high temperature behavior of materials
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• Mechanical degradation
• Chemical Degradation
• Gas Turbine and jet Turbine
• Nuclear reactors
• Power plants
• Spacecraft
• Chemical processing
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• Homologous temperature:
• Th = (tcreep+273)/(tmelting +273)
• Th > 0.5 Creep is a concern
• Creep test: measure dimensional changes
Focuses on early deformation stagesCreep conducts: Const Load Engineering purpose
Stress Rupture test: effects of Temp on long timeload bearing characteristics, tr.
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• Andrade’s Model
• 1.Sudden strain, 2.Transient creepwith
strain rate decrease with time,
• 3. const rate creep
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• Garofalo Model:
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• Elevated Temperature Tensile Test (T > 0.4 Tmelt).
• Generally,
ssceramics ss
metals sspolymers. . .
MEASURING ELEVATED T RESPONSE
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• Occurs at elevated temperature, T > 0.4 Tmelt • Deformation changes with time.
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Adapted from
Figs. 8.26 and 8.27,
Callister 6e.
CREEP
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• Most of component life spent here.
• Strain rate is constant at a given T, s --strain hardening is balanced by recovery
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stress exponent (material parameter)
strain rate
activation energy for creep
(material parameter)applied stressmaterial const.
• Strain rate
increases
for larger T, s
10
20
40
10 0
200
Steady state creep rate (%/1000hr)
10 -2 10 -1 1
s
Stress (MPa)427C
538 C
649 C
Adapted from
Fig. 8.29, Callister 6e.
(Fig. 8.29 is from Metals
Handbook: Properties
and Selection:
Stainless Steels, Tool Materials, and Special
Purpose Metals, Vol. 3,
9th ed., D. Benjamin
(Senior Ed.), American
Society for Metals,
1980, p. 131.)
s K2sn exp Qc
RT
.
SECONDARY CREEP
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• Failure:
along grain boundaries.
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time to failure (rupture)
function of
applied stress
temperature
T(20 log t r ) L
applied
stress
g.b. cavities
• Time to rupture, tr
• Estimate rupture time
S 590 Iron, T = 800C, s = 20 ksi
T(20 log t r ) L
1073K
24x103 K-log hr
Ans: tr = 233hr
Adapted from
Fig. 8.45, Callister 6e.
(Fig. 8.45 is from F.R.
Larson and J. Miller,
Trans. ASME , 74, 765
(1952).)
From V.J. Colangelo and F.A. Heiser, Analysis of
Metallurgical Failures (2nd ed.), Fig. 4.32, p. 87, John
Wiley and Sons, Inc., 1987. (Orig. source: Pergamon
Press, Inc.)
CREEP FAILURE
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• Most of component life spent here.
• Strain rate is constant at a given T, s --strain hardening is balanced by recovery
24
stress exponent (material parameter)
strain rate
activation energy for creep
(material parameter)applied stressmaterial const.
• Strain rate
increases
for larger T, s
10
20
40
10 0
200
Steady state creep rate (%/1000hr)
10 -2 10 -1 1
s
Stress (MPa)427C
538 C
649 C
Adapted from
Fig. 8.29, Callister 6e.
(Fig. 8.29 is from Metals
Handbook: Properties
and Selection:
Stainless Steels, Tool Materials, and Special
Purpose Metals, Vol. 3,
9th ed., D. Benjamin
(Senior Ed.), American
Society for Metals,
1980, p. 131.)
s K2sn exp Qc
RT
.
SECONDARY CREEP
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The Creep Test:
• a typical creep curve showing the strain produced as
a function of time for a constant stress and temperature.
Apply stress to a material at an elevated temperature
Creep: Plastic deformation
at high temperature
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The Creep Test:
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Microstructure of a Creep resistant steel
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Microstructure of a Creep resistant steel
Heat Resisting Steel
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Heat Resisting Steel
Precipitates
M 23 C 6 , M 7 C 3 , M 2 X ,
M 3 C , M
6 C , M X
IntermetallicsLaves Phase, Z-Phase
Alloying Elements
Substitutional :
Cr, V, Nb, Mo,W, Cu,
Mn
Interstitial :
C, N
Creep Resistant Steel
MicrostructureTempered Martensite,Bainite
R i t t C
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Resistance to Creep
Solid solution hardening
Precipitate hardening
Microstructure
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