Even in the best workshops, a piece can crack. While a defect or crack is obvious, the cause is often not. The cause of the crack could be due to the behavior of the specific alloy or the history of its processing. Many different phenomena can lead to similar-looking cracks.
Consequences of cracking
For metallurgists, a crack is a puzzle to solve, but for a jeweler, it has many undesirable consequences:
Item is unusable – it must be scrapped
Costly repair or replacement
Loss of time and perhaps reputation
The concern of repeated occurrence if left unsolved
The issue of defect and cracking is not unique to jewelry; it is a problem that pervades all metallurgical manufacturing, from submarines to medical implants.
Types of cracks
There are two main types of cracking:
The cracks grow along grain boundaries, so the fracture surfaces are rough.
This is a ductile metal fracture, and the fracture surfaces are dull.
The cracks cut in straight lines across and through grains. The surfaces are quite smooth.
This is often associated with the fracture of very brittle metals, and the fracture surfaces are shiny.
Cracking of jewelry materials can occur during and after manufacture. The problem is that cracking in jewelry materials is usually intergranular in nature (cracks grow around the grains along the boundaries). Still, the cause can vary widely, making tracing the cause difficult.
Why do jewelry materials crack?
The answer is simply that the imposed stresses exceed the material's mechanical strength, or the material is strained so quickly that the material can't deform fast enough.
Cracks often occur and begin at defects. These reduce the stress value at which failure occurs. A defect disrupts the crystal lattice structure, so there are fewer atomic bonds to bear the load at a given point in the lattice – the stress is concentrated. Cracking will occur when the stress concentration is high enough to break the bonds.
Hard particles such as inclusions or second phases at the surface (Figure 4) or within the alloy can act as stress raisers, i.e., they amplify the local stresses in the adjacent metal, which means that cracks can initiate and grow more easily around them, even when the imposed stresses are lower than those normally leading to crack formation.
Causes of cracking
Cracks can be due to several causes, including:
Embrittlement by impurities, including gases
Casting and working defects and inclusions
Stress corrosion cracking
Quench cracking in castings
The various causes of defects, many of which manifest themselves as cracking during working and manufacturing operations, can be attributed to the following factors:
Poor quality start materials
Recycled scrap often causes contamination and possible embrittlement.
Poor melting practice
Casting defects such as pipes and/or gas porosity and blisters, incorporation of inclusions, excessive shrinkage porosity, and chemical segregation
Poor ingot or material working practice
Incorrect working procedure for the characteristics of the specific alloy
It can also lead to surface defects, such as laps, that develop into cracks.
Incorrect annealing practice
Residual (internal) stress is possibly linked to a corrosive environment.
This can be generated mechanically or thermally and lead to phenomena such as stress corrosion cracking, quench cracking and fire cracking.