Duplex steel for deck fittings?

[DoubleWhisky]

I intend to replace the bow fitting on my boat, changing somewhat its design at the same time.
I wanted it to be made of 316L stainless steel (material used for old one), but it was suggested to me to consider the use of duplex (or rather super duplex) steel, like S32750 or S32760.

Can I ask for opinions (costs aside) regarding use of duplex steel for deck fittings?

Best regards

Tomasz

[Pauls]

Hi Tomasz,

Duplex stainless steel comes in several alloys. The standard duplex stainlesses are significantly stronger than 316 and they are significantly more corrosion resistant. The super duplexes are even stronger and more corrosion resistant.

The only downside to the duplex stainlesses is cost. Their physical properties are far superior to 316.

Regards, Paul

[DoubleWhisky]

Originally Posted by Pauls

Hi Tomasz,

Duplex stainless steel comes in several alloys. The standard duplex stainlesses are significantly stronger than 316 and they are significantly more corrosion resistant. The super duplexes are even stronger and more corrosion resistant.

The only downside to the duplex stainlesses is cost. Their physical properties are far superior to 316.

Regards, Paul

Thank You
My impressions from Internet search were similar.
But still I'm looking for possible disadvantages (except cost) of using duplex steel.

[Pauls]

Duplex machinability, as I recall, is similar to 316. It is weldable with common processes. I am unaware of any downside except for cost.

I researched using duplex on the boat I am building, researched the alloys and their properties (I'm an engineer and this is familiar territory) and priced various items. Many metal suppliers will not know what you are talking about when you ask for the duplexes. But they are available, you'll need to do a net search.

To the best of my understanding the duplexes are superior in all respects to 316 with the exception of cost and fewer suppliers. Far greater strength, significantly better corrosion resistance, good machineability and weldability.

What are you planning to build with them?

[DoubleWhisky]

Originally Posted by Pauls

What are you planning to build with them?

Complete new pulpit - bow fitting assembly - double bow roller included.

Thank You for help

[Pauls]

Your're very welcome! Good luck with your project.

[Saltyhog]

I've been considering having new chain plates made from duplex 2205. It seems to me that chain plates are a perfect application of a stronger and more corrosion resistant material. I've even considered a super-austenitic SS, AL-6XN. I've not been successful at finding either a source of rectangular bar stock or a fabricator who could make them. If you know of a fabricator, on the east coast, I would appreciate that information. All fabricators I've contacted tell me they know how to work with 316(L) and that's what I should use. I'm thinking the better steels would be worth it in this application.

[Pauls]

I agree. I don't have a high opinion of 300 series stainless chainplates. We all know they have a limited life expectancy. A couple suppliers I have quoted for 2205 are Rolled Alloys and MegaMex.

[Jim Cate]

FWIW, our 2205 chainplates are now over 23 years old. I recently pulled them for inspection and there were no signs of corrosion whatsoever. I believe that 2205 is not terribly expensive these days, but have no specific data to pass on.

One possible disadvantage in some applications is that 2205 is highly magnetic. Not likely a problem in deck fittings, but could be elsewhere.

I would personally use 2205 again with no worries.

Cheers,

Jim

[Pauls]

Thanks Jim. I appreciate the feedback.

Paul

[Saltyhog]

Thanks Paul, I'll look into those suppliers. I also found 2205 at PennStainless, although I have not checked for available sizes in flat bar stock. Is there anything special about cold working 2205? I have bends in my chain plates. I imagine using 316 hardware is ok. I've never seen threaded hardware in 2205.

[Emerald Sea]

Effectively 22% Cr DSS and 25% Cr SDSS have much higher strength than 316L SS - 450 MPa and 550MPa respectively versus typically 248 MPa for SS. DSS and SDSS also have excellent resistance to pitting and erosion corrosion. They are typically used in offshore pipeline applications where there are high temp corrosive (sour service) fluids as an internal lining under a carbon steel pipe. Welding is extremely difficult over 316 and this may drive your decision if you see this in your plans – you will find that your typical boat yard or local fabricator s/s welder is unlikely to have the skills. Also, don’t let surface corrosion overly concern you as all stainless’ will exert some discolor and give you worry. Having said that, it's prudent to do a thorough regular inspection, perhaps with a magnifying glass, of your s/s pieces and inspect hidden areas for pitting and any erosion. I wouldn’t see any concern in using duplex or super duplex for applications as described above but IMHO it seems overkill over 316L s/s.

Ive attached a paper that provides some insights.

Steve
Emerald Sea

[HappyMdRSailor]

I can only pass on that I have read and heard nothing but fantastic reviews such as Jim Cate's experience with the 2200 series alloy....

It would be a godsend if builders started incorporating this material in critical component areas... A wishful side effect also being bringing down the cost and availability....

[DoubleWhisky]

Originally Posted by Emerald Sea

Effectively 22% Cr DSS and 25% Cr SDSS have much higher strength than 316L SS - 450 MPa and 550MPa respectively versus typically 248 MPa for SS. DSS and SDSS also have excellent resistance to pitting and erosion corrosion. They are typically used in offshore pipeline applications where there are high temp corrosive (sour service) fluids as an internal lining under a carbon steel pipe. Welding is extremely difficult over 316 and this may drive your decision if you see this in your plans – you will find that your typical boat yard or local fabricator s/s welder is unlikely to have the skills. Also, don’t let surface corrosion overly concern you as all stainless’ will exert some discolor and give you worry. Having said that, it's prudent to do a thorough regular inspection, perhaps with a magnifying glass, of your s/s pieces and inspect hidden areas for pitting and any erosion. I wouldn’t see any concern in using duplex or super duplex for applications as described above but IMHO it seems overkill over 316L s/s.

Ive attached a paper that provides some insights.

Steve
Emerald Sea

The use of the duplex (or superduplex) was suggested by engineer I asked to design new bow fitting. The reason was not corrosion resistance but strength. There is very limited room and I want to have elongated double bow roller (to avoid the anchor bumping the bow upon retrieval).
Thank You for info and for remarks regarding possible welding and fabrication problems!

[Dale Hedtke]

I have worked some with duplex stainless steels and the comments about them being suitable seem correct. You need to find a commercial fabricator that has experience with these materials. (Certified welders.)
I suggest that you look for a fabricator that services either the pulp & paper industry or the oil & gas industry, as they are major consumers of these alloys. Look for a "boiler shop" or a shop that builds ASME pressure vessels.
You might even check with the local unions (pipefitters, iron workers) as they may be able to direct you to a member who has the skills.
I had never considered using a duplex SST, but it's a great idea. I do like 2205.