OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Research methods Predicting impact toughness based on the microstructural characteristics of weld metals is diffi cult due to the large number of parameters involved [1, 11–18]. The common practice of relating impact toughness to the microstructure of the last bead of a multi-pass weld turned out to be unsatisfactory since the amount of acicular ferrite, the most desirable component, may not always make the main contribution to the impact toughness [20–32]. Parameters such as the recrystallized fraction, the presence of micro-phases and inclusions can also play an important role [32–36, 37–48]. Thus, in order to take into account the infl uence of all these parameters, the method [38, 39] proposed by the International Institute of Welding (IIW) is not comprehensive enough and therefore additional methods are needed. This situation is more relevant for high-strength steel weldments where very fi ne microstructures cannot be clearly identifi ed, resulting in incorrect microstructure identifi cation. The use of scanning electron microscopy as an auxiliary method to optical microscopy has been successfully used for many decades to study C-Mn and low-alloy metal welds, mainly in the assessment of refi ned microstructure. Recently [49–61], in addition to the previously mentioned methods, electron backscatter diff raction (EBSD) has also been used to provide a more effi cient analytical procedure. This method, which provides valuable grain boundary information, is useful for refi ned microstructures to confi rm constituents such as acicular ferrite, bainite and martensite. The mechanical properties of high-strength low-alloy pipe steels largely depend on its complex microstructure. However, the precise quantitative infl uence of individual microstructural elements (e.g., dislocations, grain boundaries, phase boundaries, volume fractions of the corresponding microstructure components, phase types, dispersion and shape of martensite islands, etc.) [2, 3, 11] is usually not easy to measure with traditional optical microscopymethods. Thus, it is a general question how to obtain quantitative values of the types and quantities of these diff erent microstructural components and its topological features. Various electron diff raction techniques, mainly used in scanning electron microscopy (SEM), are capable of providing comprehensive answers to these questions. Modern scanning electron microscopes with thermal fi eld emission guns, various sensitive detectors and fl exible stages are extremely versatile tools for detailed and quantitative analysis of the microstructure of bulk materials’ specimens with high resolution, with large statistics, in 2D and 3D, as well as provide the opportunity to implement various types of fi eld observations. The most important signals to be detected for microstructural analysis are backscattered electrons (BSE) for electron channel contrast imaging (ECCI) and orientation microscopy based on electron backscatter diff raction (ORM), as well as characteristic X-rays for compositional analysis using X-beam spectroscopy (XEDS) and secondary electron (SE) to observe surface morphology. The purpose of the work is to evaluate the various microstructures of metal welds of C-Mn and highstrength steels based on the analysis of various studies carried out by optical microscopy, scanning electron microscopy and EBSD methods, taking into account the infl uence of recrystallization in multi-pass welds, microstructural components, micro-phases, and inclusion. The objective of this analysis is to establish the relationship between the microstructure and toughness of some experimental results obtained over the past decades for weld metals with tensile strength from 400 to 1,000 MPa. The analysis was carried out using the methodology proposed in [32], to verify its eff ectiveness and explanation impact toughness behavior. Research results of several authors and discussion Infl uence of carbon equivalent on tensile strength and impact toughness of weld metals Figure 1 shows the eff ect of carbon equivalent on the strength and toughness of the weld metal from the review work [32]. It was shown in [32] that Ceq has a high dependency on the tensile strength of the weld metals (fi gure 1, a), and some studies have shown an almost linear increase in the ultimate strength of the weld metal with increasing Ceq. It can be seen that with an increase in the strength of the metal, a high scatter of values is observed, which may be due to diff erent cooling rates, since the high hardenability of the alloys contributes to the same microstructure of the entire weld metal. However, small deviations in cooling rates cause signifi cant
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