Processes weakening joints in major pipelines

Abstract

Strength loss in basic parts of the major pipeline joints is related to deterioration in the structure of the metal [1]. A basic condition for good reliability in the parts of these joints is the elimination of sudden failure, including brittle failure. A distinctive feature of this is that a failure focus (crack) arises at a low degree of strain, and this propagates rapidly.The probability of brittle failure in a joint increases at high stress levels (in zones where there are stress concentrators due to the design or technology or where there is elevated loading).A particular danger from the viewpoint of brittle failure is represented by welded joints, which have a tendency to give rise to defects. Local distortions occur in the welded zone on account of the melting during welding and give rise to residual stresses, which often attain the yield point of the material. Such high stresses are unrelated to the external applied stresses or may combine with them to give rise to unstable crack growth. Also, the heating and strain during welding may embrittle the metal around the weld [2].Rapid macrobrittle failure is characterized by unstable crack propagation: if a crack enlarges, the existing stresses favor its accelerated growth. Here brittle failure is almost always caused by stresses less than the design values. On that basis, such failure is considered brittle, i.e., there is unstable or uncontrollable crack growth at stresses less than the yield point of the material.The toughness of the material should be selected to prevent possible brittle failure arising from the largest defecf occurring at a given working stress level. The winking stresses in the pipes are made up of those flom the hydrostatic pressure, the bending moments transmitted from the tubes to the ring welds, and also stresses from any additional stress concentrators (including metallurgical defects).The design, manufacture, and use of these joints at present make inadequate use of calculations and experiments, and also of appropriate nondestructive test methods, in the elimination of brittle failure in welds, for which the following measures are recommended:-the design should avoid as far as possible shm-p transitions, sharp angles, grooves, and other stress concentrators, and it should not allow welded joints to occur around stress concentrators arising from the design; -welded joints must be heat-treated appropriately to remove internal stresses and stabilize the structure in and around the weld; -one should use materials for the joints having high stress intensity coefficients Kic; -one should use metals and technologies for the parts of a joint that rule out any form of structural instability during use; and -the materials for making the joints should be virtually free from metallurgical defects, i.e., one should use refined steels and alloys (vacuum-arc production, vacuum induction, and electroslag processes).If the scope for brittle failure is eliminated, and thus with it the probability of sudden failure, the working lives of any components...