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**Unclenick** · April 3, 2013, 10:23 AM

Yep. You can't chrome line because it will reduce the bore diameter about four thousandths, making a .224" groove diameter into a .220" bore. When AR type barrels are designated for chrome lining, they make their groove diameters .228" to allow for that.

Part of the reason for that much thickness is that you can't put chromium directly on steel, as the plating solution is strongly acidic and etches carbon steel away faster than the layer can deposit. As a result, you have to first plate nickel onto the steel, to resist the acid, and then the chromium actually is plated onto the nickel. The nickel, IIRC, is about half a thousandth thick to seal off direct contact with the steel, then the hard chromium as about one and a half thousandths thick. That is about the right thickness to prevent heat stress cracking from extending down to the steel substrate, and is probably as thick as they think they can go without barrel heating causing the plated layers to separate due to differential expansion. You end up with about two thousandths of plated metal, total, but since it is all around the inside of the bore, that subtracts four thousandths from the diameter.

Nitriding is more similar to case hardening in that it penetrates into the steel, forming a case rather than thickening the surface. It can be accomplished at a temperature of 900°F-1200°F. That's the temperatures range in which barrel steel is normally stress relieved, so it doesn't tend to distort the steel the way heating and quenching it can. Sizing dies for reloading are typically nitrided rather than conventionally heat treated for this reason. This page describes the gas nitriding process, and the hardness range examples it has for 4130 and 4140 steel are right where 4150 barrel steel will fall, too: a Rockwell C scale 28-32 for the steel itself and Rockwell C scale 52-58 for the nitride case hardness at the surface.

The page points out that the very surface, which is the most brittle part, is sometimes lapped off. If you are planing to firelap a barrel you also plan to have nitrided, I would do the nitriding first for that reason.

April 3, 2013, 10:23 AM	#3
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Yep. You can't chrome line because it will reduce the bore diameter about four thousandths, making a .224" groove diameter into a .220" bore. When AR type barrels are designated for chrome lining, they make their groove diameters .228" to allow for that. Part of the reason for that much thickness is that you can't put chromium directly on steel, as the plating solution is strongly acidic and etches carbon steel away faster than the layer can deposit. As a result, you have to first plate nickel onto the steel, to resist the acid, and then the chromium actually is plated onto the nickel. The nickel, IIRC, is about half a thousandth thick to seal off direct contact with the steel, then the hard chromium as about one and a half thousandths thick. That is about the right thickness to prevent heat stress cracking from extending down to the steel substrate, and is probably as thick as they think they can go without barrel heating causing the plated layers to separate due to differential expansion. You end up with about two thousandths of plated metal, total, but since it is all around the inside of the bore, that subtracts four thousandths from the diameter. Nitriding is more similar to case hardening in that it penetrates into the steel, forming a case rather than thickening the surface. It can be accomplished at a temperature of 900°F-1200°F. That's the temperatures range in which barrel steel is normally stress relieved, so it doesn't tend to distort the steel the way heating and quenching it can. Sizing dies for reloading are typically nitrided rather than conventionally heat treated for this reason. This page describes the gas nitriding process, and the hardness range examples it has for 4130 and 4140 steel are right where 4150 barrel steel will fall, too: a Rockwell C scale 28-32 for the steel itself and Rockwell C scale 52-58 for the nitride case hardness at the surface. The page points out that the very surface, which is the most brittle part, is sometimes lapped off. If you are planing to firelap a barrel you also plan to have nitrided, I would do the nitriding first for that reason. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle Last edited by Unclenick; April 4, 2013 at 06:43 AM.