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ENMAT101A Engineering Materials and ProcessesAssociate Degree of Applied Engineering (Renewable Energy Technologies)Lecture 6 – Mechanical Deformation of Metals

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Mechanical Deformation of Metals

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Reference Text Section

Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann

Ch 6

Additional Readings Section

Sheedy, P. A, 1994. Materials : Their properties, testing and selection Ch 1

Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Ed, Wiley, New York.

Ch 3

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Elastic / Plastic Strain

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ELASTIC strain: At first, crystals within the metal are distorted elastically and strain increases proportionally with stress. If the stress is removed during this stage, the metal returns to its original shape, which is elastic. PLASTIC strain: As stress increases beyond the yield point, the binding forces between atoms are overcome, and layers of atoms begin to slide over each other. This process of 'slip', is not reversible, so if the stress is removed, deformation remains. This is plastic.

You Tube Offline (mp4)

From TLP: Introduction to dislocations, http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation_motion.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Slip (plastic strain). Higgins 6.2

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A demonstration of slip. Polish some pure copper, deform it, then look at the surface under a microscope. A large number of parallel, hair-like lines on the polished and etched surface show that layers of atoms within each crystal slid over each other (Figure 6.2). These lines are called slip bands.

Higgins Fig 6.1doitpoms.ac.uk

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A scanning electron micrograph of a single crystal of cadmium deforming by dislocation slip on 100 planes, forming steps on the surface

http://www.doitpoms.ac.uk/tlplib/slip/intro.php

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The formation of slip bands: (i) indicates the surface of the specimen before straining and (ii) The surface after straining. The relative slipping along the crystal planes produces ridges(iii) Microscope view of shadows cast by the ridges on the surface.

Higgins Fig 6.2

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Slip Plane

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Watch the bubble raft carefully:

Can you see a slip plane?

What angles does the slip plane make to the axis of the applied stress? (Note: The applied stress is vertical)

This is not unusual. The same happens with ductile failure (slip) in metals.

(Bubble raft represents a plane of HCP. i.e. honeycomb, not square grid).

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From TLP: Introduction to dislocations, http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation_motion.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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45o Slip planesAn examination of the fracture surface of a tensile test piece can show whether the part was ductile or brittle.

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Ductile specimen showing “cup-and-cone” failure, where shearing occurs at 45o to the applied force. SLIP !

Brittle specimen displays an almost flat fracture surface, perpendicular to the applied force.

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DislocationHiggins 6.2.1

Animation of slip by dislocation glide. Dislocation glide allows plastic deformation to occur at a much lower stress than would be required to move a whole plane of atoms at once.A perfect crystal (in theory) would be 1000 times stronger.

http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation_glide.php

Courtesy of DoITPoMS, The University of Cambridge.

Released under Creative Commons Attribution-Non-Commercial-Share Alike licence

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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A particular slip plane will not stop unless it hits something - a fault or obstacle within the crystal. That slip plane is now locked up.

As stress increases, another slip plane moves until it locks up too.

This process continues until all the available slip planes in the piece of metal are used up. The metal is then said to be work-hardened.

It is no longer ductile, any further increase in stress will fracture it.

In this condition, the metal is hard and strong; but it has lost its ductility, and, if further shaping is required, must be softened by annealing.

Work Hardening (Higgins 6.2.3)

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Image: Tim Lovett

Tensile test on 0.4% C steel in as-drawn state (already has some work hardening).

Work hardening occurs each time the test piece is stretched beyond the (new) yield point.(Compare YS@1 to YS@3).

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A cold-worked metal becomes work hardened and has internal stresses due to elastic strains internally balanced within the distorted crystal structure.

Annealing removes these stresses. The heat makes the atoms more mobile, so they move (migrate) into more comfortable positions, improving ductility.

Annealing progresses in three stages:1 The relief of stress2 Recrystallisation3 Grain growth

Annealing (Higgins 6.3)

Wikipedia: Annealing

Wrinkling of steel pipe that was not annealedhttp://www.sciencedirect.com/science/article/pii/S0924013605003225

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Annealing of gas cylinders to reduce cracking at welds: http://metallurgyfordummies.com

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Recrystallisation

Recrystallisation temperatures of some metals.These are well below the melting point.

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Higgins Table 6.1

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Grain growth (Higgins 6.3.3)

Both the time and temperature of annealing must be controlled, in order to limit grain growth.

Highly work-hardened material has more internal stress points. These form many nuclei of the new grains, hence grains will be smaller.

Alloy elements can also help to initiate grain formation during recrystallisation – hence finer grains.

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Recrystallisation

Properties and Grain Structure BBC (1973)

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Cooling rates and grain size

Slow cooling = more time to form = larger grains.

Rapid cooling = fine grains.

For the same metal grain, finer grains are stronger and tougher. Grains are typically 0.1 to 100 microns.

Note: This is NOT referring to quenching of Carbon steel. Quenching produces a different type of grain - Martensite.

Grain size vs yield strength. Low C steel. W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J. Bradbury: Essential metallurgy for engineers, p63-71. 1985. Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5

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Grain Growth

If solid metal is above a certain temperature (recrystallisation), certain grains will grow at the expense of their neighbours.

This is BAD!

Growth of a grain structure http://www.doitpoms.ac.uk/tlplib/grainGrowth/2dcomputersimulation.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-

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Cold-working processes (Higgins 6.4)

• STRONGER: To obtain the necessary combination of strength, hardness and toughness for service. Mild steel and most non-ferrous materials can be hardened only by cold-work.• FINISH: To produce a smooth, clean surface finish in the final operation. Hot-working generally leaves an oxidised or scaly surface, which necessitates 'pickling' the product in an acid solution.• ACCURATE: To attain greater dimensional accuracy than is possible in hotworking processes.• MACHINABLE: To improve the machinability of the material by making the surface harder and more brittle.

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Cold-working uses (Higgins 6.4.1)

• The cold-rolling of metal plate, sheet and strip.• Spinning and flow-turning, as in the manufacture of aluminiumkitchenware.• Stretch forming, particularly in the aircraft industry.• Cold-heading, as in the production of nails and bolts.• Coining and embossing.

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Hot-working processes (Higgins 6.5)

• DEFORMATION: Large deformations possible. E.g. Rolling of billets into I beam. Breaking down initial slabs, billets and plate.• NO WORK HARDENING: Since processing above recrystallisation temperature.• LOWER FORCES: Large parts might be limited to hot rolling only since machinery could not attain adequate force for cold forming.• HARD MATERIALS: Some materials will not cold form, or will work harden excessively – e.g. forming of large springs.

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Hot-working uses (Higgins 6.5.1)

• Hot-rolling, for the manufacture of plate, sheet, strip and shapedsections such as rolled-steel joists.• Forging and drop-forging, for the production of relatively simpleshapes, but with mechanical properties superior to those of castings.• Extrusion, for the production of many solid and hollow sections(tubes) in both ferrous and non-ferrous materials.

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Grain flow and fibre (Higgins 6.6)

As metal is shaped, grains get distorted. Impurities that accumulated at grain boundaries now form 'flow lines' or 'fibres' in the direction of deformation. When new crystals grow (independently of these fibres), they do not weakens the structure as much as the original inter-crystalline impurity films.

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This makes the material stronger and tougher, particularlyalong the direction of the fibres. At right angles to the fibres, the material is still weaker.

Higgins

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Grain flow and fibre examples (Higgins 6.6)

See discussion of forged bolt design vs machined bolt:Higgins 6.6

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Failure of replacement gears: Discussion:

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Diffusion

The two main diffusion methods are: Vacancy Diffusion: A new atom works it’s way into the metallic lattice by taking vacant positions.

Interstitial Diffusion: A new (small) atom migrates between atoms.

Temperature increases the rate of diffusion.Stress encourages diffusion by opening up gaps. Diffusion of new type of atom into a metallic lattice.

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Diffusion: Carburising

Mild steel cannot be hardened unless there is carbon in the lattice.

Adding carbon to steel is called carburising.

There are several ways to do this, but the oldest and simplest is to heat the mild steel in the presence of carbon (charcoal) – for a long time at high temperature.

This allows carbon to diffuse into the surface for a mm or so.

Pack Carburising. A few minutes excerpt from BBC Video Heat Treatment:Heat treatment [videorecording] / producer Brian Davies. [B.B.C.], 1981. Video: Discusses the use of heat which changes the properties of metals. Outlines different techiques including hardening, tempering, annealing, normalising as well as a non-heat process, coldworking.

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Diffusion toDislocationsDislocation slip can be hindered:

Here, an interstitial atom migrates into the stress zone, hindering dislocations.

Otherwise the slip continues to the grain boundary, which distorts the grain, hence plastic deformation.

Dislocation and the effect of migration of interstitial atomsYou Tube

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Metallurgical Furnaces (Higgins 6.7)

See text. Higgins 6.7

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Graphical comparison of materials properties.

Wikipedia: Materials properties

Online Properties Resources.

Failure of replacement gears

DoITPoMS: Dissemination of IT for the Promotion of Materials Science

Video: Heat Treatment: BBC 1981Pack Carburising only – or salt and plasma it time allows

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GLOSSARY

Binders Hot formingCold formingRecrystallisationAnnealingWork HardeningStress RelievingGrain GrowthSlipDislocationElastic vs Plastic StrainInterstitial atomSubstitutional atomDiffusionCarburising

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QUESTIONSCallister: Ch3 Moodle XML: 10107 Processing and 10105 Steel

1. Define all the glossary terms.2. Define the glossary terms 3. Explain the difference between elastic and plastic deformation in terms of

movements of atoms. 4. Why are smaller grains stronger and tougher? 5. Describe 2 ways to reduce slip in a crystal. 6. What happens if annealing is done at excess temperature and/or for too long? 7. What can be done to keep grains small when heat treating? 8. Why does forging make a stronger part than casting?9. What is the composition, in atom percent, of an alloy that contains 98 g tin and

65 g of lead?10.What is diffusion? Describe two types of diffusion.11. List several factors that encourage diffusion.12.Describe what is Carburising does to mild steel and how it can be achieved.

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