Difference between AH36 and DH36 steel

Discussion in 'Materials' started by Davor75, Jan 24, 2013.

  1. Davor75
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    Davor75 Junior Member

    Does anyone know what's the difference between AH36 and DH36 steel plates? When comparing DH36 and AH 36 chemical composition I can't see any difference. Comparing mechanical properties, only difference is in minimal impact energy. What is the difference than?? I'm student writing paper on heavylift ships, need some usable information.

    Tnx.
     
  2. murdomack
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    murdomack New Member

    I think the difference is in the finishing and the hardness testing, but I'm not an expert, even if I've worked with these for years. It may be that you are confusing AH36 with ABS A or A36. Some info here,

    http://www.chapelsteel.com/abs-ah36-dh36-eh36.html
     
  3. Davor75
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    Davor75 Junior Member

    The only difference I've found is its toughness. Tnx anyway
     
  4. murdomack
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    murdomack New Member

    Chapel Steel stock AH36 in the control rolled condition, DH36 in both the control rolled and normalized condition and EH36 in the normalized condition.
     
  5. Davor75
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    Davor75 Junior Member

    I noticed that, I have to find someone who can translate it correctly into my language
     
  6. murdomack
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    murdomack New Member

    Normalizing is a heat treatment that stress relieves the steel.

    I'm not 100% certain but I think that only DH steel 1" and thicker gets normalized. All thicknesses of EH get normalized and none of AH. It's something along these lines, I've read it somewhere years ago.
     
  7. Davor75
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    Davor75 Junior Member

    Thank you
     
  8. french44
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    french44 Junior Member

    the difference is the temperature that the ship will be used, for normal ship you use a grade for ice breaker you use d or e grade . you can find class rules BV DNV ABS .....
     
  9. Davor75
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    Davor75 Junior Member

    This is in in some relation with impact toughness (not sure if I translate it well, I am talking about that toughness which is result of Charpy test)?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Davour

    It is a tad complex since it relates, as noted above to the thickness and temperature of operation. In addition, where the steel is located on the vessel. Such that is the location Class I or II or III, and is it primary or secondary structure, and is the location within 0.4L of amidships or outside 0.4L of amidships. And finally within these ‘zones’ is it side plate, sheerstrake and stringers, or exposed deck etc.

    Once you have identified all this, you look at a matrix of Classes versus exposed temperature versus thickness.

    For example a Class I with a average external air temperature of -15 to -19c with a thickness less 10mm, if using Higher Tensile (AH or DH), then AH is fine up to 10mm thick. But if the thickness is 25mm or greater than it must DH grade.

    If this is now Class III, the cross over from AH to DH occurs above 10mm. Thus where the steel is located and what class, is the driver. Which relates to the way the steel is manufactured, i.e. quality control.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Learned something new today. Thank you, Ad Hoc. :)
     
  12. murdomack
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    murdomack New Member

    Part of my job is sending plate sections out to repair and make modifications to an FPSO, a converted oil tanker. For all the hull plate and all the frames and stiffeners they always ask for EH36. We have some DH36, existing stock only and none on order, they seldom use it although it is allowed in certain areas of the deck. We have never stocked AH36, any plate not directly welded to the above mentioned parts will normally be ABS 'A', unless a higher strength is required, when they will revert back to their old faithfull, EH36.

    The OP asked what the difference was between AH and DH and like he himself said at the beginning, I only know of the lower temp. charpy testing and the normalizing of the thicker of the DH plates. There are other finishing processes that can be asked for but normalizing is always done where applicable.

    http://steel.keytometals.com/Articles/Art81.htm
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    As noted by several, the chemical composition of AH and DH is the same, indeed this is also true for EH grade too. It all relates to the thickness and mechanical properties at that thickness of the plate you wish to use. Since the thicker you go, “as-rolled” is different from that of say thermochemically treated which is of course again different from quenched or normalised.

    So guaranteeing properties of basic quenched steel in say 10mm thick, is not difficult. But when using say 50mm thick, very difficult to ensure a consistent grain size. Thus different treatments are used to ensure with an ever increasing thickness, so the mechanical properties can be guaranteed. This is supplement by the Charpy V-notch test. So at an ever increasing thickness the Charpy V-notch test has a different temperature and energy requirement, depending upon the AH, Dh etc grade. Thus, it relates to the quality control of the steel with an ever increasing thickness, which is dictated by the temp range and notch toughness range.

    LR and DNV requirements are shown below. As you can see, for an AH grade testing is done at 0c less than 50mm thick. But in DH it is done at -20c. The values change as you increase the thickness (as in different Class values and locations noted above) from less than 50mm up to 100mm.

    LR Steels.jpg DNV Steel.jpg
     

  14. murdomack
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    murdomack New Member

    I think I am begining to understand why we don't re-order DH36, given that we are three months from the steel supplier.
    AH is basically "controlled roll" finish and EH has a defined minimum amount of finishing and quality control, wheras DH can vary depending on the needs of the manufacturer and customer. The Engineer/Designer would need to study the MTR's and select a sheet from the stack.
    They purchased a lot of DH when they had a major life extension project going on many years back, so I imagine that it was to a special order. Now that it is smaller repairs and upgrades, buying only EH probably makes more sense, even if it is more expensive.
    When I go back I am going to study these DH mtr's to see what they say with regards to finishing.
     
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