The results of the study of the mechanism of origin and growth of corrosion cracks under stress on the pipes of the gas lines is presented in the article. It was found that the elements of corrosion cracking, which are most susceptible to defect formation and have a high level of internal stresses, are related to weld-affected zones of the trunk gas lines welded joints. Laboratory study was conducted concerning the fragments of the welded joints of weld-affected zones of the trunk gas lines, which are tentatively loaded with an internal pressure of 60, 75, 94 and 130 kgf/cm2. The nature of metal destruction is described, and the assessment was made of the state of the crack-like defects in the pipeline metal using optical and magnetic measurements.
Based on the results of visual inspection of fracture boundaries of the trunk line weld-affected zone, it was identified that the defects are representing interaction and merge, which dimensions under the relevant pressure could become critical, and the cracks themselves could form the source of destruction. In the laboratory studies the internal critical load on the pipe wall (experimental fragment) was simulated at which the growth and merge of the corrosion cracks take place. The study made allowed to identify the level of critical load at which happens the destruction of the elements of the welded joint. The results of the study will allow to increase the reliability of diagnostic examination of the process of defect formation and to predict the behavior of the crack-like defects in the pipe metal. This will result in significant reduction of the probability of the trunk lines accidents, and, therefore, to increase safety of gas transfer. The methods of the laboratory research described in this article, as well as its results can be used when defining the causes of the trunk lines destruction, which will allow to significantly reduce their accident rate in future.