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Module - 4
Terms in this set (61)
What causes fatigue factor?
Repeated or fluctuating stresses that may be quite low compared to the normal tensile fracture stress. The actual failure mechanism in metals may be one of a combination of the several common modes of fracture.
What are the stages of fatigue fracture?
1) Damage Accumulation
2) Crack Initiation
3) Crack Propagation and Failure
What are the 4 kinds of fatigue?
1) Thermal - due to temperature variation
2) Mechanical - due to mechanical loading
3) Vibration - due to dynamic loading due to vibration, water hammer or unstable fluid
4) Corrosion - due to environment assisted cracking
What is thermal fatigue?
Cracking caused by swinging temperatures.
What are the affected units or equipment for thermal fatigue?
Mix points, coke drums, dissimilar weld metals (thermal expansion)
Where do cracks usually originate for thermal fatigue and how do they look like?
They originate at the surface of the component and are generally wide and oxide filled (due to the high temperature)
What are some thermal fatigue mitigation techniques?
Good design and controlled rates of heating and cooling
Removal of stress concentrators (blend grinding)
Evaluation of dissimilar materials.
What are some of the inspection techniques that can be used to detect thermal fatigue?
VT, MT, and PT
External SWUT can find internal cracks under repads.
What is endurance limit?
It is the amplitude of the stress that can be applied to a material without causing fatigue failure.
What does not having an endurance limit mean and what are some metals that do not have it?
Not having an endurance limit means that if exposed to repetitive cycles of stress, the material will fail regardless of the magnitude of the stress.
300/400 SS and Aluminum do not have endurance limits.
What is corrosion fatigue?
Cracking due to corrosion and cyclic loading vibration. This fatigue failure is from vibration or cyclic operation acting on a corroded or thin area.
How does corrosion promote fatigue?
Corrosion serves to promote failure at lower stress and/or lower number of cycles than would normally be expected in the absence of corrosion.
What is Stress corrosion cracking?
Is a brittle type failure of a normally ductile alloy by the simultaneous presence of:
1) Tensile strength
2)A specific corrosion medium
How does a SCC look like?
Fine cracks penetrate deeply into the metal while the surface exhibits only fain signs of corrosion and often, a brittle fracture may occur in what would normally be a ductile material.
What are some examples of residual stresses?
Bending or welding or uneven heating or cooling
What are applied stresses?
Working stress from internal pressure of structural loading.
Between applied stress and residual stress which one is the major factor in SCC?
What is chloride SCC?
It is the cracking of Austenitic (300SS) by chlorides
Which materials are affected by Cl SCC?
All 300SS (Austenitic Steels) are highly suscetible
Duplex can be more resistant but not immune
Ni based alloys can be highly resistant (if Ni content is >35%)
What are the critical factors of Cl SSC?
What effect does temperature has on the susceptibility to Cl SSC?
300 SS is susceptible to Cl at almost any concentration at temperatures above 140F
Increasing the temperature increases the susceptibility to cracking
Increasing the concentration of Cl also increases the susceptibility to cracking.
Cases of cracking have been documented at temperatures lower than 140F
For Ni alloys, which concentrations of Ni are most susceptible of Cl SSC?
From 8% to 12%
At what pH does Cl SSC usually occur?
At pH values above 2 and susceptibility decreases in the alkaline pH region.
What equipment is affected by Cl SSC?
All 300SS equipment
Equipment associated with hydrogen services (hydrogen can contain chlorides)
External Cl SSC can a problem in marine environments.
What is the morphology of Cl SSC?
Branching of the cracking is transgranular. In sensitized 300 Series SS, the cracking can be intergranular.
What is a characteristic of stress corrosion cracks?
Cracks may have many branches (craze-cracks)
What are some ways to prevent Cl SSC?
Use of low Cl water for hydrotesting and flush adequately
Use resistant materials of construction
What are some inspection techniques that could be used to detect Cl SCC damage?
VT - if cracks are in the surface
PT - Extremely fine cracks may be difficult to find with PT
What is Caustic Stress Corrosion?
It is the cracking of materials when exposed to caustic
What materials are affected by Caustic Stress Corrosion?
300 SS (Austenitic Steels)
What are some of the critical factors for Caustic Corrosion Cracking?
Why does caustic cracking happen in CS and low alloy steels?
The caustic attacks the grain boundaries of the metal.
The cracking is intergranular
Where does Caustic SSC usually happen?
They are usually located in the welds in the base metal at the HAZ
What is ammonia SCC?
It is cracking of the material due to ammonia.
What are the susceptible materials to ammonia SCC?
Aqueous may crack some copper alloys
Anhydrous or concentrated ammonia may crack CS
For ammonia SCC, what are the critical factors for copper alloys?
Residual Stress and chemical components
Ammonia needs to be diluted in liquid water
Presence of O2 (small concentrations are enough)
pH must be above 8.5
For ammonia SCC, what are the critical factors for steels?
Anhydrous ammonia (dry) with less than 0.2% water must be present.
PWHT will eliminate susceptibility to most CS
Where do Dry ammonia cracks have been found in Steel?
In spheres and other pressure vessels (in the vapor space) that store or handle anhydrous ammonia, but have seldom occurred in piping systems.
What is liquid metal embrittlement?
It is the cracking when certain molten metals contact specific metals.
What are the critical factors for liquid metal embrittlement?
Molten metal/metal combination
Stress - cracking under load can be very fast
Temperature - Cracks will not happen at lower temperatures and the liquid metal has to become liquid.
What metals are susceptible to liquid metal embrittlement?
Susceptible Metal-> Molten metal
300SS -> Zinc
Cu alloy -> Mercury
Alloy 400 -> Mercury
Ni based Alloy -> Mercury
Aluminium Alloy-> Mercury
CS, Low alloy SS and HSS -> Cadminium and Lead
What is Polythionic Acid SCC?
It is the stress-accelerated intergranular corrosion of "sensitized" stainless steels.
What materials are affected by Polythionic Acid SCC?
300 SS that have been sensitized
What are the critical factors for Polythionic Acid?
A combination of the following must occur:
1) Sulfur must be present
2) Materials must be in a susceptible or "sensitized" condition.
3) Stress - Residual or applied
Why is the presence of sulfur important for Polythionic Acid SCC?
Sulfur reacts with air (oxygen) and moisture to form sulfur acids (Polythionic Acid)
What is sensitization?
At temperatures between 700 and 1500 chrome carbides are removed from the metal and precipitate along the grain boundaries. This depletes de metal of Cr leaving Fe. This is a composition/time/temperature dependent formation.
What are some other factors for Polythionic Acid SCC?
304/304H and 316/316H are particularly susceptible.
Low carbon grades "L" (0.03% C) are less susceptible
What are chemically stabilized SS?
For 321 and 347 grades they have Ti and Nb added and have improved resistance to PASCC
What is the appearance PASCC?
Cracking is intergranular
Occurs next to welds
There is no apparent loss of thickness.
What is amine SCC?
It is cracking in the presence of amines.
Which materials are affected by amine SCC?
CS and low alloy steels.
What are the critical factors for amine SCC?
A combination of the following:
1) Amine type - cracking is more likely to occur in lean MEA and DEA
2) Temperature - Cracking can occur from ambient temp to higher temp, but is more likely at higher temperatures.
3) Stress - Residual stressed from bending or welding.
What is the appearance of amine SCC?
Cracking is intergranular
Cracking develops parallel to the weld. In the weld metal, the cracks are either transverse or longitudinal with the weld.
True amine cracking is associated with __________
lean amine. Cracking associated with rich amine is another corrosion mechanism.
Which materials are susceptible to carbonate SCC?
CS and low alloy steels only
Non PWHT materials are especially susceptible.
What are the critical factors for Carbonate SCC?
Water chemistry (pH and carbonate level)
What is the appearance of Carbonate SCC?
Cracking typically develops parallel to the weld
Cracks are similar to SSC (Sulfide Stress Corrosion) and SOHIC (Stress Oriented Hydrogen Stress Corrosion) but the carbonate cracks are usually further from the toe of the weld and have multiple parallel cracks.
The cracking is intergranular.
Carbonate SCC is another form of ____________________
Which unit is affected by Carbonate SCC Cracking?
FCCU, carbonate is produced by the catalytic cracking process. The areas susceptible are the main fractionator overhead piping, reflux system, downstream wet gas compressor system and the sour waters emanating from these areas.
What are some ways to monitor and inspect for Carbonate SCC?
Monitor the water chemistry (pH and carbonate level)
WFMT and ACFM - Are best for detecting the cracks
PT is usually not effective because of the small cracks that are scale filled.
What are the conditions for Carbonate SCC?
Non stress relieved CS
pH > 9 and Carbonate levels >100ppm
8<pH<9 and Carbonate levels >400 ppm
The presence of 50 wppn H2S or more increases susceptibility
Cyanides increase susceptibility
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