Effect of aging time on the microstructure and mechanical properties of the SP2215 steel welded joint

SP2215 tube welded joints (WJ) were prepared by manual gas tungsten arc welding (M-GTAW) and ERNiCr-3 welding wire. The microstructure, mechanical properties and tensile fracture mechanism of the WJ after aging for different times at 650 ℃ were studied. The results show that the microstructure of SP2215 base metal (BM) is composed of austenite, a small amount of twins and primary NbN and Z phase in the as-welded condition, the weld metal (WM) was solidified into columnar dendrite with a completely austenitic structure, and Nb element was segregated between the dendrites and forms the NbC phase, which was distributed in a chain shape. After aging at 650 ℃ for 50 h, M₂₃C₆ began to precipitate at the austenite grain boundary of SP2215 BM and gradually coarsened with the extension of aging time, at the same time NbC phase gradually precipitated between dendrites and gradually formed into a cluster shape with the increase of aging time. The microhardness of BM and WM increases with the increasing of aging time, and reaches the maximum value at 500 h. The hardness of BM is always higher than that of WM. All the WJ with different aging time were fractured at the weld seam in a ductile manner at room temperature tensile test, however the fracture initiation position and fracture direction changed with the extension of aging time. When the aging time was 0-114 h, the fracture direction was transverse to the columnar dendrite direction, and the crack initiation position was the primary NbC particles between dendrites. When the aging time was 500-2012 h, the fracture direction was parallel to the columnar dendrite direction, and the crack initiation position is the dendritic core. The fracture position of the WJ changed from the weld seam to the SP2215 side with the increase of aging time at high temperature tensile test at 650 ℃. The WJ aging for 0-114 h fractured at the weld seam in a ductile manner, however the WJ aging for 2012 h fractured at the SP2215 side in a quasi-cleavage manner.