XM16E1 barrel erosion study (1966)

[Slater]

This is one of many historical documents available on DTIC. I found the following interesting:

"During the test, it was noted that the phosphate-coated test bolts averaged more than 10,000 rounds each and only one failed. The remainder were removed due to the conditions of the test bolt carriers . This increase in bolt life is attributed to use of phosphate coating instead of hard chromium plate, thereby avoiding a condition wherein ·the hard chromium can be fractured easily leading to rapid propagation of the crack through the hard case and· soft core of the bolt."

So, some 53 years later, is this still a valid conclusion - that Parkerizing the bolt led to increased life over chrome plating?

https://apps.dtic.mil/dtic/tr/fulltext/u2/483309.pdf

[ABNAK]

Chrome plating, like most other processes, has improved with time. So no, 53 years later I don't believe that is an accurate statement.

[ViniVidivici]

Yeah, I'd like to see more recent data on the subject, if I were to be making a purchase based on that info.

I mean, for example, aren't 9310 bolts better than they used to be, due to better heat treatment processes?

[1168]

Doesn’t LMT use hard chrome on their enhanced bolt? I also seem to remember the Mk46 bolt being plated with some shiny stuff; don’t know what.

[lysander]

This is one of many historical documents available on DTIC. I found the following interesting:

"During the test, it was noted that the phosphate-coated test bolts averaged more than 10,000 rounds each and only one failed. The remainder were removed due to the conditions of the test bolt carriers . This increase in bolt life is attributed to use of phosphate coating instead of hard chromium plate, thereby avoiding a condition wherein ·the hard chromium can be fractured easily leading to rapid propagation of the crack through the hard case and· soft core of the bolt."

So, some 53 years later, is this still a valid conclusion - that Parkerizing the bolt led to increased life over chrome plating?

https://apps.dtic.mil/dtic/tr/fulltext/u2/483309.pdf

Chrome plating, like most other processes, has improved with time. So no, 53 years later I don't believe that is an accurate statement.

Yes, it is a true, and accurate statement, even 53 years on. Sadly, hard chromium plating is not recommended for things subject to fatigue. That's been known for a long, long time, and it is still a problem.

On the Effect of Hard Chromium Plating on the Fatigue Limit of Steel

The Effects of Chromium Plating on the Fatigue Strength of Steel

Effect of Residual Stress and Microcracks in Chrome Plating Layer to fatigue Strength of Axle Shaped Parts

Effect of Chromium plating on the Endurance Limit of Steels used in Aircraft

There is some good news, however. For bolts and bolt carriers, you still should not go with hard chromium plating, but you can use thin dense chromium (TDC) plating. TDC goes under many proprietary names, such as Electrolizing, Armoloy, TDC-1, etc.

If you want a shiny chrome bolt, go with one that was plated with TDC.

BTW, the original Armalite coating for the bolt and bolt carrier was not hard chromium plating, but the proprietary TDC process known as Electrolizing.

[1168]

Lysander (or whoever knows metallurgy), do NP3 and other popular coatings/surface treatments also have this downfall?

[sinister]

Electroless nickel and teflon (NP3) do not have the hydrogen embrittlement problems chrome does.

[Slater]

So chrome-lined bores are another animal altogether?

[sinister]

Nope, same problem.

[Slater]

Then, nitriding avoids all these issues?