I appologize for being offensive.
The thing about steel, and gunpowder of any sort, is that things occasionally go wrong. When that happens - and I am aware of a number of such events - then the steel barrel/breech will burst in some manner. If it bulges and splits a bit one may be injured, but the thing must show some ductilty when it fails. Or, if it just shatters, ones wounds may be severe.
I hear people speak of "Tensile Strength, "Hardness" & cetera. What matters in a metal for a reasonably safe barrel i that it be tough, above all else.
That is, it must bend or tear a little when it comes apart, and not just break like a file does.
I say "when" it comes apart, and not "if" because there are so many ways to get into trouble with a gun that accidents do happen. When something does go wrong, which no one expects, it is good to survive with only the need for fresh clothing. Loading with the ball off the powder some distance can ring the muzzle loading barrel, or it can shatter it. In a modern centerfire rifle one nasty problem is when one decides to go for light loads in his, say .30-'06. So the guy loads with a greatly reduced chgarge of rifle powder (here I am talking smokeless). For reasons I do not understand, but I think some Army artillery guys know, this reduced charge gets presssure waves bounching back & forth & where they meet the rifle disintegrates. There ws one on display in Clarkston Michigan at the Flint & Frizzen shop. Think the shooter survived OK but the rifle is a mess.
That was a digression. ANyway, it is good, normal practice to caseharden steels that are low in carbon content. Century or so ago that meant wrought iron. Then for early cartridge guns it meant low carbon steel. Low being, say, 0.20%carb or or less, nominally. "Medium carbon steels", say about 0.40% carbon can get very hard all the way through if they see a normal case hardening heat treatment.
I guess it can be done by somone who knows their business, and this would involve heating up to rather low temperatures in the 1300-something F range. Me I'd distinctly rather not be present when someone brings out his beautifully color hardened Ruger or 1911 to pop off a few rounds.
Think the first thing I wrote about steel and muzzle loaders was for the Buckskin Report, probably early 1980'. John Baird was the only one preaching design/metals gun safe manufacturing back then, that has not and will not change. At that time some fine fellows were selling breeches investmentcast of high carbon steel, specifically 1095. Never heard of anyone getting hurt by them but damnifIknow why. No, no no I'd not color harden one. Neither would I use one in any condition. This is the stuff from which files, and in Ancient Times, star drills (for concrete) were made.
43 years employed as a metallurgist I have analyzed many, many machine failures. Along the way a few muzzle loading failurs that changed the shooters' lives. One was, finally, the reason Douglas Barrel Company stopped advertising muzzle loading barrels in the mid-1980s.
Just don't harden your 4140 breech.
It is not easly to communicate this to muzzle loading shooters. I have tried close to four decades now. Hence the chip. Sorry to be offensive but that is the way it is. And it will continue to be so.
