Coupling Effect Mechanism of the δ-Ferrite and M23C6 on the Mechanical Properties of 9Cr-Steel Deposited Metals

9Cr ferritic/martensitic (9Cr F/M) steels are considered ideal structural materials for various nuclear energy systems. However, δ-ferrite (δ), as a controlled phase, may occur in its welds. Three deposited metals with different carbon contents (0.04, 0.07, and 0.10 wt%) were investigated using experimental and finite element simulation methods. The results showed that the incomplete peritectic reaction, the incomplete δ to austenite phase transition, and the segregation of ferrite-stabilized elements led to the residual δ. The amount and morphology of δ significantly influence the mechanical properties. After increasing the carbon content, the increase in strength comes mainly from precipitation strengthening and dislocation strengthening, the presence of δ will reduce the strength. During the impact process, δ affects the absorbed energy for the stable crack growth through its morphology, and M₂₃C₆ affects the crack formation energy through its quantity. By decreasing the carbon content to a certain extent, the reduction of M₂₃C₆ content and the generation of large polygonal δ can effectively improve the toughness of 9Cr-steel deposited metals.