Posting on AR is usually a humbling experience as I must admit technical assistance is needed. A client has a Hagn-actioned rifle & the action was gas nitride hardened. The surface finish looks like crap & he wants me to fix it.

However, I know little about gas nitriding to include:
1. Is gas nitriding sufficient hardening for an action made from annealed steel?
2. What is the depth of gas nitriding hardening?
3. How is a gas nitrided surface best prepared for traditional hardening like color case hardening?
4. Can the surface hardening of gas nitriding be eliminated simply by annealing the part?

I hate to sound like a complete dumb ass, but gas nitriding is a new technique outside my realm of experience. I would appreciate responses from those actually knowledgable about gas nitriding. Uninformed opinions need not be posted. My goal is to properly harden and improve the appearance of my client's Hagn action.

Thanks for you consideration.

Roger

1: No. Not if it is not modern alloy steel.
2: Very thin.
3. Do nothing; the heat from the Case Hardening process will eliminate the nitride layer. But if it was, or needs, case hardening, why was it gas nitrided in the first place?
4. Yes, but why would you want to do that?
Gas nitriding is, for me, a useless process for rifles. They either need true case hardening, or need to be heat treated per the alloy steel that they are. Gas Nitriding is to add a hard, but thin, layer of nitride, just to make the surface slick; not for strength. I have talked to the engineer at a place that does it; forgot the name. He knew little about guns; but a lot about steel.
Since Hagn actions are modern, I would assume they are made from a CM steel; unless they are truly case hardened, then it is 8620.
But since they are also nitrided, I will go with the 4140 idea.
So, if you are going to really case harden it, be careful. Or don't.
Find out what steel they are made from.
Now, Turnbull can Color 4140 steel, without making it hard. I have had many Ruger single actions, etc, done by them.

I just read that Hagn actions are made from 8620, in which case they can either be truly case hardened, or nitrided, and still be super strong. I don't know if the internet source is reliable or not. 8620 cannot be hardened without adding carbon. So you can just anneal it, work on it, and then have it nitrided, or case hardened. No need for both; that is redundant.

Nitriding was discussed at length on another discussion here;
http://forums.accuratereloadin...9411043/m/7531037172

If you read my post in in that discussion you will see some information from the Browning/Miroku website in respect of what nitriding is and what it can do.

I am not promoting this process for any particular benefit for firearms as I personally have no experience with it as a hardening process. My only experience has been that it provides a first rate anti corrosion surface in so far it protected my Miroku shotgun action when it lay in battery acid for sometime after an accidental spill.

The Japanese are pretty switched on with manufacturing processes and I'm sure they have very good reason to use their nitriding process on the Browning and Miroku firearms they make.

Whether this is what you are wanting for the Hagn-actioned rifle you are wanting to fix there should be enough info from these posts for you to decide.

Just for you Roger, I actually just got a personal note from Martin Hagn. His receivers are made from 8620. So they can be handled any way you want; CCH, blued, nitrided.
Read that other thread.
Nitriding is only for modern steel; that is what is important.
For 4140, which has already been heat treated but is not hard, then GN might be ok. (not for me)
For 8620 which can and must be, case hardened anyway, why do it?

I am mot a metallurgist. In fact I know just enough to be very careful.

I do have some limited experience with nitriding for industrial applications, and somewhat more experience with normal (not colour) case hardening.

Nitriding is a process where nitrogen is diffused into the surface, and the steel is heat-treated, leaving the surface hard.
Normal case hardening is a very similar process but carbon is used instead of nitrogen. The thickness of the affected layer is similar and can be varied/controlled to some extent in both processes.

What I don't know (and what worries me a little bit) is what happens when both elements are found in elevated quantities in that layer (such as would happen if a previously-nitrided item was later case-hardened. I would not do it on a highly-stessed component without consulting a metallurgist professionally, or testing it, both of which approaches would probably cost money.

Two questions arise:
Firstly, what is wrong with the existing finish? If it is just the texture or colour that is objectionable, could it be blued? Will the customer accept that (I assume he would since nitriding is supposed to give an even colour)?

Secondly, should the affected components of this Hagn action which need to be re-finished be considered highly stressed? The thickness of the affected zone of both these processes are probably only a couple of thou at the most. The receiver and most areas of the breech block in a falling block action will typically be a lot lower than on the average bolt action of similar caliber. For example, the total contact surface between the breech block and the receiver on a Hagn is nearly 8.25 times larger than that of a K98 Mauser. Now I am not suggesting that the Hagn is 8 times stronger, because there will certainly be other areas where there are much higher stresses. But I suspect the receiver itself would be just fine no matter what finish is applied. However, this is an opinion for which I am not prepared to accept any liability, and thus my advice would remain to either seek professional advise, or create a test piece, do both processes to it and then see how it behaves, or to select a finish treatment that does not affect the heat treatment.

One point; the hardness thickness of the two processes are NOT similar, which is why nitriding alone cannot be used on steel too soft for a rifle component.
As for the single shot contact area; sure, it is helped by that, and single shots are super strong by design, but still needs some hardness due to cartridge case setback into the bolt/block face.
K98 Mauser; you do realize that you are referring to only the Polish Small Ring 98 here.

First and foremost, Roger, ask Hagn if the action was provided to the customer in the annealed state. Then ask the customer to contact the company who preformed the gas nitriding and ask for exactly what they preformed.

Just to clarify, because I think dpcd's posts are a little confusing (at least for me).

1. 8620 cannot be gas nitride effectively, nor can any annealed steel. Gas nitriding requires a part to have a relative hardness prior to the process. 8620 in its hot rolled/annealed state has (in my experience) 7-10 Rc, in this softened state the material "absorbs" the nitrogen gasses and will not harden.

A steel like 4140 Pre Hard, ranging from 25-32 Rc will have enough hardness to accept gas nitride. The rule of thumb my heat treater gave me was AT LEAST 20Rc.

2, 3, 4 cannot really be answered until you know for sure the action was provided to the customer in the annealed state.

Martin Hagn specifically recommends gas nitriding for his actions. He even asked me for Paulo's contact info for his US customers. My only experience with the process is with my Hahn mini action in .223. There were some uneven surfaces that in retrospect, Paulo could have removed with glass beads and low air pressure. The new owner of that rifle is discussing this with Paulo now. I have a .30/06 Hagn MHK4 medium action gun that will get the same treatment. It's not for decoration, like Turnbull's color casehardening, but is for wear resistance. My rifle was treated to be file hard. Gas nitriding won't do much for old soft steel Mausers.

These threads are always interesting and mix and match the information.

Carburizing is a high temp process that results in a high carbon martensite layer at the surface of the steel. It is typically done at 1450F - 1650F depending on the steel and part geometry. This is why it is not suitable for low alloy and tool steel that also require a temper treatment to hit a specific hardness range. Carburizing typically has a depth of 0.015-0.030". Although color case treatments may only be 0.005" deep since they are for looks, not really wear resistance. Low alloy steels like 8620 were developed specifically for carburizing to give the case a better depth of hardness and allow for a tough pearlite/bainite core. Works great to make gears.

Nitriding is done at low temperature, typically 800-1100F. Carbonitriding is also done at these lower temps. The treatment simply packs as much nitrogen into the steel surface as the atomic structure will hold and creates an iron nitride layer. It's typically thin, and less than 0.001" in depth. It can be applied to any carbon steel and it will harden that surface. However, because it is so thin, the base metal strength dictates the end result. Whatever the strength was before nitriding will be the final strength after, you just get a hard surface for wear resistance. Nitriding is used on quite a few "soft" steel parts to make things like clutch discs where you need little strength, but good wear properties. When you use nitriding on previously hardened parts, like 4140 pre-hard, you choose a temp lower than the last tempering temperature used on the part so you don't screw up the parts temper too much. This also means that it isn't suitable for just any hardened part.

With that out of the way, here are the answers:

1. No, you get what you started with under the case layer, so not suitable for annealed parts.
2. Answered above, but generally less than 0.001".
3. You must grind it off. The nitride layer is iron nitride, or a mix of base metal and iron nitride. And, no you can't re-nitride a previously nitrided part. Well, you can, but it won't work as intended.
4. No. You aren't going to heat it high enough to dissolve the iron nitride. Best to abrade it off.

Is it suitable for a Hagn action? Maybe. It depends on the condition that it is sent to you in. Meaning, is it already heat treated or is it dead soft? Or something in between.

You could carburize the part and then follow with a nitriding treatment to apply a "finish".

Jeremy