pre heat

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    i have a 65 mm plate , to be joined to another 65mm plate , the prepartion is is singklr v. combined thickness is 130mm. I am fine with that . However a third plate is to be welded to the  butt welded plate to form a t section on the bottom of the root gap which has a thickness of 25 mm . In W.T.iA. in doesnt mention a  joining  caluclation for this 3rd member. How is this caluclated . Any feed back would be greatly appreciated. P.S. Loathed to contact you know who  about this question. Thanks Bulldog54


    if the two 65mm plates are being joined 1st the combined joint thickness would be 130mm and then when 25mm plate is to be attached the combined joint thickness would be 155mm, adding the 25mm. If all plates are to be joined/welded at once the combined joint thickness is 155mm, that is i’m assuming that all the plates are longer than 75mm. if they are shorter the equation changes.
    Preheat is required because without preheat the heat from welding is rapidly quenched from the welding zone through the  parent material thickness which can cause undesirable microstructures in the HAZ. so any material that is in sufficient contact to act as a “heat travel path” within 75mm of the joint centre when the weld is being laid can be added to the combined joint thickness.

    mmm yes forgot to add backing bars are not usually added to the C.J.T

    Slag in Sleeve

    As I read AS/NZS 1554.1, if your 25mm plate is acting as a backing bar I’d say t1 + t2 + t3 = 155mm. If its being added after welding your two 65mm plates together I’d say t1 + t2 = 90mm.

    What Control said 😉


    Hey Bulldog
    Keep in mind the preheat calc for AS1554.1 does not take into account restraint on the joint and the preheat calculation is not infallible
    Firstly if you do a hardness test on the PQR specimen you do not have to preheat the joint at all (not that I am recommending it)
    Secondly where do you think the highest hardness will be, from my experience it will be in the haz of the last run in the cap, this is due to the grain refining (normalising affect) of the subsequent runs
    Preheat can be expensive on large projects so you need to work out from hardness testing what you need and make sure you temper this thought with how much joint restraint there is and what the material type and welding process is

    All that said If it is grade 250/300, not highly restrained, I would be confident in saying do not allow for the backing bar in combined thickness and do a hardness and you will find it is well below 350Hv

    A note that hydrogen controlled consumables at this thickness is certainly recommended

    My opinion only but verify this by testing and give us some feedback


    Hi Lads,
    If you use  multiple strong backs on your weld procedure qualification to prevent angular distortion, then this surely would be considered restraint! and if verified the absence of cracks by MT and UT (although NDT not required by standard, but backs up your weld procedure under restraint), then you have carried out a weld procedure under restraint.
    Your thoughts please.


    whilst this could be considered restraint, it is only uniaxial (one direction)
    the forces involved here are nothing compared to those experienced in a rigid fabricated component, triaxial (three directions)
    in a rigid fabrication with multiple runs the weld has significant contraction forces, longitudinal, transverse and angular
    a free plate as experienced in a test plate is free to move in most if not all directions, the relatively small plate will heat and cool fairly evenly
    imagine a box beam made out 60mm plate being welded between two columns, that is not able to expand and contract evenly, what is going to give, generally nothing, but you will end up with a huge amount of residual stress, a precursor to HACC

    Preheat can reduce residual stress caused by contraction and allow more time for hydrogen to diffuse from the weld therefore reducing the risk of HACC

    my opinion only

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