Experimental investigation of fracture behavior and microstructure of API 5LX60 line pipe

Abstract

This paper deals with fracture and failure analysis of API5LX60 pipeline steels (micro-alloyed steels) and understand adverse consequence of corrosion on physical mechanical properties of line pipe steels. To analyze fracture behavior of API5LX60 pipeline steels, micro-structural analysis, hardness tests, tensile tests, crack tip open displacement (CTOD) tests, J-R tests, and failure analysis have been conducted in various regions of materials i.e., base, HAZ, and weld. Micro-structure reveals that heat affected zone (HAZ) have finer grain due to which it has higher hardness than weld, and parent materials. Also welded zone shows lower fracture toughness due to presence of impurities i.e., inclusions, porosity etc. The micro mechanism of fracture in the weld case consists of river lines, quasi-cleavage fracture and cracks due to influence of environment thus resulting in lower toughness (Kmax, CTOD, and JIC). Fractography analysis is done to examine the origin of fracture, the mode of fracture (intergranular, cleavage, or shear), location, and nature of flaws that may have propagated failure. In addition to this dye penetrate non-destructive testing (NDT) is done to find out surface-breaking imperfection in all close-grained materials in fractured materials of various regions. Through fracture and failure analysis, it is found that the welding toughness of pipeline steels is decreased i.e., the CTOD of the weld metal is reduced by 68% and JIC by 74% in comparison to the base metal due to hydrogen environment during welding. These studies grant the technical bases for supporting the expansion of methodology for evaluating the protection and durability of pipelines in services and for supporting the expansion of enhanced materials and safeguard systems.