The purpose of this part 5 of the UNE 9-300 standard is to provide complementary requirements to those indicated in the remaining parts of the UNE 9-300 standard, relating to the qualification of welding procedures and welding operators, preparation of welds, its execution, and thermal and mechanical treatments of welded parts, subjected to pressure, of boilers, as well as their testing. This part must be applied jointly with the others that make up the standard, and in particular, the particular specifications of materials (see part 2) and manufacturing (see part 6) must be taken into account.