| Flaws nucleated during operation of structures such as nuclear power plants, petroleum facilities and gas equipment raise many safety problems, and thus the integrity assessment of crack-like flaws in structures has been very important. Since large-scale structures include weldments with complex micro-structures and inhomogeneous material properties, it is essential to consider both metallurgical and mechanical factors. In this study, a methodology for integrity assessment in welded structures was developed based on API 5L X65 grade natural gas pipeline (an approximately 2400 ㎞ pipeline in Korea). A failure assessment diagram(FAD), which includes all failure modes from linear elastic fracture to plastic collapse, was used for integrity and fitness-for-service(FFS) assessment of the pipeline. Since tensile properties and fracture toughness of base metal, weld metal and heat affected zone(HAZ) are required for the FAD, tensile tests and fracture toughness tests were performed on each region. In particular, the evaluation of representative properties of HAZ make it possible to assess FFS accurately by using the HAZ-based FAD when crack-like flaws exist in HAZ. This HAZ-based FAD is different from current codes, which assess integrity by the material properties of weld metal or base metal even when flaws exist in HAZ. |
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