Cleavage fracture micromechanisms in simulated heat affected zones of S690 high strength steels

High strength steels are widely used for structural applications, where a combination of excellent and ductile-to-brittle transition (DBT) properties required. However, such high toughness can be deteriorated in the heat affected zone (HAZ) after welding. This work aims to develop relationship between microstructure cleavage fracture most brittle areas welded S690 structures: coarse-grained intercritically reheated HAZ (CGHAZ ICCGHAZ). Gleeble thermal simulations were performed generate three microstructures: CGHAZ ICCGHAZ at 750 800 °C intercritical peak temperatures. Their microstructures characterised, tensile investigated − 40 °C, is dominant. Results show that despite larger area fraction martensite-austenite (M-A) constituents with lowest toughness. Although M-A responsible triggering fracture, their small size (less than 1 μm) results local stress insufficient fracture. Crack propagation found crucial step. Consequently, harder auto-tempered matrix leads The main crack propagates transgranularly, along {100} {110} planes, neither necklace structure prior austenite grain boundaries ICCGHAZs nor observed as preferential sites growth. profile shows other high-angle (e.g., packet block) different neighbouring Bain axes effectively divert crack. Moreover, M A internal sub-structures, which have kernel average misorientation boundaries, deflect arrest secondary cracks. As result, multiple pop-ins load-displacement curves during bending tests HAZs.