Hull made out of 2205 duplex steel

Discussion in 'Boat Design' started by rosiner, Sep 1, 2020.

  1. rosiner
    Joined: Sep 2020
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    rosiner New Member

    Hello and thanks for this very useful forum. I've now registered to shamelessly ask you about your thoughts on this subject.

    I have a chance to possibly buy a boat with a hull made out of 2205 duplex steel. However I'm very wary especially after the things that I've read about hulls built out of 316. I know that 2205 is better than 316 in many ways, and I've tried read the information that I've found in various scientific articles online... But I'm having trouble digesting all of it in practical terms. And more importantly, I have not found any other examples of hulls made out of 2205. Do you know of any, and if yes then how are they faring?

    The things that I'm worried about most are crevice corrosion and pitting. If 2205 gets attacked by these (for example if there is an area in the bilge with stale sea water), then can it corrode through quicker than mild steel? Are there any pitfalls for 2205 in boat construction?

    The hull is painted (also on the inside). Boat is 40ft and plate thickness is 3 mm, which seems a tad thin to me, but maybe compensated by the strength of 2205? It was built by a professional welder at a small yard, however they only built one of this kind of hull.

    All thoughts appreciated... and if you know an expert in this area then I'm also ready to pay for expert advise. Thanks!
  2. JSL
    Joined: Nov 2012
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    JSL Senior Member

    Not familiar with this alloy but there may be other items to qualify/disqualify this boat.
    First build ? (is builder experienced in boatbuilding???) internal structure (framing method,)
    Sail boat? power boat?
    3mm pl.does seem a bit thin. 10 ga (3.5mm) minimum) more common for this size boat) etc.
    Some photos might help.
    Last edited: Sep 1, 2020
  3. gonzo
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    gonzo Senior Member

    This is what the manufacturer says about the steel: "Although 2205 has good high temperature oxidation resistance, this grade, like other duplex stainless steels, suffers from embrittlement if held for even short times at temperatures above 300C." I am sure it was heated over 300C while being welded.
  4. rosiner
    Joined: Sep 2020
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    rosiner New Member

    There are of course many things to consider when buying a boat, but currently I'm specifically interested in a discussion about 2205. The boat is built by a small but experienced yard but as far as I know this was their first and only 2205 boat. The welder was licensed by the supplier of the metal (Outokumpu/Sandvik as far as I understand), so I'd expect the welds to be OK. The original owner specifically wanted a stainless steel boat, and the yard advised to build out of 2205 instead of 316.
  5. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    SSY has only 1,6 mm in the hull, although not the same stainless alloy, so 3 mm may well be sufficient.

    What does empirical evidence suggest?: How many hours on the hull och how many years since manufacture? What kind of operation and where?

    Theory is one thing, a proven design can be another. Maybe this boat is a bumblebee.
  6. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    There are numerous variants of duplex steels used in the marine environment. You will find them in the offshore industry, where the high resistance to chloride stress corrosion and pitting corrosion, combined with high sigma0,2 strength is highly appreciated. Welding of duplex steels is different from welding of general steels and of 300-series ss steels, but not ovely complicated. Just as with all welding, you have to know what you are doing, what filler material to use and how to control heat aso. There is a time factor for the HAZ; the weld must be done with low heat input, making some welding methods inappropriate, and generally speaking, a slight "overmatching" of Ni in the filler makes the weld usable "as is".

    Provided the standard procedure for welding is followed, the corrosion aspects in your case should not be of any concern. With the high strength of the duplex steel, even in welded state, the skin thickness may be reduced BUT to keep it from flexing the load-carrying structure must be comparatively "dense".
    fastsailing likes this.

  7. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    "3mm pl.does seem a bit thin. 10 ga (3.5mm) minimum) more common for this size boat) etc."

    In the 1960s, working for the US Army Corps of Engineers (Yes, the Army has more boats than the Navy, being assigned to all the USA inland waterways), the USACE had a 119 ft long common tugboat, swinging an 8 ft diameter prop, pushing barges and steamships around for dredging operations. This tug had been built during WWII, and its hull was welded steel with 10 gauge plating. That also appeared (to my thinking) way too thin for this kind of service, far tougher conditions than a yacht or pleasure boat would experience, however, this tug had been in service for over 20 years, and was still functional, and didn't suffer from the thin plating.
    bajansailor likes this.
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