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

CONTENTS

INTRODUCTION CREEP DEFINATION MECHANISMS OF CREEP DESIGN OF MICROSTRUCTURE ON THE

BASIS OF CREEP RESISTANCE OF STEEL FATIGUE DEFINATION MECHANISMS OF FATIGUE DESIGN OF MICROSTRUCTURE ON THE

BASIS OF FATIGUE RESISTANCE OF STEEL CONCLUSION

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.

CREEP

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).

Creep experiments are done either at constant load or constant stress and can be classified based on Phenomenology or underlying Mechanism.

Power Law creepCreep tests can be carried out at

Constant stress

Constant load (easier)

Creep can be classified based on

Mechanism

PhenomenologyHarper-Dorn creep

CREEP MECHANISMS

Creep

Dislocation related

Diffusional

Grain boundary sliding

Nabarro-Herring creep

Coble creep

Lattice diffusion controlled

Grain boundary diffusion controlled

Climb

Cross-slip

Glide

Phenomenological descriptions of creepOne 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 (> 103G) the power law breaks down. At high stresses the mechanism changes from climb controlled (creep) to glide controlled (slip).

DESIGN THE MICROSTRUCTURE ON THE BASIS OF CREEP RESISTANCE

CREEP GENERALLY MINIMIZED IN MATERIALS WITH THE FOLLOWING PARAMETERS:

1. High melting Temperature2. High elastic modulus3. Large Grain size 4. precipitation Hardening 5. Dispersion Hardening

Creep resistance

Dispersion hardening → ThO2 dispersed Ni (~0.9 Tm)

Solid solution strengthening

High melting point → E.g. Ceramics

Single crystal / aligned (oriented) grains

PrecipitatesM23C6 , M7 C3 , M2X ,

M3 C , M6 C , M X

IntermetallicsLaves Phase, Z-Phase

Alloying ElementsSubstitutional :

Cr, V, Nb, Mo,W, Cu, Mn

Interstitial : C, N

Creep Resistant Steel

MicrostructureTempered Martensite, Bainite

References MTDATA: Metallurgical Thermochemistry Group, 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 and V. Vodarek: Materials Science and Technology, 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 of Transformations in Metals and Alloys, Pergamon

Press, Oxford, 2nd edition, part I (1975) 6. H. K. D. H. Bhadeshia: Materials Science and Technology 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 and Technology 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)

FATIGUE

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.

1. Cyclic stresses, the S—N curve

2. Crack initiation and propagation

3. Factors that affect fatigue behavior

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

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 and Ti 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

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.

DESIGN THE MICROSTRUCTURE ON THE BASIS OF FATIGUE RESISTANCE

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.

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.

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.

MICROSTRUCTURE

For specimens having comparable strength levels, resistance to fatigue depends somewhat on microstructure.

A tempered martensite structure provides the highest fatigue limit.

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.

References

R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, John Wiley & Sons, 1976

2. D.J. Wulpi, Understanding How Components Fail, American Society for Metals, 1985 3. Fatigue and Microstructure, in Proceedings of the ASM 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 and T. 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 and Steel Research, International. 18: 8. doi:10.1016/S1006-706X(11)60102-7.

Jump up^ N.A. Fleck, C.S. Shin, and R.A. Smith. "Fatigue Crack Growth Under Compressive 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 of Civil Engineers, Minutes of Proceedings, 105-108.

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