Why do they use 4140 for gun barrels?

That steel can get incredibly hard if it gets hot, but the barrels are not very hard at all.
Why pay extra for tool steel?
TIA
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On Sat, 23 Feb 2013 13:56:37 -0800 (PST), clark
I'm not sure what you mean by that; 4140 softens with heat just like every other steel.
As for gun barrels, 4140 has relatively high tensile strength even when tempered at high temperatures (for example, temper at 800 deg. F, and yield strength is still 120,000 psi).
The result is a strong steel that has high elongation (13%, in this case). That means it's strong but not brittle; its ductility is quite high for such a strong steel. It's relatively safe with high chamber pressures.
That makes a good gun barrel. It has good wear resistance, too.

4140 is not really a tool steel. It's a medium-alloy, medium-carbon steel. Because it contains chromium and molybdenum, it will harden with only 0.40% carbon to a higher degree than, say, 1040 steel.
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clark wrote:

Why indeed? Maybe they looked at the real issues and decided it was the proper steel, rather than using old galvanized water pipe? Maybe they don't like a barrel splitting when it fails to contain the pressure from the rounds, when fired?
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t very hard at all.

Don't want HARD, want tough and more important, want a material that resists erosion from very hot, very high velocity and very high pressure gas. And what the commercial barrel makers order for steel isn't 4140 that you get from the local steel merchant. Since they order tons at a time, they can get an alloy that does the same or better for physical properties, but is easier to machine and pull buttons through. If you had a super-hard barrel, it'd probably grenade, too brittle to take the near-instantaneous pressure load. Some army officer had the same bright idea, I've got a book that shows the results of putting case-hardened throats in cannon. Didn't last more than a few rounds before they shattered. And 4140 is hardly "tool steel". High chromium and nickel content cuts gas erosion considerably, back in the early days of smokeless powder, they found that plain carbon steel barrels that were fine for black powder and lead bullets just had their rifling go away in a few hundred shots. Winchester used nickel steel for the first time in their 1894 .30-30 barrels for that reason. A big deal in the late 1890's, using an alloy steel.
There are a few books out there on barrel making, most are short run printings, but can be had. I've got a couple.
Stsn
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 And 4140 is hardly

4140 is what i would call a low alloy steel. Chrome and Moly, but no Nickel.
SAE grade      % Cr      % Mo      % C *      % Mn      % P (max)      % S (max)      % Si
4140      0.80?1.10     0.15?0.25     0.38?0.43     0.75?1.00 0.03 5      0.040     0.15?0.35
Dan
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On Sat, 23 Feb 2013 22:29:08 -0800 (PST), Stanley Schaefer

I wonder how P-7 would work for gun barrels?
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That maybe related to its bollet material. And maybe the properties of <a href="https://steelpurchase.com/4140-scm440-17225-steel /">4140 steel</a> after heat treatment can handle that.
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