Effect of intergranular failure on the critical fracture stress and the fracture toughness of degraded reactor pressure vessel steel

The fracture mechanisms in the ductile-to-brittle transition (DBT) region of the undegraded and degraded A533B steel have been studied. Two mechanisms were observed in the initial stage of the DBT region: transgranular cleavage fracture and intergranular fracture. Factors affecting the proportion of intergranular fracture on the fracture surface have been investigated by four-point-bending (4PB) mechanical tests, SEM observation and statistical fractographic analysis. The effect of intergranular fracture on the critical fracture stress has been discussed and an empirical equation for the mixed critical cleavage/intergranular fracture stress is proposed. Finally, attention is also drawn to the interesting difference in the intergranular fracture percentages shown on the fracture surfaces of fracture toughness test specimens.

► Effect of intergranular on σF can be neglected when it is below a threshold value.
► Linear relationship between σF and intergranular fracture percentage is suggested.
► Effect of intergranular fracture on fracture toughness has been discussed.
► DBTT rise due to embrittlement is explained considering intergranular effect.