c-g-designs

The brass used forcartridges case making is the CuZn30, also named

‘cartridge brass’.

It is a binary alloy, composed of 70% Copper and 30% Zinc.

Its matallurgical conditions are :

---Annealed (O condition), Vickers Hardness (HV)= 80.

---1/4 hard (H11 condition), HV 105.

---1/2 hard (H12 condition), HV 125.

---3/4 hard (H13 condirion ,for memory), HV140.

---Hard (H14 condition), HV150.

Elongation (%) :

--- O condition = 60%.

--H11 condition= 42%.

---H12 condition=22%.

Processing :

The cases are produced :

Either from circular blanks cut into strips of annealed brass and cut by the first step punch of the multi-stations press tooling. This is the most common mass-production method.

Or from hot stamped cups also supplied in O condition. The use of cups already on the case bottom dialmeter of the case to produce the number of tools forming steps of the tool (at least 2), thus reducing the work-hardening. It also produce cases of better quality and regularity.

More foring operations follow, and everything goes without problems until the case is drawn to full lenght cylindrical as, up to then, the drawing was made between die and punch .. Primer pocket and marjkings were made during those operations.

But all it comes less simple when it is to form the shoulder angle and neck, as those operations are to be made without internal support. No more question of punches, the forming being by means of dies only, and concentricity become more uncertain. The metal has work-hardened and quality become function of the tooling and number of steps used for this operation. The metal has hardened at this point up to H12, sometimes H13 condition, and a recristallisation annealing becomes necessary

This annealing js a delicate metallurgical operation. A typical protocol to bring it back to O condition consist :

Heat slowly to 550°C (dark red just appearing).

Maintain in temperature for 30minutes.

Slow cooling down to 100°C.

Quench.

Therefore, nothing to do with what we name here ‘The plumber’s annealing’, often recommended who is just good enough to to transform copper tubes into chewing gum. This annealing annealing temperature is in the order of 700°C (dark red, to better observe in dark environment). For the annealing fans,the temperature can be controlled using a laser thermometer (50 to 900°C) cost about 120 USD in tool shops.

This too violent annealing has for consequence, on successive shots, to have the shoulder and neck flowing forward ,leading to cut-to-lenghts. Also,this metal flowing causes a bulge inside the neck at shoulder cone internal base needing to be removed and who tend to reform after each shot.

Why to not anneal ?

In the course of successive resizings, the metal will work-harden slightly to pass successfully from O condition to H11 and exceptionally to H12 if it is strongly resized and after a certain number of shots .

Uf shot in a precision chamber and if the neck outside diameeter is well adapted to it, one can consider that, during the resizing, the neck will be reducced in diameter by only a few hundreds of a millimetre. Considering for instance the .284 calibre (7,20mm), this diameter reduction will represent less than 0,5% in extension.

If we take for basis the elongation capacity H11 who is 42¨%, we are well under capacity. Even with the 22% of the H12 condition, we are still far from the count !!.

One point however of importance, and this is valid whatever annealed or not, the necks need to be cleaned and carbon-free before and after resizing to prevent leaks to the rear, as the slightly harder metal need a little more attention at this level.

10/2016

R.G.C