Abstract: An apparatus and method for making a seam-welded steel pipe free of untempered martensite without seam anneal. The method includes selecting a steel containing a carbon concentration below a predetermined level, for example, 0.14% or 0.12% by weight. The method also includes flooding both outside and inside of the strip with a coolant while the weld seam is being formed, and continuing to immerse the welded strip for a sufficient time after the weld seam is formed to prevent the formation of untempered martensite. The apparatus includes a heater capable of heating the strip to a temperature sufficient to form a welded seam, a cooling module configured to supply a coolant to the welded seam both inside and outside of the strip as the weld seam is being formed, and another cooling module configured to immerse the welded strip in a coolant after the weld seam is formed for a sufficient length of time to prevent the formation of untempered martensite.

Abstract: A pipe connecting structure incorporated in the male and female ends of the pipes rigidly secures pipes in sealed end-to-end relation. Each end has axially spaced tapered cylindrical surfaces with a continuous thread extended between the surfaces. Each thread has an axially inward directed leading wall extended from an annular inner wall to an annular outer wall. A radial trailing wall at the rear of the outer wall extends inwardly to the inner wall. The grooves between the threads have the same profile as the threads.