So, your saying that these barrels you work on that result in the hardness ratings aren’t heat treated at all anywhere in the process? I’ve seen where barrels range from R20 up to R60....they got there by doing nothing at all? I know about SLS and what it produces. It is nowhere near that ability. MIM may be another story, but I’d have to see it first.
I perhaps over-simplified this: The steel for the barrels is quenched/tempered at the mill, but the bar stock, as delivered to the barrel makers, isn’t usually heat treated beyond that by the barrel makers. It gets drilled, then reamed, then the rifling is cut/broached/etc and then (on the high end barrels), the bore is hand-lapped with a lead lap.
So you’re right in that it came from the mill Q&T, but my point is that the firearms industry doesn’t do a bunch of heat treatment on it - the way they do on receivers (for example). The bolt head and locking lugs on bolt action receivers are heat treated after they’re machined, for example. Those areas, even with MIM or 3DP, are going to absolutely need heat treatment. Or you’ll need to 3D print a different action than a bolt or AR-type action.
Even dead soft 4140 would be more than strong enough to contain modern smokeless powder, if you used enough radial thickness around the chamber. The gun buyer would have a hissy the first time he dropped his rifle, but the barrel wouldn’t let go under fire.
But let’s say you did want to heat treat the result. No big deal. Use an air-quenching alloy for the print, put an induction heating loop on the output side of the process, push the result through and wha-la, you’re done.
One of the reasons why barrels are made from 4140 or 416 is the ease of machining, not just the strength or heat treat properties. If we get rid of the machining requirements because we’re squirting goo out of a printer... well then, we could use all sorts of new alloy or material technology that might not require a heat treatment because we don’t need to worry about machining it.
This 3D printing stuff is going to radically change the manufacturing sector. Once you liberate your mind from the requirements of “subtractive” machining, and start thinking of material science without regard for the older ways of turning out a product... you suddenly see the skies part and trumpets blair. But more importantly, the entire process becomes as very repeatable for people who are more computer-savvy than machine savvy. In the future, it could be as easy as “pour 3D ‘ink’ in here, load program, press ‘go’ button, and wait.”
Let’s just say, as a retired engineer, I used to be a skeptic. Two years ago in a machine tools technology department at the local college, someone called me into their office and said “We can print plastic ball bearings that roll with this printer. We’re just doing this as a prototype, soon we’ll be able to make real bearings.”
I said “Bullcrap.”
He said “OK, sit down, let’s have a cup of joe while the program runs and I’ll show you some product lit from some companies...”
This we did.
30 minutes later, I had to eat my words.
It’s coming. For real. It might not be fully here yet for the average home hacker with a less than $10K budget, but it’s sure as heck coming - and quite quickly.