5.7x28 brass resists inductive heating

[galexander]

I did read a few of the other threads on inductive heating and didn't find anything that could explain this. I have been reloading 5.7 for about six years now and generally have only reloaded 2x at most due to the large amount of deformation from firing in blow backs that start to extract the case while the pressure is still high enough to blow the shoulders forward. I recently got a DBX (dual gas piston operated) which doesn't blow the shoulders forward and have experimented and found that the gun functions fine without any of the lacquer on the brass. This lead me to consider that I could anneal the brass and get a longer life from it. I have used a hand held bolt heater with a timer switch to heat the brass for a consistent time. It has worked fine for every other cartridge case that I have tried it on. It will melt the neck of a .223 case in 4-5 seconds. However when I try it with 5.7 brass (that I have removed the lacquer from) the brass doesn't even change color after 24 seconds of induction. Does anyone here understand inductive heating issues enough to speculate on why 5.7 brass isn't heated anywhere near as much as other brass. I have made a smaller diameter coil to see if that helps but it doesn't but it does heat .223 brass even faster. Could it be that 5.7 brass is of a different composition than most cartridge brass? Maybe more zinc? Maybe less zinc? Maybe tin included? I am baffled and would rather not go back to using a torch. Thanks for reading my long babble.

 

[Hoot]

= Does not compute.

Take one of the cases that you removed the lacquer from and place it across a fresh 9V battery terminals to see if it conducts electricity. Of course, if you own a multimeter, use that to see if it conducts. I'm no 5.7x28 person but did they ever make them using a polymer case? That would explain your experience. Just throwing something up to see if it sticks.

Hoot

 

[Ggmac]

no idea ! But Ive never gotten more than 2 reloads from them in a bolt action. Just not enough meat around the primer . After you subtract the extractor cut depth , you dont have much brass . Im glad you can get it working, Ive not played with it since the cartridge came out . Maybe its time again

 

[Dave M.]

Even a polymer or lacquer coating wouldn’t prevent eddy currents from heating the brass via hysteresis. Does the coil effectively heat any other brass? If not, the frequency could be way off. I suspect if it will heat other brass in the same coil, the issue must be with chemical makeup of the “brass”, because the response that you are describing is one of a non conductive material.
dave

 

[galexander]

The brass is conductive it just doesn't heat as fast as say a .223 case. The multi-meter indicated pretty much the same conductivity of a .223 case. There have been some changes to my method after I first posted this. I can get it to start to glow after about 14 seconds now. In the beginning since it was taking so long to heat up I set the head in about 3/16" of water to make sure it didn't make the head or bottom half of the body as soft as the neck and shoulder. I made that change before I made the coil a smaller diameter. I think heat was just moving as fast to the water as it was being generated by the induction. The .223 cases are much longer so even though they were sitting water too they melted quickly since the heat had to travel a lot farther to get to the water. The speed at which a .223 case melted made me think the water didn't leech the heat significantly but with the much shorter 5.7x28 it apparently it does. The 5.7 clearly still heats slower but it does heat now. For reference a .223 not in water melts at the neck in just under 4 seconds now - about 3.5x as fast. My ignorance of thermal conductivity issues was part of the reason for my initial failure, but 5.7 neck and shoulder still heats slower than the neck and shoulder of .223. Fired .22mag cases melt in about 2.5 seconds. I am sorry that I didn't mention the water earlier as someone would have undoubtedly figured this out faster than me. Thanks again.

 

[DDRH65PRC]

Not sure if it matters, but I've heard 5.7x28mm brass is machined brass? It was mentioned on a Vortex podcast.

I think this was the video:

 

[galexander]

I have loaded and fired many thousands of 5.7 cartridges and have two weapons that fire it but do admit that so far it has been a cartridge that I think Frankie and Willie (Billy Crystal and Christopher Guest) would have loved.

 

[Dave M.]

The brass is conductive it just doesn't heat as fast as say a .223 case. The multi-meter indicated pretty much the same conductivity of a .223 case. There have been some changes to my method after I first posted this. I can get it to start to glow after about 14 seconds now. In the beginning since it was taking so long to heat up I set the head in about 3/16" of water to make sure it didn't make the head or bottom half of the body as soft as the neck and shoulder. I made that change before I made the coil a smaller diameter. I think heat was just moving as fast to the water as it was being generated by the induction. The .223 cases are much longer so even though they were sitting water too they melted quickly since the heat had to travel a lot farther to get to the water. The speed at which a .223 case melted made me think the water didn't leech the heat significantly but with the much shorter 5.7x28 it apparently it does. The 5.7 clearly still heats slower but it does heat now. For reference a .223 not in water melts at the neck in just under 4 seconds now - about 3.5x as fast. My ignorance of thermal conductivity issues was part of the reason for my initial failure, but 5.7 neck and shoulder still heats slower than the neck and shoulder of .223. Fired .22mag cases melt in about 2.5 seconds. I am sorry that I didn't mention the water earlier as someone would have undoubtedly figured this out faster than me. Thanks again.

No offense, but you don’t have the principals of induction heating down. Lots of things are off if it takes 14 sec to heat a case. The frequency is wrong, the coupling distance is way off, the tank circuit is wrong. You are most definitely creating a VERY dangerous situation by allowing that much heat into the body of the case. Do you realize the effect of softening the brass throughout the entire case? Have you ever seen what happens when the a case splits in the main body of the case? Trust me, you don’t want to see it. There shouldn’t be any need to set cases in water to induction anneal them.
dave

 

[divingin]

However when I try it with 5.7 brass (that I have removed the lacquer from) the brass doesn't even change color after 24 seconds of induction.

Is it magnetic? I've seen some 223 brass that was (of all things) aluminum plated brass, and some that was brass plated steel.

 

[galexander]

No offense, but you don’t have the principals of induction heating down. Lots of things are off if it takes 14 sec to heat a case. The frequency is wrong, the coupling distance is way off, the tank circuit is wrong. You are most definitely creating a VERY dangerous situation by allowing that much heat into the body of the case. Do you realize the effect of softening the brass throughout the entire case? Have you ever seen what happens when the a case splits in the main body of the case? Trust me, you don’t want to see it. There shouldn’t be any need to set cases in water to induction anneal them.
dave

Dave, it is not dangerous unless I try to shoot them. I am not doing that. I am trying to get it to work better. I am trying to figure out what the difference in 5.7 that is causing this. The heater works pretty much exactly how I have seen it it work for others when cartridges other than 5.7 are heated. It heats .223 to the same dull glow in 3.75 seconds. If I wasn't aware of the fact that long heat times could soften the case too far down, do you think I would have been setting them in water? What it looks like now is that the hand held heater does work for cases larger than 5.7 but not 5.7. It also seemed to work much faster (2-3 seconds) on a smaller case (.22mag, no I am not trying to reload .22mag I was just seeing if it was just the small size that was causing the heating be slower). This thread was to ask why it wasn't working for 5.7 not to ask if the results I was getting were satisfactory. It is completely obvious that it is not.

 

[Dave M.]

Dave, it is not dangerous unless I try to shoot them. I am not doing that. I am trying to get it to work better. I am trying to figure out what the difference in 5.7 that is causing this. The heater works pretty much exactly how I have seen it others get with other cartridges. It heats .223 to the same dull glow in 3.75 seconds. If I wasn't aware of the fact that long heat times could soften the case too far down, do you think I would have been setting them in water? What it looks like now is that the hand held heater does work for cases larger than 5.7 but not 5.7. It also seemed to work much faster (2-3 seconds) on a smaller case (.22mag, no I am not trying to reload .22mag I was just seeing if it was just the small size that was causing the heating be slower). This thread was to ask why it wasn't working for 5.7 not to ask if the results I was getting were satisfactory. It is completely obvious that it is not.

I suspect outer wall of the brass in the 5.7 is closer to the coil than the standard .223 round you are heating. If so, then being too far from the coil shouldn’t be the issue. I suspect your tank circuit isn’t in tune and you have either significantly more or less brass mass in the coil. To properly balance the tank circuit you’d need to adjust the resistance and/or capacitance.
I also suspect the “brass” you are trying to heat has a much different composition than standard brass that we all anneal every day. To put it in perspective, I can completely melt an M8 bolt in half in under 10 sec. 6.5CM brass gets to a dull red glow in under 4.5 seconds.

 

[galexander]

I suspect outer wall of the brass in the 5.7 is closer to the coil than the standard .223 round you are heating. If so, then being too far from the coil shouldn’t be the issue. I suspect your tank circuit isn’t in tune and you have either significantly more or less brass mass in the coil. To properly balance the tank circuit you’d need to adjust the resistance and/or capacitance.
I also suspect the “brass” you are trying to heat has a much different composition than standard brass that we all anneal every day. To put it in perspective, I can completely melt an M8 bolt in half in under 10 sec. 6.5CM brass gets to a dull red glow in under 4.5 seconds.

Thanks. I just added another turn in the coil and tightened it to where it is maybe 2mm-3mm off the brass and more perfectly circular. It now heats to a dull glow in exactly 4.5 seconds (amusing coincidence). I am guessing (obviously, my physics is weak) that the extra turn did it since it is now maybe at most 2mm less in diameter. Feel free to correct me if this is likely wrong. I like to know when I am wrong.

 

[Dave M.]

Thanks. I just added another turn in the coil and tightened it to where it is maybe 2mm-3mm off the brass and more perfectly circular. It now heats to a dull glow in exactly 4.5 seconds (amusing coincidence). I am guessing (obviously, my physics is weak) that the extra turn did it since it is now maybe at most 2mm less in diameter. Feel free to correct me if this is likely wrong. I like to know when I am wrong.

This is correct. Also, what this tells me is you are likely running a much higher frequency than you need be. Adding the extra coil did two things: it got you closer to the workpiece letting the higher frequency skin effect heat the brass, but more important it helped to more closely match the inductance of the workpiece to the coil. Be sure the coil is sprayed with several coats of lacquer and the turns don’t touch one another.