Modern Cartridge Cases
Date: Mar 02 2012
The strength, formability, corrosion resistance, low coefficient of friction, good availability and low cost of C26000 brass has made it the material of choice for the modern cartridge case. C26000 serves well to orient components in a durable, properly dimensioned package, it functions well through firearm mechanisms, expands and seals the breech from powder gasses, and it springs back after firing to allow extraction from the chamber.
C26000 is a simple copper alloy created by adding zinc to copper and lies within the range of alloys with a minimum copper content of 63 percent termed “Alpha” phase or “cold working” brass. At solutions below 35 percent, zinc dissolves evenly throughout the copper, forming a uniform solid solution. The Alpha brasses are characterized by superior malleability and ductility at room temperature and are capable of withstanding extensive forming, such as the deep drawing necessary for creating a cartridge case.
Bracketing C26000 are the “Red” or “Low” brasses and the “Yellow” or “High” brasses. Red brasses are used in applications where ductility is paramount and required strength is low. Gilding metal, approximately 95 percent copper and 5 percent zinc is the softest common brass alloy and is well known for its use in reduced-fouling bullet jackets. Cartridge brass consists of approximately 70 percent copper and 30 percent zinc with no other alloying constituents. At this ratio of copper and zinc, there is a marked increase in the strength of the alloy and yet it retains excellent formability. The yellow brasses, with their higher zinc content, possess a solid solution mix of two phases of material, an Alpha phase and a zinc-rich Beta phase. These alloys are stronger and cheaper but are not as ductile as the alloys with less zinc. Alloys beyond 40 percent zinc, like 50 percent zinc “white” brass, must be hot worked and are too brittle for general use.
Because cartridge brass is a simple alloy, it does not respond to temperature hardening techniques but obtains its strength through the mechanisms of solution strengthening (alloying) and work hardening. Both methods of strengthening cartridge brass act to create “defects” within the crystalline lattice of the solid solution. These defects are known as dislocations. These dislocations of material create stress fields within the lattice of the material. Greater overall stress must be applied to move these fields within the material, thus showing increased yield strength (the point at which a material will start to deform plastically).
By alloying zinc in cartridge brass, the zinc acts as a “defect” within the copper structure. In a crude sense, the strength gain is similar to having a bowl full of two sizes of BBs. When the BBs are all one size, they can move amongst each other in a different manner than if larger or smaller BBs were wedged throughout the mix. Because zinc is chemically so similar to copper, sitting one position to the right of copper on the periodic table of the elements, the addition of zinc to copper results in a “substitutional solid solution” where zinc atoms replace copper atoms within the crystalline lattice, and the lattice remains uniform and ductile. The strength gain in alloys of higher than 35 percent zinc is the result of increasing the number of defects within the lattice, but the solid solution loses uniformity and ductility.
Cartridge brass is also capable of being work hardened by plastic deformation. When plastic deformation occurs, the density of dislocations also increases within the material. Material strength is closely related to grain size, with smaller size correlating to higher strength. Cold working breaks down the size of the crystal grains, increasing the number of dislocations between them. In a crude metaphor, applying enough kinetic energy to deform the material puts the material into a higher level of “self-tension.” As the density of dislocations increases, the stress levels that caused plastic deformation will now result in elastic deformation and yield strength will measure higher. Ultimately, if too many dislocations have accumulated and the limit of elastic deformation is exceeded, fractious deformation will occur. This is commonly seen when a case neck hardens and splits as the result of multiple firings.
To reverse the effects of cold working, heat is used to soften cartridge brass. Heat increases the rate of movement of the zinc and copper atoms within the material, allowing them to seek equilibrium. Starting around 500 degrees Fahrenheit, the stress fields created during cold working begin to relax between the grains. Maintaining this temperature and/or increasing it will then accelerate the process of the grain re-forming where the stresses within the individual grains relax. Continued heating will eventually cause the individual grains to grow, coarsening the microstructure and potentially causing the metal to have less than satisfactory mechanical properties. At 600 degrees F., it will take one hour to reach full anneal in cartridge brass. At 650 degrees, it will require 15 minutes. At 750 to 800degrees, the process occurs within a few seconds. After annealing, the alloy consists of homogeneous solid solution that is especially suitable for cold-working at room temperature.