The goal of this work has been to replicate the thermal history in a part-size sleeve-on-pipe mock-up for subsea pipeline repair applications, which is exposed to pre-heating and post-heating, and to examine the decay of hydrogen in and adjacent to a 10 cm test weld which closes the gap between the sleeve and the pipe. Temperature measurements and crack examinations are available. Two test cases were selected for modeling, w304.4 and w304.11. The former created a crack-free weld, while the latter did not. The main difference between the test cases is the post weld heat treatment (PWHT) temperature. Concentrations lower than 1-2 wt ppm is believed to minimize the risk for cracking due to hydrogen. Measured temperatures for both test cases were reproduced satisfactorily by the computer model. So did the cooling time Δt8/5 when the local temperature drops from 800 °C to 500 °C. A circular band in the heat affected zone (HAZ) that is dominated by martensite was predicted. Cracks typically start in the HAZ, which corresponds well to the brittle microstructure predicted by the model. Two approaches for modeling of hydrogen diffusion were tested. The first employed an apparent diffusion coefficient which accounted for the effect of trapping, while the second applied the lattice diffusion coefficient and calculated trapping directly. The latter methodology is considered to be the physical most correct way of modeling hydrogen diffusion. In the first approach, individual apparent hydrogen diffusion coefficients were used for the base metal and for the weld metal. Different diffusion coefficients were also used in ferrite and in martensite of the base metal. Based on the crack avoidance criterion of 1-2 ppm, the computed concentrations confirm the findings from the experiments, i.e. that w304.4 was crack-free, while w304.11 was not. In the second approach, which is based on calculation of trapping, the computations for w304.4 gave values for the total hydrogen concentration in and adjacent to the weld that satisfy the crack avoidance criterion. For the test case that did crack, w304.11, calculated values were generally larger. Values exceed 2 ppm both in the sleeve HAZ and in the pipe HAZ. The surface concentrations in the HAZ are of particular interest. In the sleeve they are within the crack avoidance criterion, but not in the pipe. The calculations suggest that w304.4 would avoid cracking, but not w304.11. With one exception, they therefore confirm the experiences from the experiments. Cracking of w304.11 was initiated in the sleeve HAZ, not the pipe HAZ as suggested by the calculations. Furthermore, it was initiated on the sleeve surface facing the Argon gap. The simulations shows that cracking due to hydrogen in the pipe could start both on the surface facing the Argon gap and at the free surface, most likely at the latter where the concentration is largest. All calculations performed in this study confirm that PWHT has significant effect on the reduction of the local hydrogen concentration in and adjacent to the weld.


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    Title :

    Simulation of hydrogen diffusion in a part-size sleeve-on-pipe mock-up for remote pipeline repair welding


    Contributors:


    Publication date :

    2013


    Size :

    8 Seiten, 11 Bilder, 2 Tabellen, 16 Quellen


    Type of media :

    Conference paper


    Type of material :

    Storage medium


    Language :

    English




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