Best Material for Marine Exposure + High Strength 2

[KSor]

I am looking to find a stainless steel that is suitable to marine environments, and has a yield strength as high as possible. I am hoping to find something with a yield strength of ~170ksi, if possible. The material will be used in a structural part that will be stressed axially.

I have been researching 17-4PH, but stress corrosion cracking and hydrogen embrittlement seem to be possible problems. Is there anything available with similar properties as the 17-4, but without the associated corrosion issues? Heat treating 17-4 to H1025 seems to improve resistance to stress corrosion cracking, but I am not sure about the hydrogen embrittlement.

 

[israelkk]

Where the hydrogen come from?

 

[KSor]

I am not 100% familiar with hydrogen embrittlement but from my research, in a marine environment, the hydrogen comes from the reduction of hydrogen ions to atomic hydrogen in aqueous corrosion.

 

[EdStainless]

You have to assume that you have some active corrosion. I have some slow strain rate test data that was done in NaCl, slightly acidified (CO2 sat.) and slightly elevated temp.
We ran 17-4PH, 718, K-500, SCF23, Nitronic 50 and 625. All of them had UTS in the range of 160-240ksi. The first three alloy age harden, the last three were cold worked.
The 17-4PH and 718 showed loss of strength in the corrosive environment. The other alloys showed no loss of properties.
You just got $10,000 worth of test data.

If it were my selection I would look at K-500, 625 or SCF23 (or AL-6XN).

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Rust never sleeps
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[israelkk]

US Navy aircrafts used to have 17-4PH cast parts such as lever arms atc.

 

[unclesyd]

Can you give us a little more information on the exposure?

You are calling for a very high strength material that is going to require something more exotic than the normal PH alloys.

EdStainless[/],

If the environment isn't sea water immersion what do you think of Carpenter 465?

 

[SJones]

Assuming full immersion, the hydrogen will most likely come from cathodic protection. I can only think of 718 to get you 150 ksi yield but it will cost you and, most probably, will not like the hydrogen (not a lot will at those yield strengths). What will it be attached to and how will it be attached?

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[mcguire]

In a marine environment any alloy with PREN(Cr+3.3Mo+16N) less than that of 2205, about 35, will suffer corrosion by pitting. This generates hydrogen in the pit which will be absorbed and cause SCC if hardness exceeds Rc 22.
You cannot have the strength you want and have immunity to SCC and/or hydrogen embrittlment. No stainless steel with the desired strength, PH-type or martensitic, will have enough corrosion resistance to avoid the risk of SCC.
The best stainless would be the strongest which can not experience SCC and that would be 2205. 2205 will not experience pitting but may see some crevice corrosion. In that case 2507 is needed. You would then have to settle for yield strengths in the 70ksi range with these duplex alloys. Still pretty good compared to austenitics.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[CoryPad]

What product form? If you need 170 ksi yield strength AND marine corrosion resistance, you may need to look at multiphase alloys like MP35N or a stout titanium alloy.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strider6]

If is it possible can you tell us:

1) type of item or element you're talking about
2) marine environemnts means seawater immersion?

Strider

http://www.corrosionist.com

 

[EdStainless]

My test data was slow strain tensile. (10^-6, about 2 hours)
The solution was sat. NaCl (26%), 300F, CO2 sat (pH2-3).
For materials over 160ksi UTS the ones that showed no reduction in strength or elongation were:
K-500 (CW + age)
6%Mo Superaustenitics (CW)
625 (CW)

I presume that you could use 725 and CW+age, but you may not need it. We used 625 cold worked at 185ksi UTS.

Custom 465 may work well for this. It has pretty good corrosion resistance and the strength level would be easy.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[KSor]

Thanks for your replies guys, this site is the best.

The parts are going to be load cells and links. They will not be immersed in seawater (during normal use at least), but are used on offshore platforms and other such environments.

Custom 465 has the high strength and resistance to stress corrosion cracking, but I haven't been able to find any info on hydrogen embrittlement. Is that an issue with this material?

 

[unclesyd]

You will probably have to contact Carpenter for specific information.

If it helps any it is considered a drop-in replacement for plated 4330M and 4340M plated parts for airframe components.
It is being used more and more high strength fasteners with no mention of H2 problems as you normally see with high strength fasteners.

 

[EdStainless]

C465 should be good against hydrogen as long as you are not at the peak strength condition.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[mcguire]

Carpenter 465 can barely be considered stainless with only 11.5% Cr and 1% Mo. Nor is there any reason to consider it to be free from SCC at these strength levels.
17-4 has better corrosionn resistance, but not good enough for marine environment.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[unclesyd]

mcquire,
I'll have to disagree with your post concerning both materials, 17/4 and C465. Both are used in and around marine environments though at present 17/4 has a longer exposure time than C465. 17/4 has been used for boat shafts for years. C465 has been use both for boat shafts and fasteners for marine service. There is also a lot of marine hardware made from 17/4.
I've read in Carpenter literature that C465 has received certification for arresting gear components for Naval Aircraft, which should make it essentially corrosion free and free from SCC in marine environments.

 

[mcguire]

They can be used for these things, but that's not the same as saying they will not corrode. Nothing with juust 17% Cr and no moly or nitrogen in solution is capable of resisting at least pitting corrosion in a marine environment. The corrosion may be tolerable in the application, but that's not immunity. I don't want to sound pedantic, but pitting will occur and if pitting occurs, SCC can occur if hardness levels are high enough.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[Cormet]

Best option is Duplex stainless steel type 2205.
Be advised that if component is going to be used in pressure containing part or vessel, ASME code limit use of duplex to less than 600F.

 

[strider6]

I agree with McGuire, the corrosion resistance of a 17/4 is less than a 304 SS, and the 304SS is a material banned by most company in offshore service.
But are you sure that for your service, you said normally not in contact with sea water, you need a SS?

regards

Strider

http://www.corrosionist.com

 

[btrueblood]

Ed,

From your first post, what is SCF23, or where would one obtain it? Thanks.