OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Based on the data presented in this work, all microstructural aspects shown previously should be taken into account when performing a complete toughness analysis. Thus, an appropriate methodology for characterizing microstructure to explain treatment outcomes should include an analysis of all contributing factors. However, its relative importance varies for each weld metal and experimental procedure. The authors of this paper believe that the methodology described in the steps shown below is suitable for assessing the toughness of weld metals. The Charpy-V notch in all tests should be located in the area of interest to the researcher of the specimen. Step 1. Measure the proportion of columnar and reheated regions due to recrystallization eff ects using low magnifi cation optical microscopy. However, this this is not applicable to single pass weld metals; Step 2. Qualitative and quantitative analysis of the main microstructural constituents, namely primary ferrite, acicular ferrite, polygonal ferrite, second phase ferrite and martensite, using optical microscopy (1,000× magnifi cation). However, for stronger weld metals containing a mixture of acicular ferrite, secondphase ferrite, and martensite, SEM analysis is sometimes necessary to clarify the major constituents (~1,000–3,000× magnifi cation). In addition, the EBSD method can be used as a complementary one. In this case, useful results include eff ective grain size (EGS) and high-angle boundary (HAB) frequency obtained from grain boundary disorientation profi les; Step 3. Qualitative and quantitative analysis of micro-phases, carbides and components of MA using SEM (~ 2,000–5,000× magnifi cation). Although some studies claim that EBSD is an excellent method to confi rm the presence of MA components, it is important to remember that statistical results depend on the number of points measured, and in this regard, quantitative analysis using SEM is easier and faster. The authors of this paper believe that the software available for EBSD is still not reliable enough for this task due to its complexity. Step 4. Qualitative and quantitative analysis of non-metallic inclusions using SEM/EDS (~1,500× magnifi cation). This analysis is useful for higher energy levels and when comparing diff erent welding processes. In addition, this may confi rm the potential of inclusions as acicular ferrite nuclei. There are more detailed studies in the literature, the full analysis of which is not required. Using all the steps above involves more complex analysis. Our analysis of various sources of information on the assessment of various microstructures of C–Mn and high-strength steels welds and the establishment of the relationship between microstructure and impact strength based on experimental results obtained over the past decades for metals of welds with tensile strength from 400 to 1,000 MPa allowed formulate conclusions for further research on this topic. Summary 1. It is shown that high-strength low-alloy (HSLA) steels have a good combination of strength, toughness and weldability and are widely used in long-distance oil and gas transportation systems [2–4]. Pipeline steels Cr80, 100, 120 are produced using thermo-mechanical control process (TMCP) followed by accelerated cooling to achieve excellent mechanical properties. An important consideration when preparing pipeline welds is to achieve equal or higher strength and toughness of the weld metal compared to the base metal to avoid failure of the weld metal. 2. Based on an analysis of experimental data from various authors, it is shown that it is extremely important to have an optimal microstructure of the weld metal, which largely depends on the composition of the electrode wire. Major alloying elements such as Cu, Ni and Mo, as well as micro-alloying elements such as V, Nb, Ti and B, are widely used to optimize the microstructure and properties of pipeline steels. 3. The predominant microstructure of acicular ferrite (AF) with MA islands as the second phase is shown to be the optimal microstructure for pipeline steel weld metal. Extensive research into the mechanisms of acicular ferrite formation in weld metals shows that elements such as C, Mn, Si, Ni, Al, Ti, Nb and Mo infl uence the nucleation of acicular ferrite within austenite grains. The eff ect of Ti addition on the microstructure and formation of inclusions in steel pipeline joints welded by automatic submerged arc welding showed that the best combination of microstructure and toughness can be obtained by adding Ti in
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