Cyclic Tests of Stress-corrosion Cracks in Steel Gas Pipelines with No Corrosive Fluid

Abstract

Stress-corrosion defects in the output pipeline of a compressor station are subjected to metallographic analysis and mechanical tests. The pipeline consists of pipe imported in the 1980s. Diagnostic data obtained in nondestructive monitoring of such pipelines are analyzed, and the effectiveness of various diagnostic methods in detecting stress-corrosion defects in large-diameter pipe is assessed. The grade of steel in the pipeline is identified. The cracks are identified by type and morphology, and their development is determined. Nonmetallic sulfide inclusions have no influence on pipeline failure in the stress-corrosion conditions considered. The sulfur content in the corrosion products is no more than the sulfur content in the metallic sample. In some samples, the sulfur content may be decreased except for local sections with nonmetallic sulfide inclusions. Electron-microscope images show that such nonmetallic inclusions do not promote failure. The results of cyclic tests of cracked samples cut from the pipeline are presented. The test conditions are selected in accordance with recent operating conditions of the compressor station. According to the results, pipe with defects at the initial stage of development exhibits considerable durability in the tests. In fact, the cracked samples withstand (1.6–7.5) × 106 cycles under transverse cyclic flexure in a single plane, in the absence of corrosive fluid. In practice, the number of such cycles is no more than 120–200 per year, in normal pipeline operation. Hence, a pipeline with stress-corrosion defects in their initial stage of development will last a long time if its metal wall is protected from the action of corrosive fluid.