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ME 215
ENGINEERING MATERIALS I
CHP.8
HIGH TEMPERATURE
PROPERTIES OF MATERIALS
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INTRODUCTIONWhile technolgy is advancing many engineering applications
require materials working at high temperatures.
High temperature means how close is the working temp to
melting temp of the material itself.
For example, could a material with 500C melting point be used
in an application where working temp is 300C for a relatively
long period of service?
The development of materials to resists high temperatures
therefore has been an important technological advence of
recent years.
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Demands for alloys to resist high temperatures
come from many new applications, including
– space vehicles,
– rockets ,
– gas turbines,
– improvements in conventional power plants for increased thermodynamic effeciency,
– chemical processes,
etc.It can probably be said that high temperature
application represents one of the ultimates in
engineering desing.
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• The corrosion and creep resistance limits of the avaliablematerials restricts the contemplated high pressurereactions and high temperatures conditions in chemical
processing.• In air craft and space vehicles, high temperatures arisefrom the heat generated in the engines and from theaerodynamic heating of airframes at supersonic speeds.
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The point of choosing correct material for hightemp application is therefore very important.
Because there is the greatest variety of practicalconsiderations which determine the choice of material for high temperature applications .
These include: – Environment
– Stress system and range
– Permissible creep deformation
– Production requirement
– Weight and size restrictions
– Cost and availability
– Service temperature range
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1. Environment;
it covers a wide range from an inert gas to completely
hostile environmental conditions. In the latter casethis becomes a prominent factor in the choice of materials.
2. Stress system and range;
this can vary from heavy loads in bulky structures tozero applied loads in covers and heat shields.
Fatigue,under fluctuating stresses or the thermalstresses is always of prime concern.
3. Permissible creep deformation;
this again is completely dependent upon the funtionalrequirements.
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4. Production requirements; it should be noted thatsome of the high temperature materials are not
amenable to regular methods of production to producethe desired shapes.
5. Weight and size restriction; these are of importancein aircraft and,especially, space like applications.
6. Cost and availability; there are many excellent hightemperature materials but their cost limits their use inmost conventional applications
7. Service temperature range; probably the mostimportant factor in deciding the essantial properties.”
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STATIC CREEP PROPERTIES
• Creep is a progressive deformation of a metarial under
stress.
• It is a time dependant permanent deformation
• In creep, at constant stress, strain continues to
increase by time
Time
Stress
Strain
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There are three main kinds of creep:
• Logarithmic creep
• Diffusion creep• High-temperature creep
The logarithmic creep
occurs at low
temperatures where norecovery of mechanical
properties is possible.
It is given the name
“logaritmic creep”because the
deformation is
proportional to the
logarithm of time.
•The diffusion creep is observed
at very high temperatures.
•High rates of diffusion permitreshaping of crystals to relieve
stress
•Diffusion is significant at both
grain boundaries and in the bulk
•High energy and weak bonds
allow dislocations to “climb”
around structures that pin them
at lower temperature
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The Critical Temperature
• Contrary to the general belief, creep is not
exclusive to high temperatures.
• Creep may occur from very low temperatures,
probably from 0oK (-273oC) ,to the melting point.
• The Critical Temperature for Creep is usually
40% of the Melting Temperature in Kelvin.
• If T > 0.40 TM ; Then creep is likely
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Example for creep risk at room temp.
Will products made from Lead Creep at Room Temperature?
• TM = 327 C = 600 K (melting point for lead in Kelvin)
• TROOM = 23 C = 296 K (room temp in Kelvin)
• TR/TM = ?
• (296 / 600)*100 = 49.3 % > 40% Yes They Will Creep
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• Creep is important in the engineering sense becauseit causes faiulure by excessive of deformation or loss
of strength.• Excessive creep deformations may also lead to
buckling of structural members or complete distortionof structural components which requires withdrawal
from service.• For most metals and alloys creep occurs at stressesbelow the yield point only when the temperature iselevated.
• Some nonferrous alloys and metals such as lead
exhibit creep at low stresses and at roomtemperatures.
• For nonmetallic materials,such as plastics andelastomers,creep may also be produced at lowstresses and room temperatures
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CREEP TESTING OF
MATERIALS
The basic creep testingarrangement consist of three pieces of major equipment;
1-An electric furnace withsuitable temperatureregulation
2-a loading device toapply stress
3-an extensometer tomeasure elongationagainst time
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Time Extension
0 ?
1 ?
2 ?
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In making a creep test at a specified temperature, the
unloaded tensile specimen is first heated to the
required temperature.
When the temperature of the specimen is steady, the
gauge length is observed and the predetermined load
is applied quickly without shock.
The resulting instantaneousextension under load is largely
an elastic strain.
Measuremets of the
subsequent creep (elongation)
are taken at sufficiently
frequent intervals to define the
strain – time curve till rupture.
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Creep time curves have four stages:
• İnitial instantaneous stage in which total deformation may be fullyelastic,or partly elastic and partly plastic;
• A transient stage where the rate of creep deformation decreaseswith time as a result of strain hardening (primary creep )
• A stage of constant minimum creep
rate where the effect of strain-hardening
is balanced by an annealing influence
(secondary creep )
• A final stage where the creep rate
accelerates. The acceleration can have
many causes.It is attiributed to the
progressive increase of the true stress
acting on the cross-sectional area of the
specimen,and to metallurgical changes
and the formation of internal cavities
(tertiary creep).
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Sample deformation at a constant stress (s) vs. time
Primary Creep: slope (creep rate)
decreases with time.
Secondary Creep: steady-state
i.e., constant slope.
Tertiary Creep: slope (creep rate)
increases with time, i.e. acceleration of rate.
s
s,e
0 t
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Creep usually occurs at elevated temperature, T > 0.4 Tm
And effect of temperature incerase on the curve is seen infigure for constant stress condition
Adapted from Figs. 8.29,
Callister 7e.
elastic
primary secondary
tertiary
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Creep tests can be done under different stresses too.
When tension-creep tests are conducted at various stressvalues (and a constant temperature), a fam ily of creep
curves is obtained as shown in figure
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Both temperature & applied stress adversely affect thecreep strains.
Usually under the same temperature different stresslevels are applied & the creep strain vs time curves
are determined.
Creep
Strain
T1 or σ1
Time
T2 or σ2
T3 or σ3T4 or σ4 T1<T2<T3<T4
σ1<σ2<σ3<σ4
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Creep can be a big problem in connection of parts like bolt.
And creep in constant load tensile specimen and a bolted
connection are not the same.
While the stress is constant in tensile test specimen due to
constant load, stress decreases (relaxes) in a bolt since
there is no external load to keep a constant stress
The figure shows that the initial rate of relaxation is high, but
it levels off as the stress level decreases creep rate
increases.
time
e
ep
eel
s
time
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Stress relaxation
time
e
ep
eel
s
time