Initiation and early propagation of the short fatigue cracks on weld metal of 1Cr18Ni9Ti pipes are studied with a replicating
technique. Nineteen smooth hourglass shaped specimens are observed. Results reveal that the effectively short cracks are always initiated from the delta ferrite bounds inlaid in the material matrix in the local weakest zone of specimen surface. This zone is located in one of districts on the specimen surface, which is orientated in accordance with the inner or outer surface of the pipe. Early propagation of the cracks has a significant character of micro-
structural effects. The maxi靘 mi-cro-structural barrier size is 40 mum, which agrees with the distance between the neighboring rich delta ferrite bounds in the characterized microstructure of weld metal, i.e. the columnar grain structure. Within the size range, density of the cracks is higher and increases gradually. The cracks grow irregularly till a coalescence occurs between some longer cracks to form a
dominant short crack which will finally result in failure of the specimens. This process is generally referred as 搈icro-structural short crack (MSC) stage? The dominant crack and the cracks on the growth path of dominant crack contribute directly to fatigue damage and thus, are referred appropriately as a 揹ominant effectively short fatigue crack (DESFC)?and 揺ffectively short fatigue cracks (ESFCs)? respectively. A virtual crack which size is equal to the average size of the ESFCs, the DESFC in the MSC stage, should be introduced to describe the irregular behavior. The crack size increases steadily and its growth rate decreases constantly in the stage.