Cleavage fracture micromechanisms in thick-section quenched and tempered S690 high-strength steels

J Mater Sci METALS & ORROSION Metals &Ccorrosion Cleavage fracture micromechanisms in thick-section quenched and tempered S690 high-strength steels Virgı́nia Bertolo1,* Vera Popovich1 , Quanxin Jiang1, Urša Tiringer1, Carey L. Walters2,3, Jilt Sietsma1, and 1 Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands Structural Dynamics, TNO, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands 3 Maritime and Transport Technology, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands 2 Received: 7 June 2022 ABSTRACT Accepted: 9 October 2022 For structural assessment and optimal design of thick-section high-strength steels in applications under harsh service conditions, it is essential to understand the cleavage fracture micromechanisms. In this study, we assess the effects of through-thickness microstructure of an 80-mm-thick quenched and tempered S690 high-strength steel, notch orientation, and crack tip constraint in cleavage nucleation and propagation via sub-sized crack tip opening displacement (CTOD) testing at -100 °C. The notch was placed parallel and perpendicular to the rolling direction, and the crack tip constraint was analysed by varying the a/W ratio: 0.5, 0.25, and 0.1. The notch orientation does not play a role, and the material is considered isotropic in-plane. Nb-rich inclusions were observed to act as the weak microstructural link in the steel, triggering fracture in specimens with the lowest CTOD values. While shallow-cracked specimens from the top section present larger critical CTOD values than deep-cracked ones due to stress relief ahead of the crack tip, the constraint does not have a significant influence in the middle due to the very detrimental microstructure in the presence of Nb-rich inclusions. Some specimens show areas of intergranular fracture due to the combined effect of C, Cr, Mn, Ni, and P segregation along with precipitation of Nb-rich inclusions clusters on the grain boundaries. Several crack deflections at high-angle grain boundaries were observed where the neighbouring sub-structure has different Bain axes. Ó The Author(s) 2022 Handling Editor: Megumi Kawasaki. Address correspondence to E-mail: https://doi.org/10.1007/s10853-022-07841-1 J Mater Sci Introduction Cleavage fracture, as the most dangerous form of fracture, raises concern in structural components under harsh operating conditions. Although highperformance materials with an optimal combination of strength and toughness, such as high-strength steels (HSSs), are often selected for structural applications, severe operating conditions can make them susceptible to brittle fracture [1–3]. For example, thick-section HSSs are applied in offshore structures, including equipment necessary for installing green energy farms and decommissioning old structures that had been used for the exploitation of oil and gas. In such applications, the service environments account for the presence of high stresses and the potential of very low temperatures. As body-centred cubic (BCC) HSSs are known to transition from ductile to brittle when the temperature is reduced or high strain rates are applied, the potential for catastrophic cleavage fracture exists. Cleavage is the governing micromechanism in the ductile-to-brittle transition region and the lower shelf [4]. Hence, for integrity assessment and optimal material design of these structures, including modelling approaches, the investigation of cleavage initiation and propagation micromechanisms of thick-section HSSs is required. The influence of microstructural factors on cleavage fracture toughness and micromechanisms has been investigated in the literature [5–10]. However, there are still some knowledge gaps to be filled. First, from the microstructural point of view, thicksection HSSs display complex and multiphase microstructures with a significant degree of inhomogeneity through the thickness (e.g. grain size, inclusions, and phase fractions) [5, 11, 12]. Consequently, a considerable scatter of mechanical and fracture properties is observed along the thickness [5, 6, 12, 13], resulting in great consequences for accurate structural integrity evaluation and definition of structural design specifications. To assess cleavage fracture for such steel plates, many experimental studies found in the literature use Charpy testing, which has limitations regarding specimen plastic constraint–blunt notch instead of a sharp crack–strain rate–dynamic loading instead of quasi-static loading– and specimen size and size-dependent property–relatively small, not representing metallurgical structural variations and material true property [5, 12, 13]. Another limitation of these studies is that they are mostly focused on modified or experimental steels that are not used in industrial practice. Thus, they do not necessarily have the actual heterogeneous microstructures observed in operating structural components [12, 13]. Moreover, they lack sufficiently detailed material characterisation to allow for an accurate and comprehensive analysis of cleavage fracture [5, 6, 12, 14]. Hence, these studies do not provide a representative understanding of the involved fracture micromechanisms, crack initiation and propagation, in structures. For instance, Pallaspuro et al. [13] carried out a similar study where a comprehensive and statistical microstructural characterisation is provided. However, the steel is experimental, where material from different positions is separately hot-rolled and heat-treated, resulting in a different microstructure and mechanical and fracture properties representative of real commercial steels. Liu, Zhang and Li, and Wang et al. [5, 12] also investigated the scatter of properties through the thickness of thick-section steels connecting it with microstructural features. These studies were carried out in steels that are not commercial but are subjected to similar industrial processing routes. However, the authors do not provide sufficient statistical microstructural information for the assessment of various microstructures features on the onset and propagation of cleavage cracks. Moreover, fracture is evaluated in terms of impact toughness via Charpy tests. Studies related to micromechanism-driven modelling of cleavage fracture share this knowledge gap. In a multi-barrier theory, cleavage fracture of ferritic steels is regarded as the result of the successive occurrence of three microscale events: nucleation of a slip-induced crack at a brittle second-phase particle (i.e. carbides in steels) or inclusion; propagation of the microcrack across the particle/matrix interface under the local stress state; and propagation of the grain-sized crack to neighbouring grains across the grain boundary under the local stress state [15–18]. Hence, the use of a multi-barrier theory requires an understanding of the relationship between representative and stati (...truncated)