I agree with Mr Pieper's on this to consult the plate mill for manufacturer recommendations (for e.g Dillinger has product range called Dillimax)
Preheating leads to a delay in the cooling of the component after welding, which means that the hydrogen has enough time to diffuse out. This phenomenon mainly takes place in the temperature range between 100 °C and 300 °C. Preheating not only refers to the heating of the weld area at the beginning of the welding process, it also means adherence to a certain minimum temperature throughout the whole welding process (interpass temperature), and even recommended for thermal cutting. The preheated area should extend to at least 100 mm on either side of the seam.
For plate thickness above 30 mm, as well as welding techniques which introduce a large amount of hydrogen into the work piece (e.g. submerged-arc welding), it is advisable to carry out low hydrogen annealing at 200 °C immediately after welding. The annealing time depends on the thickness of the component and should not be shorter than two hours. The risk of cracking occurring in welded joints as a result of residual stresses is particularly great for an only partially filled seam cross-section. Therefore, cooling down below the prescribed interpass temperature must be avoided during the entire welding process. To keep residual stress as low as possible, harsh cross sectional transitions and concentration of welds must be avoided. Make sure as well that the components to be welded form a good fit and that the welds are free from notches as far as possible. An advantageous weld sequence can also reduce the level of residual stresses. Basically, the weld sequence should be selected to ensure that the individual components can shrink freely for as long as possible.
Recommended preheating temperature for Dillimax 690 (equivalent to S690QL) are given below, source: DILLINGER HÜTTE GTS
Stress Relieving
These steels and their appropriately welded joints have a degree of toughness sufficient for them to be used in highly loaded components, in general without necessity of a stress relieving treatment. If stress relieving is necessary due to regulations or for constructive reasons, the plate manufacturer should be consulted. In general, the highest stress relieving temperature should be 40 °C below the tempering temperature of the quenching process. If such a heat treatment is to be carried out by the fabricator, it must be specified when ordering that the respective tempering temperature at plate mill has to appear on the works certificate. The holding time when annealing should not exceed 60 minutes. If longer holding times are prescribed, the stress relieving temperature is to be further reduced in relation to the tempering temperature. In case of high level of residual stresses or for very thick plates, care must be taken to avoid pronounced differences in temperature in the component while heating up to annealing temperature. In case the stress relieving temperature is laid down, i.e. the customer cannot take into account the tempering temperature during plate manufacturing, plate mill should be consulted as early as the inquiry stage. Due to their chemical composition and heat treatment, these steels have a relatively high strength at elevated temperatures. Their stress relief during stress relieving is thus less complete than for ordinary structural steels.
Note: another manufacturer Voestalpine recommends temperatures≤ 560 °C, if necessary. You may also find some info at another manufacturer website Rautaruukki,
Dillimax 690 (~S690QL) recommended H.I during welding is given below
t8/5 : Is the time in which the temperature of a bead drops from 800 °C to 500 °C is generally used to describe the heat cycles occurring during welding. This cooling time is primarily determined by the heat input per unit length of weld, the preheating or interpass temperature and, particularly for thin plates, by the plate thickness and the weld seam configuration. The t8/5 time has to be calculated according to the standard EN 1011-2.
-----Original Message-----
From:
Sent: Saturday, April 04, 2009 8:46 PM
To: Materials & Welding
Subject: [MW:1873] Re: Welding Thick S500QL & S690QL plates for Hydro Power
Dear Erhan,
I agree fully with Sanjay that it is not advisable to execute PWHT on
QT materials in order to avoid changes in materials microstrucre by
which it's loosen the original mechanical properties.
Most manufactures have datasheet available which give recommandations
and/or guidelines for welding such materials, I would advise to take a
look at the website of your supplier to see if you can find this
information, if not than take a look at the site of SSAB Sweden
(S690QL is similar to Weldox 700) and see what they advise for welding
these materials.
Always use heat imput as low as possible for same reason as why
omitting PWHT, especially in case where fatigue also can become a
problem.
Best Regards,
Herman Pieper
On 3 apr, 16:17, Erhan Turgut <ertur...@gmail.com> wrote:
> Hi,
> We're working on 2700 mm dia. pipes with a thickness range of 26 - 40 mm
> for S500QL and 30 - 48 mm for S690QL.
> These penstocks will be used in hydro power plant with a pressure of
> aprx. 140 bar
> We have two methods for welding: SAW on GTAW (root GTAW, fill & cap SAW)
> and SMAW on GTAW. We will try to achieve max. 12 kJ/cm heat input.
> So shall we apply PWHT (Stress relief)? If yes what is the thickness
> limits for PWHT.
> We planned to apply Soaking (Hydrogen Releasing) with a temperature of
> aprx 250 oC for 2.5 - 3 hours. So if we apply Stress relief
> heat-treatment with a temp. of 550 oC for 1 hour do we have to apply
> soaking anyway?
>
> Thanks in advance
> Regards,
>
> Erhan TURGUT
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