Microstructure and Charpy Impact Properties of FCAW and SAW Heat Affected Zones of 100 mm Thick Steel Plate for Offshore Platforms

In this study, heat affected zone (HAZ) specimens were fabricated by applying flux-cored arc welding (FCAW) and submerged arc welding (SAW) processes to steel plate with a thickness of 100 mm and yield strength of 460 MPa for use in offshore platforms. The correlation between microstructure and Charpy absorbed energy was investigated, and fracture mechanisms were analyzed. As distance from the fusion line increases, heat input and cooling rate decrease, making it difficult to form low-temperature transformation microstructures in HAZ specimens and increasing the grain size of quasi-polygonal ferrite. The FCAW process is advantageous for low-temperature transformation microstructures because it has a lower heat input and a faster cooling rate than the SAW process. Plastic deformation and ductile fracture occurred at low temperatures in fine acicular ferrite and quasi-polygonal ferrite regions of HAZ specimens, but brittle fractures occurred in bainitic ferrite, granular bainite, and coarse quasi-polygonal ferrite regions. The unit crack path in the brittle fracture region was similar to the grain size. In other words, as the volume fraction of fine acicular ferrite and quasi-polygonal ferrite increased in HAZ specimens, Charpy absorbed energy at low temperature increased.