The present invention relates to welding seams of and in longitudinally, seam, welded tubes or pipes of large diameter and having wall thicknesses from about 10 to about 40 millimeter. More particularly, the invention relates to welding seams of microalloy steel containing niobium and/or vanadium.
As a consequence of the increasing energy consumption, the construction of pipelines for the transport of gas and oil has accelerated in recent years. It is usually desired to use pipes of large diameter in order to permit a throughput that is as large as possible. Also, the thickness of the pipe increases because of high pressure of the medium to be transported. Of particular interest here is pipe material made of a very strong microalloy steel. The pipes made from such a steel are usually submerged arc welded to obtain a longitudinal welding seam, and the seams are welded in position and counter position, i.e., with a backing member. The welding seams have, as a particular feature, a cast-like solidification texture with well-developed primary crystallization. Such pipes are, for example, used in arctic zones and the strengths and ductility of such a seam under such conditions becomes of particular importance. The conditions for welding are rather unfavorable in the beginning because of the stratification of the material to be welded; moreover an additional problem exists in that the welding seam is comprised of about 2/3 of molten pipe material and about 1/3 of molten welding wire or rod. The metallurgy of the resulting welding seam is also influenced by the powder for the submerged arc welding process.