CREEP of METALS

Creep of metals

1. CREEPOF MATERIALS
-Elevated service temperature + staticmechanical stresses may cause creep
deformation. For metals creep deformation is usually plastic. For polymers,
on the other hand, it may be time dependent elastic deformation which is
known as anelasticity.
-Following the deformation, if the material breaks it is calledCREEP RUPTURE
-Creep is undesirable. Even if there is no rupture, too much plastic
deformation beyond a certain limit is accepted to be a FAILURE.
Creep is the time dependent deformation of a material under
constant load at high temperatures
MetE230 Prof. Dr. Rıza Gürbüz, METU 1

2. MetE230 Prof. Dr. Rıza Gürbüz, METU 2
Strain
Time
e
1
AT HIGH TEMPERATURE:
STRAIN INCREASES BY TIME
MATERIAL CREEPS
Strain-Time
Low-T
AT LOW TEMPERATURE:
THERE IS NO CHANGE IN STRAIN BY TIME:
NO CREEP
σ
1
Stress-Time
e
1
σ
1
Stress-Strain
σ
1
SUPPOSE A MATERIAL IS LOADED TO A STRESS BELOW ITS YIELD STRENGTH and then STRESS IS KEPT
CONSTANT FOR AN EXTENDED PERIOD OF TIME.
TIME DEPENDENT DEFORMATION

3. CREEP FAILURES
Creep failures are mostly seen in:
•Rocket engines, rocket motor nozzles, ballistic missile
nose cones
•Turbine rotors of jet engines
•Steam generators(steam turbines)
•High pressure steam pipelines
•Nuclear power plants
•Gas turbine engine(800-1000
o
C)
MetE230 Prof. Dr. Rıza Gürbüz, METU 3

4. STEAM POWER PLANTS and HEAT EXCHANGERS
MetE230 Prof. Dr. Rıza Gürbüz, METU 4
CREEP RUPTURE
CREEP CRACKS

5. JET ENGINETURBINE BLADES and ROCKET
NOZZLES
MetE230 Prof. Dr. Rıza Gürbüz, METU 5

6. Generally, creep becomes of engineeringsignificance at
a temperature greater than 40% of melting
temperature in
o
K.
T
APP(
o
K) > 0.4T
M(
o
K)
Below this temperature creep may still exist but it is
insignificant..
AT WHICH TEMPERATURE WILL
MATERIALS CREEP?
MetE230 Prof. Dr. Rıza Gürbüz, METU 6

7. •Tm of solder(Pb-Sn alloy): 183
o
C=456
o
K
0.4x456= 182
o
K= -90
o
C
So at room temperature solder will creep!
•Most metals, however,do not suffer from creep at room
temperature, since they have much higher melting points
than solder.
•Tm of iron: 1537
o
C=1812
o
K
0.4x1812= 725
o
K= 451
o
C
So iron will creep above 451
o
C !
Different materials will creep at different
temperatures
MetE230 Prof. Dr. Rıza Gürbüz, METU 7

8. What may happen at high temperatures?
•Atoms move faster
•Greater mobility of dislocations by the mechanism of climb
•Increased amount of vacancies
•New deformation mechanisms may come into play(such as
g.b. sliding)
•Slip systems may change or additional slip systems are
introduced
•Deformation at grain boundaries
•Metallurgical changes, i.e., phase transformation,
precipitation, oxidation, recrystallization and grain
coarsening.
•Oxidation and penetration of oxides to grain boundaries
MetE230 Prof. Dr. Rıza Gürbüz, METU 8

9. CREEP CURVE
During loading under a constant stress, the strain often varies as a function of time in
the manner shown below:
έ
s= dε/dt
ε
o: Instantenous
Strain
t
R
MetE230 Prof. Dr. Rıza Gürbüz, METU 9
Strain
Time
SECONDARY CREEP
(Steady State Creep)
TERTIORY
CREEP
PRIMARY CREEP
ε
o
Rupture

10. CREEP PROCESS is an interplay between
SOFTENING and HARDENING
SOFTENING is due to RECOVERY
Time dependent diffusional
processes(such as climb, cross-slip and
vacancy diffusion) that are accelerated at
high temperature increases the
dislocation mobilityand makes the slip
easier.
HARDENING is due to STRAIN
HARDENING
Plastic deformation causes strain
hardening
SOFTENING HARDENING
Balance between the softening rate and the hardening rate determines the slope of creep
curve (i.e.creeprate)
MetE230 Prof. Dr. Rıza Gürbüz, METU 10

11. CREEP RATE, dϵ/dt
PRIMARY CREEP
•Work hardening is faster than rate of recovery!
•Thus, creep rate continually decreases.
STEADY STATE CREEP
*The stage of creep that is of the longest duration
•Rates of work hardening and recovery are approximately equal.
•Creep rate is constant: έ
s
(Steady State Creep Rate)
TERTIARY CREEP
•Creep rate increases until rupture due to cracking, necking and
dominancy ın softening rate. Time to rupture is t
R.
MetE230 Prof. Dr. Rıza Gürbüz, METU 11

12. MetE230 Prof. Dr. Rıza Gürbüz, METU 12
Effect of Stress and Temperatureon Creep
Curve
As the Stress or Temperature increases: Creep rate increases
and rupture lifetime shortens. So creep rate is the function of
stress and temperature.
t
Rα1/(T, σ)
έ=f(T, σ)

13. STEADY STATE CREEP RATE
•Dependence of creep
rate on TEMPERATURE
•Creep occurs faster at
higher temperatures.
έ
sαexp( -Q/RT)
•Dependence of creep
rate on STRESS
•Creep occurs faster at
higher stresses.
έ
sασ
n






RT
Q
K
cn
s
exp
2

MetE230 Prof. Dr. Rıza Gürbüz, METU 13

14. The blades in jetengines can be exposed to hot gases at up to
about 1400°C.They are also under stress as a result of the high
centrifugal forces.
Example: TURBINE BLADE OF JET ENGINE
Centrifugal
Force
MetE230 Prof. Dr. Rıza Gürbüz, METU 14

15. MetE230 Prof. Dr. Rıza Gürbüz, METU 15
Example: Determination of έ
sfor TURBINE BLADE OF JET
ENGINE
•Working temperature is 1400°C+ Subjected to high centrifugal forces (Max
Centrifugal Stress ≈ 100MPa)
•Turbineblades must withstand this environment without excessive creep, which
would cause them to strike the turbine enclosure.Suppose, the max allowable
elongation is 2 mm for 1000 hours of flight time
•For these conditions we can calculate the max allowable creep creep
rate and select the blade material accordingly.
Choose an alloy having έs< 1x10
-5
h
-1
for this temperature and stress
•Initial Blade Length: 20 cm
•Max allowable elongation: 2 mm for 1000 hours
•Max allowable strain ≈ 2/200 = 0.01
•So MAX ALLOWABLE CREEP RATE, έ
s= 0.01/1000h
(Strain rate = de/dt = Strain /time)έ
s=1x10
-5
h
-1
2 mm

16. CREEP DESIGN and PRESENTATION OF
ENGINEERING CREEP DATA
•Two design parameters for creep are:
Steady state creep rate, έ
s
Rupture lifetime, t
R
•Two common diagrams for creep design are:
MetE230 Prof. Dr. Rıza Gürbüz, METU 16
σvs έ
sdiagram
σvs t
Rdiagram

17. STEADY STATE CREEP RATE vs DESIGN STRESS
DIAGRAM
Low Carbon Nickel Alloy
For a given material, design stress and temperature, the creep rate can be
determined and the creep design is made accordingly.
MetE230 Prof. Dr. Rıza Gürbüz, METU 17

18. RUPTURE LIFETIME VS DESIGN STRESS
DIAGRAM
For a given material, design stress and temperature, rupture lifetime can be
determined and the creep design is made accordingly.
Low Carbon Nickel Alloy
MetE230 Prof. Dr. Rıza Gürbüz, METU 18

19. MetE230 Prof. Dr. Rıza Gürbüz, METU 19
Columnar
grains
Conventional
casting
At high temperatures new deformation mechanisms like grain boundary sliding
come to play. So, coarse grain structure will be better than the fine grain structure
for decreasing the creep rate.
CREEP RESISTANT MICROSTRUCTURE
Materials for the blades of the rotor of a jet engine
19
Only one
grain