Back to the nitriding topic....

AR/M4 AR General Discussion
1

There has been much discussion, some good, some bad, on M4C regarding nitriding.

Some of the best technical information came about in the following thread:

https://www.m4carbine.net/showthread.php?t=106680

But it was still lacking in some details.

Today I get to unfortunatly add some real world experience to some of the theory discussed in the above thread.

It has been established that nitriding steel has the benifits of increasing surface hardness, lowering the coefficient of friction, increasing the fatigue strength of the base material, and creating corrosion resistance superior to chrome plating.

On enegative aspect that has been barely touched upon and so far only so by Bill Alexander, is the reduction in impact resistance caused by the high hardness of the effected zone on the steel.

Logic dictates that if a part is properly heat treated and tempered, that its core toughness should easily remain stable and be uneffected by nitriding. As it were, this is an absolute function of the alloy used, the quality of the treat/temper process, and of course the size of the part in question.

That last part is where I failed to logically extrapolate the effects of nitriding on gun parts when I sent my HK MR556 parts to be nitrided.

In cases where a small part will not be subjected to impacts during the normal course of use, nitriding is still not a bad idea, especially so long as any non-impact forces imparted on the parts do not exceed its load bearing capabilities. Most of the pins and detents in an AR15 are still fine to have nitrided.

On the other hand, small parts that can allow nitriding to reach a near or even a full penetration into the part and are subject to repeated impact, are NOT good to have nitrided.

Case in point, the firing pin.

On Saturday I went out with a buddy to do some pre-deployment (he’s heading out on a contract in the sandbox) training/practice and when we started, I was able to get of a single round before shit stopped working.

A quick inspection showed that the tip of the firing pin broke off and was never to be found.

This also leads me to some other information that I have shared on the site.

I stated that the full DI bolt assembly is compatible with the HK 416. This is true. I have actually swapped out the entire bolt assembly and firing pin from a DI AR into the HK and it worked fine.

BUT, while the extractor and ejector, and bolt cam pin are also each individually cross compatible with the HK, it turns out that the firing pin from a DI AR will not swap into the HK bolt and still work.

The geometry at the very tip of the pin and the inner channel of the HK bolt are different enough that the DI pin will not protrude far enough to strike the primer.

So, for folks who want to have parts nitrided, my advice would be to only nitride large assemblys that may be impacted, and small parts that will not be subject to repeated impacts.

Thus far, bolt carriers, barrels, etc are find, most small pins in the lower are fine, but due to the difference in size vs. the temperatures involved in the nitriding process, the jury is still out on the bolt cam pin and the bolt (at least as far as i’m concerned). I had mine nitrided and shoot it as is, but if the bolt breaks at anything less then 10K rounds, i’ll give nitriding bolts the thumbs down.

I hope this helps somebody eventually.

2

Yes, it is helpful.
Thanks for taking one for the team.

3

Thanks, you bring up some good points; I think Nitriding the bolt, and definitely the firing pin is a bad idea. Let us know how that bolt works out.

Question about Nitriding a Stainless Steel barrel…
I’ve read from people on this forum that Nitriding a Stainless Steel barrel is a bad idea because it can actually be corrosive or harmfull to SS.

Does anyone have a Nitrided SS barrel??

Where are people getting this info from, saying it’s bad for SS? (Any company or reliable source do a test on Nitriding SS…Link)

4

I do coatings for a living, and sometimes its just “gilding the lily”. I’m not against improvements, but a top shelf weapon is probably GTG already. Thanks for the info.

5

Interesting info, thanks for following up. We all benefit from this sort of knowledge sharing.

I tend to agree that coatings aren’t generally worthwhile, and we’ve all seen or heard of problems reported in various applications using them, but treatments like nitrocarburizing are more compelling. Still, I wouldn’t consider the firing pin and other parts of that nature for surface treatment.

6

I’ve nitrided 3 BCM SS410 barrels and a Kreiger SS bbl, no issues thus far.

7

good info to know

8

Superior barrels sells nitrided SS barrels and seem to have consistently good reviews (they use Douglas blanks profiled by compass lake). I have almost picked up one of their barrels to check out.

9

thanks for the info Grumpy, keep us updated on the bolt.

10

I have had 2 barrels done thru Rock Creek and both are going strong. One has 3,400 rounds of 5.56, the other 300 of 6.5/284. I have experienced no issues with the metal so treated.

11

I’ve got a nitride treated carbine test barrel that I’ve been shooting since 2002 or so, lots of rounds and never any problems. As already mentioned, there is a large difference between barrels, and small components.

There is a reason why you don’t see glock treating its smaller pieces, this is it.

Grumpy, I hope you are able to get HK parts quicker than our teams were. I’m not saying that 18 months is a long time for basic MP5 parts or anything…

12

Stainless steel is “stainless” because it contains enough chromium to form a protective chromium oxide layer.

The nitriding process converts the chromium into a nitride, so it can reduce the corrosion resistance of stainless steel.

13

https://sites.google.com/site/freeballisticcalculator/black-nitride-melonite-and-qpq

i have nitrided 9 or so SS bbl’s… no issues…
NEVER nitride a chrome lines bbl… it will trash it…

we had 1 CBI/keriger 223 barrel that i did for a buddy that seems to be shooting poorly, but we need to send it to CBI to find out why

on a side note, i do BC’s, but not bolt heads

14

Thin sections are a no go for the treatment as Te diffusion depth will will be too great a percentage (not sure what it should be or what the limits are; can be found in technical documentation though).

I must have half read the OP’s posts prior to him sending his parts in as I would have cautioned him.

For that I apologize, brother.

I’d stay with the barrel, bolt carrier and bolt only.

15

I’m going to do a group response here, so please bear with me.

As has been covered by others in this thread, this is a “Yes and No” thing. Yes, nitriding of stainless will “reduce” its corrosion resistance.

As Todd K. pointed out, Chromium is one of the uber nitriding elements along with others such as Aluminum, TItanium, and a few others. Meaning that they form extremely hard nitrides easily. What this means in relation to stainless is that stainless has corrosion resistance better then nitridided OS (Ordinary Steel) in its natural state. When nitrided, it now has a corrosion resistance equal to that of Nitrided OS, which is still lightyears better then untreated OS and even better then chrome lining.

Also, don’t forget though that if your part is left in a corrosive environment long enough to corrode through the nitride layer, it’s right back to stainless steel and hence any corrosion will be self limiting to the penetration depth of the nitriding. Which, on stainless, is far less then on OS because of the chromium.

A visit to CenterMass Firearms website and 5 minutes later of ordering and entering CC info, topped off with $84 bucks (including shipping) and two 416 firing pins are on their way to me now.

Because of the work I’ve done to my gun, i’m so far off the reservation that I don’t even bother trying warranty work on any of this so I don’t deal with HK at all.

No need to apologize. Not your fault. I researched nitriding like a mofo prior to having it done and found nothing that mentioned lowered inpact resistance, the size of parts vs. penetration depth and possible damaging of the core heat treat/temper based on the size of the par, tec. I also talked with the guys at H&M on the phone about it for quite some time. Told them the parts I was sending in, etc. and even they didn’t say anything about small parts and impact. Honestly, I’m curious as to why.

Next time I have a “whole gun” nitrided, I will still do all the non-impact small parts simply for the corrosion resistance. Plungers, detents, etc. But that firing pin is a no go. I’m on the fence bout the hammer and trigger pin.

Only good thing is that if my bolt shits the bed on me, I already have a replacement.

16

Wouldn’t the bolt lugs suffer? Honest question since they are thin as well.

17

Nitriding doesn’t penetrate deep enough to full penny the lugs.

The possible issue with lugs though, from what I understand, is that the nitriding makes the surface of the steel excessivly hard for the amount of stress the part is under, leading to stress cracks/fractures, which could grow and eventually lead to sheared lugs.

This is theoretical though. If the bolt was properly heat treated/tempered, etc. prior to nitriding and the nitriding doesn’t penetrate far enough to effect this, any surface cracks in the nitriding should not continue on into the untreated steel and it should still be just as strong as it was prior to nitriding.

Lots of factors to consider and little or no data existing on the subject.

Being that i’m playing guinea pig with an HK, any failures will be an expensive learning experience, and unfortunatly a fair amount of any “data” or experience I collect might not actually transfer over to regular DI ARs due to the use of different materials, specifications, and manufacturing processes that HK uses compared to DI guns.

18

What temperature was the nitriding done at and which process was it?

As I understand it, The general problem with nitriding is the temperature.

Through hardening steels like the 4140/4150CMV used for barrels can retain good core hardness and unchanged grain structure during the nitriding process if they are initially tempered just above the nitriding temperature.

Case hardening steels like 8620 /C158 normally are tempered at a relatively low temperature.

Re-tempering at a higher temperature (via the nitriding process ) can change the grain structure, usually for the worse, and result in brittle fracture.

19

Process was QPQ nitriding as done by H&M Metal Processing.

Highest temp involved is around 1075F.

Some specs on the QPQ process:

http://www.finishing.com/kolene/qpq/

20

So what’s better NP 3 or nitriding BCG and so forth. I have some LMT BCG that have been MP3’d. They have done well.