While laminations that lie parallel to the pipe surface can fail under unusual circumstances, as was the case for the feature considered in Figure 3 in the Materials and Construction (M&C) Defects Chapter, they usually are not a problem. This is because they do not cause a net decrease in the pipe’s cross-sectional area that resists the pressure-induced hoop stress. Other laminations, however, can cause structural problems if they attract hydrogen and lead to cracking and/or form blisters if they are in sour service or are in a hydrogen-charging environment. Other laminations can be problematic when they lie inclined to the pipe wall, as this leads to a decrease in the net area resisting the pressure-induced wall stress.
Given the nature of steel production and its hot rolling, laminations if the form are expected to lie parallel to the surface. Such features when present in sour service acted as traps for atomic hydrogen which combined to form hydrogen gas that was much less mobile within the microstructure and so became trapped. Whether they led to blisters or not, the cross-sectional area is not otherwise reduced by such features, such that they generally have not been considered an issue. However, on occasion inclined laminations have formed. Figure 1 shows about one half of such an inclined lamination.
The inset to Figure 1 clearly indicates that as the lamination approaches the surface of the pipe wall the cross-section area carrying the pressure loading diminishes in one ligament whereas it increases in the other, as is illustrated in the schematic shown in Figure 2.
Figure 2. Schematic illustrating the net area reduction due to an inclined lamination
For the case shown in Figure 1, considering the inclined lamination to be symmetric through the wall means that t1 = t2. Given that the inset in Figure 1 indicates that each of t1 and t2 comprise about 25% of the wall means that the stress acting on each of these ligaments is twice that acting on the full wall thickness, which for the case shown led to a leak.
Inclined laminations can lead to a significant increase in the net wall stress, such that it is not surprising that when they do occur they cause failures. Care should be exercised in managing such features.