Fractography of Heat-Affected Zone Fracture Toughness Specimens

Recent advances in the steel industry have resulted in high fracture toughness steels becoming common in many structures in the United States, including bridges. However, structures are only as strong as their weakest link, and in terms of fracture behavior in steel structures, this is often the welded connections. Heat-affected zones (HAZs) of steel surrounding welds have altered mechanical properties and potentially reduced fracture toughness. Recent work performed at the University of Kansas utilized notched and fatigue pre-cracked single edge bend specimens to attempt to quantify the difference in fracture toughness between typical highway steel base metal and the HAZ. ASTM A709 Gr. 50W and HPS Gr. 70W steels were welded using two submerged arc welding (SAW) procedures and one narrow gap improved electroslag welding (NGI-ESW) procedure. The SAW weldments featured double bevel groove welds in an attempt to produce vertical HAZs to facilitate sampling of the HAZ across the entire specimen crack front. While the results of this study made it clear that specimens notched at or near the fusion line of all three weld procedures exhibited significant reductions in fracture toughness compared to base metal, it was noted that no fusion lines or HAZs were perfectly straight, leading to variations in the material sampled by the crack front. In order to quantify the amount of weld metal, HAZ, and base metal present in the crack front, a portion of the fractured specimens were chosen for further microscopic investigation. Fracture surfaces were examined under an optical microscope and a scanning electron microscope (SEM) to evaluate the microstructures present. Potential cleavage fracture initiation sites were also identified on a number of fracture surfaces, providing further insight into the critical locations of fracture within a weldment.