Steel alloys for AR bolt heads ?

[Waterworks]

Can this alloy be used for an AR style bolt head and barrel extension with multiple locking lugs for a 5.56 bolt action rifle ?

EN8
Technical Datasheet
Service. Quality. Value.
Applications
• Automotive parts
• Connecting rods
• Studs, bolts
• Axles, spindles
• General engineering components
Product Description
EN8 is an unalloyed medium carbon steel which is used in
applications where better properties than mild steel are
required but where the costs do not justify the purchase of
a steel alloy. EN8 can be heat treated to provide a good
surface hardness and moderate wear resistance by flame
or induction hardening processes. From the automotive
trade to wider general engineering applications, EN8 is a
popular steel in industry.
Chemical Composition (weight %)
C Si Mn P S
min. 0.36 0.10 0.60
max. 0.44 0.40 1.00 0.05 0.05

Engineering Steel
• Unalloyed medium carbon steel
• Reasonable tensile strength
• Can be flame or induction hardened


Mechanical Properties (in "R" condition)
Max Stress 700-850 n/mm2
Yield Stress 465 n/mm2 Min (up to 19mm LRS)
0.2% Proof Stress 450 n/mm2 Min (up to 19mm LRS)
Elongation 16% Min (12% if cold drawn)
Impact KCV 28 Joules Min (up to 19mm LRS
Hardness 201-255 Brinell
Related material specifications
BS970: 1955 EN8
BS970/PD970: 080M40
1970 onwards
European C40, C45, Ck40,Ck45, Cm40, Cm45
Werkstoff No. 1.0511, 1.1186, 1.1189
US SAE (AISI) 1039, 1040, 1042, 1043, 1045

 

[GGaskill]

I am no authority but EN8 seems to be less than 4140 and I would not really like to make those AR parts from 4140. I think they are made from a maraging steel which has much better physical characteristics.

 

[mhajicek]

In med device design we try to avoid having two parts of the same material and hardness move against each other, so my instinct would be to use two slightly different alloys. I think I'd make the bolt head slightly softer, since it's the easier part to replace. But we have to design for perfectly clean, dry, passivated moving parts, so maybe I'm over-thinking / over-engineering. A barrel isn't going to last forever anyway.

 

[wesg]

I think AR bolts are 9310 ... off the top of my head. Teeny little lugs... unlocking under pressure... wear... galling ... has to be considered.

8620 maybe a useful substitute ... life span for you to figure out.

Bigger question ... bolt action ???

What you gonna use for extraction? Need more than 22 deg of rotation for any useful camping. Even 4 lugs ... RPA ... can require a fair slap of the palm to open. I've had a couple occasions with my 3 lug Barnard and Gilkes to push me back to 2 on my own projects.

 

[john.k]

Both 8620 and 9310 will need a costly carburize and quench/temper ,which will make one offs very expensive.

 

[wesg]

Yeah, no idea about 9310. But 8620 certainly needs 'casing'. And best done in vacuum.

Why I stick with the easy s'...

4140 pre-hard ... which means lugs of a useful size.

Anecdotal... AR breech extensions have been made from 4140 at 52RC. I doubt anywhere near the wear resistance ... but if they'll get thru the useful life of a match barrel who cares?

I had a couple thru my hands that DID NOT. Nowhere near 52 ... perfectly good Krieger barrels gone to waste ...

State team scrambling to get back on track. New... 'improved' ... is all well and good...until it isn't. And it's back to the supplier stepping in to correct the problem...

Or not ...

 

[Cole2534]

Original spec is for Carpenter 158, but many new bolts are made from 9310.

My KAC bolts are Aermet100. Neener neener.