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.

This paper investigated the effects of welding and alternating wet-dry seawater on the hydrogen embrittlement (HE) susceptibility of a new nanoparticle-reinforced high-strength steel, dispersion-strengthened-high-strength (DSHS) steel. The results show that welding led to the transformation of granular bainite to tempered martensite. It also promotes grain coarsening as well as an increase in inclusions size and irregular changes in shape. This results in increased susceptibility to HE at the welded joint (WJ). After wet-dry cycle corrosion in seawater, corrosion products impede the oxygen transfer and promote the hydrogen evolution reaction, so the hydrogen production increases. The decrease in the protection of corrosion products at the WJ and the decrease in NbC content led to an increase in hydrogen production but a decrease in hydrogen trapping capacity. Therefore, the WJ HE sensitivity of DSHS steel is further increased.

A technique developed recently in our Institute to investigate crack initiation, rapid crack propagation and crack arrest with a single specimen was applied to two low alloy steels (16MND5 and 15CD910) with a tempered bainitic microstructure. This technique is based on tests on cracked ring specimens submitted to a compressive load applied to the poles of the ring while a crack is located in the equatorial plane at the outer surface of the specimen. The main interest of this geometry lies in the variation of the stress intensity factor, K with crack length which follows a bell-shaped curve. The experiments were carried out at -196 degrees C. This condition produced brittle cleavage fracture. It is shown that the fracture toughness at crack arrest, K-1a, may largely be lower than the fracture toughness at crack initiation, K-1c. Moreover K-1a is a decreasing function of the crack velocity.