Tim Boot

This work studies the hydrogen embrittlement (HE) behaviour of Dual-Phase steels with varying martensite content. Steels with martensite contents of 25 ± 5, 50 ± 4 and 78 ± 7% were realised by intercritically annealing an as-received DP steel. These steels were charged with hydrogen and consequently subjected to an in situ slow strain rate tensile test to characterise the embrittlement. It was found that the steel with 50% martensitic content showed the most ductility in air, but the highest embrittlement of 86 ± 10%. The extent of embrittlement does not increase further from the point that martensite forms a continuous network in the microstructure. The presence of martensite on the surface is linked to the formation of brittle crack initiation sites in these steels. Furthermore it was found that the anisotropic banded structure in the annealed steels promotes brittle crack propagation along the direction of banding, which originates from rolling process. This research shows that anisotropic martensite distributions as well as surface martensite should be avoided when developing rolled steels, to maximise HE resistance.

The effect of hydrogen charging during plastic deformation was investigated on a ferritic steel containing TiC nano-precipitates. Specimens were subjected to a slow strain rate tensile test (SSRT) up to 0, 1, or 3% plastic engineering strain, held until a total duration of 2 h to saturate with hydrogen, then fast fractured. The specimens pre-strained elastically absorbed 2.36 wppm of hydrogen, which increased to 3.69 wppm for 3% plastic strain. Only 0.72 wppm is stored in non-dislocation traps such as precipitates, grain boundaries, and lattice sites, which makes dislocations the main contributor to hydrogen trapping. The increased hydrogen uptake did not lead to a decrease in fracture strain, which remained between 6 and 10% for all pre-strains, compared to 60% for full SSRT tests that were charged for a shorter time. This research highlights the necessity of high plastic strains and the presence of hydrogen in the environment during crack growth to cause HE in ductile steels.

The effect of TiC and VC nano-precipitate size on the hydrogen embrittlement of ferritic steels was studied in this work. Steels containing two size distributions (10 nm or less and 10 - 100 nm) of TiC and VC carbides are subjected to tensile tests in-situ in an electrochemical hydrogen charging environment. Hydrogen is found to be trapped in interstitial matrix sites on the precipitate/matrix interface with activation energies of 14 - 20 kJ/mol and inside misfit dislocation cores with energies of 27 - 37 kJ/mol. All steels are embrittled by 15 to 20%, except the TiC steel with semi-coherent carbides up to 100 nm, which is embrittled by 37%. This is caused by accelerated intergranular fracture as a result of hydrogen trapped in dislocation pile-ups around grain boundary precipitates. The steel with coherent VC nano-carbides retained the highest strength and ductility during in-situ testing. This is therefore the optimal carbide configuration for use in hydrogen environments.

This work studies the effect of TiC and VC precipitate sizes on hydrogen trapping and embrittlement. Two experimental ferritic HSLA steels containing either TiC or VC carbides for precipitation strengthening are annealed in nitrogen and hydrogen gas. This results in a hydrogen uptake of up to 0.91 and 0.44 wppm in the TiC and VC steels, respectively. TEM and TDS analysis indicate that semi-coherent TiC particles trap hydrogen in misfit dislocations with an activation energy of 43 kJ/mol. Coherent VC particles are suggested to trap hydrogen in interface carbon vacancies, with an energy between 53 and 72 kJ/mol. Carbon vacancies are the likely trapping site in incoherent precipitates, where SIMS imaging confirms that incoherent TiC precipitates trap preferentially near the interface, whereas incoherent VC precipitates trap throughout their bulk. Neither alloy is embrittled in SSRT tests after hydrogen absorption, which shows that these precipitates can be used as both a hydrogen sink and a strengthening mechanism in steels. Graphical

In this work, the hydrogen fatigue of pipeline steel X60, its girth welds and weld defects were investigated through in situ fatigue testing. A novel in situ gaseous hydrogen charging fatigue set-up was developed, which involves a sample geometry that mimics a small-scale pipeline with high internal hydrogen gas pressure. The effect of hydrogen was investigated by measuring the crack initiation and growth, using a direct current potential drop (DCPD) set-up, which probes the outer surface of the specimen. The base and weld metal specimens both experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life manifested solely in the crack growth phase; hydrogen accelerated the crack growth by a factor of 4. The crack growth rate for the weld metal accelerated by a factor of 8. However, in contrast to the base metal, the weld metal also experienced a reduction of 57% in resistance to crack initiation. Macropores (>500 µm in size) on the notch surface reduced the fatigue life by a factor of 11. Varying the pressure from 70 barg to 150 barg of hydrogen caused no difference in the hydrogen fatigue behavior of the weld metal. The fracture path of the base and weld metal transitioned from transgranular and ductile in nature to a mixed-mode transgranular and intergranular quasi-cleavage fracture. Hydrogen accelerated the crack growth by decreasing the roughness- and plasticity-induced crack closure. The worst case scenario for pipelines was found in the case of weld defects. This work therefore highlights the necessity to re-evaluate pipelines for existing defects before they can be reused for hydrogen transport.

This work discusses the design and demonstration of an in-situ test setup for testing pipeline steels in a high pressure gaseous hydrogen (H₂) environment. A miniature hollow pipe-like tensile specimen was designed that acts as the gas containment volume during the test. Specific areas of the specimen can be forced to fracture by selective notching, as performed on the weldment. The volume of H₂ used was minimised so the test can be performed safely without the need of specialised equipment. The setup is shown to be capable of characterising Hydrogen Embrittlement (HE) in steels through testing an X60 pipeline steel and its weldment. The percentage elongation (%El) of the base metal was found to be reduced by 40% when tested in 100 barg H₂. Reduction of cross-sectional area (%RA) was found to decrease by 28% and 11% in the base metal and weld metal, respectively, when tested in 100 barg H₂. Benchmark test were performed at 100 barg N₂ pressure. SEM fractography further indicated a shift from normal ductile fracture mechanisms to a brittle transgranular (TG) quasi-cleavage (QC) type fracture that is characteristic of HE.