In modern world the material has to be high creep resistance and high fatigue resistance. So, Design the material in such a way that material has to be high creep resistance and high fatigue resistance.

1. DESIGN THE
MICROSTRUCTURE OF
STEEL TO ACHIEVE HIGH
CREEP RESISTANCE AND
HIGH FATIGUE
RASISTANCE
NAME :- RAJDEEP MONDAL
ROLL :- 16MT61R51

2. CONTENTS
 INTRODUCTION
 CREEP DEFINATION
 MECHANISMS OF CREEP
 DESIGN OF MICROSTRUCTURE ONTHE
BASIS OF CREEP RESISTANCE OF STEEL
 FATIGUE DEFINATION
 MECHANISMS OF FATIGUE
 DESIGN OF MICROSTRUCTURE ONTHE
BASIS OF FATIGUE RESISTANCE OF STEEL
 CONCLUSION

3. INTRODUCTION
The is a growing need for materials to operate
at high temperatures and to operate at large
number of cycles.(and in some applications
for long times). Hence, there is a need to
design materials which can withstand high
temperatures as well as it can operate safely
at large number of cycles.

5. What is Creep?
Creep is permanent deformation (plastic
deformation) of a material under constant
load (or constant stress) as a function of
time. (Usually at ‘high temperatures’ →
lead creeps at RT).

6.  Creep experiments are done either at
constant load or constant stress and can be
classified based on Phenomenology or
underlying Mechanism.
Power Law creep
Creep tests can
be carried out at
Constant stress
Constant load (easier)
Creep can
be classified
based on
Mechanism
Phenomenology
Harper-Dorn creep

7. CREEP MECHANISMS

8. Creep
Dislocation related
Diffusional
Grain boundary sliding
Nabarro-Herring creep
Coble creep
Lattice diffusion controlled
Grain boundary diffusion controlled
Climb
Cross-slip
Glide

9. Phenomenological descriptions of
creep
One of the important descriptions of creep is using the power-law
formula. The shear strain rate is a power function of the shear
stress.
Power-law behavior can arise from:
 Only glide at low temperatures (~0.3TM). Here the exponent n ~
3.
 Glide + climb (referred to as climb controlled creep) occurs at
higher temperatures.
Above ~0.6TM climb is lattice-diffusion controlled.
 At high stresses (> 103G) the power law breaks down. At high
stresses the mechanism changes from climb controlled (creep)
to glide controlled (slip).

10. DESIGNTHEMICROSTRUCTUREONTHEBASIS
OFCREEPRESISTANCE

11. CREEP GENERALLY MINIMIZED IN
MATERIALSWITHTHE FOLLOWING
PARAMETERS:
1. High meltingTemperature
2. High elastic modulus
3. Large Grain size
4. precipitation Hardening
5. Dispersion Hardening

12. Creep
resistance
Dispersion hardening → ThO2 dispersed Ni (~0.9 Tm)
Solid solution strengthening
High melting point → E.g. Ceramics
Single crystal / aligned (oriented) grains

13. Precipitates
M23C6 , M7 C3 , M2X ,
M3 C , M6 C , M X
Intermetallics
Laves Phase, Z-Phase
Alloying Elements
Substitutional :
Cr, V, Nb, Mo,W, Cu, Mn
Interstitial :
C, N
Creep Resistant Steel
Microstructure
Tempered Martensite,
Bainite

14. References
 MTDATA: MetallurgicalThermochemistryGroup, National Physical Laboratory,
Teddington, London (1998)
 2. S. D. Mann, D. G. McCulloch and B. C. Muddle: Metallurgical andMaterials
Transactions A 26A, 509–520(1995)
 3. A. Strang andV.Vodarek: Materials Science andTechnology, 12,552–
556.(1996)
 4. J. D. Robson and H. K. D. H. Bhadeshia: Mat. Sci.Tech. 13, 631–644(1997)
 5. J.W. Christian:Theory ofTransformations in Metals andAlloys, Pergamon
Press, Oxford, 2nd edition, part I (1975)
 6. H. K. D. H. Bhadeshia: Materials Science andTechnology 5, 131–137.(1989)
 7. N. Fujita and H. K. D. H. Bhadeshia:Advanced Heat Resistant Steels for Power
Generation, San Sebastian, published by the Institute of Materials, London, in
press.(1998)
 8. R.G. Baker and J. Nutting: Journal of the Iron and Steel Institute 192, 257–
268(1959)
 9. F. Brun,T.Yoshida, J. D. Robson,V. Narayan and H. K. D. H.Bhadeshia:
Materials Science andTechnology submitted(1998)
 10. D. Cole and H. K. D. H. Bhadeshia: Unpublished research, Universityof
Cambridge (1998)
 11. D. J. C. MacKay: Neural Computation 4, 415-472(1992)

16. WHAT IS FATIGUE?
Fatigue is caused by repeated application of
stress to the metal. It is the failure of a
material by fracture when subjected to a
cyclic stress.

17. 1. Cyclic stresses, the S—N curve
2. Crack initiation and propagation
3. Factors that affect fatigue behavior

18. CYCLIC STRESS
 Characterized by maximum, minimum and mean Range of
stress, stress amplitude, and stress ratio
 Mean stress m = (max + min) / 2
 Range of stress r = (max - min)
 Stress amplitude a = r/2 = (max - min) / 2
 Stress ratio R = min / max

19. SN CURVE
S- Stress , N- Number of cycles
 Low cycle fatigue: small # of cycles
high loads, plastic and elastic deformation
 High cycle fatigue: large # of cycles
low loads, elastic deformation (N > 105)
 Fatigue limit (some Fe andTi alloys)
 S—N curve becomes horizontal at large N
 Stress amplitude below which the material never fails, no matter
how large the number of cycles is

20. Fatigue: Crack initiation
and propagation
 Crack initiation: Quality of surface and sites of stress
concentration (microcracks, scratches, indents, interior
corners, dislocation slip steps, etc.).
 Crack propagation
 I: Slow propagation along crystal planes. Involves a few
grains.
Flat fracture surface
 II: Fast propagation perpendicular to applied stress.
Crack grows by repetitive blunting and sharpening
process at crack tip. Rough fracture surface.

22. DESIGN THE MICROSTRUCTURE ON
THE BASIS OF FATIGUE RESISTANCE

23. To secure satisfactory
fatigue life
 Modification of the design to avoid stress
concentration eliminating sharp recesses and
severe stress raisers.
 Precise control of the surface finish by
avoiding damage to surface.
 Control of corrosion and erosion or chemical
attack in service and to prevent of surface
decarburization during processing of heat
treatment.
 Surface treatment of the metal.

24.  Solutions:
 Polish surface
 Introduce compressive stresses (compensate
for applied tensile stresses) into surface layer.
 Case Hardening: Steel - create C- or N- rich
outer layer by atomic diffusion from surface
 Optimize geometry
Avoid internal corners, notches etc.

27. Grain Size
Fine-grained steels have greater fatigue
strength than do coarse-grained steels.
Composition
An increase in carbon content can increase the
fatigue limit of steels. Other alloying elements
may be required to attain the desired
hardenability.

28. MICROSTRUCTURE
For specimens having comparable strength
levels, resistance to fatigue depends
somewhat on microstructure.
A tempered martensite structure provides the
highest fatigue limit.

29. conclusion
For high creep resistance the grain size must be
large and the microstructure consists of
Bainite o Martensite.
For high fatigue resistance, the grain size
should be small and the microstructure
consists of tempered martensite structure.

30. References
 R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, JohnWiley &
Sons, 1976
 2. D.J.Wulpi, Understanding How Components Fail, American Society for Metals, 1985
 3. Fatigue and Microstructure, in Proceedings of theASM Materials Science Seminar, American
Society for Metals, 1979
 4. Metallic Materials and Elements forAerospace Vehicle Structures, MILHDBK-5B, Military
StandardizationHandbook, U.S. Department of De
 . Bäumel, Jr andT. Seeger (1990). Materials data for cyclic loading, supplement 1. Elsevier. ISBN 978-
0-444-88603-3.
 Jump up^ S. Korkmaz (2010). Uniform Material Law: Extension to High-Strength
Steels.VDM. ISBN 978-3-639-25625-3.
 Jump up^ Korkmaz, S. (2011). "A Methodology to Predict Fatigue Life of Cast Iron: Uniform Material
Law for Cast Iron".Journal of Iron andSteel Research, International. 18: 8. doi:10.1016/S1006-
706X(11)60102-7.
 Jump up^ N.A. Fleck, C.S. Shin, and R.A. Smith. "FatigueCrack Growth UnderCompressive
Loading". Engineering Fracture Mechanics, 1985, vol 21, No 1, pp. 173-185.
 ^ Jump up to:a b Schutz,W. (1996). "A history of fatigue". Engineering Fracture Mechanics. 54: 263–
300. doi:10.1016/0013-7944(95)00178-6.
 Jump up^W.J.M. Rankine. (1842). "On the causes of the unexpected breakage of the journals of
railway axles, and on the means of preventing such accidents by observing the law of continuity in
their construction". Institution ofCivil Engineers, Minutes of Proceedings, 105-108.

31. THANKYOU