A few thoughts which I have been musing on given some of the comments in this thread...
The effects of fully annealing, or stress relieving at lower temperatures, of brass, is not something easily quantified. Brinell hardness is not directly related, though relatively easily measured with simple tools. It's the change in ductility that matters.
Well yes, it is easily quantified...
The significant change in ductility occurs above stress relieving temperature when recrystallisation has occurred at full annealing temperatures above 450˚C
As the graph shows stress relief annealing has variable hardness outcomes depending on the initial hardness...the stress history of the brass...between 250˚C and 350˚C these can vary dramatically. There is much more tolerance for our crude systems around 400˚C where they come together and thus give a reasonably consistent hardness whilst maintaining factory neck tension before the grain size rapidly increases during recrystallisation above 500˚C.
By a curious coincidence just "twiddling" a case in the flame until a smear of laundry soap on the neck and shoulder goes black enables you to achieve around 400˚C repeatedly...case to case, batch to batch including for neck thickness and maker, irrespective of either timing or position in flame or variable ambient light.
The big advantage of the lovely semi-automated devices that
@Yorric and
@1066 have shown and described are that you are not having to load each case...the carousel types that you have to load by hand do not seem much of an advantage over hand held with soap labour-wise to me. The carousels though should produce more consistent case to case results than hand-held and counting because of the constant position of the case in the flame.
All the timed-to-determine-temperature systems whether flame, lead or salt depend on the initial setting up and constancy of the heat source and timer for each session. How many degrees does each case cool the salt bath say over 20? The lead chilled and solidified inside the cases when I tried and failed with that so it does have a cooling effect.
The soap turning black is a constant temperature, session to session, week in week out.
Any of the timed systems with a heat source capable of melting the brass, rely on precise timing to hit that 400˚C and not get into the recrystallisation temperature where you will reduce neck tension. I stress that I do not think that even full annealing of the case neck and shoulder ruins the brass...it will have a reduced neck tension until it has been stressed a few times by resizing and firing...but the batch will all be similar and should therefore yield similar POI results. Running it through a Lee full length sizer a few times would toughen it up. 400˚C will just be closer to the original neck tension.
The quoted fact that annealing is dependant on temperature and time should not be confused with the use of timing to achieve a temperature, especially given our situation of the thin material of a cartridge case neck with a 1400˚C flame. Time at a given temperature is significant of course when you are stress relieving a tonne or two of brass stampings or castings in a kiln and the heat source will not get that mass up to a stress relieving temperature instantly or through the actual thickness of material.
If anyone with a timed machine does not agree the near instantaneous nature of heat transfer through the thin material, perhaps they could have a go trying try to achieve a Tempilaq change on the outside without affecting the Tempilaq on the inside of the neck. Paint the Tempilaq inside and out and try and get the timing just right in order to create a temperature difference across the thickness of the neck material and just turn the Tempilaq transparent on the outside and not the inside.
This then leads to the significance of whether the Tempilaq is painted on the inside or the outside of the neck which again again I think is minimal...Tempilaq is designed to indicate when the surface it is painted on has reached or exceeded the specified temperature. It is obviously true that Tempilaq on the outside of the neck is in the direct flame but for the full thickness of Tempilaq to go transparent, the metal surface it is painted on must also reach that temperature. Below the specified temperature, the brass is acting as a heat sink and preventing the Tempilaq in contact with it from changing.
Even if it was possible to have the Tempilaq on the outside go transparent because it was in direct contact with the flame, the flame will also be swirling into the mouth of the neck and playing on the Tempilaq inside anyway so the end result is still going to be much the same.
Alan
