PSI - Issue 61

Shahriar Afkhami et al. / Procedia Structural Integrity 61 (2024) 53–61 Shahriar Afkhami et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Low-alloy carbon steels that are thermomechanically processed to achieve ultra-high strength levels, known as ultra-high strength (carbon) steels (UHSS), have a dominant role in the fabrication and assembly of contemporary industrial components and structures. Fabrication and assembly of metal parts and structures are costly, resource intensive, and can cause significant material waste and pollution; hence, dealing with cost issues and controlling material usage on the industrial or mass-construction scale is a common concern. Therefore, UHSSs, in numerous cases, seem a reasonable choice to replace relatively more costly and sophisticated metals and alloys for such purposes. UHSSs reduce material consumption (and, consequently, material waste) required for parts or structures to reach any specific strength level due to their ultra-high strength levels; these steels are also effectively economical since they are iron-based and low-alloy. In conclusion, UHSSs are a suitable solution to save energy, produce relatively more sustainably, and reduce pollution and unwanted emissions, e.g., CO 2 emissions [1,2]. Welding UHSSs, as a standard joining method in manufacturing metal parts and erecting metallic structures, is inevitable. However, most welding processes can expose UHSSs to complications such as cold cracking, toughness reduction, brittleness, reduction in the load-carrying capacity, and heat-affected zone (HAZ) softening [3]. Furthermore, steel is typically used under different forms of cold-forming, e.g., hollow or tubular sections and bent profiles, to be efficiently used in various designs (providing more freedom to designers) and applications. However, like welding, cold-forming can adversely affect UHSSs; cold-forming imposes plastic strains (pre-strains in the literature) in deformed UHSSs, potentially changing their mechanical properties, microstructure, or fracture mechanism. These pre-strains increase the dislocation density in deformed areas, altering the deformation and fracture mechanisms. Direct results of such changes typically are an increase in material strength and hardness and a simultaneous decrease in ductility and fracture toughness. Furthermore, pre-strains can facilitate crack initiation and growth through the microstructure and increase ductile-to-brittle transition temperature [4]. Ultimately, combinations of complexities associated with welding and cold-forming of UHSSs can cause cold-formed UHSSs, such as their hollow sections, to have lower weldability than their original sections [5]. According to the literature, areas in the vicinity of five times the thickness of cold-formed steel plates can suffer from the adverse effects of cold-forming and are preferred not to be welded to avoid brittle fractures or catastrophic failures unless they satisfy specific criteria proposed in the literature and related standards [4,6,7]. However, currently available standards and codes such as Eurocode 3 (EN 1993-1-8) cover steel types with strength levels barely up to 1000 MPa for such guidelines [8]. Hence, further research is required on the weldability of cold-formed UHSSs and their welded joints’ failure and fracture mechanism. Such studies can improve the reliability of welded UHSSs in contemporary industry and construction and help expand the application domain of such metals and their cold-formed profiles. As a step forward to the authors’ ongoing research, this article continues the study on the weldability of cold formed UHSSs published in [4]. The primary incentive of these studies is the significant role of welded cold-formed UHSSs in modern structures, vehicles, and cranes based on industrial and constructional needs [9]. In the authors’ previous work [4], signs of HAZ softening and notch fracture toughness reduction were detected in the welded cold formed S1100 UHSS. Hence, the current study further investigates the subject matter from the fracture mechanics perspective. Compared to [4], a modified joint design has been utilized to alter the welding heat distribution throughout the joint area and discourage HAZ softening caused by welding heat input. Also, different degrees of cold-forming are investigated to increase the comprehensiveness of the overall research by combining the two studies. Finally, the current study is thoroughly focused on the deformation behavior of the material, its plastic deformation, hardening behavior, and fracture mechanisms. 2. Experimental procedure Quenched and tempered S1100 steel plates with a thickness of 8 mm were used as the base metal in the welded joints for this research. For the cold-formed side of the welded joints, the plates were bent for 45° via air bending with bending radii ( R ) of 10, 20, and 30 mm to achieve different degrees of cold-forming (DOC) or plastic pre-strains. Regarding the straight side of the joints, the raw steel plates were machined to reach 6 mm thickness, and a 45° bevel