Episode 52 - Cleavage Fracture in Heat-Affected Zones of High Strength Steel скачать в хорошем качестве

Episode 52 - Cleavage Fracture in Heat-Affected Zones of High Strength Steel

Presenters: Dr. Virgínia Bertolo, Dr. Quanxin Jiang Delft University of Technology - Materials Science and Engineering Department Abstract: High strength steels are widely used for structural applications, where a combination of excellent strength and ductile-to-brittle transition (DBT) properties are required. However, such a combination of high strength and toughness can be deteriorated in the heat-affected zone (HAZ) after welding. This work aims to develop a relationship between microstructure and cleavage fracture in the most brittle areas of welded S690 high strength structures: coarse-grained and intercritically reheated coarse-grained HAZ (CGHAZ and ICCGHAZ). Gleeble thermal simulations were performed to generate three microstructures: CGHAZ and ICCGHAZ at 750 and 800 °C intercritical peak temperatures. Their microstructures were characterised, and fracture properties were investigated at –40 °C, where cleavage is dominant. Results show that despite the larger area fraction of martensite-austenite (M-A) constituents in ICCGHAZ 750 °C, the CGHAZ is the zone with the lowest fracture toughness. Although M-A constituents are responsible for triggering fracture, their small size (less than 1 µm) results in local stress that is insufficient for fracture. Crack propagation is found to be the crucial fracture step. Consequently, the harder auto-tempered matrix of CGHAZ leads to the lowest fracture toughness. During propagation, M- A constituents with internal sub-structures, which have high kernel average misorientation and high-angle boundaries, are observed to deflect and arrest the secondary cracks. As a result, multiple pop-ins in load-displacement curves during bending tests are observed for the investigated HAZs. Based on the experimental results, a microstructure-based method with input from finite element analysis is used to model the cleavage behavior of the HAZ. It is found that the resistance to micro-crack propagation is more effective in heat-affected zones, which can be explained by the residual compressive stress in M-A constituents. This study informs the trade-off between microstructural parameters (e.g., M-A constituents and inclusions) and helps a designer choose a process for controlling cleavage toughness.