Weldability of cold-formed high strength and ultra-high strength steels

Abstract

Hollow sections and cold-formed steels have a key role in modern structures and machinery. In addition, to benefit from full potentials of cold-formed steels, it is usually required to weld them to other steel parts of structures. However, data provided by relevant standards, such as Eurocode 3, do not cover newly developed high strength and ultra-high strength grades of this material. Thus, further study is critical to complete available data in literature and standards. Regarding this matter, having a good weldability for cold-formed high and ultra-high strength steels is vital for development of contemporary steel structures. Thus, newly developed steels S700MC and S1100 were selected to be investigated in this study. To do so, bended base metals with different degrees of coldforming were welded to their straight (virgin) counterparts. Next, welded joints were investigated via microstructural analysis, hardness measurement, tensile test, and Charpy impact test to assess the weldabilities of the cold-formed base metals. Results show that the final joints had acceptable characteristics, and the cold-formed base metals showed good weldability. However, bending and pre-straining criteria recommended by the manufacturer must be satisfied to have an acceptable joint after welding. Beyond that criteria, fracture elongation and notch toughness of the welded joints decreased, and some welded samples failed from their cold-formed base metals.

Effects of heat input on the mechanical properties of butt-welded high and ultra-high strength steels

“…The aforementioned filler materials create a weldment in the state of matching strength with the BM. Even though the nominal yield and ultimate strengths seem to be lower than the virgin base plates, the previous experiments show that these filler materials create matching joints after reheating and dilution with the base material [12,20]. The chemical compositions and mechanical properties of the filler materials are presented in Table 3.…”

“…After welding, the softened HAZ shows a mixture of tempered martensite and granular bainite [42]. The hardness drop is due to the existence of granular bainite, since this feature is naturally softer than bainite and martensite [12,43]. Due to the higher cooling rate, the LW specimen shows a finer texture of bainite and M/A at its softened HAZ compared to the GMAW specimens, which resulted in less hardness drop.…”

“…Even though these grades of steel have been in the market for several decades, the terminology of HSS/UHSS is not well established yet due to the continuous developments of such steel grades [12]. According to the current definition of HSS/UHSS, advanced high strength steels with nominal yield strength up to 700 MPa are to be considered as HSS, while higher yield strengths are categorized as UHSS [13].…”

Microstructure and Mechanical Properties of Inertia‐Friction‐Welded Fe–Cr–Ni–Mo High‐Strength Steel

“…[4] According to the current definition of UHSS, AHSSs with nominal yield strength (YS) exceeding 780 MPa are categorized as UHSS. [5] To mitigate the manufacturing complexity of large-scale structures, many such applications require UHSS to be welded. Thus, weldability of these steels was investigated using various welding processes (including laser welding [LW], [6][7][8][9] tungsten inert gas [TIG] welding, [10][11][12] gas-metal arc welding [GMAW], [13,14] electron beam welding [EBW], [15,16] and plasma arc welding [PAW] [17,18] ).…”

“…[10,13] However, it is usually possible to reduce these defects by controlling heat input, cooling rate, and heat flow during a welding procedure. [5,10] The high heat input of fusion welding deteriorates mechanical properties of the welded joints. Therefore, these problems associated with fusion welding restrict its further development.…”