WARNING - Bubba alert.
I was explaining my $50 heat treating setup/process to someone this weekend, and he was hovering between amazement and calling BS on me, so I figured I would post this up to get some reactions. If I had planned to post before, I would have taken more pics, but this is all I have for now. Maybe I'll take more pics when I do my next one (M92 SBR!).
SB281 has led us all in unexpected directions. It brought me into the AK fold. As I scrambled to buy a couple pre-ban guns, I discovered that I really liked the platform... But it was too late to really explore it. The guns were vaporware with the Obama scare, and quality receivers were out of stock everywhere. With the deadline approaching, I bought a couple dozen flats in various configurations, a bending die, and folded them all before October 1. Then I tooled up to build my budding wish list of com bloc goodness.
The problem was that I had no idea about receiver hardening issues when I hatched my plan. So I went to the inter webs for knowledge... And was disappointed by the AK community. According to the internet forum "experts," I had three options:
1 - Send it away to a professional heat treater, who may or may not warp it to uselessness (and who specifically warns you that it will probably come back warped).
2 - Spot harden the FCG holes, ejector and rails with a little MAPP gas torch.
3 - Spend mucho dinero on a ceramics kiln.
Well, none of that seemed attractive to me. Sending to a pro goes against everything I hold dear. I'm a DIYer. If I can't do it, it can't be done (so I say). Option 2 seemed grossly inadequate (I know this will tick some people off). Even if spot hardening proved durable, I would not be happy. Annealed 4130 (what the flats I bought are made of) is some seriously soft and ductile metal... Without hardening, you can literally bend your ak in half at the mag well without too much difficulty (no, that's not exaggerating). Finally, buying a kiln was out... Too much money. Too few uses.
So I started researching and experimenting... And after a few destroyed receivers, I settled on the following method.
I built a small furnace out of refractory bricks on a wheeled base I welded up. Only the bottom, sides and back are mortared in, so I can change the configuration.
Stage one of the process is stress relief. I'm convinced the stresses of bending, dimpling, welding, punching, etc are partly responsible for warping issues some experience on quench. To relieve these stresses, I put extra bricks in the furnace (for thermal inertia) then heat the furnace up with my propane weed burner until the bricks are glowing orange. Then I put the receiver in and heat that up to glowing. Finally, I slide the top brinks to seal the vent, and seal the front of the furnace up with an extra brick. This is to retain heat. It takes a LONG time to cool, but slower is better for this. I leave it overnight. Before sealing, I put a few wood chips in. This, in theory takes up the oxygen by burning and helps avoid scale. Does this process work to relieve stresses? I have no clue, but it probably does, and it can't hurt.
Stage two is hardening. The receiver goes back into the preheated furnace and gets heated until the whole receiver is EVENLY glowing orange. Preheating the bricks really helps a lot to give me even heating. Given how thin the metal is, there is no need to hold temperature for any length of time. To the contrary, I take it out as fast as possible to avoid scaling.
Stage three is the quench. For 4130 I've read that you need a very violent quench. I've also read that, oil is too slow. You must use water. Saturated salt brine is better because it raises the boiling temp such that you avoid a boundary layer of steam. Adding a bit of soap is supposed to help in the process as well. I use a 5 gallon bucket kept right under the furnace. You need to get the receiver into the water FAST. Don't give it time to cool. And for gods sake, put it in perfectly vertical. Anything else will banana your part (think katana).
After the quench, my testing files usually show between 60-65R... Which is physically impossible for 4130 (as quenched, it should never get harder than very low 50's). I suspect that means the steel is carburizing from the propane, but that really isn't a bad thing here. In any case, it is definitely HARD. It is also brittle. Trying to bend it at this point will shatter the steel, so don't try (don't ask me how I figured that out).
Stage four is tempering. Anywhere between 650F and 850F for one hour will easily put you within the 37-44R range... which is what I have read is the soviet spec. I've also read that soviets found hardnesses above 45 would tend to develop cracks... That may be something to be concerned about, so make sure you get the required temperature. That said, the 1050 steel the soviets used isn't nearly as tough as 4130 chromoly generally used for US flats... So it may not be an issue. In any case, I found that my propane grill with all burners on high will hold 725-750F after a preheat... So that's what I used. Another option may be the cleaning cycle of a self-cleaning oven... That's usually 800-850F. After tempering, you should let it cool slowly... I just turn the grill off and come back an hour or two later.
After tempering, my test files show hardness between 40-45R, which is where it should be for the tempering temperature I used (it should be 41).
Issues:
1 - Scale. Yes, a little scale builds up. It's not a problem. It comes off VERY easily in the blasting cabinet.
2 - Warping. Yes, despite my best efforts, I get a little warping sometimes... Not a lot, but some. And never more than a dead blow hammer can't fix in a half dozen whacks (after tempering!!!). Between that and the support of the trunnions, there are no straightness issues. I included some pics of the last one I did to show how straight the rails are.
3 - Temperature control when hardening. Precise control is not really needed for this steel. You can tell temperature close enough by color. If you go too hot, it really doesn't hurt unless you melt it. I tried 1600F tempilaq as an indicator, but found that it etched the steel badly and didn't materially refine the process.
4 - Long term durability. No clue. I have 660 rounds through one gun with no issues. If anything bad happens, I'll update.
Now for the pictures... These are pics from two different receivers.
The "furnace".
In action.
After quench... Note the scale.
Same gun, scale cleaned up.
Tempering.
Perfectly straight.
That's all for now.
I was explaining my $50 heat treating setup/process to someone this weekend, and he was hovering between amazement and calling BS on me, so I figured I would post this up to get some reactions. If I had planned to post before, I would have taken more pics, but this is all I have for now. Maybe I'll take more pics when I do my next one (M92 SBR!).
SB281 has led us all in unexpected directions. It brought me into the AK fold. As I scrambled to buy a couple pre-ban guns, I discovered that I really liked the platform... But it was too late to really explore it. The guns were vaporware with the Obama scare, and quality receivers were out of stock everywhere. With the deadline approaching, I bought a couple dozen flats in various configurations, a bending die, and folded them all before October 1. Then I tooled up to build my budding wish list of com bloc goodness.
The problem was that I had no idea about receiver hardening issues when I hatched my plan. So I went to the inter webs for knowledge... And was disappointed by the AK community. According to the internet forum "experts," I had three options:
1 - Send it away to a professional heat treater, who may or may not warp it to uselessness (and who specifically warns you that it will probably come back warped).
2 - Spot harden the FCG holes, ejector and rails with a little MAPP gas torch.
3 - Spend mucho dinero on a ceramics kiln.
Well, none of that seemed attractive to me. Sending to a pro goes against everything I hold dear. I'm a DIYer. If I can't do it, it can't be done (so I say). Option 2 seemed grossly inadequate (I know this will tick some people off). Even if spot hardening proved durable, I would not be happy. Annealed 4130 (what the flats I bought are made of) is some seriously soft and ductile metal... Without hardening, you can literally bend your ak in half at the mag well without too much difficulty (no, that's not exaggerating). Finally, buying a kiln was out... Too much money. Too few uses.
So I started researching and experimenting... And after a few destroyed receivers, I settled on the following method.
I built a small furnace out of refractory bricks on a wheeled base I welded up. Only the bottom, sides and back are mortared in, so I can change the configuration.
Stage one of the process is stress relief. I'm convinced the stresses of bending, dimpling, welding, punching, etc are partly responsible for warping issues some experience on quench. To relieve these stresses, I put extra bricks in the furnace (for thermal inertia) then heat the furnace up with my propane weed burner until the bricks are glowing orange. Then I put the receiver in and heat that up to glowing. Finally, I slide the top brinks to seal the vent, and seal the front of the furnace up with an extra brick. This is to retain heat. It takes a LONG time to cool, but slower is better for this. I leave it overnight. Before sealing, I put a few wood chips in. This, in theory takes up the oxygen by burning and helps avoid scale. Does this process work to relieve stresses? I have no clue, but it probably does, and it can't hurt.
Stage two is hardening. The receiver goes back into the preheated furnace and gets heated until the whole receiver is EVENLY glowing orange. Preheating the bricks really helps a lot to give me even heating. Given how thin the metal is, there is no need to hold temperature for any length of time. To the contrary, I take it out as fast as possible to avoid scaling.
Stage three is the quench. For 4130 I've read that you need a very violent quench. I've also read that, oil is too slow. You must use water. Saturated salt brine is better because it raises the boiling temp such that you avoid a boundary layer of steam. Adding a bit of soap is supposed to help in the process as well. I use a 5 gallon bucket kept right under the furnace. You need to get the receiver into the water FAST. Don't give it time to cool. And for gods sake, put it in perfectly vertical. Anything else will banana your part (think katana).
After the quench, my testing files usually show between 60-65R... Which is physically impossible for 4130 (as quenched, it should never get harder than very low 50's). I suspect that means the steel is carburizing from the propane, but that really isn't a bad thing here. In any case, it is definitely HARD. It is also brittle. Trying to bend it at this point will shatter the steel, so don't try (don't ask me how I figured that out).
Stage four is tempering. Anywhere between 650F and 850F for one hour will easily put you within the 37-44R range... which is what I have read is the soviet spec. I've also read that soviets found hardnesses above 45 would tend to develop cracks... That may be something to be concerned about, so make sure you get the required temperature. That said, the 1050 steel the soviets used isn't nearly as tough as 4130 chromoly generally used for US flats... So it may not be an issue. In any case, I found that my propane grill with all burners on high will hold 725-750F after a preheat... So that's what I used. Another option may be the cleaning cycle of a self-cleaning oven... That's usually 800-850F. After tempering, you should let it cool slowly... I just turn the grill off and come back an hour or two later.
After tempering, my test files show hardness between 40-45R, which is where it should be for the tempering temperature I used (it should be 41).
Issues:
1 - Scale. Yes, a little scale builds up. It's not a problem. It comes off VERY easily in the blasting cabinet.
2 - Warping. Yes, despite my best efforts, I get a little warping sometimes... Not a lot, but some. And never more than a dead blow hammer can't fix in a half dozen whacks (after tempering!!!). Between that and the support of the trunnions, there are no straightness issues. I included some pics of the last one I did to show how straight the rails are.
3 - Temperature control when hardening. Precise control is not really needed for this steel. You can tell temperature close enough by color. If you go too hot, it really doesn't hurt unless you melt it. I tried 1600F tempilaq as an indicator, but found that it etched the steel badly and didn't materially refine the process.
4 - Long term durability. No clue. I have 660 rounds through one gun with no issues. If anything bad happens, I'll update.
Now for the pictures... These are pics from two different receivers.
The "furnace".
In action.
After quench... Note the scale.
Same gun, scale cleaned up.
Tempering.
Perfectly straight.
That's all for now.
