OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Although MA has been widely studied in recent decades, the eff ect of cooling rate on its volume fraction remains controversial [23–29]. Some researchers have shown that increasing the cooling rate increases the MA fraction, while others have shown, on the contrary, [32–35] that a slower cooling rate decreases the MA fraction. Works [38–48] showed that for various steels there is an increase in the fraction of MA at a lower cooling rate. In addition to the eff ect of cooling rate on MA fraction, the eff ect of MA grain size, morphology and distribution on impact toughness has also not been established. This is largely due to the complex factors that determine impact strength, including the fraction, size, substructure and morphology of MA. It is generally accepted that MA reduces the impact strength of pipeline steel [4]. A slower cooling rate results in a coarser MA structure, resulting in poor toughness properties. In the work [43] it is reported that the formation of lath type (thin MA), associated with poor toughness, occurs at slower cooling rates, while block MA is formed at higher cooling rates. It is important for further analysis to interpret the microstructure of steel after welding because this is controversial because constituents that are part of the same primary structure may appear morphologically diff erent depending on the viewing plane (fi gs. 8, 9), and some structures may have similar morphological features but present diff erent mechanical properties [44–46]. Fig. 7 shows an overview of the evolution of the major components present in the weld metal as observed by optical microscopy (OM). This fi gure shows that the microstructure changes continuously with increasing carbon equivalent (Ceq). A mixture of acicular ferrite (AF), primary ferrite (PF) and ferrite with a second phase (FS) in the columnar region has lower strength values. In contrast, the reheat region is dominated by polygonal ferrite. In addition to the tendency to have a mixture of martensite and bainite with a higher content of alloying elements due to increased hardenability, it is worth noting the presence of similar components for both columnar and reheated areas. The terminology of microstructural constituents observed in weld metals has been very confusing [35], with diff erent terms being used to refer to the same constituent. This lack of clarity prompted the International Institute of Welding (IIW) to develop a general framework for microstructure quantifi cation [36] in the 1980s, where components were easily identifi ed using optical microscopy (OM). Another critical issue relates to the low resolution of optical microscopy for refi ning the constituents of refi ned weld metals, even when using higher magnifi cation than recommended by the IIW [38, 39]. To solve this problem, scanning electron microscopy (SEM) has been widely used in recent decades, mainly to separate bainite and martensite and evaluate micro-phases. However, sometimes even this method has limitations in distinguishing the overall microstructure. This occurs mainly for weld metals with a tensile strength greater than 600 MPa, where a mixed microstructure consisting of acicular ferrite, bainite (ferrite with a second phase) and martensite predominates. To ensure proper resolution in the study of microstructure, the EBSD technique is used as an additional tool [11, 12, 32]. This method has been considered as an interesting alternative [32–40] to overcome the shortcomings of optical microscopy. 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 high clarity provided by EBSD, especially at grain boundaries, is useful for separating acicular ferrite and bainite (second-phase ferrite). Regarding the assessment of MA components and inclusions, SEM analysis is more suitable for this task [36–39]. Thus, it is believed [11, 12, 24, 25, 32–40] that a combination of OM, SEM and EBSD methods provides the best methodology for the study of metal welds in C-Mn steel in the presence of a refi ned microstructure. In [38], the author examined the IIW scheme for the main structures that develop during reduction and shear transformation in steels. However, he noted that questions remain to be resolved regarding reaction kinetics, especially elucidating the growth mechanisms of bainite, which could lead to greater precision in distinguishing bainite from other phases. In [38], a critical review is presented to clarify the existing confusion in the literature regarding bainite and acicular ferrite due to the similarity in appearance of these two microstructural constituents observed under an optical microscope. The works [44-48] present a description of the microstructural components in relation to low-carbon pipe steels.
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