Metallography of non-metallic inclusions in pipe metal and impact assessment of defect structures on non-destructive testing outcomes

Abstract

Low carbon steel pipes represent a core component in the gas pipeline system, so it is necessary to improve constantly diagnostic operations and assessment methods of their technical state. Studies on objects operated in diverse conditions within a long period provide reliable data on the real behavior of materials. A key issue is the investigation of characteristics and behavior of products with primarily incorporated defective structures undetected while manufacturing and constructing. It is difficult to forecast the behavior of such defects, in addition, considerably more work need to be done to determine their development, detection methods, and more importantly, their hazards. The reported study investigates the phases, structure and sizes of inclusions detected in the defective pipe metal in the natural gas pipeline. The property that characterizes defects of interest is their concentration within one pipe section in combination with diverse thickness, composition, and location in the pipe wall. Defects are found at various depths, forming one, two and more layers. The difference in structure and phase composition is outlined. The research has pointed out principal inclusions represent fragments of charge materials — fluorite and broken fireclay brick, a number of defects result from casting defects, e.g. gas bubbles and shrinkage holes. Non-destructive ultrasonic testing is implemented; the importance of thickness and structure of inclusions for measurement data has been realized.