Best type of finish for an MP5 clone?

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Originally Posted by NoScoE30

This is quoting H&M Metal Processing. I am waiting on the heat treat spec for the trunnion.

"Q: What is the effect on the core hardness?
A: If the core region's depth is not influenced by diffused nitrogen, the degree of tempering associated with time and temperature during the Black Nitriding cycle determines change in core hardness as follows:

1. Additional tempering occurs during Black Nitriding if the pre-hardened component has been pre-tempered below the subsequent Black Nitriding temperature. Reduction in core hardness depends primarily on the steel grade used and attendant-tempering characteristics.

2. If the pre-hardened component is pre-tempered, or non-pre-hardened above subsequent temperature, core hardness will not change significantly as a result of the Black Nitriding process."

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So if an HK trunnion is a zone tempered piece of something like O1 at 60 HRC, then it was tempered at 400° F. If you put it in a 900° F salt bath for even 5 minutes the internal hardness will drop to 40HRC while the surface hardness will jump to 68 HRC. That would completely ruin the trunniun.

There are some steels that get to high hardnesses above 900° F, but you would have to know what specific alloy your trunnion was made of, and it is unlikely expensive steels that temper in that range would be used for gun parts. Most steel used in guns are low alloy or tool steels like 4140 or O1.



I can't see how nitriding an entire HK rifle upper could possibly be safe.

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Originally Posted by Handy

So if an HK trunnion is a zone tempered piece of something like O1 at 60 HRC, then it was tempered at 400° F. If you put it in a 900° F salt bath for even 5 minutes the internal hardness will drop to 40HRC while the surface hardness will jump to 68 HRC. That would completely ruin the trunniun.

There are some steels that get to high hardnesses above 900° F, but you would have to know what specific alloy your trunnion was made of, and it is unlikely expensive steels that temper in that range would be used for gun parts. Most steel used in guns are low alloy or tool steels like 4140 or O1.



I can't see how nitriding an entire HK rifle upper could possibly be safe.

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Hence me questioning bretheren armament.

But, we dont know what the trunnion is made out of, or its heat treat condition.

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Originally Posted by NoScoE30

Hence me questioning bretheren armament.

But, we dont know what the trunnion is made out of, or its heat treat condition.

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We do know how hard they are because a group of HK builders have tested them. And we can make some pretty good guesses about what kind of steels will hit 60 HRC after tempering and were available in the late 1950s, as well as making some assumptions about what kinds of tool steels are likely to be used and what kinds are prohibitively expensive, difficult to machine or hard to heat treat.

So unless you're suggesting that all HK trunnions are made of stainless steel or some oddball tool steel, it is fair to say that we are talking about some common grade of oil quenching steel that is NOT going to end up much above 40 HRC if re-tempered at over 900° F. Especially if it is from a 40 year old surplus rifle or a company supply parts to make $600 clones.

Instead of directing your hard ass towards me, how about you bring brethren armament back to this thread. They actually got some guns nitrided.

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Originally Posted by NoScoE30

Instead of directing your hard ass towards me, how about you bring brethren armament back to this thread. They actually got some guns nitrided.

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I thought we were talking heat treating temperatures. What's the problem?

Here, I brought them back in:

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Originally Posted by kmcintosh78

Complete nitride.

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The RCM trunnions are 9310, and I am waiting to hear back with the heat treat spec. It might only be case hardened.

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Originally Posted by NoScoE30

The RCM trunnions are 9310, and I am waiting to hear back with the heat treat spec. It might only be case hardened.

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9310 is weird stuff. I would not know how to even begin figuring out what happens when you nitride something that has been previously pseudo-carburized. I think RCM trunnions are 52 HRC.

Hopefully Bretheren will reply and explain how it worked and how it came out-maybe pics?

3k rounds without any issues. I am not a metallurgist. We tried it out, and it turned out great.
I can get pics up in a bit.

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Originally Posted by kmcintosh78

3k rounds without any issues. I am not a metallurgist. We tried it out, and it turned out great.
I can get pics up in a bit.

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Depending on the type of steel that you started with, a nitride salt bath is going to take a very strong piece of carbon steel that is hardened all the way through to 60 HRC and drop the structural hardness (and also strength) down below 40 HRC while surface hardening it to 68 HRC. So the trunnion surface will wear well for awhile but the structural steel holding the trunnion together will be 1/3 to 1/2 has strong as it should be. A trunnion that was 60 HRC and has been nitrided is now like a hard boiled egg with the eggshell still on.

I am also not a metallurgist. But I don't sell metal guns that I have re-heat treated in a 1000° tank of molten salt as if I have a staff metallurgist.

For anyone wondering what happens to steel that is only surface hardened with nitriding:

https://encrypted-tbn0.gstatic.com/i..._avRznStz7BLh0