Component-like cold cracking test for thick plates under consideration of the minimum waiting time (MWT) before NDT: a) influencing factors on hydrogen assisted cracking, b) geometric dimensions of specimen, weight approx. 350 kg, c) cross-section of 50 mm multi-layer submerged arc welded joint without detectable cracking.
Source: BAM
Offshore wind turbines (OWTs) are a key element of the energy transition for generating green electricity. OWTs are becoming ever larger and heavier. Accordingly, suitable offshore foundation concepts such as monopiles or jackets must be manufactured as anchoring structures. The latest developments in monopiles are moving in the direction of 100 m in length with diameters of up to 8 m. Sheet thicknesses of up to 200 mm must be joined to maintain buckling stiffness, usually using highly efficient submerged arc welding (SAW). One associated risk is the so-called delayed hydrogen-assisted cracking (HAC). Hydrogen introduced into the weld seam can cause a crack to form even days later. To rule this out, current regulations stipulate that non-destructive testing (NDT) of the weld seam may only be carried out after 24 hours, the so-called minimum waiting time (MWT). The reason for this is the slow diffusion of hydrogen into susceptible areas of the weld seam, such as the heat-affected zone (HAZ), with the simultaneous effect of high (welding) residual stresses.
Due to the large sheet thicknesses, only multi-layer welding is used, which creates a complex microstructure and residual stress distribution in the weld seam due to the repeated tempering effects. The large dimensions and tonnages of the monopiles also make integral testing of a component difficult. Empirical experience to date indicates that the MWZ concept should be critically discussed. For this reason, a prototype demonstrator of a real component was developed, which, due to its special geometry, enables high stiffness/mechanical stress, but with dimensions and weights that are manageable at workshop level. For this purpose, a 1,000 mm long submerged arc weld seam was created on 50 mm thick sheets of offshore grade S420ML. In order to achieve the most critical HAC conditions possible, a second demonstrator was welded with a high hydrogen input. Both demonstrators were then monitored by phased-array ultrasonic testing (PAUT) for 48 hours to rule out the possibility of HAC within the MWT. The residual stresses were then determined using a robotic XRD goniometer.
This resulted in residual tensile stresses up to the yield strength of the material both in the weld metal and in the HAZ. Numerical modeling enabled the qualitative estimation of hydrogen diffusion in the weld seam. The “interesting” events indicated by PAUT were further investigated using metallographic cross-sections and revealed only internal imperfections in the weld seam (mainly pores or isolated micro hot cracks). Despite the selection of the most critical parameters possible (high mechanical stress, high hardness in the HAZ and high hydrogen concentration), no HAC was found to occur. This indicates the high crack resistance of the investigated material. Summarized, the demonstrator concept presented has great potential for cold crack testing of welded joints at component level. The results also show that the MWZ concept should be critically discussed with the materials available today.
Component test for the assessment of delayed hydrogen-assisted cracking in thick-walled SAW joints for offshore applications.
Michael Rhode, Arne Kromm, Tobias Mente, Daniel Brackrock, Denis Czeskleba & Thomas Kannengiesser
Welding in the World 68, 621–635, 2024