The contribution outlines the results of measurements of the mechanical and fracture mechanics properties of the base metal and the heat affected zone of longitudinal welded ø720/8 mm pipes made of 17GS steel under GOST 19281 specifications (equivalent to S355J2G3). The pipeline transported crude oil from Russia to the Ventspils terminal (Latvia) on the Baltic Sea coast in the past. Now, following pipeline revalidation, it is expected to be used for gas transmission. The identified values of mechanical and fracture mechanics properties have shown that the heat affected zone does not have to be the weakest area of a weld in terms of resistence to fatigue crack growth and to fracture, as is generally assumed in technical practice; on the contrary, the fracture properties of the heat affected zone can surpass those of the base metal. This is borne out by the heat affected zone's higher resilience to fracture than that of the base metal, all other strength properties being virtually the same, and also by the lower value of the exponent in the Paris equation.