CYCLIC TESTS OF STRESS-CORROSION CRACKS OF STEEL GAS PIPELINE PIPES IN THE ABSENCE OF CORROSIVE ENVIRONMENT

Abstract

The paper presents the results of metallographic studies of stresscorrosion defects (SCC defects) detected on the output gas pipeline of the compressor station. The diagnostics statistics of defects in such pipelines and the data on the effectiveness evaluation of various diagnostic tools during the diagnosis of SCC are given. A presumptive steel grade corresponding to the pipeline metal was identified. The species and morphology of cracks are described. Their character of development is defined. To detect traces of the influence of metallurgical impurities and the external environment on the defects development, a spectrometry of corrosion products and base metal was made. The authors have made the maps of elements distribution over the sample controlled surface. The absence of influence of nonmetallic sulphide inclusions on the development of metal destruction in this is described for the particular case of SCC. It is shown that the sulfur content in corrosion products does not exceed the sulfur content in the sample metal. In some samples, reduced sulfur content can be observed, with the exception of local sites with nonmetallic sulphide inclusions. Electronic images were obtained, which show that these nonmetallic inclusions (in this case) are not sources of cracking development. The results of cyclic tests of samples cut from the pipeline and containing cracks are given. The choice of the cyclic test mode was carried out in accordance with the analysis of the compressor station operating mode for the current year of operation. According to the data received, pipe materials with defects at their initial stage of development have shown considerable durability under test loads. The samples with cracks have withstood from 1.6·106 to 7.5·106 loading cycles under conditions of cyclic transverse bending in the same test plane in the absence of a corrosive medium. In a real gas pipeline under normal operation conditions, the number of such cycles does not exceed 120 – 200 per year, therefore, it can be concluded that the pipeline with defects in their initial stage of development has a significant residual life, provided that its metal wall is reliably protected from effects of corrosive environment.