Study of Defects on the Outer Surface of Large Diameter Pipes

Abstract

The results of metallographic study (including optical and scanning electron microscopy, x-ray diffraction and chemical analyses) of defects on the outer surface of the large diameter pipes (LDP) are presented. There are two types of surface defects present in the problem-plagued zone: steel-making sliver and stress crack. Elucidation of the true cause of defect formation on the surface of metal products is rather difficult especially if the defects are unrelated in their features and do not have an explicit nature. Analysis of the phase and chemical composition of the flawed area provided determination of the true cause of their formation, i.e., local carburizing of the slab surface layers during production. Formation of the local areas of extra hardness on the metal surface results in appearance of slivers and stress cracks upon rolling. Hence, formation of defects is first attributed to violation of the steelmaking technology (carburization of the slab surface layers).