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Fine-grain, high-strength, low-alloy (HSLA) structural steels with yield strengths > 600 MPa are now the state of the art in construction applications such as mobile cranes and civil engineering. HSLA grades derive their strength from a combination of specific heat treatment and the underlying chemical composition. In this context, Ti or Nb are essential to obtain a fine-grained microstructure as well as the necessary carbides or nitrides for precipitation strengthening. In this context, the specific effect of Ti or Nb-rich compounds on hydrogen trapping and diffusion is well known for special laboratory cast alloys, but unknown for realistic steel compositions. For this reason, a series of S690Q-based alloys were synthesized, close to a real steel composition, but with well controlled Ti or Nb additions in different amounts. Specimens were obtained from these alloys by electrochemical discharge machining (EDM). The specimens were tested using the well-established electrochemical permeation technique. From the experimental results, the hydrogen diffusion coefficients and the analytical subsurface hydrogen concentration were calculated. In addition, the hydrogen trapping behavior at elevated temperatures was interpreted by thermal desorption analysis (TDA) using different heating rates of hydrogen charged samples. The results showed that in contrast to metallurgically "pure" laboratory cast alloys, realistic chemical compositions were similar in their hydrogen trapping behavior, despite some small differences. All investigated steel grades exhibited shallow and reversible hydrogen trapping, regardless of their chemical composition. Of course, the experiments only allowed the calculation of effective diffusion coefficients and trapping energies, which represent an average of the entire microstructure. Nevertheless, HSLA steels are typically joined by arc welding, which includes the risk of delayed hydrogen assisted cracking. From the point of view of welding practice, however, a more or less identical hydrogen diffusion behavior means that no special "metallurgically specific", justifiable measures need to be considered, despite the well-established processes such as "soaking" or dehydrogenation heat treatment.
Weldability of Historic Mild Steel: Challenges, Characterization, and Structural Implications
(2026)
The DeWet headgear (1949, Tsumeb, Namibia) is one of the few surviving mid-20th century mine headframes subject to heritage preservation requirements. This study presents a comprehensive materials characterisation of nine structural steel samples extracted from the structure, with detailed investigation of three primary samples (NE-2, NE-5, N3-L5). The steel is identified as a Si-Mn semi-killed open-hearth (Siemens-Martin) mild steel, consistent with production practices of the late 1940s. Chemical composition determined by optical emission spectrometry (OES) confirms low carbon content (0.18–0.25 wt.%), moderate manganese (0.44–0.89 wt.%), and carbon equivalents CE(IIW) in the range 0.29–0.34 — below the critical threshold for mandatory preheating. Vickers hardness traverses (HV1) reveal a consistent hierarchy N3-L5 > NE-2 > NE-5 across web and flange positions, with estimated yield strengths of 375–509 MPa derived from hardness-strength correlations. Optical metallography after Nital etching reveals a ferrite-pearlite microstructure with grain sizes between ASTM 9.5 and 11.3. Baumann sulfur printing on NE-5 confirms macro-level MnS segregation at the flange extremities, consistent with ingot-cast production. A spatially resolved depth-profile OES campaign on NE-5 (31 measurement points) reveals mild surface decarburisation and sulfur banding. Weldability is assessed as conditionally acceptable, with NE-5 identified as the most sensitive sample due to elevated sulfur and copper content. Planned next steps include tensile testing, thermal desorption analysis (TDA) for hydrogen quantification, and preheat protocol development to support heritage-compliant repair welding.
