It seems that people anneal case necks to restore neck tension to enhance uniformity, as well as to prevent neck cracking. I've taken to retiring cases that lose neck tension - LC .223 brass after about 8 firings - and putting it in my "practice brass" pile which is for single-load / single-fire standing practice where poor neck tension has minimal effect.
This is not complete - for one thing there is no micro of new, unshot brass because I did not feel like sacrificing a brand new case, and didn't have a new case from the same year / lot anyway.
The two cases I did use were LC 84 which had been fired at least 10 times, from my "practice brass" pile.
I did not get a composition on these cases I used so that is another point of incompleteness. I believe it to be C26000 70-30 Cartridge Brass but it might be something else that is close.
I cut one case into 4 strips on a Buehler Isomet diamond saw, kept one strip as it was, took one and dipped the neck (only the neck) into a molten salt bath at 800º F for 10 seconds, put a third piece into the molten salt at 800ºF and held for 1 hour, heated the salt bath to 1,000ºF and put a fourth piece in for 1 hour.
The molten salt bath was 50% sodium nitrate / 50% potassium nitrate, which is Class 2 per MIL-S-10699B (free from everyspec.com). Bath temp was monitored with a type K thermocouple and verified with a Fluke infrared thermometer.
The second case - also LC 84 with poor neck tension - I held the case head and twirled the case neck in a propane torch flame for ~ 8 to 10 seconds (count of ten) - until my fingers started to get hot. It did not get red or glow. Cut a strip out of it to mount with the first four.
All the strips were mounted in styrene, ground flat on 180 grit silicon carbide paper, then 240 grit, 400 grit, 600 grit, followed by 6 µ diamond on a nylon cloth, and finished on 0.05 µ aluminum oxide on Buehler MicroFloc cloth to remove scratches. It is true that grinding produces damage layers, but sequential grinding on finer media removes each damage layer and produces a shallower one.
Aside: there are 25.4 µm (microns) to 0.001" so the solid particles in 6 µm diamond paste are around 0.00024" and the 0.05 µm alumina is about 0.00002" if I did the arithmetic right. The point is that you are cutting, not burnishing, but you cut really shallow layers at the end so the damage is removed and true structure revealed upon etching.
By now any Physicists reading this are upset. I apologize for using the term "microns" for "micrometers" - but to everyone but Physicists, a micrometer is a tool used to measure things, and a millionth of a meter (10^-6 meter) is a micron. I know "micron" is not a really word, but everyone uses it.
The mount was etched in ASTM E407 # 29 - 1 gram potassium dichromate + 4 mL sulfuric acid in 50 mL water followed by #34 - 5 grams ferric chloride + 50 mL HCl in 100 mL water.
Vickers microhardness indents were made at 500 gram load per ASTM E 384. All the measurements and micros are at the neck region of the strips, not the shoulder or any other part of the case.
Another aside: Everyone knows that grams are mass and force is in Newtons, but I'm sorry, all the instruments are calibrated in "grams load" and the reporting convention is e.g. HV500 (hardness Vickers @ 500 gram load). This is because on Earth and one gravity is one gravity, so the force (in Newtons) exerted by 500 grams mass is the same everywhere. Different on the moon or Mars.
You can use Table 4 in ASTM E140 to convert Vickers hardness to Rockwell B (HRB) but there is no tabulated conversion to yield strength or tensile strength.
This is the case as it was, after firing at least 10 times and no annealing. Fine grain, about 189 HV500
This is the strip that saw 10 seconds in 800º F salt. Not much different, 176 HV500 so it did see a little recovery, no recrystallization or grain growth
This one was held for one hour at 800ºF. Lots of grain growth, softened to 89 HV500 I would call it ruined.
This on was held on hour at 1,000ºF. More grain growth, softened to 67 HV500; even more ruined.
This was held in the torch flame ~10 seconds, until my fingers on the case body/head started to get too hot. It is down to 112 HV500 and the grain growth is not excessive.
I don't think any of this proves much of anything but it was fun. My boss was amused that I wanted to do it, gave his OK for me to use the lab on the weekend.
PS JeepHammer someone did let me try a USB microscope they had and I was crushed - I couldn't see anything very well. I have to keep looking for something to pinch hit for an actual metallograph.
This is not complete - for one thing there is no micro of new, unshot brass because I did not feel like sacrificing a brand new case, and didn't have a new case from the same year / lot anyway.
The two cases I did use were LC 84 which had been fired at least 10 times, from my "practice brass" pile.
I did not get a composition on these cases I used so that is another point of incompleteness. I believe it to be C26000 70-30 Cartridge Brass but it might be something else that is close.
I cut one case into 4 strips on a Buehler Isomet diamond saw, kept one strip as it was, took one and dipped the neck (only the neck) into a molten salt bath at 800º F for 10 seconds, put a third piece into the molten salt at 800ºF and held for 1 hour, heated the salt bath to 1,000ºF and put a fourth piece in for 1 hour.
The molten salt bath was 50% sodium nitrate / 50% potassium nitrate, which is Class 2 per MIL-S-10699B (free from everyspec.com). Bath temp was monitored with a type K thermocouple and verified with a Fluke infrared thermometer.
The second case - also LC 84 with poor neck tension - I held the case head and twirled the case neck in a propane torch flame for ~ 8 to 10 seconds (count of ten) - until my fingers started to get hot. It did not get red or glow. Cut a strip out of it to mount with the first four.
All the strips were mounted in styrene, ground flat on 180 grit silicon carbide paper, then 240 grit, 400 grit, 600 grit, followed by 6 µ diamond on a nylon cloth, and finished on 0.05 µ aluminum oxide on Buehler MicroFloc cloth to remove scratches. It is true that grinding produces damage layers, but sequential grinding on finer media removes each damage layer and produces a shallower one.
Aside: there are 25.4 µm (microns) to 0.001" so the solid particles in 6 µm diamond paste are around 0.00024" and the 0.05 µm alumina is about 0.00002" if I did the arithmetic right. The point is that you are cutting, not burnishing, but you cut really shallow layers at the end so the damage is removed and true structure revealed upon etching.
By now any Physicists reading this are upset. I apologize for using the term "microns" for "micrometers" - but to everyone but Physicists, a micrometer is a tool used to measure things, and a millionth of a meter (10^-6 meter) is a micron. I know "micron" is not a really word, but everyone uses it.
The mount was etched in ASTM E407 # 29 - 1 gram potassium dichromate + 4 mL sulfuric acid in 50 mL water followed by #34 - 5 grams ferric chloride + 50 mL HCl in 100 mL water.
Vickers microhardness indents were made at 500 gram load per ASTM E 384. All the measurements and micros are at the neck region of the strips, not the shoulder or any other part of the case.
Another aside: Everyone knows that grams are mass and force is in Newtons, but I'm sorry, all the instruments are calibrated in "grams load" and the reporting convention is e.g. HV500 (hardness Vickers @ 500 gram load). This is because on Earth and one gravity is one gravity, so the force (in Newtons) exerted by 500 grams mass is the same everywhere. Different on the moon or Mars.
You can use Table 4 in ASTM E140 to convert Vickers hardness to Rockwell B (HRB) but there is no tabulated conversion to yield strength or tensile strength.
This is the case as it was, after firing at least 10 times and no annealing. Fine grain, about 189 HV500
This is the strip that saw 10 seconds in 800º F salt. Not much different, 176 HV500 so it did see a little recovery, no recrystallization or grain growth
This one was held for one hour at 800ºF. Lots of grain growth, softened to 89 HV500 I would call it ruined.
This on was held on hour at 1,000ºF. More grain growth, softened to 67 HV500; even more ruined.
This was held in the torch flame ~10 seconds, until my fingers on the case body/head started to get too hot. It is down to 112 HV500 and the grain growth is not excessive.
I don't think any of this proves much of anything but it was fun. My boss was amused that I wanted to do it, gave his OK for me to use the lab on the weekend.
PS JeepHammer someone did let me try a USB microscope they had and I was crushed - I couldn't see anything very well. I have to keep looking for something to pinch hit for an actual metallograph.
