Relationship Between Martensite Islands and Haz Fracture Toughness in Welded Ni-Cu Structural Steels

The relationship between high carbon martensite island constituent (M*) and the HAZ CTOD fracture toughness has been evaluated in four HSLA plate steels containing varying amounts of C, Ni and Cu. Test pieces with plane fusion boundaries were made by first pass welding and the M* constituent was produced by thermal simulation of second pass welding, intercritically reheating the HAZ to 1043 K. M* was evaluated by quantitative light microscopy and the CTOD fracture toughness was measured in the coarse-grained region of the HAZ. The volume fraction of M* increases with the carbon content of ths steel, and also with the combined content of nickel and copper, but the latter occurs at a much lower rate. The amount of M* does not correlate well with the IIW carbon equivalent, being lower than expected at high Ni and Cu contents. The CTOD fracture toughness decreases with increasing volume fraction of M*. The weldability of steels containing nickel and copper is significantly better than indicated by hardenability based formulae, such as IIW Ceq