Achieving 1 GPa Tensile Strength with Extraordinary High-Cycle Fatigue Resistance in the Welded Ultra-high-strength Low Alloy Steels

초록

The present study investigated the influence of Ni content in filler metals on achieving 1 GPa tensile strength and superior high-cycle fatigue resistance in welded ultra-high-strength low alloy (UHSLA) steels. UHSLA steels with a grade of 2 GPa (base metal) were welded by gas metal arc welding using filler metals containing 2 and 3.5 wt pct Ni. The weld zone exhibited a refined bainitic structure with acicular ferrite (AF) and MC-type carbides, and the degree of refinement increased with Ni content. Notably, welded steels with 3.5 wt pct Ni filler metal exhibited yield and tensile strengths of 997 and 1047 MPa, respectively. Moreover, the high-Ni specimens displayed a fatigue limit of 740 MPa, comparable to conventional wrought high-strength low alloy steels. For the steels investigated, fatigue cracks initiated within the weld metal rather than from heat-affected zone defects, which is uncommon in conventional welded steels. This highlights a distinctive fatigue fracture behavior governed by weld microstructure. Microstructural analysis confirmed that refined bainite, together with acicular ferrite and finely dispersed MC-type carbides, effectively impeded dislocation motion and delayed fatigue crack propagation. Based on these results, the tensile and fatigue deformation mechanisms underlying the superior performance of welded UHSLA steels are discussed.

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