Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr‐1 Mo pressure vessel steel

Abstract

Tests were performed on a 2¼ 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 °C 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 (∼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 (∼100 °C) between K Ic and K Ia in 2¼ Cr–1 Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼ 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