Nina Schroeder

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.

High-strength, quenched and tempered steels, such as S690QL, require specific microalloying concepts to achieve the necessary balance of strength, toughness and weldability. As the mechanical integrity of welded structures is significantly impacted by the behaviour of the heat-affected zone (HAZ) during welding, a thorough understanding of microstructural evolution is crucial. This study systematically investigates the effect of Ti or Nb microalloying as well as a combination of both on three-layer gas metal arc welded joints. Metallographic characterisation, hardness mapping and thermophysical simulations are employed to determine the interaction between precipitate stability, austenite grain growth and softened HAZ formation. Cross-weld tensile tests combined with digital image correlation (DIC) enable the in situ monitoring of local strain accumulation in critical microstructural zones. A mirror-assisted optical setup enables the simultaneous evaluation of strain on multiple specimen surfaces, providing comprehensive insight into constraint effects and the mechanisms controlling strain localisation and fracture initiation. Complementary impact toughness measurements further highlight the influence of microalloying and dilution on fracture behaviour in both the HAZ and the weld metal. The combined findings demonstrate that the microalloying strategy governs the extent of HAZ softening, the distribution of local strain and the failure mechanisms within the weldment. Specifically, the interaction between Ti- and Nb-containing precipitates plays a significant role in HAZ microstructure formation and local mechanical response enhancement. In summary, the results indicate that a combination of Ti and Nb microalloying is the most effective approach to enhancing the local mechanical performance and toughness, improving the integrity of the HAZ in S690QL steel welds.

For reasons of sustainability and cost efficiency, there is an increasing trend in the steel-processing industry to retrofit existing structures, avoiding costly shutdowns or dismantling. Although welding is a cost-effective joining method, it is rarely applied to old steels, whereas riveted or bolted connections are often uneconomical. Repair and refurbishment frequently require the replacement of damaged material or the creation of dissimilar old–new steel joints. Due to the varied manufacturing processes of historical steels, not all twentieth-century steels are inherently weldable, making an initial assessment of weldability essential. In this study, a historical non-deoxidized mild steel (produced by Siemens–Martin processing) originating from the construction of the Berlin Radio Tower (erected in 1926) was investigated using dilatometry to analyze its welding behavior. A database of welding CCT diagrams and HAZ simulations was established to support practice-oriented welding experiments, providing key insights into their weld-metallurgical behavior and weldability. Furthermore, initial welding trials were conducted, and the local residual stress states in dissimilar old–new steel joints were determined. These foundational investigations are critical for the development of innovative, load-adapted welding concepts for the repair and refurbishment of existing old-steel infrastructure in Germany.

High-strength low-alloyed (HSLA) steels with yield strength/proof stress ≥ 600 MPa are the basis of modern light-weight steel constructions. Indeed, the economic and ecological benefits strongly depend on their processability in terms of welding. In this context, the use of highly productive welding processes and suitable welding consumables is of vital interest and requires a fundamental understanding of the microstructural changes in the HSLA steel and especially the heat-affected zone (HAZ) of the welded joint. Microalloying elements, such as Ti or Nb, are essential to achieving the desired mechanical properties. To analyse the weldability, three-layer welds were performed using gas metal arc welding (GMAW) and critical microstructures, such as areas of the HAZ that exhibit significant softening or hardening, were identified. The effect of the softened HAZ region on failure was evaluated using cross-weld tensile specimens. Digital image correlation (DIC) was used for in situ monitoring of the development and accumulation of the local strains in different HAZ regions during tensile testing. Using a specially designed mirror system, the local strains of the microstructure zones on the top and bottom of the weld were recorded simultaneously. In addition, the analysis of the local deformation helps to understand the effects of the softened HAZ on the global strain, the reduction in area, the fracture position, and the overall fracture behavior.

Microalloying elements such as Nb and Ti are essential to increase the strength of quenched and tempered high-strength low alloy (HSLA) structural steels with nominal yield strength ≥ 690 MPa and their welded joints. Standards such as EN 10025–6 only specify limits or ranges for chemical composition, which leads to variations in specific compositions between steel manufacturers. These standards do not address the mechanical properties of the material, and even small variations in alloy content can significantly affect these properties. This makes it difficult to predict the weldability and integrity of welded joints, with potential problems such as softening or excessive hardening of the heat-affected zone (HAZ). To understand these metallurgical effects, previous studies have investigated different microalloying routes with varying Ti and Nb contents using test alloys. The high-strength quenched and tempered fine-grained structural steel S690QL is the basic grade regarding chemical composition and heat treatment. To evaluate weldability, three-layer welds were made using high-performance MAG welding. HAZ formation was investigated, and critical microstructural areas were identified, focusing on phase transformations during cooling and metallurgical precipitation behavior. Isothermal thermodynamic calculations for different precipitations were also carried out. Mechanical properties, especially Charpy notch impact toughness, were evaluated to understand the influence of different microalloys on the microstructure of the HAZ and mechanical properties.

Modern arc processes, such as the modified spray arc (Mod. SA), have been developed for gas metal arc welding of high-strength structural steels with which even narrow weld seams can be welded. High-strength joints are subjected to increasingly stringent requirements in terms of welding processing and the resulting component performance. In the present work, this challenge is to be met by clarifying the influences on hydrogen-assisted cracking (HAC) in a high-strength structural steel S960QL. Adapted samples analogous to the self-restraint TEKKEN test are used and analyzed with respect to crack formation, microstructure, diffusible hydrogen concentration and residual stresses. The variation of the seam opening angle of the test seams is between 30° and 60°. To prevent HAC, the effectiveness of a dehydrogenation heat treatment (DHT) from the welding heat is investigated. As a result, the weld metals produced at reduced weld opening angle show slightly higher hydrogen concentrations on average. In addition, increased micro- as well as macro-crack formation can be observed on these weld metal samples. On all samples without DHT, cracks in the root notch occur due to HAC, which can be prevented by DHT immediately after welding.