PSI - Issue 36

Sergiy Kozulin et al. / Procedia Structural Integrity 36 (2022) 247–253 Sergiy Kozulin et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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The investigation of metal microstructure of multi-pass electroslag welds showed that in the overheating zone of base metal not exposed to reheating, the value of grains reaches 0.157 mm and 0.222 mm according to the GOST 5639-82 (Figure 5, a-c ). In the reheating zones (self-heat treatment) the growth of grains corresponds to 0.0267 mm and 0.0196 mm (Figure 5, e , Figure 6, d ).

Fig. 5. Microstructure of multi-pass weld: a, b, c, d - weld metal of the 1st, 2nd, 3rd and 4th welds, respectively; e - line of fusion of the second weld with the first one; f - the same of the third weld with the second one; g - the same of the fourth weld with the third one (×100) .

Fig. 6. Fusion zone microstructure: a - the interface between HAZ and the metal of the 1st weld; b – the same the 2nd weld; c – the same the 3rd weld; d – metal of the 2nd weld, heated above the temperature of A c3 point while performing the 3rd weld (×100) .

Analysis of the macro- and microstructures of multi-pass electroslag welds showed that parts of the previously made welds and their HAZ were heated above the temperature corresponding to A c3 , i.e. underwent heat treatment in the form of normalization, while others underwent heat treatment within the intercritical temperature range A c1 – A c3 . For 34L-ESH steel critical point temperatures were A c1 = 730°С, A c3 = 802°С. Therefore, the metal of th e welds and their HAZ fallen in the normalization zone had to undergo complete recrystallization and had fine-grained structure, as well as improved mechanical properties. Analysis of the results of the measurements of metal hardness of the central part of welds and heat-affected zone between the base metal and the clad metal showed that the nature of metal hardness changed in multi-pass electroslag welds was of general pattern. It is shown that the hardness in the superheat area of each weld that has not been reheated reaches maximum value corresponding to the hardness of the superheat area of the last weld (HB 160...179). The areas subjected to automatic heat treatment and located in the central part of the multilayer weld have the lowest hardness values (HB 143...156), which actually correspond to the hardness of the base metal. The difference between the hardness values at the superheated area and the central part of the weld metal is on average HB 20...30.The hardness of the weld metal in the layers of multi-pass weld, subjected to reheating, changes with the same difference of values (HB 5...10) and has smaller value compared to the metal hardness of the last welds not subjected to reheating (Fig. 4). It is found that the metal hardness of seams and sections of the secondary heating is on average 20% lower than that of sections of the secondary heating with base metal not subjected to reheating. The results of impact bend tests of characteristic sections of welded joints (Fig. 2) show that the impact toughness of the welded joint has clearly increased values, especially in the areas subjected to the accompanying heat treatment. The impact toughness KCU in the weld metal areas, undergoing the accompanying auto thermal treatment, exceeds the test results of the base