A technique is described for establishing the microstructural features associated with the initiation of brittle fracture in weld metal and heat-affected zone (HAZ) crack-tip opening displacement (CTOD) test specimens. The technique involves detailed optical and scanning electron microscope examination of fractographic features to establish the local directions of fracture propagation at various locations on the fracture surface. This information is used to trace fracture propagation back to the region of initiation. In many instances, the precise initiating feature can be located and identified. Once identified, the fracture initiation position is marked and the specimen sectioned through this point to allow the region in which fracture initiated to be examined metallographically and related to the surrounding microstructure. Details of the technique are discussed.
Examination of steel weld metal and HAZ regions has revealed nonmetallic inclusions, pearlite colonies, second-phase regions (martensite, retained austenite, or carbide), and localized regions of intergranular fracture as the initiating features. The significance of these findings is considered with regard both to the size of features at which cleavage fracture initiates and to scatter in CTOD values.