Forming defects that affect serviceability are generally found in tubes. But
beyond this simple fact, nothing can be said about locations, as defective
material can be inadvertently placed in any tubed region.
Failures resulting from manufacturing defects are relatively rare. In fact,
manufacturing defects account for less than 1% of total failures examined.
Of those that do occur, the two types of defects that are fairly common
include seam or lap defects in seamless tubing, and deficient welds in
Seam defects are crevices in steel that are closed but not metallurgically
bonded. They may occur in unwelded tubes as a consequence of the pres-
ence of internal voids (pipe) or cracks in the ingot from which the tube was
formed. Seam defects can also be caused by faulty methods of steel rolling
in the steel mill.
Deficiencies in welded tubes are often caused by incomplete fusion of the
weldment during the manufacturing process.
The critical factors resulting in the use of construction materials having
significant forming defects include insufficient adherence on the part of
the manufacturer to specified fabrication practices or quality control prac-
tices and, possibly, insufficient adherence on the part of the boiler manu-
facturer to specified quality control practices.
Seam or lap defects appear in a transverse cross section as straight or
gently curving cracks, which may run longitudinally for some distance
along the tube wall (Fig. 19.1). Although the defect commonly originates at
a surface, it may be difficult to see since the surfaces of the defect are
commonly covered with a layer of iron oxide or nonmetallic inclusions.
Deficient welds in welded tubing can often be identified visually once
coverings of corrosion products or deposits are removed. The deficiency
will be apparent as an intermittent or continuous opening or crevice that
runs in a straight line down the tube wall.
Such defects in boiler tubes c