Welding historical steels: a material name is not enough to establish weldability
Old mild steels may contain severe segregation and elevated P, S or N. New research supports material-specific assessment before repair or strengthening.
An old section may not be homogeneous
Steel bridges and buildings from the late nineteenth and early twentieth centuries may contain steels produced by the Thomas, Bessemer or Siemens–Martin processes. Compared with modern structural steels, they can show pronounced centreline segregation, alternating cleaner and contaminated zones, and elevated phosphorus, sulphur or nitrogen.
A single chemical analysis from an accessible surface may therefore fail to represent the complete section. Repair planning should combine material identification, chemical analysis at meaningful locations, metallography and mechanical testing appropriate to the significance of the structure.
What the new experiment showed
Researchers joined an early-twentieth-century Siemens–Martin steel to modern S355J2+N and S355ML steels. GMAW with austenitic solid wire G 18 8 Mn was compared with FCAW using basic flux-cored wire T 42 6 1Ni B M21 1 H5.
In the investigated system, the basic flux-cored wire produced full-penetration joints without relevant indications in the applied NDT. The austenitic solid-wire condition contained inclusions and lack of fusion exceeding the applied criteria. Nevertheless, tensile specimens fractured in the unaffected old base material.
This is an important warning: fracture outside the weld does not prove that the weld contains no unacceptable imperfections. Tensile testing, NDT, toughness testing and metallography answer different questions.
Boundary of applicability
The result is not a general authorization to use FCAW on every historical steel. It supports case-by-case assessment, a qualified repair procedure and inspection appropriate to the actual structure.
Source
- Uhlig, M. et al.: Reinforcement of old mild steel structures using fine-grained steel with austenitic solid wire and ferritic flux-cored wire, *Welding in the World*, 21 October 2026.
This is original technical commentary. It reproduces no photographs, micrographs, tables or protected wording from the publication.