The article discusses the effect of annealing temperature applied during heat treatment as well as the suitability of preheating before the welding of butt joints in pipes (having a diameter of 33.7 mm and a wall thickness of 4.5 mm) made of steel X10CrMoVNb9-1 (P91). In the article, the structure and properties of a joint subjected to heat treatment performed in accordance with manufacturing standards concerning power unit elements are compared with those of a joint not subjected to heat treatment. The welding process discussed in the paper was based on the TIG method and involved the use of filler metal Thermanit MTS 3 (W Cr Mo 91). The material of steel X10CrMoVNb9-1 after welding and not subjected to heat treatment is both very hard and brittle. Because of the fact that the power engineering steel of the above-presented characteristics cannot be exposed to the effect of a high-pressure and high-temperature medium (due to possible crack formation), the welding of such steel should be followed by appropriate post-weld heat treatment.