Influence of Ti-, Nb- and Ti + Nb-based microalloying concepts on HAZ behaviour and local mechanical performance of welded S690QL steel

Abstract High-strength, quenched and tempered steels, such as S690QL, require specific microalloying concepts to achieve the necessary balance of strength, toughness and weldability. As the mechanical integrity of welded structures is significantly impacted by the behaviour of the heat-affected zone (HAZ) during welding, a thorough understanding of microstructural evolution is crucial. This study systematically investigates the effect of Ti or Nb microalloying as well as a combination of both on three-layer gas metal arc welded joints. Metallographic characterisation, hardness mapping and thermophysical simulations are employed to determine the interaction between precipitate stability, austenite grain growth and softened HAZ formation. Cross-weld tensile tests combined with digital image correlation (DIC) enable the in situ monitoring of local strain accumulation in critical microstructural zones. A mirror-assisted optical setup enables the simultaneous evaluation of strain on multiple specimen surfaces, providing comprehensive insight into constraint effects and the mechanisms controlling strain localisation and fracture initiation. Complementary impact toughness measurements further highlight the influence of microalloying and dilution on fracture behaviour in both the HAZ and the weld metal. The combined findings demonstrate that the microalloying strategy governs the extent of HAZ softening, the distribution of local strain and the failure mechanisms within the weldment. Specifically, the interaction between Ti- and Nb-containing precipitates plays a significant role in HAZ microstructure formation and local mechanical response enhancement. In summary, the results indicate that a combination of Ti and Nb microalloying is the most effective approach to enhancing the local mechanical performance and toughness, improving the integrity of the HAZ in S690QL steel welds.