Black Oxide Coating of Nitronic 60 1

[tbuelna]

I am considering using moderately strain hardened (160ksi uts) Nitronic 60 bar for a shaft design. The shaft body will be loaded in shear and may experience a very small amount of sliding at the contact with the mating bore surface. The mating part is 17-4PH H1150 cres. I need a cosmetic black finish on both parts that will not produce a dimensional change. I know I can use MIL-DTL-13924, cl. 4 black oxide on the 17-4PH part, but the spec is not clear about using it for Nitronic 60, which I believe is an austenitic alloy.

I came across an old post from kenvlach that described some anti-chafing/wear benefits of AMS 2485 black oxide coating. This interested me so I purchased a current copy of the spec. But after reading AMS 2485K it was not clear to me whether this process is suitable for use with corrosion resistant steels like Nitronic 60. Sec. 8.5 states "Assemblies containing parts made of metals other than steel or iron alloys should not be processed....", so would this exclude stainless steels?

I'd like to use the black oxide process described in AMS 2485, so can someone tell me if the process is suitable for use with Nitronic 60? If not, is MIL-DTL-13924, cl. 4 black oxide suitable for use with Nitronic 60? And if neither process is suitable for use with Nitronic 60, does anyone know of another compatible coating that will produce a cosmetic black finish with minimal dimensional change?

Thanks in advance for any help.
Terry

 

[CoryPad]

No, the AMS does not exclude stainless steel. Either the AMS or the MIL standard should be capable of working on your Nitronic part.

One possible alternative is manganese phosphate, which can be blackish.

 

[BiPolarMoment]

Might be cost prohibitive but: PVD Coating? (AlTiN or DLC)

Link

 

[tbuelna]

BPM- Thanks for the reply, but PVD would not be economical. All I need is a cosmetic black finish. The base material provides all the corrosion resistance and surface hardness I need.

CoryPad- Thanks for your response. While I have used phosphate coatings on alloy steel parts before, I hadn't considered it for this part because I wasn't aware it was suitable for corrosion resistant steels. After reading your post I scanned through a copy of MIL-DTL-16232G which covers both zinc and manganese phosphate. As I mentioned I'd prefer a nice cosmetic black finish, and MIL-DTL-16232 type M cl.4 is a manganese phosphate base that can be dyed black, so that is something I would consider. But I found the same issue when reading this MIL spec that I did with the AMS spec in that it was not clear to me that the process is suitable for corrosion resistant steels. Both AMS 2485 and MIL-DTL-16232 mention "ferrous metals" and "carbon and alloy steel", but I did not see specific mention of corrosion resistant steels like MIL-DTL-13924 does. Nitronic 60 has >60% iron content, so I guess it qualifies as a ferrous metal. My knowledge of chemical processes is a bit limited, but are there any specific processing instructions I should provide when applying either AMS 2485 black oxide or MIL-DTL-16232 manganese phosphate to Nitronic 60 at 160ksi UTS, other than embrittlement relief?

Thanks again for the help. Now that I have a bit more information I'll make some calls to plating vendors that perform these processes and see what they have to say.

 

[CoryPad]

Obviously you need to inform the vendors about Nitronic 60 (austenitic stainless steel with 17 % chromium) and need for embrittlement relief. Additionally, you will need to discuss the condition of the stainless steel prior to surface treatment. Can they treat a part that has been passivated? An active surface may be preferred (especially for phosphating), I am not completely aware of all of the details. An active surface part with phosphate may not meet your applciations requirements. I know 18 % chromium stainless steels are phosphated and electrocoated for automotive applications.

I forgot that the aerospace industry is still using chromate conversion coatings. Stainless steel will turn black when immersed in hot sodium dichromate. Another option for you to consider.

 

[TVP]

In general, phosphating of stainless steel does not result in good, adherent deposits. I definitely suggest black oxide for cosmetic black coating on stainless steel like Nitronic 60. Either SAE or MIL spec can be used.

 

[tbuelna]

TVP- As I noted in my OP, I would prefer the AMS 2485 black oxide process over MIL-DTL-13924 since it apparently seems to provide some marginal benefit in terms of surface chafing/wear. But after reading thru the AMS spec it is not clear to me exactly how one determines the exact chemical solution compositions and processing required for different "ferrous metals". MIL-DTL-13924 describes 4 different process classes for specific metals, so I have a good idea which process class I should specify for my particular material. But AMS 2485 provides no similar guidance. How are the AMS 2485 process variables for a given material determined by each vendor?

I'm not that great with chemistry, so could you maybe give me a brief explanation of how the single process described in AMS 2485 can work for all ferrous metals, while MIL-DTL-13924 describes 4 different processes?

Appreciate the help.
Terry

 

[TVP]

Terry,

Essentially there is no difference in the final black oxide layer when specifying either SAE AMS 2485 and MIL-DTL-13924. Corrosion and wear resistance will be the same. The SAE spec was written to be somewhat generic, with the key section being 8.3 (in J revision, I don't have K in front of me). Before the proprietary chemical formulations were developed, the molten salt bath (Class 3) procedure was more common. Since this would soften previously hardened martensitic stainless steels and strain hardened 3xx grades, Class 2 was developed, but it had limited effectiveness. Hence the development of Class 4. Use the following links for more information on the history, etc.

CORROSION RESISTANCE OF BLACK OXIDE COATINGS ON MILD AND CORROSION RESISTANT STEELS

http://www.dtic.mil/dtic/tr/fulltext/u2/615941.pdf

MIL-HDBK-205

http://www.everyspec.com/MIL-HDBK/MIL-HDBK-0200-0299/MIL_HDBK_205A_2071/

 

[tbuelna]

TVP-

Thanks for the help. As you noted, the first sentence of AMS 2485 sec 8.3 caught my eye- "No specific composition is given for the black oxide processing bath."

Once I find a vendor, I'll have them run some sample parts to be sure.

 

[tbuelna]

Just an update. I got the parts noted above back from being black oxide coated and they look fantastic. However I opted to use 17-4PH cond. H1025 rather than Nitronic 60 for these parts. Had them black oxide coated per MIL-DTL-13924 cl.4 with a hydrogen embrittlement relief. I'm going to assemble the parts and do some qual testing over the next 2-3 weeks. I'll let you know how it goes.

Thanks for the help.

 

[TVP]

Glad it worked out for you. I would assume that the parts would look equally as good in Nitronic 60, in case you need that in the future.

 

[MagBen]

Just for curiosity, why did you switch between Nitronic 60 to 17-4PH H1150 to H1025? what was the max temp used for oxidation?

 

[tbuelna]

MagBen-

This is a shaft that will experience lots of the small, local back-and-forth surface sliding that typically produces fretting, so that's why I initially chose Nitronic 60. I made the switch from Nitronic 60 to 17-4PH after concluding the initial amount of raw material I required would make Nitronic 60 cost prohibitive. I started with 17-4PH H1150 cond. because the end of the shaft needs to be swaged to retain another part, and I didn't want the shaft material to be so hard that it would crack when it is swaged. After doing some test swage samples, I found that 17-4PH in a H1025 cond. would work OK. So that's what I'm currently using for the shaft material.

The shafts were processed per MIL-DTL-13924 cl.4, and the vendor told me that 17-4PH H1025 was right at the hardness limit where hydrogen embrittlement becomes an issue. In service the shaft will never see temps above about 400degF.

The shafts are being laser marked right now, but I'll post a picture of the finished product in a couple days.

 

[EdStainless]

Did you re-age to stress relive after the swaging?
Otherwise that end will be significantly harder and more susceptible to cracking.

What size shaft is this?
There is a lot of Nit60 shafting made for marine applications. You can buy single pieces.
Look up Aquamet 22 (this is not an endorsement of a supplier, just an example).

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Plymouth Tube

 

[tbuelna]

EdStainless-

It is not possible to re-age or stress relieve the end of the 17-4PH H1025 pin after swaging. The swage joint fixes a retainer flange that keeps a cylindrical roller from coming off the end of the shaft. The roller is made from 4340 and is liquid nitro-carburized (AMS 2753 QPQ). The swaged end of the shaft starts out at .14" OD with a .03" wall that is flared out to a 50deg included angle over a .08" length. The bend radius of the retainer ID the swaged end of the shaft is formed over is .040".

I made a couple samples of the swage joint and looked at them with a microscope, and I did not see any evidence of cracking at the outer edge of the flared pin material. To be safe I probably should have done a dye-pen inspection of the sample parts, but this is a garage project so money is tight.

I'll post more details in a few days when the parts are finished.

 

[israelkk]

I would not trust the statement that hydrogen embrittlement is not an issue. The hardness can vary for up to 10RC hardness points for same H1025 heat treatment. Therefore, do not be surprised that for future batches you may find that the hardness will be higher than 40RC. According to MIL-HDBK-C (metric) Table 2.6.9.0 (c) the nominal hardness for 17-4PH H1025 bars, forgings, rings, plates and strips the hardness variation is 35/42 RC. Note that this is "nominal" only. Actually it can varies more. PH steels are ONLY guarantied for tensile and yield properties, never for hardness.

 

[israelkk]

Correction, MIL-HDBK-5C (metric).

 

[MagBen]

while 17-4 at both conditions can be fabricated by cold working, you might go to the wrong direction by switching from H1150F to H1025F. H1150 is more ductile, softer and less susceptable to cracking when swaging.

SCC could be improved by increasing aging temperatures, while the general corrosion resistance might decrease. Since the black oxide would satisfy the general corrosion reisstance, it seems the main concern for your project is the SCC/H2 embrittlement, so if the strength for H1150 is good enough, 1150 seems to be a better choice.

 

[EdStainless]

I agree with Ben on this.
The lower strength level has some real advantages.
If you do it again this would be advisable.

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Plymouth Tube

 

[tbuelna]

MagBen & EdStainless-

I agree that 17-4 in an H1150 cond. would be less likely to crack during a cold forming operation than in an H1025 cond. And I also agree that 17-4 in an H1150 cond. would be less susceptible to SCC than in an H1025 cond. But as noted, I finally opted for using the material in an H1025 cond. because the main body of the pin benefits from the harder surface.

I've swaged a couple of pin assemblies with pins made from the harder H1025 cond. material. I'll try to take some high-res, close-up pictures of the flared pin edges so you can see what I'm describing.