Duplex Threaded Components

[SteveyB]

Hi,

I was wondering if someone could advise. Basically I have two super duplex (UNS S32760) components that are required to be threaded together with a M40x1.5 thread, torqued to 750Nm. Hence I was wondering how susceptible the threads will be to galling issues, as there seems to be little or no information available. Is duplex as prone to galling as the austenitic grades of stainless? Due to he application I could not use a different mating material of different hardness to reduce the galling effect as generally recommended.

Any advice on this would be greatly appreciated. Thanks in advance.

 

[TVP]

It should not be as bad as austenitic SS, but you still need to use a suitable thread lubricant. If the surface is cold drawn, that will reduce the tendency for galling relative to turned surfaces.

 

[SteveyB]

Thanks for your response. Do you know what the galling threshold stress is for 25Cr?

 

[1gibson]

Polishing the threads with abrasive impregnated rubber flapper wheel (name escapes me, dremel recently came out with some) can reduce the chances of galling. They have small fingers that can get inside the threads with low risk of damage compared to other methods.

 

[SteveyB]

Unfortunately the threads are turned. From what I have calculated I have a thread surface stress of 115MPa(16.7ksi). From what I have found so far this seems quite worrying for a super duplex stainless.

 

[OGMetEngr]

Unfortunately, there isn't any data I've found on super-duplex SS, only standard duplex SS. Duplex SS has a low threshold galling stress (<2ksi except for mated to Nitronic 60 which is 30ksi). Since both super duplex and duplex have similar microstructures of both austenite and ferrite (just higher strength for super duplex), I would imagine both would have similar galling tendencies and thresholds.

Here's a reference for 2205 duplex:

http://www.specialtysteelsupply.com/brochure/nitronic-60-technical-data.pdf

 

[OGMetEngr]

Page 30 of 32 of this attachment has testing for 2507:

http://namis.alionscience.com/conf/tscc/search/pdf/AM026041.pdf

Just as I suspected, low galling threshold for 2507 in the dry and salt-water sprayed condition, but surprisingly better when submerged in salt water for 90 days prior to testing. Maybe has to do with a change in surface roughness or passive layer strengthening after submersion? Regardless, surface treatments (nitriding, carburizing, TiN, etc.) and lubrication (PTFE impregnated coatings, MoS2 coatings) will help, as well as keeping the surface roughness between 10-70 microinches.

 

[SteveyB]

Thanks for that, great info. I guess as I can't change anything else but lubricate it with fett paste or similar lubricant. I guess my question is what is the likely hood of this galling considering I'm a factor of 8 greater than the dry galling threshold stress? Also what coatings/surface treatments could I use that don't decrease the corrosion resistance or increase the risk of hydrogen embrittlement and stress corrosion cracking. Also are there any coatings or treatments that dont increase the size of the component/thread.

 

[OGMetEngr]

Nitriding doesn't increase dimensions all that much. I would use gas nitriding and keep the "white" layer nonexistent or thin as possible to prevent a brittle top layer. It also puts the top layer in compression (if I remember correctly, might want to double check that) and would reduce or eliminate any chloride SCC if used at elevated temperatures. I don't know your environment so I'll leave it at that.

Even if nitriding, it would be a good idea to use a PTFE/MoS2-impregnated coating as well.

 

[EdStainless]

Just make sure that your lube has no metals or halides in it.
I just got done with one of those failure analysis.

You really need to try to roll the male thread. It will be smoother and harder, both will help you greatly.

Why such a high torque?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[SJones]

Keep away from the MoS2 as well as it can break down to give H2S. The hardness will be a trade off as these bolts are going subsea with cathodic protection.

If the purpose is solely joining, and the joint does not require a break out capability, is galling an issue anyway?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[SteveyB]

Thanks again for your replies. The actual component is a custom made locking nut as well as the body it screws into. So hence I would find it difficult to roll the threads as it isn't a bolt. The torque required is high due to the required preload to stop two sealing surfaces from separating under pressure. Ideally I would of made the thread more coarse and longer but due to space restrictions this was not feasible. The actual lubricating grease I was going to use is Exmar ASW grease (fett paste) that has a pressure load of 230MPa and a pressure resistance of approx 4000N.

 

[CoryPad]

There are rolling methods for parts other than bolts. If you can chuck the part in a lathe to turn threads, chances are good that a rolling attachment can be added to the lathe to roll the threads instead.

 

[TVP]

Options for improved internal threads:

1. Use forming tap instead of turning, etc. I would investigate Cut and Form taps from Emuge:

http://www.emuge.com/technical/pdf/Threading/zp10029_gb_cutundform.pdf

Using a forming tap in super duplex to create 100% of the thread form is not optimal (high forces, low tool life, etc.), which is why Cut and Form may be a good option. Emuge has a standard 5-flute forming tap called Robust 2X-VA that is available in M38x1.5 and M40x2, so you might be able to get a custom M40x1.5 tap made.

2. Use thread milling instead of turning, etc. Thread milling produces far better surface quality, especially in difficult to machine materials like SDSS. Emuge has a number of options in this category as well, maybe their GIGANT-IC product. It will be custom, so you would need to contact them for application assistance:

http://www.emuge.com/technical/pdf/2013-EMUGE-LEADING-SOLUTIONS-IN-THREAD-MILLING-TECHNOLOGY.PDF