OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Our analysis shows that, in relation to low-carbon pipe steels, the weld metal can have the following microstructures [11–49]: – primary ferrite, which is nucleated at the boundaries of the initial austenite grains (allotriomorphic ferrite) and to a lesser extent inside the austenite grains (euhedral ferrite), where non-metallic inclusions (NI) are presented [39–42]. Primary ferrite with nuclei at grain boundaries is formed during cooling in the temperature range of 1,000 and 650 °C [20–34]; – side plates of ferrite [34, 39–42] (separated by low-angle boundaries) are formed during cooling at temperatures from 750 to 650 °C, also at the boundaries of primary austenite grains [29]; – acicular ferrite [34, 39–42] is heterogeneously nucleated on the surface of non-metallic inclusions during the austenite-ferrite transition. As the transformation proceeds, ferrite grains diverge in diff erent directions, creating a chaotic structure [29, 30] of crystallographically misoriented plates approximately 5–15 μm long and 1–3 μm wide [17–29, 39–42]. The temperature range, over which acicular ferrite is formed, depends on the overall composition and cooling rate across the transformation temperature range, but is typically in the range of 750–560 °C [34, 35]. – bainite grows in the form of individual plates or subunits [48], which can form bundles of parallel ferrite laths [34]. It can be classifi ed as upper or lower bainite depending on the transformation temperature. In upper bainite, carbon is deposited as cementite (Fe3C) between bainitic ferrite plates (bundles) [48]. In lower bainite, the ferrite becomes supersaturated with carbon, and some carbide precipitation occurs within the ferrite subunits as well as between it [43]. The initial temperature of bainite depends on the composition and cooling rate, but is usually on the order of 560 °C [48–67]. The nucleation effi ciency of nonmetallic inclusions in modern low-alloy steel weld metals is such that the colony size of intragranular bainite is similar to the size of acicular ferrite in C-Mn steel weld metal [29]. Consequently, when examined under an optical microscope, the colonies within granular bainite are very similar to the type of acicular ferrite with which it is confused in the literature [41–44]. Some authors [44, 45] use the term granular bainite, which does not diff er from lath bainite in terms of the transformation mechanism, although granular bainite packets form at relatively higher temperatures and mainly consist of wide parallel laths, while lath bainite packets form at relatively lower temperatures and consist of thin parallel slats; – transformation of pearlite can occur at the boundaries of austenite grains or in such inhomogeneities as inclusions. At high transformation temperatures, pearlite forms nodules of alternating plates of ferrite and cementite, which can be quite large. As the transformation temperature decreases, the pearlite sheets become increasingly thinner until the structure becomes indistinguishable under a light microscope. Alternatively, distorted pearlite plates may appear as a virtually indistinguishable ferrite/carbide aggregate [53–56]. Lamellar pearlite, FC(P) in the IIW classifi cation scheme [35], can be confused with martensite if the ferrite/cement laminae are indistinguishable under the optical microscope [2, 41–44]. – martensite is formed as a result of a rapid and diff usion-free transformation, in which carbon remains in solution [43]. Martensite can occur in the form of laths or plates. The substructure of lath martensite is characterized by a high density of dislocations located in cells, where each martensite plate consists of many dislocation cells. The substructure of lamellar martensite consists of very small twins, i.e. twinned martensite [42–45]. The mechanisms of formation of the components are not discussed in this work, since there is extensive material on this topic in the literature [33–67]. It is noted in [67] that, unlike metals of single-pass welds, metals of multi-pass welds contain in each bead (except for the last bead) a large proportion of overheated areas, which, due to subsequent beads, are reheated to a temperature above Ac3. The eff ect of multiple welding passes on C-Mn and low-alloy steel deposits is very complex since the proportion of columnar and recrystallized regions and its corresponding microstructures depend on various parameters such as heat input, temperature between passes and chemical composition [29]. The previous columnar morphology changes during the reheating process, resulting in a heterogeneous microstructure that aff ects the performance of the welded joint [4, 29, 32].
RkJQdWJsaXNoZXIy MTk0ODM1