The Significance of Softened HAZs in High Strength Structural Steels

Abstract

The development of lean alloyed high strength steels of 500–800 MPa yield strength has shown that conventional welding processes and procedures can result in significant HAZ softening. Previous work on the significance of softened HAZ regions is reviewed and a recent investigation will be described in which the importance of HAZ softening in a 550 MPa QT steel has been assessed using fracture toughness and surface notched mini-wide plate tests. Submerged arc welds at 2.4 kJ/mm were made using consumables of different strength levels, which both overmatched and undermatched the parent plate yield strength. Softening of approximately 45HV10 was observed in the HAZ and the CTOD fracture toughness tests showed that the transition temperature of the HAZ for the overmatched weld was 60°C higher than the equivalent HAZ in the undermatched weld. The work shows that overmatching weld metals can concentrate strain into softened HAZs and if flaws are close to the HAZ, low cleavage resistance can be measured. This was confirmed by the results of the mini-wide plate tests. The implications of these findings are discussed in the context of welded steels of 500–800 MPa yield strength.