Kinetics of phosphorus segregation and its effect on low temperature fracture behaviour in 2.25Cr-1Mo pressure vessel steel

Low alloy steels operating for long times at temperatures of similar to 500 degrees C may be sensitive to embrittlement owing to the segregation of various trace elements to prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation can adversely affect the resistance of the steel to fracture; this is associated with a change in fracture mode from transgranular to intergranular. The present paper describes research carried out to examine the effects of aging at 520 degrees C following 'full' tempering at 650 degrees C on the kinetics of phosphorus segregation in 2.25Cr-1Mo steel, and any subsequent effects on fracture resistance and fracture mode. Fracture toughness tests have been conducted at low temperatures and the fracture surfaces have been examined using scanning electron microscopy (SEM) with EDX. Auger electron spectroscopy (AES) and some limited microanalysis in a field emission gun scanning transmission electron microscope (FEGSTEM) have been employed to measure phosphorus segregation at grain boundaries It is observed that full tempering changes the kinetics of phosphorus segregation during aging from that associated with a steel aged directly at 520 degrees C after quenching. This observation is ascribed to the formation of molybdenum rich alloy carbide precipitates at 650 degrees C, which enables phosphorus atoms to be released from Mo-P clusters. Subsequent aging at 520 degrees C allows the released phosphorus to segregate to grain boundaries and carbide/matrix interfaces. No segregation induced embrittlement is observed in the quenched steel, when aged directly at 520 degrees C for a comparable period of time.