They are considered by the present authors to be more appropriate for the assessment of welds than those given in standards (e.g. BS 7448, ASTM E1290), because they are valid for a wider range at a/W (0.05 to 0.7), and J is more accurately estimated because it is derived directly from CMOD rather than from an estimate of load line displacement. Although the equations are for homogeneous material, they have been shown to be applicable to the assessment of welds and HAZs for a wide range of strength mismatch conditions. The JCMOD values have been transformed into more familiar stress intensity factor (K) values using the standard plane strain transformation. An important feature of the testing procedure was post-test metallography conducted on the fracture toughness specimens. This ensured that results analysed only represented those from specimens in which the fatigue crack tip was located in grain coarsened HAZ (GCHAZ) or fusion boundary. In addition, post-test metallography enabled identification of the microstructure in which fracture initiation occurred. RESULTS AND DISCUSSION HAZ Hardness and Tensile Properties Hardness traverses were conducted across the welds at approximately midplate thickness and the results are presented in Fig.1 and 2. In the QT1 steel, HAZ softening had taken place in the overmatched weld (M=1.51), see Fig.1. In both the QT2 (M=0.95) and QT2N (M=2.15) steels, hardness in the GCHAZ close to the fusion boundary was higher than in the weld metal or parent plate, see Fig.2. Some HAZ softening is observed in the outer, intercritical HAZ of the QT2 steel. Gleeble simulation of the GCHAZ was undertaken so that a direct estimate of the tensile strength could be made. The peak temperature achieved was 1350°C and the cooling time between 800°C and 500°C was 32s for the SAW welds (in QT1 steel) and 8s for the FCAW welds (in QT2 steel). Tensile properties, measured after post-weld heat treatment, are compared with the parent plate and weld metals in Table 2. Despite the weld metal strength overmatch in the QT1 steel, the HAZ strength undermatched the strength of the parent plate by a factor of 0.8. In contrast, the HAZs in the FCAW welds overmatched both the strength of the QT2 and QT2N steels by factors of 1.07 and 2.4, respectively.
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