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.