Hydrogen as an alternative energy source has risen in popularity due to increased environmental awareness. Existing natural gas infrastructure is considered as a means to transport hydrogen, due to practicality and financial aspects. However, hydrogen can deteriorate the fatigue behaviour and thus induce premature failure. Since fatigue is a common failure mode in pipelines, a more thorough understanding of the effects of hydrogen on fatigue behaviour is required. In this work, the hydrogen fatigue of X60 pipeline steel and its girth welds was investigated through a combined approach of modelling and 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 specimen geometry involved an internal circumferential notch that induces a stress concentration factor (Kt = 3.0) related to the worst case scenario for pipelines. The effect of hydrogen was investigated by measuring the onset of crack initiation and growth using a newly designed direct current potential drop setup which probes the outer surface of the specimen. A FEA modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens, as well as to determine the stress states in the material. Results showed that both materials experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life (37%) manifested solely in the crack growth phase; hydrogen accelerated the crack growth (factor 4). In contrast, the reduction in fatigue life (68%) of the weld metal was due to accelerated crack growth (factor 8) and a decrease in resistance to crack initiation (57%). Varying the hydrogen gas pressure from 70 barg to 150 barg did not cause any differences in the fatigue behaviour. The presence of hydrogen influenced the fracture mechanisms of both materials. The fracture path of the base metal transitioned from transgranular and ductile in nature, to a mixed-mode transgranular and intergranular quasi-cleavage fracture. The weld metal exhibited a similar transition, however in the inert environment some intergranular features were observed at the prior austenite grain boundaries. The presence of hydrogen reduced the crack tortuosity. This is associated with a decrease in roughness- and plasticity-induced crack closure, thereby accelerating the crack growth. It is inferred that hydrogen-enhanced localised plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) were the dominant types of hydrogen embrittlement mechanisms during fatigue of this pipeline material. It was concluded that the weld metal is more susceptible to hydrogen fatigue than the base metal in a gaseous hydrogen environment. The worst-case scenario for pipelines is in the case of weld defects. The weld defects involved in this work were macropores (0.5-1.0 mm) with a spheroid morphology. When these defects were located at the notch surface, the resistance to crack initiation decreased by 92% compared to non-porous specimens in nitrogen. The existing natural gas infrastructure could have accumulated similar flaws during service life, which would make them unreliable for safe hydrogen transport. The costs associated with the repurposing of these pipe segments could raise unexpected economic hurdles, hindering the transition to a hydrogen economy.

To mitigate global warming, society must reduce greenhouse gas emissions. The transportation sector is one of the largest energy consumers and greenhouse gas emitters. One way to mitigate the emissions of this sector is to reduce the weight of vehicles using Advanced High Strength Steels (AHSS), particularly dual phase steels (DP). However, these steels are subject to hydrogen embrittlement which hinders their full potential and application inside the automotive industry. The effect of pre-strain level and in-situ pre-straining on hydrogen embrittlement was investigated for DP1000. Therefore, in-situ tensile test was performed at 0.5% and 2% plastic pre-strain to study hydrogen embrittlement by measuring strain at fracture. This test was done under ex-situ and in-situ pre-strain conditions. In addition, sample charging time was varied from 15 to 60 minutes for 2 % plastic pre-strain. Thermal Desorption spectroscopy was performed to measure hydrogen uptake. Fracture surface was studied by scanning electron microscopy. Results showed that pre-strain plays an important role in hydrogen embrittlement, namely 0.5% and 2 % plastic pre-strain reduced the final strain to fracture by almost 50 % for both in-situ and ex-situ charging cases. However, no significant increase in hydrogen embrittlement occurs for the increment from 0.5% to 2% pre-straining, except for the 0.1 wppm increase in hydrogen content. Differences in results for in-situ and ex-situ testing were only observed for 2% pre-strain at 60 minutes charging. Increase in charging time resulted in increased hydrogen uptake, however, no severe effects were observed in hydrogen embrittlement. For future work it is recommended to study different extra pre-strain levels, further increase the charging time and investigate the effect of various DP1000 microstructures, such as ferrite, martensite and retained austenite ratios.

Carbon dioxide emissions from the transportation sector account for roughly one-third of world-wide CO2 emissions. Governmental pressure to tackle this problem has forced the industry to adapt with regard to material development. One such adaption is the development and application of advanced high-strength steels (AHSS), offering high specific strength combined with improved ductility. This enables lightweighting of consumer cars, as well as improving the overall occupant safety and crashworthiness. Despite these apparent advantages of AHSS, sheet metal forming operations can cause unexpected edge-cracking. In this research the occurrence of edge-cracking is investigated and rationalized with fracture toughness testing. The Essential Work of Fracture (EWF) methodology is applied on double-edge notched (DENT) specimens, an increasingly applied method to characterize the crack propagation resistance of thin sheet metal. A novel sample preparation method is evaluated here, based on sheared notches instead of fatigue pre-cracked specimens. The obtained fracture toughness parameters are validated against laboratory-scale deep-drawing experiments to estimate its predictive capabilities. Scanning electron microscopy (SEM) was employed to examine the fractured surfaces and to identify the relevant micro-mechanisms of fracture. It was found that the obtained EWF results are in good agreement with the observed cracking behaviour during deep-drawing. Both the established method (fatigue pre-cracking) as well as the novel method of sheared notches were able to reveal the intrinsically low fracture toughness, otherwise undetected with conventional tensile testing. From the fractured surfaces of the EWF samples the relevant micro-mechanisms of failure were identified that govern the fracture toughness properties. Transgranular cleavage fracture was responsible for brittle behaviour while failure ductile failure occurred via microvoid coalescence. Although the fatigue pre-cracking method for EWF testing seems to show a higher accuracy in predicting the cracking susceptibility, the shearing method can be employed as a rapid routine test to identify deterioration of fracture toughness properties early on.

To decrease vehicle emissions, automotive manufacturers aim to decrease materials usage. Dual-phase (DP) steels are used in safety critical locations of a vehicle and with stronger DP steels, less material is needed. However, DP steels are known to be affected by hydrogen embrittlement (HE), which decreases the ductility and is thus of significant concern. This work investigates the effect of heat treatment on the microstructure and HE of DP steel. Heat treatments are developed and conducted using dilatometry, with obtained microstructures characterized using scanning electron microscopy (SEM), optical microscopy (OM), electron backscatter diffraction (EBSD) and hardness measurements. Tensile specimens are heat treated using a Gleeble 3800-GTC thermo-mechanical simulator, before evaluating the HE effect by measuring the total absorbed hydrogen content using thermal desorption spectroscopy (TDS), and determining the static mechanical properties using slow strain-rate tensile (SSRT) testing. The fracture micromechanisms are investigated with fractography using SEM and EBSD with interrupted SSRT and fractured specimens. The critical transformation temperatures are determined at 686±1 ⁰C and 571±10 ⁰C for Ar3 and Ar1 respectively. Intercritical annealing at 590 ⁰C, 630 ⁰C, and 670 ⁰C result in a martensite content of 24.97±6.43 %, 40.41±4.21 %, and 77.63±6.97 %, respectively. Hardness values of 240±31, 300±9, and 380±20 HV1 are obtained for the respective intercritical annealing conditions and are an accurate method for comparing the martensite content and microstructure of dilatometry and Gleeble specimen. The martensite distribution changes from a discontinuous network of lath martensite with small martensite islands at 590 ⁰C, to a continuous network of lath martensite without martensite islands at 670 ⁰C. TDS yield an increase in the total absorbed hydrogen content of 1.16±0.11 wppm, 1.29±0.13 wppm, and 1.58±0.27 wppm at increasing intercritical annealing temperature, which is attributed to an increase in the ferrite/martensite interphase. Through SSTT the optimal combination of ultimate tensile strength (UTS) and elongation is found at 630 ⁰C, with UTS of 1025±8 MPa and elongation of 12.6±0.6 %. For all specimens significant HE is observed, with an embrittlement index of 58.9±7.1 %, 89.2±1.1 %, and 85.8±2.5 % for increasing intercritical annealing temperature. The highest embrittlement at 630 ⁰C is attributed to dislocation pile-up around martensite islands, severely increasing embrittlement. Fractographic investigation revealed the presence of quasi-cleavage fracture bands across the specimen cross-section. This is attributed to preferred crack propagation along martensite bands in the cross-section, with preferred crack nucleation at the martensite surface layer of the specimen short edge. EBSD of the fractured specimen revealed an increase in the kernel average misorientation (KAM) surrounding the ferrite/martensite interface and martensite islands, indicating an increased HE action due to the HEDE and HELP mechanisms. It is concluded that there is considerable embrittlement of the heat-treated DP steel, which is significantly influenced by the martensite content and distribution. Since DP steels are used in safety critical components, controlling the distribution of martensite in the microstructure is vital, and avoiding surface layer martensite and martensite bands can reduce the HE effect.

To decrease vehicle emissions, automotive manufacturers aim to decrease materials usage. Dual-phase (DP) steels are used in safety critical locations of a vehicle and with stronger DP steels, less material is needed. However, DP steels are known to be affected by hydrogen embrittlement (HE), which decreases the ductility and is thus of significant concern. This work investigates the effect of heat treatment on the microstructure and HE of DP steel. Heat treatments are developed and conducted using dilatometry, with obtained microstructures characterized using scanning electron microscopy (SEM), optical microscopy (OM), electron backscatter diffraction (EBSD) and hardness measurements. Tensile specimens are heat treated using a Gleeble 3800-GTC thermo-mechanical simulator, before evaluating the HE effect by measuring the total absorbed hydrogen content using thermal desorption spectroscopy (TDS), and determining the static mechanical properties using slow strain-rate tensile (SSRT) testing. The fracture micromechanisms are investigated with fractography using SEM and EBSD with interrupted SSRT and fractured specimens. The critical transformation temperatures are determined at 686±1 ⁰C and 571±10 ⁰C for Ar3 and Ar1 respectively. Intercritical annealing at 590 ⁰C, 630 ⁰C, and 670 ⁰C result in a martensite content of 24.97±6.43 %, 40.41±4.21 %, and 77.63±6.97 %, respectively. Hardness values of 240±31, 300±9, and 380±20 HV1 are obtained for the respective intercritical annealing conditions and are an accurate method for comparing the martensite content and microstructure of dilatometry and Gleeble specimen. The martensite distribution changes from a discontinuous network of lath martensite with small martensite islands at 590 ⁰C, to a continuous network of lath martensite without martensite islands at 670 ⁰C. TDS yield an increase in the total absorbed hydrogen content of 1.16±0.11 wppm, 1.29±0.13 wppm, and 1.58±0.27 wppm at increasing intercritical annealing temperature, which is attributed to an increase in the ferrite/martensite interphase. Through SSTT the optimal combination of ultimate tensile strength (UTS) and elongation is found at 630 ⁰C, with UTS of 1025±8 MPa and elongation of 12.6±0.6 %. For all specimens significant HE is observed, with an embrittlement index of 58.9±7.1 %, 89.2±1.1 %, and 85.8±2.5 % for increasing intercritical annealing temperature. The highest embrittlement at 630 ⁰C is attributed to dislocation pile-up around martensite islands, severely increasing embrittlement. Fractographic investigation revealed the presence of quasi-cleavage fracture bands across the specimen cross-section. This is attributed to preferred crack propagation along martensite bands in the cross-section, with preferred crack nucleation at the martensite surface layer of the specimen short edge. EBSD of the fractured specimen revealed an increase in the kernel average misorientation (KAM) surrounding the ferrite/martensite interface and martensite islands, indicating an increased HE action due to the HEDE and HELP mechanisms. It is concluded that there is considerable embrittlement of the heat-treated DP steel, which is significantly influenced by the martensite content and distribution. Since DP steels are used in safety critical components, controlling the distribution of martensite in the microstructure is vital, and avoiding surface layer martensite and martensite bands can reduce the HE effect.