customer return a ring due to prong failure. In some cases, the stone has fallen out and has been lost. This causes additional problems such as replacement cost, possible damage to the jeweler’s reputation, and the likelihood of liability. Also, many pieces have sentimental value, which cannot be restored by even the most skilled craftsman.

Having been approached with such problems in the past, our company has conducted studies and metallurgical examinations under extreme magnification to establish the cause of the problem. These studies have indicated that the underlying cause of prong failure has frequently been due to residual stress. This is an internal condition within the metal, which, if not relieved, leads to cracking and eventual failure of the prong.

The nickel white gold alloys are the most popular choices for diamond settings in the United States. While they look attractive, they have a tendency to create problems, which lead to prong failure. Nickel, while acting as a whitening agent for gold alloys, can make the metal brittle. This is due to the nickel being insoluble with gold. As it is melted, it mixes with other base metals to form nickel rich boundaries, which become brittle when stressed. Consequently, the whiter the alloy the more prone to brittleness it will be.
Most cracks are not visible to the naked eye. They are intergranular fractures, which form along the grain boundaries of the metal itself. While cracking through some of the grains has been infrequently observed, the majority of these faults occur along the grain boundaries. Once they are formed, these cracks act as a conduit for corrosive elements, which hasten prong failure.

While the causes of these stress related problems are very subtle, most are within the control of the jeweler. The processes used in the assembly of a piece of jewelry can cause changes in the metal, which will contribute to prong failure. Care must be taken when performing such operations as soldering the setting to the shank, cutting in the seat for the stone and bending the ends of the prongs over the stone.

Heating problems commonly develop during soldering when the setting is overheated or the piece is quenched prematurely. Subjecting a setting to excessive temperature will result in cracking. Samples submitted for evaluation have frequently shown one side of a setting to be broken and brittle, while the opposite side remains very ductile and shows no signs of cracking. This would indicate uneven heating where the broken prongs were overheated while the uncracked prongs remained cooler. Quenching a piece which has been heated to a high temperature will result in a rapid cooling of the external material and a slower cooling of the core. This uneven thermal contraction can cause stress and even cracks in the setting. Excessive heat can also be generated while cutting the seat for the diamond into the prong.

Bending or forming a setting causes a change in its grain structure. If overworked, this can lead to increased residual stress and eventual prong failure.

One answer to preventing or minimizing the chances of breakage in nickel alloys is to take extreme care when performing soldering and setting operations. Uniform heating and cooling are vital to a successful piece. Once a prong is exposed to excessive heat or a setting is quenched prematurely, the damage is done. Evenly heat the mounting, bearing in mind that the prongs, which have less mass, will heat up more rapidly than the shank. Avoid overheating the prongs by directing the flame away from them. Prevent overworking by annealing when needed and reduce residual stress by heat-treating the mounting at 600°F for at least 15 minutes, letting it air cool.

Another solution to soldering problems is to have Hoover & Strong assemble the mounting for you. This is done in a furnace with a controlled atmosphere where even heating and cooling can be maintained. This will provide a clean mounting which is free of the problems associated with overheating. It will also yield a piece, which is properly annealed and is very workable.

Copyright 2006 Fred Klotz.