OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Mechanical properties The authors [4] stated that only a few studies have examined the mechanical properties of reheated weld metals. The results are still inconsistent as it depends on several factors such as the amount of acicular ferrite and the presence of MA components. The authors of [29–33] noted that understanding the variation in toughness in the multi-pass weld metal of C-Mn steels is very diffi cult, even if the eff ect of reheating due to multiple passes is taken into account. Similarly, the authors of [29, 38–42] suggested that signifi cant changes in the toughness of C-Mn weld metals are due to the microstructural features existing in the Charpy-V notch, which are the combined result of the chemical composition, welding procedure, deposition sequence and specifi c welding methods. In addition to the factors mentioned above, it is critical to consider the position of the Charpy-V notch in relation to the proportion of weld metal reheated. A specifi c assessment should be made for each case. The author [48] observed that although complete recrystallization was observed for two intersecting regions per layer and the proportion of reheated regions was about 75–80 % for three layers, the same toughness was obtained for both sequences, depending on the Mn content. In general, toughness increases when the fraction of recrystallized region increases due to the predominance of polygonal ferrite, microstructure refi nement, or tempering eff ects during subsequent deposition [41–58, 67]. However, some data suggests that this property deteriorates with extensive segregation [29, 30] or the presence of micro-phases located along the grain boundaries of the previous austenite [47, 48]. Another negative contribution is associated with a decrease in the proportion of acicular ferrite due to the smaller size of the precursor equiaxed austenite grains in the reheated weld metal [46–53]. Figure 10 shows an OM image of a Charpy-V notch, where the ratio of columnar and reheated regions can be easily determined for C-Mn weld metals since these regions are well defi ned. For more alloyed weld metals, this distinction can be more complex. In this case, several polishing and etching steps may be required to enhance the contrast between the areas. The authors of [54] noted that only a few studies have focused on the microstructure and toughness of actual weld metal. This is because it is very diffi cult to analyze its correlation using a real weldment, and the precise determination of the correlation between the ring-type MA component in the weld metal reheat zone and the toughness is still uncertain. At the same time, after metallographic studies, when an accurate characterization of the microstructure has been obtained, it is possible to assess impact strength based on the following criteria. (1) Reheat. This criterion is less representative in many of the studies analyzed because the same proportion of recrystallization was obtained for all deposits; (2) Microstructure. EBSD results confi rm this trend, showing that fi ner microstructure has a higher frequency of high-angle boundaries (HABs), which can eff ectively cause the propagation of cleavage cracks to defl ect or stop [32–46]. The same behavior was noted for the reheating region of the refi ned grains, where polygonal ferrite predominates; (3) No metallic inclusions. It is known that non-metallic inclusions can have two opposite eff ects on impact toughness [11, 12]. One of it is that inclusions act as crack initiation sites, both plastic and cleavage. Secondly, it can promote the formation of acicular ferrite. It has been observed that increasing the Ti content promotes the formation of inclusions suffi cient to support the formation of purifi ed acicular ferrite, in agreement with other works [3, 11, 12, 32, 36]. Fig. 10. Optical microscopy at low magnifi cation of the Charpy-V notch position for C-Mn weld metal after etching with Nital 2 % [32]
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