Strength model for softened heat-affected zone of S700 butt-welded connections

The growing use of high strength steel in construction necessitates updates to the weld design rules, particularly for the heat-affected zone (HAZ), where welding induces softening and potential strength reduction. This study examines gas metal arc-welded joints of S700 high-strength low-alloy steel, focusing on the softened HAZ. Tensile tests show that small-scale specimens have ∼5 % lower strength than standard-sized counterparts, emphasizing the impact of specimen size on strength assessment. The thermally simulated wide HAZ soft zone has ∼10 % lower strength compared to the welded specimens with narrower soft zone, indicating the magnitude of the constraint effect. The lowest strength of the simulated HAZ was reached with ∼18 s t8/5-time and peak temperature of 912C°, leading to ∼17 % decrease in strength compared to the base material. The welds with a long (∼18 s) t8/5-time have approximately 8 % lower strength than the welds with a short (∼6 s) t8/5-time. Based on the experiments, the study introduces a novel semi-empirical equation with two damage parameters to predict the softened HAZ tensile strength. The effect of a triaxial stress state on the tensile strength was considered indirectly, by considering the relative width of the soft zone and the relative hardness values of the soft zone and the stronger adjacent zones, including weld metal. Using the developed equation, the relative width 0.52 of the soft zone provides the same strength as the base material.