Method for welding two edges of one or more steel parts to each other including a heat treatment step after the welding step: penstock obtained with such a method

A method for welding two edges of one or more parts together, the or each part being produced from thermomechanical high yield strength steel whose composition simultaneously meets the following conditions:

• 0.02%≦C≦0.12%, C being the mass carbon content of the steel, expressed as a percentage by weight, and
• 0.20%≦C+(Mn+Mo)/10+(Cr+Cu)/20+Ni/40≦0.505%, with C, Mn, Mo, Cr, Cu and Ni being respectively the mass carbon, manganese, molybdenum, chromium, copper and nickel contents of the steel expressed as a percentage by weight,
• said welding method comprising a welding step during which a weld bead is created, produced from a filler metal, said weld bead producing the secure attachment of the two edges together, the creation of said weld bead inducing the appearance of a heat-affected zone, or HAZ, borne by the steel of the part or parts in proximity to said weld bead, and
• a heat treatment step after the welding step, said heat treatment step comprising:
• a heating step, during which at least a portion of the weld bead and of the HAZ is gradually heated to a treatment temperature lower than the recrystallization temperature of the steel of the or each part, and higher than the austenitization temperature of said steel, then
• a holding step, during which the portion of the weld bead and of the HAZ is held at the treatment temperature, then
• a cooling step, during which the HAZ and the weld bead are gradually cooled and pass from the austenitic transformation end temperature to the martensitic transformation start temperature of the steel of the parts in a time less than 10 s, and preferably substantially equal to 8 s, and pass from the austenitic transformation end temperature to the martensitic transformation end temperature in a time less than 15.5 s, and preferably equal to 15 s.