Shaft for chloride rich enviroment

[linares]

Hi,

I am designing a shaft that needs to be submerged in water with something like 500 ppm of chlorides (KCL, NaCL, etc). PH is around 8.0. Temperature is ambient. We have used SST 416 and evidenced pitting (stress corrosion cracking?). We moved to 17-4 H1025 with great results but currently we need something with higher strength. I am thinking to try 440C but haven't found any good information on its performance under chloride environments. Do any of you have used 440C for this environment? Do you know of any good source that I can check?

Many thanks,

 

[metengr]

linares;
The concern with the martensitic stainless steels is that above 180 Ksi you are asking for stress corrosion cracking problems because of the strength level and exposure to aqueous environment.

If you have been using the precipitation hardening stainless steels with great success, what is the problem? Why do you need such high strength? Also, the 17-4 PH grade can be supplied as H925, but I would have very serious concerns of using such high strength in water, over 17-4 PH H1025.

 

[linares]

metengr,
We have been using 17-4 in the same environment but this application is more demanding strength-wise. We have also maxed out the diameter to match the largest ID from off the shelf components that go on the shaft. Ideally, we could choose a stronger material and stick with the current geometry.

Thanks again,

 

[EdStainless]

I would worry about a PH grade at higher strength in a chloride environment. The risk of SCC increases rapidly.
For high strength shafting there are two options that I have experience with.
1. K-500 in the cold draw and direct aged condition
2. 625 in the cold drawn condition

There are other options that might work such as either a superduplex (2507) or super austenitic (AL-6XN) that has been heavily cold worked.

I would avoid martensitic SS, PH SS, and any PH nickle alloy.

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Plymouth Tube

 

[linares]

I perhaps should add that the maximum stresses are around 120 ksi.

Due to shaft features (shoulders, keyways, etc) and I am not getting an infinite life (a requirement here).

Ed, I appreciate your advice. I checked with 2507 and wouldn't work. I haven't check k-500 and 625. Can you asses me in how difficult to find (or expensive) are these alloys?

Thanks,

 

[linares]

Also, Ed, would avoid the use of 17-4 PH H1025? The literature I found mentioned that it has good corrosion resistance to chloride environments.

Are there any mathematical models that could aid me in predicting what levels of stress are acceptable under certain chloride concentrations?

 

[EdStainless]

If the stress is 120k what shaft strength are you looking for? About 160ksi or so?
You need to find someone with a rotary forge to cold work the 2507 or AL-6XN, unless you know someone that cold draw it (doubtful).

I have seen a lot of pitted 17-4, not high on my list.

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Plymouth Tube

 

[TVP]

linares,

If you are confident about the Cl- concentration (500 ppm) and the stress level (120 ksi, 830 MPa), then a titanium alloy may be suitable for this application. Ti-3Al-8V-6Cr-4Zr-4Mo (UNS R58640) has higher strength than the Ti-6Al-4V (UNS R56400) alloy:

http://www.parrinst.com/doc_library\dealer\Titanium.pdf

http://www.npl.co.uk/upload/pdf/stress.pdf

http://www.allvac.com/allvac/pages/PDF/tech/TI-50938-644.pdf

http://rtiintl.s3.amazonaws.com/RTI-Reports/tiguideWeb.pdf

 

[metengr]

In addition to corrosion and strength as mentioned above, stiffness of the shaft needs to be carefully evaluated for Ti alloys.

 

[linares]

TVP,

Thank you for your recommendations. That NPL guide on SCC is very nice. Loved (and hated) this part:

"Finally, of course, Murphy’s Law dictates that the materials
that are resistant to SCC will almost inevitably be the most
expensive (and that they will be found to be susceptible to
SCC in your environment as soon as you have used them!)."

It is unfortunate that it seems that trial an error will be the only way forward as there are too many variables to control. We have no control over the environment.

From what I've heard from other industry experts is that 416 sst is very susceptible to SCC. On the other hand I have received very conflicting information on 17-4PH. I read some data sheet. (

http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-4_PH_Data_Bulletin.pdf)

that 17-4PH H1025 exhibits really good chloride resistance and SCC resistance at ambient temperature. This is also what we have witnessed. The shaft is particularly special since it has alternating stresses. I guess it is important to mention that the form of chloride that we are witnessing is in a salt form, not acid (KCl or NaCl, not HCl).

I have no doubts that titanium alloys will do. I am sure Ed's recommendations will do. I will proceed to obtain quotes on these options but I am afraid they could prohibitively expensive (but I've been surprised before). But unless I am surprised I would go with what I know is best and find ways to keep stresses under control (as metengr suggested).

Many thanks all of you for your valuable advice,

 

[EdStainless]

I have seen 17-4 listed as having good CSCC resistance when heattreated above 1024F, but never any data. It is a martensitic structure so I am cautious.

Never use 416 for highly stressed parts. It is a free machining grade and has poor fatigue resistance.
410 is very resistant to CSCC, but has poor pitting resistance.

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Plymouth Tube