OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 The proposed methodology for the proper description of the microstructure to explain the impact toughness of weld metals is as follows: – The mechanical properties of weld metals are a consequence of the microstructure, mainly related to its alloying elements and the cooling rate. Regardless of the chemistry, for decades a cooling time from 800 to 500 °C (Δt8/5) has been used as a guideline for achieving desired welding performance and, in certain cases, a limited interval is recommended to ensure superior performance. For example, in some works a range of 5–20 s is recommended for high-strength weld metals [4, 17, 29–31]. – Although Δt8/5 does not take into account any microstructural transformations such as lower bainite formed at temperatures below 500 °C, the author [48] notes that this indicator can be used for high-strength steels because it refers not only to the time spent on cooling in the temperature range of 800–500 °C, but also to the entire thermal cycle, including the time spent at high temperatures. Usually, in order to achieve the recommended maximum value of Δt8/5, the welding energy input is limited, which leads to a decrease in the deposition rate of the weld metal and the need for a greater number of welding passes [4]. In general, longer cooling times due to higher heat inputs result in a coarser microstructure [38–57] and ultimately the presence of undesirable components such as granular bainite, fused bainite, or aggregate ferrite-carbides [48]. Although decomposition of MA components can improve mechanical properties, replacement with large carbides does not necessarily provide positive results [45–50]. To overcome these problems, suppliers can change the basic composition of welding consumables; this is because welding consumable standards allow a wider range of alloying and micro-alloying elements, and therefore each manufacturer off ers its own chemistry to achieve qualifi cation requirements. Conclusion In accordance with the purpose of this work and the objective of the review study, our analysis of numerous studies shows that for pipe steel weld metal, acicular ferrite (AF) is the most desirable component due to its fi ne grain size and interlocking structure with high-angle boundaries, providing high impact toughness [39–46]. It has also been reported [46–48] that AF is the weld metal component that best improves the toughness of HSLA steels with a yield strength of about 600 MPa. The structure with smaller grains has more boundaries and changes the direction of crack propagation, acting as eff ective barriers since it has diff erent crystallographic orientations [46–48]. Therefore, in recent decades, much work has been aimed at identifying the factors that control the formation of acicular ferrite [43–52]. According to the study [47, 52], conducted using electron backscatter diff raction analysis [48–51], polygonal ferrite also acts as a strengthening phase because its boundaries are high-angle ones and there is a relatively low dislocation density within the grains. As mentioned, a large amount of acicular ferrite is critical to the toughness of weld metals. Weld metal with a signifi cant amount of acicular ferrite can more eff ectively control other important parameters such as inclusions and MA components. This is because acicular ferrite refi nes the microstructure, which improves the size and distribution of MA, which determines the level of brittleness caused by MA [18]. In addition, a large amount of acicular ferrite, which is favored by small inclusions, minimizes the harmful eff ects of inclusions acting as initiation sites for both ductile and cleavage failures [28–44]. The combination of good toughness with a high proportion of acicular ferrite in the top bead of weld deposits is not the most suitable solution, even in single-pass welding [37–43]. In this regard, it is important to emphasize the position of the Charpy-V notch relative to the appearance of columnar deposited or reheated weld metal [43–53]. Moreover, it is necessary to take into account the infl uence of inclusions, which is directly related to the results of Charpy-V tests at higher temperatures. This situation may be diff erent for higher strength levels of steel, since the microstructure is dominated by bainite and martensite rather than acicular ferrite, and its relative amount and morphology are critical to toughness. Even if the microstructure is more uniform in both the columnar and heated regions, multiple welding passes are also relevant due to recrystallization. Clearly, all of these factors contribute to the results obtained from Charpy-V tests and make its analysis signifi cantly more complex than that associated with tensile tests.
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