Gunsmithing Metallurgical/engineering help

[molonlave]

I'm working on building a clone of a canadian C3A1 sniper. I found a Parker hale M84 action that I'm modifying to accept a detachable magazine. I recently found an unfinished bolt body. However, after some machining, it became clear that it had not been heat treated. Got some testing done and the receiver is coming in at 43 HRC, while the bolt came in at 9 HRC. XRF analysis determined that bolt alloy is 4140. Found a local shop that specializes in heat treating to heat treat, oil quench, and temper my bolt. The report states an HRC of 53. My target was 46+/-

So, the big question: Does 53 HRC for a rifle bolt pose any potential problems. The rifle will be chambered for .308 win.

 

[ken226]

I would make an effort to try and stay around hrc 40-44 if possible.

HRC 53 isnt bad in general, and doesnt totally kill the ductility but likely is too hard for something subject to repeated shock. I seem to remember a batch of chinese made m14 receivers that were treated to around hrc54, had some cracking issues near the bolt locking lugs. A bolt would be under similar shock loads.

It may work fine at hrc53, but it may not.

For 4000 series alloy steels, hrc44 is about the right combination of hardness/ductility for a bolt head.

Hrc53 is getting closer to knife blade hardness. A bolt head is subject to shock loads, and tends to have un-filleted inside corners which are notorious for starting and propogating cracks. Stick with your original plan for hardness.

HRC40-44 is what you want.

An example of the change in ductility per hardness:

Bhn 388 is about 42hrc and shows a ductility of 12% enlongation, whereas bhn223 is about hrc20, which has a ductility of 24% enlongation. So about a 100% change in ductility for a 100% change of hardness. A pretty linear relationship.

From 43hrc to 53hrc would show a similar reduction in ductility of about 20%.

https://www.quora.com/What-is-the-r...-strength-compressive-strength-and-elongation

If they can heat treat it to hrc53, im sure they can heat treat to hrc40-44 just by increasing the temper temp.

Timken steel provides a hardening/normalizing temper temperature chart in rockwell c here:

https://www.google.com/url?sa=t&sou...FjAAegQIBBAB&usg=AOvVaw2zFUTnbfi2h2enjVML9Ft6

The chart on page 3 might be helpful.
According to Timken Steel, austenizing at 1625f, with a water quench, then a 60 minute soak at 1175f for temper should get you to hrc42.

 

[308pirate]

Agree with Ken on mid 40s HRc for 4140 or 4150. Another combination would be through harden to mid to high 30s HRc and gas nitride a case .010 - .020 deep to mid 50s HRc.

Be aware that any heat treatment involving heating, quenching, and tempering is highly likely to induce significant distortion, most of it happening during the quench phase. Sometimes quench jigs/plugs can be effective in reducing that distortion but that might be cost prohibitive for a one off job. BTDT

 

[molonlave]

Thank you, that exactly what I needed to know. Can they simply temper it or must it be annealed and re-heat treated?

 

[ken226]

They could re-temper, but the needed temperature would depend on variables that only they would know, and only if they know them. The austenizing temp, type of quench, original temper soak time and temperature would be factors.
They may or may not be able to do this depending on how carefully they keep records and if they have an engineer on staff that can figure out what temp/time to use for re-temper.

Typically after quenching, the temp is held above a set value before tempering. This part would be impossible as it has already been done, and cooled below this value. So, an engineer whos really up on heat treating would have to factor in the effects of not doing this.

But, since your acceptable range is 40-44, it might not be that hard. A pretty big target to hit.

At 53hrc, your really close to the full austenizid hardness of 4140, which indicates that your heat treater probably didnt do much, if any tempering. If they oil quenched, but didnt temper, it may explain why your a little under the full hardness of quenched 4140.

If it was just heated and quenched in water, it would have ended up at about 54-55hrc.

If you could heat it for 60 minutes to 700f, then let it cool slowly i think you'd be end up close to 44hrc.

In my opinion though, if you want accuracy in the heat treat, you should start over.

And keep in mind, my info is all from booklearnin. I have a degree in engineering, but im not employed as an engineer. I found a better paying job doing less. I've never heat treated shit.

 

[308pirate]

And this is why I don't buy any rifles, from anyone, that have been nitrided in any way after the fact. 99% of the time the owner has no idea what has happened to the original metallurgy.