This is long and full of tech that is likely boring to many so you may just want to skim or skip it.
I have gleaned any knowledge I have from annealing case necks using various heat sources and then spending hours searching for and reading thru studies and papers trying to glean nuggets of pertinent info and making deductions based off it and observations. AMP early on added a nice amount of case neck specific technical data on what there final state goal is. What I am not is someone with any professional or academic institutional study of metallurgy. Below I'm only going to reference a couple sources but to really explore and get a better picture it took me going thru numerous papers. Most time was reading thru papers that gave me nothing useful.
I think, since annealing has become significantly more wide spread, as the value both for performance consistency and case longevity is recognized; it would be helpful to break down the technical details to understand exactly what the ideal state is. How various forces effect the properties and structure of the case neck. I still have numerous questions. Some I think I know the basic reasons others still searching and learning.
As an couple examples:
Consider the repeated observation, that a case neck sized and seated with a bullet in a short period, hours, uses significantly less force to seat the bullet, than does one after a longer period of time, weeks to months. This shows that the brass structure and retained energy is not static after significant plastic deformation via mechanic force (resizing die) is applied to them.
Necks heated below the temp / time window, generally recognized as needed to affect structural change, seem to display properties indicative of some stress relief yet no discernable change in hardness. This has been observed with molten salt bath @ 900-1000F for period of a few (3-10 sec), too short to bring the submerged neck to temperature equilibrium with the salt solution and not affect hardness/temper.
(According to research by University of Illinios, a fully submerged piece in that 450-500°C of molten lead takes 19 second just to reach temp. In neck only annealing, with the significant amount of case mass not submerged and acting as a very effective heat sink then time would have to be significantly increased.) I think the start of the recovery phase maybe more gradual and start sooner. It may also have mini sub stages. My SWAG.
Ref:
Grain Sizes Produced by Recrystallization and Coalescence in Cold-Rolled Cartridge Brass (graph on page 25 Fig. 3. Heating Rate in Molten Bath)
Our goal is to achieve a temper and more precisely, a differential temper of a certain hardness and ductility taper. Most research that people quote, regurgitate, or frankly that is done, seems to have no correlations to time/temperatures combo we use in neck annealing as it's focused on the recrystallization phase or returning to that of the inItally pre-rolled state or further to crystal grain regrow. It requires digging thru many of these studies and finding that odd few nuggets to really be able to gain the knowledge to answer those questions.
Brass Stages of Annealing:
(1) Recover
(2) Recrystallization
(3) Coalescence
Everything we care about and the annealing stage we are keeping our case necks in is that 1st stage; Recovery. It is also the shortest of the stages in terms of time especially at high temperatures that we need to use to overcome the extremely efficient heat conductivity of brass and not effect the rest of the case.
You will see even in research dealing with actual case production statements about recylstallization happening at any temp above 750°F/400°C. The issue is that is meant in the context of a certain time window for entire object annealing which is a minimum of 30 minutes window. It has nothing to do with the goals of neck annealing to a differential temper or those of case manf when they do final neck annealing stages. This appearance of dichotomy can be seen clearly in this master thesis from 2018 done at the Norma case manufacturing facility:
Ref:
Variations in Hardness and Microstructure in Cartridge Cases at Annealing By
Richard Ohlsson
"Earlier reports by Hajizadeh et.al [6] have found that for brass annealed at 400°C, the brass has passed the recovery stage and entered the recrystallization stage."
The supporting graphs are on page 5 of the paper.
You will notice while the graph fig 3 has no reference to time and is labeled recrystallization graph fig 4 labeled for hardness denotes the time duration of 30 minutes. Makes it very murky for those looking for the infon without metallugic knowledge.
As we reach the sections on case neck annealing stages we start to see much more familiar annealing times we are accustom to, as shooters, in neck annealing.
Under Figure 10 on page 12 we find this statement that helps clear things up in terms of case necks:
"As seen in Table 2, the cases experience two different annealing's of the neck during the manufacturing processes. The annealing at Norma is made by induction heating where the Win Mag at the
first neck annealing is annealed for 14 seconds at an induction of 2.8 kW and at the
second neck annealing ca 14-15 sec at 6.1 kW. The exact temperatures have not been measured but previous measurements at other cases at Norma Precision AB has shown that the temperatures lie in the range of 350-600°C.
From Figure 5, it is seen that annealing at those temperatures does not change the phase of the microstructure for the brass."
I would not get hung up on the power kw output mentioned as its effect can vary greatly based on the coil shape dimensions etc. Instead notice the temperatures and time mentioned. Also understand they are dealing with cases that have the maximum tolerated deformation changes while keep case loss low.. Most of the shoulder and upper case body can handle some annealing as well. But as can be seen final neck annealing us done @ 600°C/1112°F for 14-15 sec. It should also be mentioned the heat produced thru induction freq is penetrative in the thin case necks compared to that of heat via flame or liquid submersion. If the actual full power of those machines were used you could forge weld steel. My point is these numbers do not directly convert to other induction machines and certainly not other heat sources for annealing. Induction of these machines heats the necks to temp in under 1 sec. Complete Submersion 19 seconds, neck only submersion with case as heatsink 30 sec??, propane flame 1800°F+ 2-4 sec
Finally, if we proceed to
page 19 figure 7 we can see how the case necks after final annealing have a deferential temper or progressive hardness as you move away from the case mouth further into the neck. Average:
1mm: 105 hv, 3mm: 108 hv, 6mm: 115 hv
What is the ideal state of the temper for most consistent bullet release force or case neck expansion / ductility balanced with minimal strength to prevent neck deformation during sizing?
We always speak of neck tension in terms of a dimension under the bullet bearing surface diameter, friction fit. But this is a unique individual/specific unit measure. What our real goal is bullet release consistency for the entire mated surface area of bullet to case neck and also cartridge neck unit to unit for a given inside diameter.
From everything I have learned the future of home case neck annealing is in induction systems. Not that torch based can not and do not work, they do andvhave for a very long time. But induction allows a much greater level of precision and far simpler setup and increased safety as there is no open flame or pot of molten solution waiting to start a fire or explode, respectively. No need for case rotation is another.
With little knowledge you can put a basic induction annealer together for around a couple hundred. If you are somewhat skilled you can add varying amounts of automation. Its the future; thats my personal opinion.