On detection of crack-like welding defects by existing quality control methods

Abstract

Introduction. The research materials devoted to the operability assurance of welded structures of hazardous technical devices — components of oil and gas production equipment are summarized. The factors determining the operational strength of welded joints and structures are systematized and diagrammed. The causes for the decrease in their service properties are described. We have defined the role of volumetric and crack-like welding defects, which were not detected during quality control at the manufacturing stage and in determining the residual life while in operation, in ensuring the life cycle of the structure. Measures to improve the practice of detecting crack-like defects under visual- and-dimensional and ultrasonic methods of quality control of welded joints are proposed. Materials and Methods. In accordance with SNiP 2.05.06–85, the structures of main oil and gas pipelines, vertical cylindrical tanks, and other oil and gas production equipment (OGPE) are made of dead-killed and semikilled low- carbon and low-alloy steels with tensile strength of up to 686.5 MPa. Regardless of the class and strength level of steel, it should be well welded by all methods prescribed by standard process documentation (SPD). At the same time, regardless of the state of supply, the carbon equivalent Сэкв should not exceed 0.46 %. The research methods are calculation-experimental ones. To calculate the stress-strain state of welded joints, the following methods were used: finite elements (FEM), fracture mechanics using the stress intensity coefficient Kt. Methods of mathematical statistics were used to estimate the geometric dimensions of crack-like welding defects. Welding defects were detected by standard quality control methods prescribed by the SPD and GOST standards. Results. Welded structures operating in the fatigue mode are considered. Some factors characteristic of welded joints are analyzed. It is shown how they affect the formation of strength performance properties. The role of dangerous crack-like defects, which with high probability can be formed in welded joints in the manufacture of structures and during operation, is established. Often, the reason is a decrease in mechanical properties due to aging and loss of plasticity caused by accidental mechanical actions. Note that it is impossible to identify these defects by existing quality control methods, both during the control process under production and during diagnostics while in operation. This reduces the accuracy of predicting the operational life of the welded structure. Discussion and Conclusion. It is proposed to include the following requirements in the SPD: to the quality of welded joints of hazardous technical devices of OGPE, to detection of sharp crack-like defects with a radius of curvature in the range of 1-0.25 mm. However, the reliability of detection of such defects by the control methods used remains low due to the human factor. To increase the reliability of detection of sharp crack-like defects, the directions of improvement of ultrasonic quality control of welded joints are determined.