Toughness of simulated HAZ was evaluated by means of CTOD and instrumented pre-crack Charpy tests. While the CTOD transition temperature represents brittle fracture initiation toughness, pre-crack Charpy transition temperature is strongly influenced by brittle fracture arrest toughness. The both transition temperatures are controlled by different microsturcutal parameters.
Property requirements for the steels used for offshore platforms are becoming increasingly severe. In particular, toughness of welded joints is one of the severest requirements to achieve. Offshore platform steel specifications, like API-RP-2Z (1992) and EEMUA 150 (1987), require the heat-affected zone (HAZ) CTOD test in addition to the conventional Charpy-V impact test. CTOD property is very sensitive to the local brittle zones (LBZ) in the HAZ (Haze et al, 1988a). HAZ CTOD value has a large scatter because size of the LBZ is small and it is difficult for fatigue pre-crack-tip to sample the LBZ with high probability.(Toyoda et al,1991). Because of this nature of HAZ CTOD, post- test examination on the sampling of the LBZ is required. In addition to the inherent difficulty in sampling the LBZ, HAZ CTOD value is also influenced by welding conditions even if steel is welded at constant heat input; local toughness of the LBZ changes with thermal cycles of the subsequent welding passes and also size and distribution of the LBZs change with them. All of these factors are influenced by welding bead placement even under constant heat input. On the stage of steel development, simulated HAZ toughness tests may be conducted. They have no ambiguity in terms of the notch location relative to the LBZ and of the variability of welding condition, although large scatter is still observed due to inherent toughness variability, which is a result of a character of the weakest-link brittle cleavage fracture (Lin et al,1987; Tagawa et al,1993).