OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 Ta b l e 3 Ultimate strength of 0.15C-5Cr-Mo steel welded joints made with different welding parameters Series no. 1 2 3 Sparking allowance, mm 12 6 12 Upsetting pressure, MPa 120 120 140 Upsetting allowance, mm 18 18 18 Upsetting current time, s 5 5 5 Ultimate strength, MPa, average of three specimens 398 405 470 to close the gap between the adjoining surfaces and remove the liquid metal and oxide inclusions. The upsetting stage occurs when the two ends are brought closer together under the influence of an axial force at a controlled speed. Meanwhile, a certain plastic deformation occurs in the joint, resulting in dynamic recrystallization and recrystallization during the upsetting process to form a strong joint. Welding thermal cycles have high peak temperatures and high heating and cooling rates. As the spark allowance increases from 6 mm to 12 mm, the heating rate (peak temperature/heating time) decreases. During flash heating, the greater the flash allowance, the longer it takes to reach peak temperature, resulting in a slower heating rate. During the cooling process, the time t8/5 increases from 26.0 s to 32.5 s, since the upsetting allowance increases from 12 mm to 16 mm, which can be explained by an increase in heat input during welding. In resistance butt welding, the interface of both pipe specimens was heated to the melting temperature, most of the resulting liquid metal splashed out from the interface, and the remaining liquid metal formed very fine grains. Peak temperatures in the HAZ were in the solidus-liquidus range, so its width was also limited. Depending on the peak temperatures and microstructural characteristics, the heated pipe specimens can be divided into four zones: 1) molten zone, 2) semi-molten zone, 3) coarse-grained zone, and 4) finegrained zone. Based on the results of metallographic studies, the following features of the evolution of the microstructure can be noted. A noticeable decrease in the content of primary coarsened ferrite is observed in the structure of the weld after tempering heat treatment. It should be noted that blocky primary coarsened ferrite along grain boundaries is always considered as the main factor contributing to the rapid propagation of cracks and a decrease in impact toughness. Therefore, it can be assumed that the overall performance of the weld can be significantly improved. In the coarse-grained heat-affected zone on the right side of the fusion line, a portion of the bainite undergoes decomposition and the characteristic lamellar structure of the ferrite becomes less distinct, resulting in a more uniform microstructure distribution. After heat treatment (tempering) of the welded joint, the grain size in the fine-grained zone increases due to the decomposition of part of the pearlite, and the ferrite increases in size (Fig. 6). In the base metal zone, there is no significant change in grain size, but the pearlite content is noticeably reduced. When analyzing the results of Fig. 7, it was found that the distribution of microhardness over the welded joints in the post-weld heat treatment modes of “tempering” and “normalization + tempering”, annealing and tempering heat treatment is significantly reduced. The general trend in the sizes of the HAZ areas is as follows: the weld area is larger than the heat-affected zone, which is larger than the base material zone. This difference in hardness across the weld joint and HAZ as a whole is reflected in variations in the content of elements, microstructure, and grain size in different microzones of the welded joint. More precisely, the weld zone has the highest microhardness due to the higher content of elements such as Mn and Si, which play an important role in solid solution strengthening. In addition, the microstructure of the weld zone consists predominantly of bainite and ferrite, which also determines the highest microhardness. During resistance butt welding, the heat-affected zone above 1,300 °C (WZ) experienced a thermal cycle with a higher peak temperature and greater plastic deformation caused by dislocation motion, resulting in dynamic recrystallization and recrystallization in this zone. It is known that the plastic deformation experienced in the weld joint zone tended to increase the number of dislocations, while the number of dislocations tended to decrease during dynamic recrystallization and recrystallization process. The heat-
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