Creep of Metals 2009

8/2/2019 Creep of Metals 2009

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Creep of metals

Section No. 5

Component/materials often subjected to high

temperatures and static mechanical stresses.

CREEP: Time dependent and permanent deformation

when subjected to constant load at elevated

temperatures.

Metals > 0.4 Tm (Tm is melting point in Kelvin)Polymers very sensitive to temperature.

We will consider metals.

Creep rupture


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See Moodle on

measurements

issues.


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constant creep rate

Steadystate creep

rate: this is

something

we can use

for design


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Creep measurements/indicators

(i) Steady state creep in stage II

(ii) Time to rupture : tR

dt

d

tII

II

I !(

(

!


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Remove instantaneous

strain I0 and measure

gradient of steady state

region

Increase in W leads to:

Increase in III

Decrease in tr

Increase in I0(elastic deformation)


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Derivation of stress function , 3

Intercept (i.e. K

constant in power

law) depends on

temperature

Gradient (n)

independent of

temperature

Highest T

Lowest T


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Increase in T leads to:

Increase in III

Decrease in tr

Increase in I0 (E decreases with increase in T)


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Gradient = -Q / R

independent ofstress

W1 (lowest stress)

W3 (highest stress)

log A1

log A3

Intercept

dependent on

stress level


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NATURAL

LOGS


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n

II TK 11 ).()(WWI !

n

II TK WWI ).()( !

)()1( TKII !I

W= W1

W=

11 log)1(log)(log WIWI nIIII !

Development of Constitutive Equation

Power law creep


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Relevant equations:

At particular8(power law creep)

At particularW

For particularW and T

n

II KWI 1!

-

!RTQKII exp2I

-

!

RTQK nII exp..3 WI

Q = activation energy for creep

Variety of creep mechanisms lead to different values of Q and n


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Data Extrapolation

e.g. power plant design for25 years lifetime

Data extrapolation allows testing at elevated temperatures

or higher stress to test for shorter times.

- enables comparison on materials/materials selection

- determine lifetime of component or operating stress

Variety of methods to deal with experimental data captures during

creep testing.

ConsiderDORN and LARSON-MILLAR approaches.


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For each stress level

NATURAL LOG


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Larson-Millar Parameter (LMP)

LMP = T ( C + log tr)

C = constant (17-23)

T = temperature (K)

tr= rupture lifetime

IfLMP is known for specific stress level and temperaturethen rupture lifetime can be calculated

LMP versus stress plotted on a single creep mastercurve


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T(20+ log tr) (K hr)


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Design Example:

For S-590 iron predict

timeto rupture for a

componentsubjectsto

stress of 140MPa at 800C


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Practical Aspects ofLMP

(i) Stress/activation energy (gradient) interaction

(ii) Time for rupture/failure ONLY, not strain rate

(iii) Relatively low cost testing no need to

measure strain.

(iv) Used in applications where strain is not aserious problem e.g. surpheater tubes in

steam power plant.


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Practical aspects ofDorn

Diffusion controlled (Arrhenius)

Measure steady state strain rate (not rupture time) at range of stressand temperatures

Expensive: Precise strain measurements of elevated temperatureand close control of temperature required (to measure Q)

Critical applications with design for minimum strain. e.g. turbineblades which require high tolerance.

See Moodle on measurements issues (I expect you to read this)

[Stress_Rupture_and_Creep_Testing_Details.pdf]


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In this case time to specific strain (1%) rather than absolute strain rate


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DORN LMP


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Case Study: Power plant failures

Steel pipes for power plant

Superheated steam to turbines

Diameter 0.75m, thickness (t) 37.5mm

Internal pressure 5MPa

Temperature of steam 811K

~0.44Tm so creep rupture/deformation

possible

Pipes constructed from flat plates androlled and welded along seam


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1980s Two failures with damage and fatalities

Failures after 10-15yrs service.

Tangental stress () is:

W= P. r / t (thin wall approximation)

W= 50MPa

Calculate expected lifetime:


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Why did it failure less than 20yrs ?

Welding changed microstructure and creep resistance of steel.

Short term solution: Regular inspection (increased costs), reduce temperature or

pressure (reduces efficiency of power plant)

Long term solution: Change/discontinue welding process (seamless pipes),

increase thickness


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Dangers of extrapolation Unexpected changes in creep behaviour at different stress or temperature

Short experimental tests require higher T Grain growth

Phase changes

Recrystallisation (new grain growth)

Precipitate formation

Oxidation

Testing uniaxial loads in reality multi-axial loading

Military turbines - few 100hrs lifetime

Steam turbine 100,000 hrs

Use extrapolation with caution, with sufficient expertise, knowledge ofmaterials and adequate safety factors.

Loss of accuracy reading off log curves.


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Creep resistant materials

(i) High melting point (~0.4Tm)(ii) Precipitation hardening or solid solution strengthening

(iii) High lattice resistance (e.g. covalent bonding)

e.g. Ni based superalloys (turbine blades)

Solid solution strengthening (Cr, Co, W)

Precipitates (carbides/intermetallics)

Used ~ 950C

Unlike fatigue, stress concentrations not as significant in early stages ofcreep as they are relaxed by creep deformation see creep

relaxation section.


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Cambridge website


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Stress rupture

curve

Stress for given

tr (1000hr)

Documents

Creep of Metals 2009