Macroscopic And Microstructural Changes in S355 Steel Welded Joints under Different Fire Scenarios

Abstract

Post-fire assessment of welded steel structures requires knowledge of whether fire exposure may cause permanent changes in the weld and its vicinity. This study examines macro- and microstructural changes in full-penetration butt welded joints made of S355J2+N steel after controlled thermal exposures representative of selected fire-related scenarios. The aim was to determine how maximum temperature, exposure time and cooling method affect the weld metal, heat affected zone (HAZ) and base material. Welded joints, prepared in 16 mm plates were heat-treated at 600°C and 800°C, held for 1 h or 10 h, and cooled freely in a muffle furnace or rapidly in water mist. Etched cross-sections were analysed by optical microscopy. Results showed that short subcritical exposure caused no significant macro- and microstructural changes, whereas prolonged heating below Ac₁ promoted cementite spheroidisation in pearlite and formation of a ferritic–spheroiditic structure. Exposure at 800°C, i.e. within the intercritical Ac₁–Ac₃ temperature range, led to extensive microstructural transformations, with the final microstructure governed mainly by cooling rate. Thus, at 800°C restructuring of the weld and HAZ becomes more likely, especially after rapid cooling. The originality of the study lies in comparing different fire scenarios for welded joints rather than base steel alone. The findings may support post-fire diagnostics of welded structures and indicate that mechanical testing is needed to relate structural changes to residual load-bearing capacity.