OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 In the transition zone to the base metal at a distance of 10–12 mm, the structure is sorbite and ferrite (Fig. 4). The microstructure is represented by equiaxed fine-grained grains due to dynamic recrystallization. At the same time, the size of the ferrite grain compared to the base metal has decreased from 25 μm to approximately 4 μm, and the hardness decreases to 180–235 HB. As we move (when examined on an optical microscope) along the HAZ to the base metal, the structure of sorbite and ferrite smoothly transforms into the structure of the base metal ferrite + granular carbides (Fig. 2). After local heat treatment of the weld in the tempering mode, a structure of ferrite + sorbite is formed along the fusion line (Fig. 5). The length of this zone is 0.5 mm. The hardness in this zone is 134–150 HB. After local heat treatment of the welded joint in the normalization + tempering mode, the structures in the joint zone (Fig. 6) and the HAZ at the distance of 15 mm from the axis consists of sorbite and a ferrite field, with a hardness in this zone of 207–212 НВ. In this area, a decrease in hardness values occurs due to the presence of ferrite. Fig. 7 shows the distribution of hardness over the welded joint after welding, after welding and tempering, after welding and complex heat treatment normalization + tempering. It can be seen that heat treatment of the welded joint significantly reduces the hardness. Fig. 8 visually shows the results of flattening and bending of welded specimens. Both the welded material and the base material specimen passed the bending test without visible defects or cracks. The bend angle of the pipes in the initial state and after local heat treatment according to the tempering mode was 180°. At the same time, after complex heat treatment normalization + tempering, the bend angle decreased and amounted to only 45–70°. Fig. 4. Microstructure of metal in the transition zone to the base one Fig. 5. Microstructure of metal in the joint area after tempering Fig. 6. Microstructure of metal in HAZ after normalization and tempering
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