READER Andrew McFarlane of Newcastle has asked for information about cartridge case annealing.
Like many thousands of Australian shooters, Andrew assembles live cartridges at home. That is, he handloads. Annealing is a core handloading skill, hence his request.
We are talking about heating and cooling specific parts of brass cartridge cases. The temperature reached and the rate of cooling govern the changes which occur within the brass itself. These changes must be the right sort. So, like it or not, we must know some basics of metallurgy. Here goes:
Our aim is to restore three properties (ductility, malleability and elasticity) to their original levels in some parts of the brass cartridge case, without robbing the rest of the case of anything.
Ductility refers to the brass' ability to stretch permanently without fracturing, while malleability refers to its ability to be permanently compressed, also without fracturing. Elasticity is linked with the brass' capacity to spring back from small deformations, such as the expansion that occurs when we fire a shot.
A brass centrefire case has three zones, each with a separate part to play in the case's functioning.
The head region (the case's rear section) includes a small part of the case walls as well as the solid head itself. If it compresses readily under pressure, the case walls next to the solid head will stretch abnormally, grow thin and rupture - all in a few milliseconds. At the same time, the head will expand sideways, opening up the primer pocket.
Therefore, the head region's job is to resist deformation under the stresses of firing, load after load.
At the other end, the neck brass must possess enough resilience to grip the bullet, expand to let it go and then spring back from the chamber neck walls.
In between is the third zone, comprising the body and, in bottlenecked cartridges, the shoulder. The body's task is threefold: to seal the breech by expanding into firm contact with the chamber walls, to resist rupturing in the vicinity of the head, and finally to relax enough of its grip on the chamber walls to allow extraction.
Both the case's shoulder and the walls immediately behind it need enough strength to resist set-back when the firing pin slams home and jams the brass against the chamber's shoulder.
Thus the brass in the head region is tough and springy, whereas the neck brass is relatively ductile and malleable, as well as seeming more elastic. The hardness of the body metal falls between these two and "shades" or grades into them.
Eventually, our handloading operations and the stresses of firing will result in strain hardening of the brass.
When such happens, annealing the case necks (and, where necessary, the shoulders and upper body) will fix it.
Ideally, we should heat the necks to 400-500C for a maximum of 5-6 seconds. See our next column (February 17) for ways to achieve this.
