The analysis involved a similar welded joint made of steel TP347HFG after operation at a temperature of 580°C. Tests revealed that the primary mechanisms responsible for the degradation of the microstructure in all areas of the joint subjected to analysis were precipitation processes within the grains and along the grain boundaries. The grain boundaries contained two morphologies forming a continuous lattice. Precipitation processes resulted in the high tensile strength of the joint and high

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... dness within the weld face area. After operation, the test joint was characterised by relatively high impact energy, which could be attributed to the fine-grained microstructure and the presence of numerous annealing twins.