Effects of Carbon and Phosphorus on Microstructure and Mechanical Properties of Friction Stir Welded Weathering Steels

Abstract

Weathering steels with the increased concentration of carbon (C) and phosphorus (P) that tend to be difficult to join without defects by fusion welding were fabricated by friction stir welding (FSW), and the effects of C and P contents on the microstructure and mechanical properties of FSWed joints were investigated. P segregation formed during solidification was present in the base material (BM) of the steels. In the stir zone (SZ) of FSWed joints, P segregation was reduced under the condition of higher welding temperature (above A3). In the tensile test, both the addition of C and P increased tensile strength. On the other hand, the increase in C content impaired the uniform elongation, while the increase in P content did not deteriorate it. It is because the work hardening rate was improved by the P addition presumably due to the suppression of cross slip of dislocations. In the Charpy impact test, the increase in C and P contents shifted the DBTT to the higher temperatures and simultaneously decreased the upper shelf energy. EPMA analysis performed on the cross-section of the ductile fracture surface of BM of 0.3 wt%P steel revealed that voids were formed at the boundary between P segregated area and the non-segregated area. Therefore, P segregation was inferred to be the main factor determining the upper shelf energy. Since the P segregation of the joint FSWed above A3 was improved, the upper shelf energy of the SZ was the highest among BM and SZ FSWed below A1.