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

Journal article (2023)

Authors

V. Morete Barbosa Bertolo Team Vera Popovich - Mechanical, Maritime and Materials Engineering
Q. Jiang Steel & Composite Structures - CEG
M. Terol Sanchez Team Vera Popovich - Mechanical, Maritime and Materials Engineering
A.C. Riemslag Team Vera Popovich - Mechanical, Maritime and Materials Engineering
C.L. Walters Ship Hydromechanics and Structures - Mechanical, Maritime and Materials Engineering , TNO
J. Sietsma Team Kevin Rossi - Mechanical, Maritime and Materials Engineering
V. Popovich Team Vera Popovich - Mechanical, Maritime and Materials Engineering
Research Group
Team Vera Popovich (Mechanical, Maritime and Materials Engineering) (TU Delft)
Copyright: © 2023 V. Morete Barbosa Bertolo, Q. Jiang, M. Terol Sanchez, A.C. Riemslag, C.L. Walters, J. Sietsma, V. Popovich
DOI
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https://doi.org/10.1016/j.msea.2023.144762
High strength steel Cleavage fracture toughness Gleeble HAZ M-A constituents Pop-in
More Info
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Published Date

2023

Language

English

Faculty

Mechanical, Maritime and Materials Engineering

Department

Materials Science and Engineering

Research Group

Team Vera Popovich

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 the tensile 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. The main crack propagates transgranularly, along {100} and {110} planes, and neither the necklace structure at prior austenite grain boundaries of ICCGHAZs nor M-A constituents are observed as preferential sites for crack growth. The fracture profile shows that prior austenite grain boundaries and other high-angle grain boundaries (e.g., packet and block) with different neighbouring Bain axes can effectively divert the cleavage crack. Moreover, 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.