In recent decades, high strength steels in thick sections have been increasingly used in offshore structures where they are subjected to harsh service conditions such as freezing temperatures and high static/dynamic loading. At these conditions they are susceptible to a transition from ductile failure to a dangerous brittle (cleavage) predominant type of failure, which occurs well before yielding. One of the main challenges in employing thick sections is the through-thickness heterogeneous variance of microstructures as a result of the processing route owing to a gradient of cooling rates from the surface to the bulk. As the material’s mechanical and fracture behaviour strongly depend on the microstructure, the through-thickness microstructural heterogeneity leads to a significant scatter in mechanical and fracture properties, which makes it difficult to predict and control cleavage fracture. From the body of literature establishing microstructural dependence on cleavage failure, several features contributing to this type failure can be identified. Phases, grain size, grain boundary misorientations and the presence of secondary phase constituents can play a major role in failure through cleavage. Additionally, cleavage failure is also sensitive to the crack depth to width ratio (a/W). This study investigates the microstructural features contributing to cleavage failure in a 100 mm thick S690QT high strength steel plate by performing mechanical and fracture toughness tests. In order to improve mechanical properties and cleavage fracture toughness, an isoparametric study employing rapid cyclic heating with the objective of grain refinement was performed. The steel plate has coarser prior austenite grain (PAG) sizes, and a larger area and number fraction of inclusions in the middle section. Additionally, segregation bands as a result of solute segregation during the solidification process were observed to be dispersed throughout the middle section. The middle section was also characterized by lower hardness compared to the top section. The detrimental effects of the middle section were evidenced by inferior low temperature tensile properties and cleavage fracture toughness. This was attributed to the larger PAG sizes, larger area and number fractions of inclusions, and segregation bands in the middle section. The specimen orientation with respect to the rolling direction was found to have no effects on the tensile properties. Additionally, different a/W geometries and notch orientations with respect to the rolling direction were used to investigate the role of constraint effect and rolling orientation, respectively, in the fracture behaviour. Shallow-notched specimens representative of the defects found in offshore structures demonstrated a higher fracture toughness than the deep-notched specimens. This was attributed to lower hydrostatic stresses at the crack tip, which reduces stress triaxiality. The isoparametric study resulted in average grain size reduction by 41% and proved to improve micro-hardness, low temperature tensile properties and cleavage fracture toughness by 5%, 13% and 41% respectively. Fractographic analysis on the fracture toughness specimens revealed the presence of O, C-rich regions which are known to promote brittle behaviour.

Thick-section high strength steels (HSS) are widely used in engineering applications with requirements of high toughness and strength. Structural components are commonly joined by welding, known to significantly degrade the mechanical properties of steel due to changes in microstructure (e.g., grain size coarsening and martensite-austenite (M-A) constituent formation). This degradation is even more prominent at sub-zero temperatures, where steels exhibit ductile-to-brittle transition behaviour and cleavage becomes the governing fracture mechanism. The most detrimental area is the part of the base material exposed to high temperatures during welding, known as the heat affected zone (HAZ). This work investigates the microstructural features affecting cleavage fracture in the most brittle areas of a quenched and tempered (QT) S690 HSS plate of 100 mm thickness, the coarse-grained and intercritical coarsegrained HAZ (CGHAZ and ICCGHAZ). The effect of intercritical peak temperature and heat input is also examined. Results show that fracture toughness is strongly affected by the matrix properties. ICCGHAZ 750 °C zone, exhibiting the lowest hardness, preserved relatively high levels of fracture toughness, while CGHAZ with the highest hardness had a substantial fracture toughness deterioration. The comparison of the two different intercritical peak temperatures, 750 °C and 800 °C, revealed a strong influence of this parameter on fracture toughness since a significant toughness reduction of ICCGHAZ 800 °C compared to 750 °C was present. The larger area fraction of M-A constituents in ICCGHAZ 750 °C and 800 °C compared to CGHAZ does not influence fracture toughness, which is attributed to the small size of M-As. Although M-As are responsible for crack initiation, the majority have a size smaller than 1 µm which is not sufficient to cause unstable fracture. Consequently, the step of crack propagation is governed by the matrix. Regarding the heat input effect, a reduction from 2.2 to 1.1 kJ/mm resulted in an increase of fracture toughness in CGHAZ and deterioration in ICCGHAZ 750 °C, however, CGHAZ remained the most critical zone. The improvement in CGHAZ is attributed to a higher fraction of smaller M-A constituents leading to less critical crack initiation sites. The fracture profile shows that the main crack propagates transgranularly and can be diverted by PAG, with high-angle grain boundaries. The investigation of secondary cracks revealed that the slender M-A constituents do not act as crack arrestors as they can easily be cut off. On the other hand, blocky M-A constituents can act as crack arrestors when the crack width is comparable to their size. This thesis provides more knowledge on the cleavage fracture process of HAZ and guidelines on how to control cleavage fracture in welded structures and improve the design of welds for structural applications.

Additive Manufacturing (AM), commonly known as 3D printing, exemplifies the recently emerging processing methodologies that aim to substitute the conventional routes, such as to produce parts with complex geometries and eliminate expensive tooling. AM also allows high degree of freedom and rapid prototyping for functional part optimization. This has led to a renewed interest in the Functionally Graded Materials (FGMs). FGMs are a class of novel materials designed to have graded compositions or microstructures with tailored properties. Selective Laser Melting (SLM) is one of the most widely used AM method showing great potential to produce parts made from Inconel 718, a Nickel-based superalloy.This study aims to investigate the microstructural gradients in Inconel 718 produced with SLM by manipulating the thermal fields acting during the production and their subsequent effect on fatigue behavior. Two different laser powers, 950 W and 250 W were used to develop coarse grained and fine grained microstructures respectively. Ungraded and graded specimens were produced to study the fatigue crack growth in individual as well as graded microstructures under cycling loading. The effect of standard post-process heat treatments on the microstructure and fatigue properties of as-printed (AP) AM Inconel 718 was also studied. The two heat treatments under discussion here are homogenisation + solution + aging (HT) and hot isostatic pressing + HT (HIPHT). Direct Current Potential Drop (DCPD) method was used to measure the fatigue crack growth rates in standard tests to identify the fatigue properties of ungraded microstructures. A new approach of using a constant ΔK procedure was employed for graded specimens to investigate the crack growth rate as a function of the crack interaction with local microstructure.The coarse columnar grains with preferred <001> texture were found elongated along the building direction (BD) and their axis of elongation in the specimens changed as a function of BD. Fine grains were found equiaxed and randomly oriented. HT had no significant effect on this observed trend, while HIPHT entirely altered the printed microstructure. The grain sizes, orientation as well as heat treatments were found to be affecting the fatigue behavior of individual microstructures. Fine grained microstructures showed a slower fatigue crack propagation (by ≈70% in AP, ≈40% in HT and ≈45% in HIPHT) than coarse grained. Fatigue cracks propagated slower in coarse grains oriented perpendicular to the crack path in AP (≈80%) while they were slower when oriented parallel to the crack path in HT (≈75%) and HIPHT (≈9%). The interfaces produced in AP and HT graded specimens were seen to introduce barriers for crack propagation and reduce the local crack growth rate. The same was not observed in HIPHT due to diminished gradients, resulting from grain coarsening.Thus, this study has successfully demonstrated the feasibility of using AM to fabricate future FGMs featuring altered fatigue response of the local microstructures.