Reinforcement of old mild steel structures using fine-grained steel with austenitic solid wire and ferritic flux-cored wire

The preservation and strengthening of existing steel structures is gaining importance, as many bridges and buildings erected between 1880 and 1940 from old mild steels remain in service today. Unlike modern structural steels, these historical materials, produced by the Thomas, Bessemer, or Siemens-Martin processes, often exhibit pronounced chemical and metallurgical inhomogeneities, including centreline segregation and elevated phosphorus, sulphur, and nitrogen contents, which may impair their weldability. Reliable guidance for joining such steels by gas-shielded arc welding, particularly for full-penetration joints, is still lacking. This work examines the weldability of an early twentieth-century Siemens-Martin steel joined to the fine-grained structural steels S355J2 + N and S355ML. The combination with S355J2 + N was welded with the austenitic solid wire G 18 8 Mn by gas metal arc welding (GMAW), and the combination with S355ML with the basic flux-cored wire T42 6 1Ni B M21 1 H5 by flux-cored arc welding (FCAW). Both combinations were welded at three energy inputs per unit length (5, 10 and 15 kJ/cm) and characterised by non-destructive testing, metallography, hardness, tensile, and Charpy impact testing. With the basic flux-cored wire, sound full-penetration joints were obtained. With the austenitic solid wire, radiographic testing and macro-sections revealed inclusions and lack of fusion exceeding the acceptance limits as well as isolated pores, indicating that the slag-free filler metal is less tolerant of the impurities of the old steel. The heat-affected zone (HAZ) of the old steel underwent marked grain refinement (from approximately 29.80 to 8 µm), and only a slight hardness increase occurred, remaining well below a critical hardness. Although the Mn/S ratio in the segregation zone of the old steel was below the recommended minimum of 4, no hot cracks occurred, which is attributed to the manganese supplied by the filler metals. In tensile testing, failure consistently occurred in the unaffected base metal rather than in the weld, while the Charpy impact energy of the old-steel HAZ at − 20 °C increased to 1.5 to 4 times that of the base metal. FCAW with a basic-flux-cored wire represents a viable technique for repairing historical steel structures, provided that careful material assessment and adapted process control are ensured.