Effect of long term service on fatigue crack growth behavior of low alloy CrMoV steel weld metals

•ΔKth 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 (rc) and prior austenite grains (rPAG).•P segregation led to local intergranular fracture and high da/dN when rc>rPAG.•P segregation had little effect on ΔKth due to rc≪rPAG in near-threshold regime.