13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- microvoid coalescence, interspersed with some secondary cracks. The difference between various specimens seems to be a slightly different amount and size of secondary cracks. These results indicated that the specimen tested in air and at Ecorr failed due to ductile overload. The specimens tested under negative potentials showed a significant amount of ductile features on the fracture surface, similar to those in air. Most of the substantial cracks on the fracture surfaces were oriented radially. With increasingly applied negative potential, cracks on the fracture surfaces grew longer, up to about 550 µm at -1550 mVAg/AgCl. However, it is hard to say that these cracks were due to hydrogen, since there were similar cracks on the fracture surfaces for specimens tested in air. The initiation of these cracks may be due to inclusions [19]. Yet the presence of hydrogen did accelerate the growth rate of these cracks. Brittle features at the edge associated with surface cracks in the neck region were deeper at a more negative potential indicating that hydrogen had a greater influence. There were brittle features in small localised areas in the presence of hydrogen, associated with final ductile fracture. However, these brittle features were all surrounded by regions of ductile fracture. Even at the most negative potential, the fracture surface was predominately ductile, indicating that the specimens tested under hydrogen charging also failed due to ductile overload. The influence of hydrogen was not significant. 4.5 Mechanistic interpretation Due to (i) no measureable influence of hydrogen on the yield stress; (ii) no surface cracks in the uniformly deformed part of the specimen gauge section for any specimen, the PD technique measured the onset of plastic deformation rather than the onset of subcritical crack growth. The influence of hydrogen was on the final fracture process after the onset of necking via two ways: (i) causing localized brittle fracture events, and (ii) accelerating crack growth rate on the fracture surface. The brittle hydrogen associated fracture events in the present study occurred simultaneously with a ductile fracture process throughout the necked region of a fracturing specimen. However, the hydrogen fracture mechanism was not comparable to the other ductile fracture mechanism attributable to the stress reaching the fracture stress, and to mechanically unstable of the specimen. The dominant fracture mechanism was ductile microvoid rupture. There were secondary cracks on the fracture surfaces not only for the specimens tested in solution, but also from the specimens tested in air. This implies that even though these secondary cracks were accelerated by hydrogen, inclusions [19] instead of hydrogen might be responsible for the initiation of these cracks. 4.6 Implications for service Considering the results discussed above, (i) the properties of the NiCrMo1 might be improved by reducing the inclusion density; and (ii) if the material will be used at a stress under the yield stress, the influence of hydrogen can be negligible, indicating that it is safe to use this material under the yield stress in an environment containing hydrogen for the H economy. 5. Conclusions The tensile and fracture properties of NiCrMo1 steel under conditions of hydrogen charging were investigated by LIST and SEM. The results showed that: 1. The influence of hydrogen on the tensile parameters (σy, σth and RA) was negligible; 2. There were no surface cracks in the region that had undergone uniform plastic deformation,
RkJQdWJsaXNoZXIy MjM0NDE=