An Investigation into the Effect of Weld Strength Mismatch on the Assessment of HAZ Fracture Toughness H. G. Pisarski1 and P. L. Harrison2 1 TWI Ltd, Cambridge, CB1 6AL, UK 2 Corus Research Development and Technology, Rotherham, S60 3AR, UK ABSTRACT The effect of weld metal strength mismatch on the fracture toughness of the weld heat affected zone (HAZ) in two ferritic steels is described. Mismatch was achieved in two different ways which ensured that the HAZs were the same, irrespective of mismatch level. It is shown that fracture toughness is more affected by the local strength of the HAZ rather than differences in strength between weld metal and parent plate. Where HAZ strength is lower than both the parent plate and weld metal, HAZ fracture toughness is reduced. On the other hand, where HAZ strength is greater than both weld metal and parent plate, differences in strength between weld metal and parent plate have little influence on HAZ fracture toughness. INTRODUCTION Opinions on the effect of weld strength mismatch on heat affected zone (HAZ) fracture toughness in ferritic steels are conflicting. Some consider overmatching to be beneficial to HAZ fracture toughness and others that it is harmful1,2,3. In order to better understand the effect of strength mismatch on HAZ fracture toughness, a systematic study was undertaken in which mismatch was achieved by two different methods. In the first method, mismatch was achieved by employing the same parent plate and welding conditions, and changing the welding consumable to provide two different weld strength levels. Since HAZ fracture toughness is primarily dependent on parent plate composition and weld cooling rate, the HAZs produced by the two welds are the same. A 550MPa yield strength roller quenched and tempered steel was employed and yield strength mismatch, M, (where M is defined as the ratio of weld metal to parent plate
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