Hydrogen diffusion in a 60 mm thick SAW joint: (left) experimental data of permeation experiments for different sample thicknesses in the weld metal, (right) cross section and numerically simulated hydrogen distribution
Source: BAM, division Weld Mechanics
Due to the need for the accelerated energy transition and future hydrogen-based economy, offshore wind turbines (OWT) are essential for generation of sustainable green electrical power. In that connection, the newly installed OWTs continuously increase in size and weight. This requires adequate heavy foundations concepts like monopiles. Due to the occurring high mechanical loads (wind, water waves, generator weight and so on), the components require steel plates with a thickness of typically up to 200 mm. The predominant joining technique for the component fabrication of OWTs is the submerged arc welding (SAW) process. In accordance with the current standards for steel constructions and welding manufacturing, the possible so-called delayed hydrogen-assisted cracking must be anticipated by either a minimum waiting time (MWT, before non-destructive testing of the welded joint is allowed) or a dehydrogenation heat treatment (DHT) or “soaking”. The DHT is typically also referred as hydrogen removal heat treatment (HRHT) in industrial practice and is carried at temperatures of 200 to 300°C for a certain dwell-time depending in the plate thickness. The effectiveness of both MWT and DHT/HRHT can be estimated by use of hydrogen diffusion coefficients for the respective temperature. These coefficients allow the calculation of the necessary waiting for MWT or dwell-time for HRHT. In the present article, the coefficients were reported, (from our knowledge) for the first time, for a S420G2+M offshore steel grade and its multi-layer SAW joint. The electrochemical permeation technique (in accordance with ISO 17081) was used for the determination of diffusion coefficients at room temperature. For that reason, the S420G2+M base material and the pure multi-layer weld metal samples were investigated. The coefficients were within a range of 10-5 to 10-4 mm2/s. Analytical calculations and accompanying numerical simulations showed that very long MWTs can occur, which would be necessary to significantly decrease the hydrogen concentration. Additional weld metal diffusion at elevated temperatures was determined by carrier gas hot extraction/thermal desorption analysis and the underlying diffusion coefficients were calculated. They were within a range of 10-2 mm2/s. The analytically calculated long HRHT dwell-times follow the same tendency of virtually very long time also at elevated temperatures. In that connection, the considered plate or welding seam thickness, i.e., diffusion path length, is a considerable factor for analytical and numerical calculations. Hence, any estimation of MWT/HRHT via diffusion coefficients should be critically discussed, especially if they are taken from literature without any further experimental verification.
Characterization of Hydrogen Diffusion in Offshore Steel S420G2+M Multi-layer Submerged Arc Welded Joint
Michael Rhode, Jonathan Nietzke, Tobias Mente, Tim Richter, Thomas Kannengiesser
published in Journal of Materials Engineering and Performance on 28 February 2022
BAM division Weld Mechanics