Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2 1/4 Cr-1Mo pressure vessel steel

Tests were performed on a 2 1/4 Cr-1 Mo steel to measure the fracture toughness at initiation, K-Ic and at arrest, K-Ia. The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure K-Ia: (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to -196 degreesC and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (similar to 500 m s(-1)) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic-plastic finite element method to determine K-Ia values. These tests reveal a greater temperature shift (similar to 100 degreesC) between K-Ic and K-Ia in 2 1/4 Cr-1 Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2 1/4 Cr-1 Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism.