Choosing correct steel for welded firearm accessory.

Hello again.

I have a medium sized mill that I haven't used enough to know what to search for as far as steel choices.

I am planning to copy a very expensive forward-assist housing for a couple of Hk rifle builds that I am working on. Basically, it is a rectangular bar a few inches long that will be radiused down its length on one of the narrow sides (probably with a rotary table) , and grooved with an end-mill or ball-mill on the other. This housing will be then be tig welded to the hk reciever. My main concern will be to achieve a decent outside finish (on the radiused and flat sides) as long as the steel is fine for the application.

Nobody seems to know the exact carbon content of the receivers, but I have seen the number 4140 kicked around as a guess. I cannot find 4140 at my online supplier, and have no local suppliers.

Any input on the correct steel? Thanks for any help!

4140 is not eminently known as weldable, because it can form a hard Heat Affected Zone next to the weld. Even with preheat, this can happen to some extent, so tapping threads through such a zone is problematic. Other than the risk of brittle failure if the HAZ is hard, you won't really notice any particular difficulty welding it. The HAZ will maybe also polish up, or blue differently than the receiver.

The advantage of using 4140 is it is more scratch and dent resistant than, say a 1018 (which is very weldable). If all the requirement is basically a gob of steel welded on which is unlikely to be highly stressed, nothing wrong with just hot rolled mild steel.

Use 8620 4140 is not high on the list for welding...Phil

don't make it out of any beater or worst then it needs to be to do its job as far as welding , machining , finishing [coatings ] 4140 and the like may work well for barrels , breach and receivers but in the gun world its not the only steel out there . as form fallows function dam that sounds like a German thing and at some point its just ego and ego don't pay the bills

Phil in Montana said:

Use 8620 4140 is not high on the list for welding...Phil

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Seconding 8620.

It isn't called "ordnance steel" by accident.

General "lack-of", rather!

This is interesting stuff! Thanks guys, I will definitely be looking into for 8620 for this project; then sitting down and learning more about metallurgy!

This is a perfect example of a thread that sure could use a photograph of the object....

GregSY said:

This is a perfect example of a thread that sure could use a photograph of the object....

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I actually attempted to. I am fairly computer savvy and despite searching the screen, saw no controls for inserting media (at least from a mobile device). I am assuming this was due to my low post count, or a flaw with the mobile site, but won't know until I get back to my desktop computer to confirm, this evening. If I can, I will add photos then.

I agree 8620 would be the safest, 4130 is weldable & cheaper but needs more preheat. I'd also be welding with E80-Ni-1 or 2 tig rod.

Good luck,
Matt

Kipling79 said:

This is interesting stuff! Thanks guys, I will definitely be looking into for 8620 for this project; then sitting down and learning more about metallurgy!

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8620 "ordnance steel" has been the "go to" for more firearms than not, major national arsenals since forever-ago.

It has the "good stuff". Nickel, Chromium, etc. Compare it to the 4XXX family. The differences are not huge.

And no, it is not the EASIEST of alloy to weld "well".

But it is waaaay easier to get good welds that 4XXX is. And it takes to "case" hardening REALLY well. Hard wear-points, tough core, fewer cracks. And hard-working machine guns and cannon lasted longer.

For which reason, many of us stash it for general-purpose use, nought to do with firearms atall.

Just stock that when in a hurry, forced to "use what is here, NOW", you have more flexibility - weldability VERY much on that list.

I wish I could go back and add these photos to the OP. Here are images of the housing to be copied:




On a side note, I have ordered quite a bit of 8620 round stock for this project and the stiffening rails, which will be next. Like @Thermite and others have said, it seems that this stuff will be good to have around even if not the final choice for these projects (for some unforeseen reason).