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• Δ K th and da/dN of weld metal (WM) of an ex-service CrMoV steel rotor were studied. • Phosphorus (P) segregation occurred in turbine part WM but not in compressor part WM. • Effect of P was controlled by plastic zones (r c) and prior austenite grains (r PAG ). • P segregation led to local intergranular fracture and high da/dN when r c > r PAG . • P segregation had little effect on Δ K th due to r c ≪ r PAG in near-threshold regime. Fatigue crack growth (FCG) behaviors of weld metal (WM) in turbine part and WM in compressor part of a gas turbine rotor after 14 years of service were investigated. Phosphorus (P) segregation at prior austenite grain (PAG) boundaries occurred in the turbine part WM but not in the compressor part WM, which resulted in local intergranular fracture and high da/dN in Paris regime of the turbine part WM. However, P segregation at PAG boundaries had little effect on near-threshold FCG and Δ K th because the cyclic plastic zone sizes were much smaller than PAG sizes and cracks mainly grew inside PAGs. [ABSTRACT FROM AUTHOR]