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 tough ness at initiation, K-Ic and at arrest, K-Ia. The results were compared wit h those obtained on another pressure vessel steel (A508) of similar strengt h. Two techniques were used to measure K-Ia: (i) isothermal compact crack a rrest (CCA) tests, and (ii) specially designed thermal shock experiments us ing an externally notched ring. These specimens were cooled to -196 degrees C and then heated by induction in the centre of the ring to produce very st eep thermal gradients. This caused crack initiation from the notch. The cra ck propagates very rapidly (similar to 500 m s(-1)) and stopped when it rea ched the warmer region of the specimen. The specimens were analysed using a n elastic-plastic finite element method to determine K-Ia values. These tes ts reveal a greater temperature shift (similar to 100 degreesC) between K-I c and K-Ia in 2 1/4 Cr-1 Mo steel than in A508 steel. Detailed metallograph ical 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 thre e-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between th e cleavage microcracks break by ductile rupture mechanism.