Page 85 - Failure Analysis Case Studies II

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spring in the line. A complete stress analysis using computer software (Autopipe) [2] was completed
using both the original design parameters and the actual pipe configurations based on field inspec-
tions.
The results of these analyses suggested that the piping at the tee was overstressed, and that two
factors contributed to that overstressing:
(1) Improper hanger selection at location 6 in the original construction.
(2) Bottoming out of the hangers at locations 5 and 6, and topping out of the hanger at 3.
In-service piping systems do not creep uniformly through time or throughout their length. Axial
stresses due to bending and dead-weight vary along the length of any system, and other factors
concentrate stresses and, thus, creep strain. At the tee block and steam chest connection, the stresses
(strains) are also concentrated due to the local geometry. When analyzed using the original design
parameters, the stresses at the location of the major flaws were found to be greater than the design
stress allowable based on the 1993 ASME B31.1 Code [3].
In addition to this, the allowable stresses used in the early 1960s for the design of piping systems
using ASTM A335 P11 materials, such as the 1.25Cr-0.5Mo steels used here, have been recognized
as being too high. At the design temperature of 1005 OF, the allowable stresses have been reduced
from 7600psi (52.4 MPa), prior to 1965, to the current level of 6090psi (42.0MPa).

5.3. Mid-wall cracking initiation
The stress analysis shows that the maximum stress occurs at the pipe’s outer surface. Given a
homogeneous material, the cracking would therefore be expected to start at that outer surface. The
evidence is that the cracking did not start there, but started within the pipe wall. To understand
why cracks would be initiated at sites other than the apparently, most highly stressed sites, it must
be remembered that the material around a weld is not homogeneous, either compositionally or
structurally. The HAZs of these particular welds, close to the fusion line, consisted of alternating
bands of grain-coarsened and grain-refined steel. These bands have very different high-temperature
mechanical properties to each other. In general, coarse-grained materials, including coarse-grained
femtic steels, have a higher creep strength, or a lower creep rate, but less creep ductility than finer
grained steels of otherwise identical composition [4, 51. There is a tendency, therefore, for a load to
be shed from the finer grained and creep soft, but ductile zones, to their coarser grained and creep
harder neighbours. These coarse-grained areas can withstand this extra load shed from their finer
grained neighbouring zones, in the sense that their creep extension is relatively slow. However, these
regions tend to be “creep brittle”. They exhaust their creep ductility, giving rise to the cracking that
is seen here.

5.4. Importance of weld defects
It should be emphasized that this cracking occurred without the involvement of weld flaws.
Whenever cracking is found in an operating plant, and that cracking is associated with a weld, there
seems to be a natural tendency, for many of the personnel involved, to lay the blame on some sort
of “weld defect” or “bad welding”. This is true when creep cracking is involved, as here. It is also
true when the cracking mechanism is fatigue cracking or stress corrosion cracking. The belief that
cracking in, or at, a weld is a result of the weld having been originally faulty can be very difficult to
displace. That belief can adversely affect the scope and type of both present and future inspections,
and can influence repair decisions. Often, however, it is not “weld flaws” or “bad welds” that are
responsible for in-service cracking at welds. The problems that occur at welds are usually more a
product of the intrinsic properties of most fusion welds: because the residual stresses tend to be high
and tensile at the surfaces of a weld that has not received post-weld heat treatment, stress corrosion
cracking may occur there. Because stress (strain) concentrations are generally associated with fusion
welds, welds tend to be potent sites for the initiation of fatigue cracks. In the present specific case,
because of the microstructural changes generated by the thermal cycle of fusion welding, some weld
zones have an increased susceptibility to creep cracking.
It is important, however, in any particular case, to try to find out whether specific weld flaws are
involved. If a weld flaw is involved, the cracking may be localized to a specific site, or to an