304 Stainless bowsprit/boomkin. Need metalurgical advice.

Discussion in 'Materials' started by jmwoodring, Jun 27, 2013.

  1. jmwoodring
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    jmwoodring Junior Member

    Apologies for the large image sizes.

    We own a Westsail 32 cutter. We have our rig off and are in the process of polishing and inspecting all of the breakable metal parts. Most of it is in good shape. We will be replacing a few of the smaller components.



    Unfortunately, when we removed and polished our bowsprit and boomkin (both constructed of stainless steel pipe), we found many hairline cracks on both. The cracks were often clustered around welded joints or high stress areas. The cracks are very thin and have been laughed off by some of our marina-mates, but our boat is an offshore boat, and the thought of a failure haunts us. The pieces are original to the boat and are 30+ years old. We are unsure of the exact alloy, but have been told they're likely constructed of 304SS.

    [​IMG]


    Here is an example of one of several cracks:

    [​IMG]

    We've had several sources tell us that trying to repair cracks in SS by welding propagates more cracks and that it's a fool's errand. Another metalworker suggested welding across the cracks in order to stop them traveling further.

    We're trying to decide whether to pay for welding repairs or to replace the peices outright. We aren't welders ourselves and we've got a lot of mixed advice from many folks who are. We are willing to replace the pieces if necessary, but, of course, don't want to waste money if they're salvageable.

    I call on the boat design gods for a sign. I will hereby make the appropriate
    sacrifice of dropping various quality tools into the water.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Hi JMW,

    Firstly when you get cracking, you really need to establish the reason for the cracks. Is it a simple manufacturer error, is it a design error, is it an overload freak event etc etc.?? So really, if you are concerned, it would be worth paying a qualified NA to go have a look.

    If the alloy is 304, then that is half the problem, probably. It should be 316L. This helps to prevent what is called carbide precipitation, commonly termed 'weld decay'. It occurs in and around the HAZ of the weld from extremely localised corrosion in the alloy.

    Selecting a SS such as 316L, the L suffix is to signify a low carbon content and welding is best done with electric arc, a submerged arc or inert gas, not basic gas welding.
     
  3. petereng
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    petereng Senior Member

    Hi JM - I'm an engineer on the Gold Coast in Oz. Firstly you need to clean your SS with a good SS cleaner. Get the rust off it and then have a good look at the "cracks" . Then find a local marine rigger/welder and ask them what they feel about it. 1) Its fine to weld over welds. 304 or 316 is readily weldable and is usually welded with a TIG process. Your only problem will be the parts are on the boat and ideally they may have to be removed to do a really good job. Another porcess that works quite well in this case is silver solder. Its easy to do, just get the right grade of solder and it will fix the cracks nicely. But I think an experienced marine type welder will answer your questions easily. The "rust" indictates a source of carbon is nearby eg from the grinding wheel or pencil or cutting tools that repaired/fabricated the tube last time. There are several nitric acid type products that clean and passivate the SS surface so rusting does not occur into the future. cheers Peter S
     
  4. jmwoodring
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    jmwoodring Junior Member

    Thank you both for your input. Seeing as we have been having so much trouble coming to a consensus with the local professionals about a repair, we are looking at having a new set fabricated. The set we currently have has lasted almost 30 years, so the material seems to have proven itself to be adequate, but since 304 seems to be priced at about 2/3 the cost of 316, we should be able to get superior quality material for not much more.

    Unfortunately, there aren't a great many marine architects in our area. Thankfully, the internet has enabled us to inform ourselves and put us in touch with knowledgeable people such as yourselves. Thanks again. I may post an update as we continue to tackle this in the future.
     
  5. jonr
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    jonr Senior Member

    When you are getting tossed around in nasty weather, I think you will appreciate a rail that won't break and dump you overboard. Ask for it to be passivated or electropolished.

    I'd consider replacing just the cracked sections. If you did repair the cracks, I'd investigate drilling "crack-stop holes" and then welding over them.
     
  6. alan white
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    alan white Senior Member

    I wouldn't replace anything. As suggested, welding the cracks is the way to go using the advice of a qualified welder. Might even be doable without removal. Stainless can be pieced in or reinforced. welds can be made invisible by grinding and polishing. A new sprit would not be cheap.
     
  7. LP
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    LP Flying Boatman

    Another thought as long as you are investigating solutions would be to have the suspect areas dye-penetrant inspected. I'm not a SS expert so I don't know if the dye-penetrant is compatible with SS, but if it is, it will also reveal cracks the are not normally visible to the naked eye. As long as fixes are being done, might as well try to get it all with one stroke.
     
  8. jamieh3
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    jamieh3 New Member

    If the Stainless Rust or Tea-staining is superficial you can clean it off pretty easily with a Phosphoric based cleaner, something like this:
    http://www.anzor.com.au/chemical-products/cleaners-and-maintenance/grunt-emergel/product

    But yeah - it should have been made out of 316L originally not 304, and after welding; pickled and passivated properly. This not done would increase your chance of it rusting. But if the cracks are more than cosmetic - getting new ones made form 316L sounds like a good plan. The smoother the finish of the new ones the better its corrosion resistance, here's a good guide:
    http://www.anzor.com.au/blog/the-dos-and-donts-of-using-stainless-steel-fasteners/
     
  9. bpw
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    bpw Senior Member

    If you do replace, might be worth looking into aluminum. May be cheaper, but more important you could take a big chunk of weight off the ends of the boat.
     
  10. daiquiri
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    daiquiri Engineering and Design

    Hi jmwoodring,
    In addition to what has been said before, if you decide to go the route of repairing the crack, first you need to stop it's propagation. So first you have to identify the exact position of the tip of the crack (where it currently ends), and then drill a hole of a generous diameter right at the cusp of the crack. You can even do it a millimeter or so beyond the end of the crack. The important thing is to make sure that no parts of the crack extend beyond the hole. After that, you can proceed with crack repair with any of standard techniques.
    Cheers
     
  11. jonr
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    jonr Senior Member

    If you don't replace any of it, be sure you have a good answer to "why won't it just crack again?".
     
  12. Titirangi

    Titirangi Previous Member

    The 304 s/steel as Ad Hoc pointed out is an issue, it suffers surface discolouring/surface rust because it lacks chrome/nickel to create passivation layer in salt environment and isn't suitable for sea vessels.

    That being said it still shouldn't fracture unless its too thin walled and or is a very poor quality combined with the forestay loads are too much for the material.

    Only sensible fix under circumstances to ensure safety in a blow is replace the unit. Welding repairs is absolutely not an option, you cannot repair by welding a failing metallurgy.
     
  13. jonr
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    jonr Senior Member

    > That being said it still shouldn't fracture unless ....

    Or the boat hit something and this isn't likely to occur again.
     
  14. Titirangi

    Titirangi Previous Member

    That's the rub, 304 isn't suitable sea enviro or for structural loads i.e. forestay, should never have been used to begin with.

    304 is a low grade cheap to produce s/steel, we don't use it for deck stanchions or for building products exposed to weather/loads.
     

  15. MikeJohns
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    MikeJohns Senior Member


    :):):)


    A magnet will stick to 302 and usually weakly to 304 pipe but not 316. Commonly your pulpit would be constructed of 316. Rust stains around the welds are because either the incorrect filler was used and or more likely it wasn't well pickled to remove the surface iron precipitates.

    It also looks like the tube is a bit undersize for the design and the crack has started at the weld run termination which means you can simply reweld it but you need to add a better stress path eg a bracket. But hard to say without a more complete picture. I think the structure needs a slightly better design in the the attachment area.



    I'd use 316 not 316L. Best practice for heavy assemblies tends to migrate unnecessarily into what we'd term 'light fabrication' such as the application you have here.

    The correct application of Low carbon varieties is in heavily welded assemblies or assemblies slowly cooled after annealing rather than being quenched. The low carbon reduces the chance of chromium carbide precipitation on grain boundaries in the heat affected zones. This depletes the available local chromium content ( the protective element).
    It’s not always an issue, even on thick and slow cooled profiles the precipitation may not form as it is dependent on a critical cooling rate. Often on large assemblies the chromium simply re-migrates back into the depleted zones while the material is still hot or afterwards when it’s annealed.
    Thinner sections cool quickly enough to avoid grain boundary carbide precipitation conditions.

    316 is also significantly stronger than the low carbon variant 316L although there's not much between them for the fatigue design limits. But for a pullpit I'd use the stronger material since buckling is the usual collapse mechanism.


    Has that confused your enough ?
     
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