Multi-layer pipe

Claim 1

An improved pipe having a multilayer, spirally-wound steel wall greater than 6 millimeters thick comprising: a plurality of wall layers including at least an inner wall layer and an outer wall layer, with adjacent wall layers having a pair of opposing longitudinal edges and opposing surfaces, each of said wall layers being joined to adjacent wall layers by substantially continuous lineal welding along fillets at their opposing longitudinal edges, with the portion of opposing surfaces of adjacent wall layers between said fillets being free of bonding to permit limited relative expansion and contraction of adjacent wall layers due to changes in temperature, said plurality of wall layers being spirally formed with abutting longitudinal edges into a relatively cylindrical configuration and defining a continuous spiral edge, and a common welded seam running from said outer wall layer to said inner wall layer continuously along the entire length of said spiral edge for joining corresponding portions of said continuous spiral edge together to form said pipe.