Effects of multiple thermal cycles on the mechanical properties of steel components

Refabrication for reuse is an important scenario in the circular economy of steel structures. This scenario may involve multiple processes of welding, thermal cutting and welding again in the same location or vicinity. The material in the heat-affected zones (HAZ) may deteriorate due to multiple heating and cooling cycles. In this paper the experimental tests were conducted to investigate the effects of multiple thermal cycles due to welding and thermal cutting processes on the mechanical properties of S355MC and S500MC steel materials. Tensile, hardness and toughness properties were studied for thermo-physically heat treated specimens using the measured temperature histories. Both engineering and true stress-strain data were recorded and analysed. The results show that the reduction in ultimate strength due to multiple thermal cycles is 5% for MAG welding and 7% for SMAW welding more than single thermal cycle. Hardness of specimens treated by multiple thermal cycles is slightly less (1–3%) for MAG welding and 5–7% less for SMAW welding than the single thermal cycle. Toughness is improved significantly due to the rapid heating-cooling cycle(s) with 11% increase in the impact energy for S355MC specimens and 39–45% increase for S500MC specimens for single welding cycle. From single to multiple thermal cycle(s), the improvement in toughness is insignificant for S355MC steel and 5–8% for S500MC of MAG welding cases. Multiple thermal cycles tend to soften the steel by reducing the hardness and strength and improve the toughness. The results can be used for the design of refabricated steel structures for reuse. • Both peak temperature and cooling rate of welding affect mechanical properties of HAZ. • Multiple welding-cutting-welding cycles tend to reduce strength and improve toughness. • Upper yield strength is higher than ultimate strength at HAZ of 900C peak temperature. • Multiple thermal cycles cause more significant reduction in S500MC than S355MC steel. • Continuous yielding behaviour is observed for thermal cycle in coarse-grained HAZ.