Structural steel?

Discussion in 'Metal Boat Building' started by Nick.K, May 8, 2011.

  1. Nick.K
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    Nick.K Senior Member

    I have an 11m steel sail hull which needs redecking. I have had conflicting advice from local professionals. Some say use only marine grade plate and others that structural steel plate is fine.
    Structural steel plate is available just minutes away from the yard, but I am told that 3mm marine grade plate will probably have to be imported especially.
    What are the issues / risks of using structural steel, is it acceptable or not and if so, what grade (european) should be used?
    Nick.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Nick

    I’m not sure what you, or rather you have been told about "marine grade".

    Steel, or rather commonly used steel on boats, buildings, bridges etc, are all basic plain carbon steel, generally referred to as mild steel.

    All mild steel rusts, so there is no "marine" aspect to it. I suspect, correct me if I am wrong, that what they really mean is approved, or Class approved plate. This means, that the plate has been made to conform to certain standards used in shipbuilding. (You can get coatings for steel, but not worth in this instance). These mild steels are generally made to BS EN 10 025.

    The type or approval grade is also where they could be telling you it is “structural steel”. But again, it shouldn't matter, but they may be refering to BS EN 10 113. Just that the designations are slightly different.

    The most basic is called either Grade A, or J235 or JR235 or S235 steel depending upon or stockist/mill supplier. This conforms to BS EN 10 025 (1993). It is this that is commonly used in shipbuilding.
     
  3. Dean Smith

    Dean Smith Previous Member

    yes there are different grades used
    say in icebreakers they will not use the same grades as in a (normal ) ship, the yield, tensile will be different
    The navigating officers on board a vessel in ice must be aware of the type of steel used in the construction of their ship. The shell expansion plan will be on board and it will show clearly the steel qualities used. If a ship has no low temperature steel, it is important to avoid impacts with hard ice when the air temperatures are very low.
    But in your case as AD Hoc has mentioned it matters not
    what does matter is the paint
    I would preblast the inside, then straight off 75 microns dft (dry film thickness) of Altex 2 pack zinc, then 200 microns of epoxy That,ll outlast your ship:)
    25 microns is one thou. Where many fall down is the lack of film thickness of the paint system
    When you get to it and want weld advice, plate prep advice give me a yodel
    oh I mean weld before the epoxy:) oR you can get preblasted with holding primer, in which case you then dont use the Zn
     
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  4. pdwiley
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    pdwiley Senior Member

    Agree. My hull including deck is 4mm mild steel plate pre-blasted & primed. I'm building undercover so rust while working isn't an issue. It is a lot less hassle to get the plate blasted & a holding prime coat on before building. You'll never get as good access to the plate as when it's nice flat sheets.

    I'm using Jotun 2 part epoxy primer over the weld-through primer and it sticks like **** to a blanket. I don't want to have to do it twice - or replace the deck due to rust sometime in the future.

    PDW
     
  5. welder/fitter
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    welder/fitter Senior Member

    Marine grade mild steels(AH36, DH36, & EH36) have higher strength than A36.

    Tensile: 70-90 KSI vs. 58-80 KSI
    Yield: 51 KSI minimum vs. 36 KSI minimum

    (ASTM A 131M)

    While I'm getting myself into possible controversy, I'd also personally prefer AH40/DH40/ EH40 over AH36/DH36/EH36. But, obviously, one has to balance costs with "best choice".:p
    Mike
     
  6. welder/fitter
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    welder/fitter Senior Member

    As well, The marine grades have small percentages of nickel & chromium, higher percentage of copper & lower carbon content.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Mike

    The above, A and AH et al are all "technically" marine garde, only in so much that if they come with a Class approval cert. Just as the other grades are too.

    The AH..is now the Higher Tensile steels. Anything above a yeild stress of 250MPa is now consdired "higher tensile", hence the AH designation, rather than A or J or S. For the even higher grades, the minimum charpy notch toughness is increased.
     
  8. welder/fitter
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    welder/fitter Senior Member

    I thought that the key difference was the nickel & chromium content, but, no? I agree that they're all structural steels, but I've never heard of the "A"s being considered as marine grade. Of course, I could be wrong.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Mike

    Attached, LR's Grade A and AH definitions. You can see the differnce in yeild strengths and chemical composition.

    Grade-steel A.jpg Grade-steel A compo.jpg Grade-steel AH.jpg Grade-steel AH compo.jpg

    Normal mild steel, ie Grade A or its variants is usually good enough. But of course it depends, like most things, what the structure is designed to whithstand, forces wise that is, and hence is this sufficient?

    Many select the higher tensile steels, like AH etc, because for a given load, the plate thickness is less because the yeild stress, or allowable design stress limit is higher, ie saving weight.
     
  10. welder/fitter
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    welder/fitter Senior Member

  11. Dean Smith

    Dean Smith Previous Member

    how fortunate they are in Holland where they have so many ship building sections in the bestest steel. in most place they would think a Dutch Flat was a place you lived in
     
  12. daiquiri
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    daiquiri Engineering and Design

    Great thread. Great answers. :)

    Hardly anything else can be added to the above. Perhaps this: whichever way you decide to go, take care you use a ductile (high elongation) steel. That's because in the inauspicious case of impact it will allow for some plastic deformation before a crack in the hull structure opens. See Ad hoc's tables where typically the required elongation is 15-20%.

    Cheers!
     
  13. tazmann
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    tazmann Senior Member

    Here in the States we pretty much use A36 on home builds, I think the equivilent in europe would be S235. On the decks I would not be to conserned and would just use the local hot rolled mild steel.
    Tom
     
  14. Nick.K
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    Nick.K Senior Member

    Many thanks for all the replies.

    Ad Hoc
    Yes, I meant class approved steel.

    "The most basic is called either Grade A, or J235 or JR235 or S235 steel depending upon or stockist/mill supplier. This conforms to BS EN 10 025 (1993). It is this that is commonly used in shipbuilding.[/QUOTE]"

    The J235, JR & S235 are grades that are sold as construction steel and grade A is the same steel that has been approved by Lloyds for ship use. Is this correct?
    Can you recommend a concise source of information on the web about what these grades mean?

    Dean
    I was planning to blast after completion of the welding and use Jota mastic direct on the blasted surface by airless spray. Will take up your offer of welding advice.
    The hull was built in Holland around 1985
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Basically yes. If you bought from a DNV stockist, for example, it would be designated "NV Grade A" steel, and so on. The only real difference being that the process to make the steel, the methods, traceability and QA of the whole mill has been witnessed and approved by Class, as part of their QA audit. Thus to differentiate this steel from any other, it is Grade A, with an LR or DNV stamp and cert.

    You can use S235 or other for shipbuilding, but, the caveat being the steel must be taken away to be independently tested to verify its properties.

    See my post above.

    Otherwise any stockist will provide the same:
    http://www.dentsteel.co.uk/specifications-introduction.aspx

    If you want welding advice, why not post here. Plenty of professional welders reading these posts, "Welder/Fitter" for one.
     
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