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Fabricate a monoblock

4575wcf

Aluminum
Joined
Apr 16, 2020
Hi Everyone
I am a new member as of this morning. I am a machinist 30+ year man and still learning. I am not much of a welder though. I work with a couple of youngsters that are welders with advanced schooling, but not advanced gun guys. My question is, can a side by side shotgun mono block be welded up from AISI 8620? Say a couple of 4" long 1 1\2 inch rounds properly fitted up, welded together, and then welded to a 4" long piece of 3\4 x 1 1\2 AISI 8620 flatbar? You see where I am going here. Of course the whole thing annealed before starting machine work. Chamber pressure could go up as high as a stock 303 Savage in a proper AISI 4140 barrel blank of course. Whaddaya think?
 
Yes ,weld no problem in low carbon ally steel with suitable filler and heat treat........however ,the weld will be very visible in case hardening colours ,and also in blueing......
 
Brownell's sells a 3 1/2% nickle rod which ment for gun work. it will blend in with blueing.

Not always. Years ago, I TIG welded up some scope mount holes in a Win. Mod 61 receiver ( low carbon steel) with the 3.5% rod. After bluing, the welds showed up as light spots in the otherwise nice blue on the rest of the receiver. The gunsmith did not know how to fix it. I have since read that allowing the hot blue bath to get up to a higher temp (over 300deg) will cause the spots to blue properly, but I have no experience.

RWO
 
Thanks so much for the input guys. You have answered my main question, the one of mono block strength. I think I can now safely say that if the job is approached in a proper fashion, then weld strength is not an issue. Now for the finish concerns, I must admit that although I am aware of the difficulty of getting a good blue job on welded\repaired areas, that had not occurred to me yet. So to take this idea to the next level. . . We have several welding methods available at work. The one I am leaning toward right now is our supercharged stick welder. It is older than dirt, came with the shop, but has been customized to be the most powerful stick welder any of the guys have used. If you want more details on this welder I will pass questions on to the kids and answer best I can. Since this mono block is aimed {no pun intended} at an LC Smith I must incorporate a locking rib extension. How about this-I cut a gap between the two 1 1/2 " rounds {barrels} large enough to accept a proper electrode, and then weld down onto a square piece of stock {rib extension} laid cornerwise between them on the bottom. The gap between is then filled with weld, then dressed out before the 3/4 inch flat bar {lug section} is welded between the rounds {barrels} over it. We are working from the top of the monoblock up, and material must of course be altered and shifted around a bit to get cleanup, but I think you can visualize the process. The welds above the water table now are largely concealed, and we got the stick welder laying electrode down thick and deep. Sorry to get so wordy but this was a bit hard to describe. Whaddaya think?
 
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No disrespect ,well not much anyhoo,but supercharged stick welder sounds like nonsense......I would advise TIG ,as being a much more controllable process......The 3 1/2 % Nickel rod sounds like a bog standard 8018 C3 ,which I used to use by the ton in Lincoln 50lb tins ....Its strong ,runs nicley ,but high amps with a small mass workpiece carries the likelihood of extensive melting and undercut ......stick weld will always have inclusions too,even if too tiny to be be noticeable without magnification,and stick also produces some strange colours in case hardening..so go with TIG and your filler of choice.
 
Sounds like sound advice. We are set up for TIG also. The reason I was leaning toward stick-I once worked with a fellow that welded all the weldments that I machined. He did some beautiful work with a big fuel driven stick welder, believe me. I was thinking along the lines of turning all those various pieces into an oversized thoroughly melted together unit like he used to make for me. The super stick welder is real enough, sits by the employee door to our shop. I'll ask the kids what is so special about it. Thanx.
 
Okay, Off to work this morning. I will see what my weld guys know about this 8018 C3. They have quite a variety of stick in their cabinet, we may have some. As for the TIG, we have a young man that is pretty handy with that, I will ask him what kind of filler he would recommend. Mono blocs don't usually get case hardened and this mono bloc project will be cold rust blued. James V. Howe goes into some detail in his book on how to treat hardened steels during this process to get them to color and blend. We are off to some sort of start.:) Thanks so much.
 
Well it is all about machine time. I am free at work to machine stuff on my own time but I prefer to saw and file at home. I can get a surface close, take it to work and square it up there in a short while. Of course I would rather start with a slightly oversized forging, but they seem unavailable. As for 8620 it is fully strong enough for the pressures involved here and it welds better than 4140.
 
Not saying you can't do this. I'm not an expert, but I've got the book and followed builds on line, and people don't make monoblocks the way your planning.

It looks to me like by the time you mill the flats on the bottom of the round bars, and mill the lumps to width, the fillet welds holding it all together are gone. I don't know. Maybe I'm missing something.
 
In response I would say that barrel assemblies, not mono blocs, have been made in this way long before the turn of the 20th century. What differs is that they were hard soldered\brazed at the breech end versus welded. As far as the fillets being all cut off, that is all about weld prep and weld penetration. In a nutshell you cut channels and chamfers in the parts to allow the weld to fill in well below the cut line. If only I could weld as well as I can do weld prep, i would be all set, but always the welding has been a bit out of my job description. I do not know if anybody has tried to build a mono bloc this way or not but I am positive that getting a decent finish on the thing is going to be most challenging.
 
RE: 8620 vs. 4140 for the monoblock.

I would go with 4140 in the monoblock, because welding 8620 to 4140 might not be as easy as welding 4140 to 4140.

Generally speaking, in gun design, there's really only one reason to choose 8620 for your steel: you want to case-harden the steel. When you make something from 8620, you can case harden the shell of the object, and leave the core of the object soft, owing to it having only 20 points of carbon. 4140 will tend to harden clear through when trying to case harden it.

OK, now as to welding the pieces together in the monoblock. Welding 4140 can be done well and easily if you preheat the pieces to 400 to 500F, and then cool them slowly. I would advise against trying to weld pre-quenched 4140 - I vastly prefer to weld 4140 that has been annealed. When I'm TIG welding 4140 on guns, I use ER80S-D2 filler. You can use ER70S2, but your weld material will be weaker than your base metal in that case.

Now, as to historical ways that this was done: In the past, barrels were not welding onto monoblocks. They were silver-soldered, or even soft-soldered. Silver soldering will require near-brazing temperatures, and you can find soft solders that melt anywhere from 500 to 900F. Older SxS shotguns had barrel assemblies that were almost completely soft-soldered. The ribs were soldered, the lumps would be soldered together, the end of the barrels would be soldered up, etc. This is why you do not want to hot-salt blue old SxS or O/U shotguns - because the hot blueing salts will attack the solder and eventually lift the ribs.
 
You sir know your business and are accurate on all counts. It is the tendency for 4140 to harden completely through that makes it tricky to weld. We avoid it at the shop in certain aps, like agriculture shafts under spring tension, hydraulic shafting, or anyplace where a welded end or button can crack and pull off. There is no question about it's superiority for receiver work however. A mono bloc, I think, can be considered a barrel component rather than a receiver component, altho it is subjected to some additional forces and wear. The casehardening ability of 8620 does enable me to harden up the extension rib latch area for the LC Smith rotary bolt. Of course the one rifle barrel silver soldered into the monobloc must be made of a suitable steel. If I was going to machine a mono bloc from a block of steel I would choose half hard 4140 every time. To fabricate up a welded mono bloc I think that there might be better choices.
 
If you are going to silver solder ,why not assemble the whole unit with silver solder?....I believe the cadmium solder is still available in the US ,and at the lower temp ,the heat treatment of 4140 HT wont be affected.....Incidentally ,ribs are normally soft soldered.............Ive made a number of single shots of the side hammer on a box type (Sharps),and the first one I slotted out a solid block ,after that I went for pieces welded together along the centreline.
 
4340 is much less critical to weld than 4140 according to some machine fabricators I have talked to in the past. I have no experience.

RWO
 
This whole project idea is something that just came to me. No one has produced a solid reason why it should not work. Strength seems to be not an issue. Granted the idea is a bit unconventional. There is nothing for it but to mock up an example in A36 HR and have that welded up to see exactly what we get, before going to the trouble and expense of deciding on and laying in a proper alloy steel. I have a variety of double shotgun receivers lying about, one old Belgian by Dumulion CIE. The A36 mono bloc could probably be fitted later to that one in strictly a black powder equivalent format, provided the idea works out.
 








 
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