This study concentrates on the fatigue crack propagation behaviour of a high-strength low-alloy (HSLA) steel and austenitic stainless (AS) steel bi-material part, as obtained by wire arc additive manufacturing (WAAM). Due to partial mixing in the weld pool, the first layer of AS steel laid onto the previously deposited HSLA steel results in a diluted interface layer of distinct chemical and microstructural characteristics. Average Paris parameters are obtained for the interface layer along transverse and longitudinal planes to the deposition direction (BD-LD plane: m = 2.79, log₁₀(C) = –7.83 log₁₀(da/dN)) (BD-TD plane: m = 3.47, log₁₀(C) = –8.39 log₁₀(da/dN)). However, it is observed that this interface layer manifests an intriguing crack propagation behaviour. FCGR consistently drop as the crack front transitions from undiluted AS steel to the interface. At ΔK = 20 MPa⋅m^0.5, the greatest Δ is −0.77 log₁₀ steps (R = 0.1). As cracks near the HSLA fusion line, rates re-accelerate up to + 0.75 log₁₀ steps (R = 0.5). The phenomenon is attributed to the interplay between deformation-induced martensitic transformation and pre-existing allotropic martensite. Our findings, derived from a series of fatigue tests in correlation with multiscale microstructural and fracture characterization, offer insights into the damage-tolerant behaviour of these bi-material structures.

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Multi-barrier cleavage models consider cleavage fracture which is characterized by a series of microscale events. One of the challenges for multi-barrier cleavage models is the strong variations of cleavage parameters across different types of steels. The source and magnitude of the variations have not been studied systematically. In the current paper, cleavage parameters corresponding to fracture initiation at a hard particle and crack propagation overcoming grain boundaries are determined for three bainitic steels, a martensitic steel, and a ferritic steel, using a recently proposed model. It is found that the particle fracture parameter depends on particle morphology and composition, while the grain boundary cleavage parameter depends on the hierarchical grain structure. The determined values of cleavage parameters present a high degree of consistency among the five different steels, which allows the further application on microstructure design to control macroscopic toughness.

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Study of the cleavage behavior of heat treated S690 steel by a microstructure-based approach combined with finite element analysis is present in this paper. Cleavage simulations of steels subjected to heat treatments that cause grain refinement or simulate heat affected zones are performed, and are compared with experiments. It is found that the experimental improvement of toughness from grain refinement is 80% of what would be expected based on the model. The 20% difference is due to the lower number fraction of high-angle misorientation boundaries. It is also 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 martensite-austenite constituents. This research assesses the balance between microstructural parameters for controlling cleavage toughness.

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The use of materials in increasingly severe service conditions raises concerns about structural safety with respect to cleavage fracture. There are three main material-related challenges that structures face under harsh environments: 1) the trade-off between strength and toughness; 2) the ductile-to-brittle transition behaviour of BCC high strength steels; 3) the inhomogeneous microstructures found in multiphase steels, thick-section steels, and welded structures. Therefore, the objective of this research is to systematically investigate the cleavage fracture micromechanisms in high strength steels considering diverse microstructures (e.g., as-received commercial steel, thermally simulated heat-affected zones, and grain refined microstructure) and experimental conditions (e.g., plastic constraint and temperature). Thereby, this study provides a thorough understanding of the effect of the microstructural details on cleavage fracture behaviour of high strength steel structures allowing for failure control and improvement of cleavage-resistant steel’s design...@en

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.

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The through-thickness heterogeneous microstructure of thick-section high strength steels is responsible for the significant scatter of properties along the thickness. In this study, in order to identify the critical microstructural features in the fracture behaviour and allow for design optimisation and prediction of structural failure, the through-thickness microstructure of thick-section steels was extensively characterised and quantified. For this purpose, samples were extracted from the top quarter and middle thickness positions, and a combination of techniques including chemical composition analysis, dilatometry, and microscopy was used. The hardness variation through the thickness was analysed via micro-Vickers measurements and the local hardness variation in the middle section was studied via nanoindentation. The middle section presented larger prior austenite grain (PAG) sizes and larger sizes and area fraction of inclusions than the top section. Additionally, cubic inclusions were observed distributed as clusters in the middle, sometimes decorating PAG boundaries. Defects associated with the cubic inclusions or the interface between the matrix and the circular and cubic inclusions were observed in the mid-thickness. Moreover, the middle section presented long interfaces with the most significant hardness gradients due to the presence of hard centreline segregation bands. Hence, the microstructural and nanoindentation analyses indicated the middle section as the most likely area to have the lowest fracture toughness and, therefore, the most unfavourable section for fracture performance of the investigated S690QL high strength steel. The detrimental effect of the middle section was confirmed via CTOD tests where the middle presents lower fracture toughness than the top section.

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Thick section S690 QT steel is modelled with a modified multibarrier model that is based on the weakest-link mechanism. Segregation bands are modelled as discrete layers which have different grain size, yield properties, and local fracture parameters from outside of the bands. The results show that embrittlement from segregation bands can only be adequately reflected if the inhomogeneities of the fracture parameters are accounted for. The present methodology quantitively captures the cooperation of complex microstructural features in cleavage and can facilitate the trade-off between the effects of various microstructural parameters in toughness control.

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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.

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In this paper, the effect of microstructural anisotropy on the fatigue crack growth behaviour of the functionally graded Inconel 718 fabricated through laser powder bed fusion (L-PBF) is investigated. Different manufacturing parameters, including low and high laser powers, were used to produce a variety of non-graded (NG) and functionally graded (G) specimens in two build directions, vertical and horizontal. In addition, a group of heat treated wrought samples was tested as a reference. It was observed that the different manufacturing parameters result in various grain size, crystallographic textures, precipitates and Laves phases, porosity, and un-melted particles. Three-point bending fatigue tests were conducted to measure the threshold stress intensity factor (ΔK_th) and fatigue crack growth rate (FCGR),da/dN. Only the lower laser power L-BPF Inconel material was found to have comparable to the wrought heat treated material fatigue crack growth behaviour. Furthermore, a new approach of automatically controlling ΔK as a function of the crack length was employed for graded specimens to investigate the crack growth rate as a function of local microstructure. The FCGR value of the vertical L-PBF samples, in which the crack direction was perpendicular to the build direction, remained constant. In contrast, the da/dN value of the horizontal samples with the crack direction parallel to the build direction increased constantly with the increase of the crack length. This behaviour is in good agreement with the hardness profile of the graded materials. Melt pool boundaries, graded interface boundaries, and grain orientations close to 〈001〉 were found to deflect the crack path. Additionally, it was found that L-PBF material is more affected (at a low stress ratio of R = 0.1) by the roughness-induced crack closure than the wrought counterparts. This study has successfully demonstrated the feasibility of using an additive manufacturing process to fabricate functionally graded materials featuring tailorable fatigue response of the local microstructures.

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Macroscale cleavage fracture toughness of high strength steels is strongly related to the fracture of hard microstructural inclusions. Therefore, an accurate determination of the local stress on these inclusions based on the matrix stress is necessary for the statistical modelling of macroscale cleavage fracture. This paper presents analytical equations to quantitatively estimate the stress of the microstructural inclusions from the far-field stress of the matrix. The analytical equations account for the inclusion shape, the inclusion orientation, the far-field stress state and matrix material properties. Finite element modelling of a representative volume element containing a hard inclusion shows that the equations provide an accurate representation of the local stress state. The equations are implemented into a multi-barrier model and compared with CTOD experiments with two different levels of constraint.

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One of the main challenges in applying thick-section high-strength steels (HSS) at arctic condition in offshore and maritime industry is to maintain a sufficient level of toughness to prevent brittle failure. An aspect that requires special attention is the through-thickness microstructural variation which may result in different local mechanical responses affecting the overall material’s fracture behaviour. This paper presents an experimental study combining microstructural investigation and sub-sized fracture toughness testing at −100 °C of different sections of 80 mm S690QL steel aimed to evaluate the effect of microstructure on cleavage fracture. In addition, different crack depth to width ratios (a/W) were used to investigate the constraint effect, while different notch orientations were applied to assess the effect of rolling orientation. Results show lower fracture toughness for the middle of the plate, which was attributed to the presence of large Nb-rich inclusions which may feature pre-existing cracks and/or defects in the inclusion/matrix interface and also often distributed as clusters. It was also observed that a/W ratio plays an important role in fracture toughness showing shallow-notched specimens with substantially higher fracture toughness than deep cracked specimens. Moreover, microstructural features such as inclusions aligned parallel to the pre-crack can ease the crack propagation and contribute to a reduction in fracture toughness.

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Offshore activity in low-temperature areas requires the use of analysis methods that are capable of reliably predicting cleavage (brittle) fracture of ferritic steels in order to guarantee the structural integrity during service. Cleavage fracture is controlled by physical events at different size scales and is influenced by the multiple microstructural parameters of the material. The prediction of fracture toughness of steels based on the microstructure has received great attention, and relevant techniques have been continuously developed. This paper is aimed at reviewing the recent development of cleavage fracture modelling in steels and identifying the existing challenges to inspire further research. The paper contains three parts aimed at explaining how methods are developed and utilized to predict fracture toughness of steel from its microstructures. (1) The complex multiparametric nature of the microstructures of ferritic steels and its influence on cleavage fracture is introduced. (2) A review is given on the main perspectives and models in micromechanisms of cleavage fracture in steels. (3) Discussion is contributed to the link between micromechanisms and the local approach in cleavage fracture modelling. As a result, the paper gives a state of the art on microstructural mechanics and local approach methods of cleavage fracture modelling in structural steels.

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