Stresses during repair welding: a) Weld seam defects (cold cracking), b) Thermal gouging, c) Mechanical preparation of the joint groove, d) Repair welding, e) Residual stresses in the repair welding area, f) Application of high-strength steels in the wind energy sector
Source: BAM
The sustainable and resource-efficient production of wind turbines requires the use of modern high-strength fine-grained structural steels. This applies to both foundation and erection structures, such as mobile or ship cranes. During the assembly of steel structures, unacceptable defects occasionally occur in the weld seam area. In most cases, the economical solution or remedy for these defects is localised thermal gouging of the affected areas and re-welding. Due to the generally very high shrinkage restraint of the joint groove in the overall structure, the residual stresses introduced by repair welding can lead to renewed crack formation and component failure, particularly in conjunction with increasing degradation of the microstructure and mechanical properties of these special high-strength steels.
However, manufacturers have little information on these issues and there is a lack of recommendations and guidelines for taking these safety-relevant aspects into account in adequate repair concepts. The aim of this study is to derive recommendations for stress and material-appropriate repair concepts that form a basis for standards and guidelines to avoid cold cracking, damage and expensive reworking, particularly in the case of high-strength steels. Part 1 of this study comprises systematic investigations into the influence of shrinkage restraint during repair welding of two high-strength steels S500MLO for offshore applications and S960QL for mobile crane structures.
The degree of shrinkage resistance of repair welds was determined by means of experimental and numerical stress analyses. In welding experiments with so-called slit specimens, the degree of shrinkage restraint and the addition of hydrogen via the welding arc were systematically varied in order to analyse the effects on the welding result, residual stresses and cold cracking. It was shown that a higher shrinkage inhibition leads to significantly greater stresses. In the case of the hydrogen addition, S500MLO showed no cold cracking regardless of the restraining conditions. However, it was found that S960QL is considerably more susceptible to cold cracking when hydrogen is introduced. The length and number of cold cracks increase significantly with increasing restraint. Part 2 of this study deals with microstructure and residual stresses due to gouging as well as stress optimisation through suitable heat control parameters during repair welding.
Stresses in repair welding of high‑strength steels—part 1: restraint and cold cracking risk
D. Schroepfer, J. Witte1, A. Kromm, T. Kannengiesser
Published in Welding in the World (2024), Volume 68, pages 685–697