ACE
This was covered in another post (which has disappeared)
8620 is more than adequate for a Highwall frame. Shilo Sharps, Ballard, Winchester/Browning all used it for their HW actions. 8620 is also considered a Case Hardening Steel which makes it ideal for Color Case Hardening.
For firearm frames you generally want steel with a hard wearing outer layer and a softer inner core to absorb shock and reduce cracking/warping.
4140 is excellent steel, but it’s a thru hardening steel (the hardness goes thru the steel, no soft inner core). 4140 is also an OIL quenching steel, quenching oil quench steel in water, cools the material too quickly. Problems with quenching oil hardening steel in water can cause non uniform hardening, warping and cracking.
Since Color Case Hardeing requires quenching in water; 4140 is an inappropriate material for this. Today’s steels purposely are designed for specific applications, 4140 was not designed for case hardening, 8620 was.
An example I used earlier: Bar chain oil is an excellent lubricant, but you’re not going to put that in you family car. Use the correct material for the job.
Now 4140 can be case hardened, by induction hardening… but neither you nor I have the money for this, and besides the colors look terrible.
Considering that Shilo’s foundry can cast either 4140 or 8620, little if any price difference between the two materials, 8620 casts nicely, easier to machine, and takes blue well, to me its an ideal frame material.
I’m not quite sure why AG Co. is making frames strictly out of 4140, they really need to consult a metallurgist. They are a barrel manufacturer, they use plenty of 4140 for barrels, more than likely they are used to dealing with it… but they don’t Color Case Harden Barrels.
A simple check in Machinery’s Handbook , American Society of Metals (ASM) Metals Handbook or quick Google search on quenching 4140 in water; answers will range from “Not Recommended” to “Don’t do it”
Mike Hunter
Hunter Restorations
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