Objective 12: Creep, Fatigue, and Corrosion Flashcards
Creep
The tendency of a solid material to deform slowly, but permanently, under the influence of long-term exposure to stress well below its elastic limit
What causes Creep tendency to increase?
Temperature increase
What can ultimately happen to metal experiencing Creep?
The metal can eventually break, or rupture, under pressure, even though the load applied is considerably lower than that required to cause rupture in the short term or in a tensile test
What factors affect Creep?
- Temperature
- Time
- Material properties
- Applied Load
Creep Rupture Test
Test used to determine the rate of deformation and the estimated time until rupture, at a given temperature
What is the procedure for a Creep Rupture Test?
- A test piece, at constant temperature, is subjected to a fixed tensile load
- The deformation of the test sample is measured during the test and the time to rupture is determined
- Several such tests are run under different loads and graphed
What is the duration of Creep Rupture Tests?
Range from 1000 to 10 000+ hours
Describe Test Piece Extension in a Creep Rupture Test during Initial Extension
Deformation caused by initial loading and depends on test conditions, usually increasing with increases in temperature and load
Describe Test Piece Extension in a Creep Rupture Test during Primary Creep
There is a decreasing rate of deformation
Describe Test Piece Extension in a Creep Rupture Test during Secondary Creep
Essentially constant rate of creep
Describe Test Piece Extension in a Creep Rupture Test during Tertiary Creep
Accelerating rate of creep, leading up to fracture
How is the practical application of Creep data simplified?
By establishing under laboratory conditions 2 values of stress (at a specific temperature) that produces 2 corresponding rates of creep: 1% per 10 000 and 100 000 hours respectively
What is indicated by rapid rates of creep elongation?
Transgranular (ductile) fracture
What is indicated by slow rates of creep elongation?
Intergranular (brittle) fracture
What is an indication of Transgranular Fracture?
Presence of surface oxidation
What is an indication of Intergranular Fracture that may or may not be present?
Visible intercrystalline oxides
How is creep rupture determined for materials at time periods longer than they’ve been tested for?
ASME Code provides extrapolation methods which depend on the behaviour of the particular alloy for which the design values are being established and on the extent and quality of the available database.
How is design data for creep usually provided?
As a series of curves for constant creep strain relating stress and time for a given temperature and specifying whether the data represents just secondary stage or also includes primary stage
What a concern vis-a-vis creep for designing plants that work at temperature well above atmospheric temperature?
The designer must consider carefully what possible maximum strains can be allowed and what the final life of the plant will likely be.
Why are Superheater tubes subject to reduced creep life?
- Higher operating temperatures
- Damaging mechanisms, e.g. erosion and corrosion
- Thermal expansion and mechanical loading
How are Superheater Tubes monitored for Creep?
- Visual Inspection
- Ultrasonic Thickness Testing
- Tube Sample Analysis
What factors affect Creep Fatigue in Superheaters and other HT Headers?
- Steam Flow
- Combustion
- Boiler Load
How are there temperature gradients created by Combustion in Superheaters and other HT Headers?
- Firing rate in multi-burners boilers is not uniform, nor is the flow of combustion air and combustion products
- Ash and slag deposits affect heat distribution
How are there temperature gradients created by Steam Flow due to Boiler Load in Superheaters and other HT Headers?
Changes in boiler load cause changes in firing rate and therefore changes in steam flow
Why is a temperature gradient in Superheaters and other HT Headers problematic?
It causes localized stresses in the header, resulting in ligament cracks
How are Superheater Tubes and Header best monitored for defects?
Combinations of NDE techniques targeted at the welds where cracks are most likely to develop (stress risers)
Stress Risers
Flaws that can amplify an applied stress in a particular location
How exactly are Headers monitored for creep?
The header diameter is measured at several locations on the header and the outlet nozzle to look for increases in diameter
What is the most effective test to find Creep Damage in HT Headers?
Metallographic Replication
Metallographic Replication
Study of the grain microstructure of a component, without taking samples from the component, by faithfully reproducing the surface topography of the metal specimen on an acetate tape to be examined under a microscope
When is Metallographic Replication especially useful?
When repeated observations are required
What is the procedure for Metallographic Replication?
- The surface of the specimen is cleaned of oxides to bare metal
- A cellulose acetate film is softened by soaking one side of the film with acetone
- The softened flim is immediately applied to the etched surface and pressed firmly into place
- After 20 minutes, the film hardens
- The film is carefully peeled from the metal for microscopic examination
- The side of the film that was not in contact with the metal is placed against a black surface to improve the contrast
How is the surface of a specimen cleaned to prepare for Metallographic Replication?
- The metal is polished with a series of grits and diamond pastes, leaving the surface scratch free
- The surfaace is etched with 2-5% Nital
What is looked for under microscope in Metallographic Replication?
The grain boundaries are examined to determine the amount of carbide speroidization and cavities that are formed
What happens to the film used in Metallographic Replication after examination?
The films are kept and compared at each outage
What are possible actions to take depending on where on the Creep Curve the results from Metallographic Replication show?
- No action required at this stage
- Schedule regular metallographic replication testing
- Limit the service of the component and schedule repairs
- Repair immediately as failure is imminent
What are the limitations of Metallographic Replication?
- The area to be monitored must be accessible
- The same area of the tube or header must be monitored each time to ensure an accurate record
- The surface temperature of the tube or header must be comfortable to the touch
Why is the temperature important for Metallographic Replication?
If it is too hot, the cellulose acetate film dries too fast and does not have time to penetrate between the grains. If it’s too cold, the water content in the Nital affects the surface
How can Fatigue Failuer occur in metals undergoing high temperature service?
Exposure to cycles of alternating stress, with or without the superimposition of a mean stress
Is Fatigue Failure common in power plants?
No
Why isn’t Fatigue Failure common in power plants?
It is possible to design away from high levels of alternating stress
Thermal Fatigue
Fatigue due to the frequency of straining due to the number of stops and starts endured during the full life of the plant
What is the basic method of reducing the likelihood of accelerated corrosion attack on the waterside of the boiler?
Good mechanical designs that minimize cracks, crevices, and high stress zones
How do most metals react when exposed to water?
They form an oxide or hydroxide corrosion film
What is the function of the oxide layer coating that forms on metal exposed to water?
It protects the metal from most types of corrosion
How do Boiler Water Treatments attack corrosion via the oxide layer?
They stabilize the protective oxide films so corrosion decreases with time
Describe the metal losses associated with oxide films that form on metals exposed to water
Uniform and occur at a predictable rate
Corrosion Allowance
The known rate of metal loss that is incorporated in the vessel design
What is the result of all corrosion?
The loss of strength of the material and the structure
What factors make corrosion inspection and detection difficult?
- The areas where corrosion occurs
- The materials in which it occurs
- The conditions under which it occurs
What is the first step in corrosion inspection?
Visual inspection
What does field inspection for corrosion entail?
- Eddy Current Inspection; and/or
2. Ultrasonic Inspection
How can corrosion inspection be carried out without impacting operations?
As part of routine maintenance
If additional inspection for corrosion is necessary, who carries it out?
Specialists under controlled conditions e.g. in a protected space or NDE laboratory
What are the factors affecting corrosion?
- The type of material selected for the application
- The heat treatment of the material
- The environment of the application
- The presence of any contaminants in the material itself
What are the 9 types of Corrosion?
- Uniform Attack
- Pitting
- Crevice
- Galvanic Corrosion
- Stress Corrosion Cracking
- Caustic Attack (Grooving or Gouging)
- Acid Attack
- Intergranular or Exfoliation
- Erosion-Corrosion
What causes Uniform Attack Corrosion?
Exposure to corrosive environment
What causes Pitting Corrosion?
Impurity or chemical discontinuity in the paint or protective oxide coating
What causes Crevice Corrosion?
Local difference in the environment composition (oxygen concentration) at mechanical joints
What causes Galvanic Corrosion?
Contact of different metals
What causes Stress Corrosion Cracking?
Mechanical tensile stresses combined with chemical susceptibility
What causes Caustic Attack Corrosion?
Concentration of salts in high heat zones found in upper surfaces of incline steam generating tubes
What causes Acid Attach Corrosion?
Low pH in boiler water
What causes Intergranular or Exfoliation Corrosion?
Presence of strong potential differences in grain or phase boundaries
What causes Erosion-Corrosion?
Flowing particles foud at restrictions, bends, or disruptions in the fluid stream damage protective films
What are 7 Corrosion Detection NDE Technologies?
- Visual
- Enhanced Visual
- Eddy Current
- Ultrasonic
- Radiography
- Thermography
- Robotics and Automation
What are the pros of Visual Corrosion Detection?
- Relatively inexpensive
- Large area coverage
- Portability
What are the cons of Visual Corrosion Detection?
- Highly subjective
- Measurements not precise
- Limited to surface inspection
- Labour intensive
What are the pros of Enhanced Visual Corrosion Detection?
- Large area coverage
- Very fast
- Very sensitive to lap joint corrosion
- Multi-layer
What are the cons of Enhanced Visual Corrosion Detection?
- Quantification difficult
- Subjective - requires experience
- Requires surface preparation
What are the pros of Eddy Current Corrosion Detection?
- Relatively inexpensive
- Good resolution
- Multiple layer capability
- Portability
What are the cons of Eddy Current Corrosion Detection?
- Low throughput
- Interpretation of output
- Operator factors
- Human factors (tedium)
What are the pros of Ultrasonic Corrosion Detection?
- Good resolution
2. Can detect material loss and thickness
What are the cons of Ultrasonic Corrosion Detection?
- Single-sided
- Requires couplant
- Cannot assess multiple layers
- Low throughput
What are the pros of Radiography Corrosion Detection?
- Best resolution
2. Image interpretation
What are the cons of Radiography Corrosion Detection?
- Expensive
- Radiation safety
- Bulky equipment
What are the pros of Thermography Corrosion Detection?
- Large area scan
- Relatively high throughput
- “Macroview” of structures
What are the cons of Thermograhy Corrosion Detection?
- Complex equipment
- Layered structures are problematic
- Precision of measurements
What is the pro of Robotics and Automation Corrosion Detection?
Potential productivity improvements
What are the cons of Robotics and Automation Corrosion Detection?
- Quality assurance
2. Reliability
What is the benefit of using NDE techiques to reliably and accurately determinine that no corrosion is present?
The huge costs associated with shutdowns could be avoided
