OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology affected zone has a microstructure similar to that of the base metal, but has relatively smaller grain sizes, resulting in higher hardness values. In addition, after heat treatment in the tempering mode, a significant decrease in microhardness occurs throughout the welded joint. The difference in hardness between different zones is reduced, especially between the heat-affected zone and the base metal zone. In combination with the observation of the microstructure, it is assumed that the decrease in hardness occurs mainly due to the partial decomposition of bainite and pearlite, the growth of ferrite regions (Fig. 6) with lower hardness and due to the formation of tempered structures. Analysis of the stress-strain curves during testing of pipe specimens showed that the final destruction occurred in the zone of the base material of the specimens both under heat treatment conditions in the normalization + tempering mode, and under the conditions of the tempering mode. There can be two reasons: firstly, the content of alloying elements in weld joint is higher than in the base metal (as shown in Table 3), which indicates that the high strength of the weld obtained during welding compared to the base metal corresponds to the strength of the base metal of the pipes. Secondly, the coarse-grained region in the heat-affected zone experiences strengthening of the alloy due to the diffusion of elements from the weld metal, while the fine-grained zone, characterized by small and evenly distributed grain sizes, promotes strengthening in the heat-affected zone. As a result, the overall strength of the heat-affected zone is higher than that of the base metal. Research has shown that the resistance butt welding process forms quenching structures of the acicular bainite type, with a hardness of 380 HB in the weld joint and HAZ, increasing strength but limiting ductility, as shown by tensile tests and optical microscopic analysis. Hardness measurements clearly confirm these results, showing an increase in hardness in the weld joint and HAZ zone of the specimens. However, as for the bending tests, no cracks were recorded, but the high hardness of the weld moves the bending center from the weld to the base material. The hardness values of the specimens after welding increased slightly due to the formation of bainite in the HAZ. Rapid cooling in the HAZ where the temperature (during welding) was above Ac3 can promote the formation of hard phases such as martensite and bainite in the weld joint. The hardness values after welding were 310 HB. This was due to the fact that the microstructure, completely converted to lath, had a high dislocation density. However, since bainite predominated, virtually no lath martensite was observed. During the normalization process, grain recovery and growth occurred, while the hardness values decreased compared to the hardness of the sample after welding (Fig. 7). After tempering, the hardness values were approximately 130–150 HB lower compared to the hardness values of the specimen after welding. Fig. 7 shows the hardness changes that occur in the fusion zone and HAZ during tempering and normalization + tempering. When the tempering temperature was increased to 600 °C the hardness decreased, but with a further increase in temperature it remained stable. Therefore, it can be understood that 600 °C may be the best tempering temperature in terms of hardness. Thus, it is obvious that after resistance butt welding of pipes made of 0.15C-5Cr-Mo steel, the bainitetype structures formed in the HAZ increase the strength properties of the welded joint and the hardness, which requires additional heat treatment of the joint. It is worth noting that no cold cracking effect was felt in the specimens due to the waiting time between the welding process and subsequent heat treatment. In this work, the effect of post-weld heat treatment on the microstructure and mechanical properties of various 0.15C-5Cr-Mo pipe welded joints was investigated. In further research, we plan to develop a practical technology with optimal welding and heat treatment conditions for the purpose of implementing it in real production. Conclusion 1. It is established that pipes made of 0.15C-5Cr-Mo steel with a size of 25×2.5 mm during resistance butt welding have satisfactory weldability due to the formation of bainitic structures of increased hardness in the joint zone. Taking into account the thermophysical properties of the pipe material, resistance butt welding should be carried out under hard modes with maximum upsetting pressure.
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