Estimating remaining life of elevated-temperature steam pipes—Part I. Materials properties

Abstract

Material data were obtained for life prediction and the development of inspection criteria for seam-welded, elevated-temperature steam pipes using time-dependent fracture mechanics concepts. Tensile, creep deformation and rupture, fracture toughness and creep crack growth tests were conducted on an ex-service 2-l/4Cr-1Mo steam pipe steel. Both composite and weld specimens were used for material testing. The composite specimen consisted of base, weld and fusion line metals. The creep deformation rate of the composite specimen was found to be comparable to that of the weld specimen. Varying the specimen diameter from 9.53 mm (0.375 in.) to 1.98 mm (0.078 in.) did not affect creep deformation rate properties. A large variability in fracture toughness properties was observed among the specimens machined from the fusion line region. Creep crack growth rates in the fusion line area were found to be generally greater than those in the weld and the base metal regions. Fracture morphology of creep crack growth in the fusion line area of a laboratory-tested specimen resembled the failure mode of an ex-service ruptured steam pipe. Thus, fast creep crack growth in the fusion line area is suggested to be the primary failure mechanism for seam-welded steam pipes. The remaining life prediction methodology and inspection criteria development for seam-welded, elevated-temperature steam pipes are contained in Part II of this paper.