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Mechanical and Metallurgical Failure Mechanisms
Description and Appearance
Terms in this set (20)
a change in the microstructure of certain carbon steels and 0.5Mo steels after long-term operation in the 800°F to 1100°F (427°C to 593°C) range that may cause a loss in strength, ductility, and or creep resistance.
b) At elevated temperatures, the carbide phases in these steels are unstable and may decompose into graphite nodules. This decomposition is known as graphitization. // Damage is not visible/only by
metallographic exam. Advanced stages of damage related to loss in creep strength may include microfissuring/microvoid
formation, subsurface cracking or surface connected cracking
a change in the microstructure of steels after exposure in the 850°F to 1400°F (440°C to 760°C) range, where the carbide phases in carbon steels are unstable and may agglomerate from their
normal plate-like form to a spheroidal form, or from small, finely dispersed carbides in low alloy steels like 1Cr-0.5Mo to large agglomerated carbides. Spheroidization may cause a loss in strength and/or creep resistance. Spheroidization is not visible or readily apparent and can only be observed through metallography. The pearlitic phase undergoes a time dependent transformation from partial to complete spheroidization b) In the case of the 5% to 9% CrMo alloys, spheroidization is the process of transforming the carbides
from their original finely dispersed morphology to large agglomerated carbides.
the reduction in toughness due to a metallurgical change that can occur in some low alloy steels as a result of long term exposure in the temperature range of about 650°F to 1070°F
(343°C to 577°C). This change causes an upward shift in the ductile-to-brittle transition temperature as measured by Charpy impact testing. Although the loss of toughness is not evident at operating temperature, equipment that is temper embrittled may be susceptible to brittle fracture during start-up and shutdown. // is a metallurgical change that is not readily apparent and can be confirmed
through impact testing. Damage due to temper embrittlement may result in catastrophic brittle
b) Temper embrittlement can be identified by an upward shift in the ductile-to-brittle transition temperature measured in a Charpy V-notch impact test, as compared to the non-embrittled or deembrittled material (Figure 4-5). Another important characteristic of temper embrittlement is that
there is no effect on the upper shelf energy
Strain aging is a form of damage found mostly in older vintage carbon steels and C-0.5 Mo low alloy
steels under the combined effects of deformation and aging at an intermediate temperature. This results
in an increase in hardness and strength with a reduction in ductility and toughness. Strain aging can result in the formation of brittle cracks that are revealed through detailed metallurgical
analyses, but damage most likely will not be identified as strain aging until fracture has already occurred.
885°F (475C) Embrittlement
is a loss in toughness due to a metallurgical change that can occur in alloys
containing a ferrite phase, as a result of exposure in the temperature range 600°F to1000°F (316°C to
540°C).//is a metallurgical change that is not readily apparent with metallography but can
be confirmed through bend or impact testing (Figure 4-6).
b) The existence of 885°F embrittlement can be identified by an increase in hardness in affected areas.
Failure during bend testing or impact testing of samples removed from service is the most positive
indicator of 885°F embrittlement.
Sigma Phase Embrittlement
Formation of a metallurgical phase known as sigma phase can result in a loss of fracture toughness in
some stainless steels as a result of high temperature exposure. //is a metallurgical change that is not readily apparent, and can only be
confirmed through metallographic examination and impact testing. (Tables 4-1 and 4-2)
b) Damage due to sigma phase embrittlement appears in the form of cracking, particularly at welds or
in areas of high restraint.
c) Tests performed on sigmatized 300 Series SS (304H) samples from FCC regenerator internals have
shown that even with 10% sigma formation, the Charpy impact toughness was 39 ft-lbs (53 J) at
d) For the 10% sigmatized specimen, the values ranged from 0% ductility at room temperature to 100%
at 1200°F (649°C). Thus, although the impact toughness is reduced at high temperature, the
specimens broke in a 100% ductile fashion, indicating that the wrought material is still suitable at
operating temperatures. See Figures 4-7 to 4-11.
e) Cast austenitic stainless steels typically have high ferrite/sigma content (up to 40%) and may have
very poor high temperature ductility.
the sudden rapid fracture under stress (residual or applied) where the material exhibits
little or no evidence of ductility or plastic deformation.//a) Cracks will typically be straight, non-branching, and largely devoid of any associated plastic
deformation (although fine shear lips may be found along the free edge of the fracture, or localized
necking around the crack (Figure 4-12 to Figure 4-16).
b) Microscopically, the fracture surface will be composed largely of cleavage, with limited intergranular
cracking and very little microvoid coalescence.
Creep and Stress Rupture
a) At high temperatures, metal components can slowly and continuously deform under load below the
yield stress. This time dependent deformation of stressed components is known as creep.
b) Deformation leads to damage that may eventually lead to a rupture. // a) The initial stages of creep damage can only be identified by scanning electron microscope
metallography. Creep voids typically show up at the grain boundaries and in later stages form
fissures and then cracks.
b) At temperatures well above the threshold limits, noticeable deformation may be observed. For
example, heater tubes may suffer long term creep damage and exhibit significant bulging before final
fracture occurs. The amount of deformation is highly dependent on the material, and the combination
of temperature and stress level (Figure 4-17 to 4-19).
c) In vessels and piping, creep cracking can occur where high metal temperatures and stress
concentrations occur together, such as near major structural discontinuities including pipe tee joints,
nozzles, or welds at flaws. Creep cracking, once initiated, can progress rapidly.
9 Thermal Fatigue
is the result of cyclic stresses caused by variations in temperature. Damage is in the
form of cracking that may occur anywhere in a metallic component where relative movement or
differential expansion is constrained, particularly under repeated thermal cycling. a) Thermal fatigue cracks usually initiate on the surface of the component. They are generally wide and
often filled with oxides due to elevated temperature exposure. Cracks may occur as single or
b) Thermal fatigue cracks propagate transverse to the stress and they are usually dagger-shaped,
transgranular, and oxide filled (Figure 4-24 and 4-25). However, cracking may be axial or
circumferential, or both, at the same location.
c) In steam generating equipment, cracks usually follow the toe of the fillet weld, as the change in
section thickness creates a stress raiser. Cracks often start at the end of an attachment lug and if
there is a bending moment as a result of the constraint, they will develop into circumferential cracks
into the tube.
d) Water in soot blowers may lead to a crazing pattern. The predominant cracks will be circumferential
and the minor cracks will be axial. (Figure 4-26 to 4-27).
Short Term Overheating - Stress Rupture
Permanent deformation occurring at relatively low stress levels as a result of localized overheating. This
usually results in bulging and eventually failure by stress rupture. a) Damage is typically characterized by localized deformation or bulging on the order of 3% to 10% or
more, depending on the alloy, temperature and stress level.
b) Ruptures are characterized by open "fishmouth" failures and are usually accompanied by thinning at
the fracture surface (Figure 4-28 to 4-31).
11 Steam Blanketing
steam generating equipment is a balance between the heat flow from the combustion of
the fuel and the generation of steam within the waterwall or generating tube. The flow of heat energy
through the wall of the tube results in the formation of discrete steam bubbles (nucleate boiling) on the ID
surface. The moving fluid sweeps the bubbles away. When the heat flow balance is disturbed, individual
bubbles join to form a steam blanket, a condition known as Departure From Nucleate Boiling (DNB).
Once a steam blanket forms, tube rupture can occur rapidly, as a result of short term overheating, usually
within a few minutes.
a) These short-term, high-temperature failures always show an open burst with the fracture edges
drawn to a near knife-edge (Figure 4-32).
b) The microstructure will always show severe elongation of the grain structure due to the plastic
deformation that occurs at the time of failure.
Dissimilar Metal Weld (DMW) Cracking
Cracking of dissimilar metal welds occurs in the ferritic (carbon steel or low alloy steel) side of a weld
between an austenitic (300 Series SS or Nickel base alloy) and a ferritic material operating at high
temperature (Figure 4-33 and 4-44). Cracking can result from creep damage, from fatigue cracking, from
sulfide stress cracking or hydrogen disbonding. a) In most cases, the cracks form at the toe of the weld in the heat-affected zone of the ferritic material
(Figure 4-36 to Figure 4-42).
b) Welds joining tubes are the most common problem area, but support lugs or attachments of cast or
wrought 300 Series SS to 400 Series SS are also affected.
A form of thermal fatigue cracking - thermal shock - can occur when high and non-uniform thermal
stresses develop over a relatively short time in a piece of equipment due to differential expansion or
contraction. If the thermal expansion/contraction is restrained, stresses above the yield strength of the
material can result. Thermal shock usually occurs when a colder liquid contact a warmer metal surface.
Surface initiating cracks may also appear as "craze" cracks.
Erosion/Erosion - Corrosion
a) Erosion is the accelerated mechanical removal of surface material as a result of relative movement
between, or impact from solids, liquids, vapor or any combination thereof.
b) Erosion-corrosion is a description for the damage that occurs when corrosion contributes to erosion
by removing protective films or scales, or by exposing the metal surface to further corrosion under
the combined action of erosion and corrosion.
a) Erosion and erosion-corrosion are characterized by a localized loss in thickness in the form of pits,
grooves, gullies, waves, rounded holes and valleys. These losses often exhibit a directional pattern.
b) Failures can occur in a relatively short time.
is a form of erosion caused by the formation and instantaneous collapse of innumerable
tiny vapor bubbles.
b) The collapsing bubbles exert severe localized impact forces that can result in metal loss referred to
as cavitation damage.
c) The bubbles may contain the vapor phase of the liquid, air or other gas entrained in the liquid
Cavitation damage generally looks like sharp-edged pitting but may also have a gouged appearance in
rotational components. However, damage occurs only in localized low-pressure zones (see Figure 4-46,
Figure 4-47 to Figure 4-49).
a) Fatigue cracking is a mechanical form of degradation that occurs when a component is exposed to
cyclical stresses for an extended period, often resulting in sudden, unexpected failure.
b) These stresses can arise from either mechanical loading or thermal cycling and are typically well
below the yield strength of the material.
a) The signature mark of a fatigue failure is a "clam shell" type fingerprint that has concentric rings
called "beach marks" emanating from the crack initiation site (Figure 4-50 and Figure 4-51). This
signature pattern results from the "waves" of crack propagation that occur during cycles above the
threshold loading. These concentric cracks continue to propagate until the cross-sectional area is
reduced to the point where failure due to overload occurs.
b) Cracks nucleating from a surface stress concentration or defect will typically result in a single "clam
shell" fingerprint (Figure 4-52 to Figure 4-56).
c) Cracks resulting from cyclical overstress of a component without significant stress concentration will
typically result in a fatigue failure with multiple points of nucleation and hence multiple "clam shell"
fingerprints. These multiple nucleation sites are the result of microscopic yielding that occurs when
the component is momentarily cycled above its yield strength.
A form of mechanical fatigue in which cracks are produced as the result of dynamic loading due to
vibration, water hammer, or unstable fluid flow.a) Damage is usually in the form of a crack initiating at a point of high stress or discontinuity such as a
thread or weld joint (Figure 4-57 and Figure 4-58).
b) A potential warning sign of vibration damage to refractories is the visible damage resulting from the
failure of the refractory and/or the anchoring system. High skin temperatures may result from
Both thermal insulating and erosion resistant refractories are susceptible to various forms of mechanical
damage (cracking, spalling and erosion) as well as corrosion due to oxidation, sulfidation and other high
a) Refractory may show signs of excessive cracking, spalling or lift-off from the substrate, softening or
general degradation from exposure to moisture.
b) Coke deposits may develop behind refractory and promote cracking and deterioration.
c) In erosive services, refractory may be washed away or thinned, exposing the anchoring system.
Cracking of a metal due to stress relaxation during Post Weld Heat Treatment (PWHT) or in service at
elevated temperatures above 750°F (399°C). It is most often observed in heavy wall sections.a) Reheat cracking is intergranular and can be surface breaking or embedded depending on the state
of stress and geometry. It is most frequently observed in coarse-grained sections of a weld heataffected zone.
b) In many cases, cracks are confined to the heat-affected zone, initiate at some type of stress
concentration, and may act as an initiation site for fatigue. Figure 4-60 to 4-63.
Gaseous Oxygen-Enhanced Ignition and Combustion
Many metals are flammable in oxygen and enriched air (>25% oxygen) services even at low pressures,
whereas they are non-flammable in air. The spontaneous ignition or combustion of metallic and nonmetallic components can result in fires and explosions in certain oxygen-enriched gaseous environments
if not properly designed, operated and maintained. Once ignited, metals and non-metals burn more
vigorously with higher oxygen purity, pressure and temperature.
a) In some cases a small component will burn, such as a valve seat, without kindling other materials
and without any outward sign of fire damage. It is noticed when the component is removed because
it is not functioning properly. Figure 4-67 to 4-68.
b) Also, external heat damage (glowing pipe or heat tint) is a strong indication of an internal fire. This
can be caused by accumulation of flammable debris at a low point or other location and combustion
or smoldering of the debris.
c) The worst situation is when the pressure envelope is breached because of fire. Oxygen fires can
cause significant burning of metal components and extensive structural damage (Figure 4-68).
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