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«The Erosion of Metals The Erosion of Metals A dissertation submitted to the University of Cambridge for the degree of Doctor of Philosophy. David ...»

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The Erosion of Metals

The Erosion of Metals

A dissertation submitted to the University of Cambridge for the degree of Doctor of Philosophy.

David Richard Andrews

Selwyn College


May 1980

The Erosion of Metals

Memorandum I

The work described in this dissertation was carried out between October 1976 and January 1980 in

the Cavendish Laboratory under the supervision of Dr J. E. Field. Unless specifically stated otherwise,

all work is the result of my own research activities and none of it has been used previously in a degree thesis submitted to Cambridge University or any other university.

David Richard Andrews Selwyn College Cambridge The Erosion of Metals Memorandum II This thesis was converted from a hard-back (paper) book into an electronic document in February and March 2015 for inclusion in the University of Cambridge’s on-line library, so that a wider public

may gain access to it. The process of conversion included:

 Optical scanning and character recognition of text.

 Optical scanning and creation of image files for figures.

 Creation of a Microsoft Word 2007 document into which text and files have been pasted.

 Mathematical equations were recreated manually using the equation editor in Word.

 Similarly, references were recreated manually using Words bibliography manager.

 Flow diagrams were recreated using Microsoft Visio and pasted into the Word document.

The creation of the Word document was done to improve the final appearance and to compress the

size of the electronic thesis. The following changes have also been made:

 Re-formatting – the greatest change to appearance by far.

 In a very few places one or two words were found to be missing from the original thesis and these have been added.

 A new, short section on historical context has been added to the section titled Importance of Erosion.

 One reference error has been corrected.

 In one equation the explanation of variables has been changed to reflect convention.

 The title of one chapter has been altered by adding a few words – to reflect a discussion with one of the examiners of the thesis in 1980.

 Equation numbers have been altered to include the chapter in which they are first created.

 Generally, shortened terms like: E.g. and viz have been converted to prose.

 In several places the use of inverted commas around words to indicate some special or unusual context has been replaced with italic letters, to conform to popular use; italic letters were not readily available to Mrs Lonzarich in 1979/80 when she typed the original thesis.

The changes above were made to improve readability of the electronic thesis and have not in any way changed the scientific content. I have performed all the above tasks myself to ensure the quality of conversion.

David Richard Andrews camben@cambridgeultrasonics.com. Cambridge Ultrasonics, Over, Cambridge CB24 5NX, UK

–  –  –

Acknowledgements I would like to thank Dr J. E. Field for his help, advice and interest throughout the duration of this work. I am also deeply indebted to him for the opportunity to join the Physics and Chemistry of Solids research group of the Cavendish Laboratory.

I would also like to thank Professor D. Tabor, Dr I. M. Hutchings, Dr M. Chaudhri, Dr D. Gorham, Dr J.

Matthewson and Mr P. N. H. Davies for stimulating conversations concerning the work described here. May I also take this opportunity to thank the many research assistants and technicians of the Cavendish for their help with innumerable pieces of equipment but in particular Mr P. N. H. Davies, Mr C. Naunton and Mr D. L. Johnson.

I wish to thank the Science Research Council and United States Air Force for financial assistance.

British Gas has been most generous in connection with this work by the loan of equipment - my thanks go to them and in particular to Dr M. Howe and Dr I. Glasgow. I would like to thank Rolls Royce (1971) for allowing me to visit their research laboratories at Hucknell.

Lastly, but by no means least, may I thank Mrs C. Lonzarich for her patience and excellence in typing the original manuscript.

David Richard Andrews Selwyn College Cambridge I would also like to thank Cambridge Ultrasonics for allowing me to use its facilities for converting this thesis into electronic format and for allowing me some time away from work to perform the conversion process.

David Richard Andrews Cambridge Ultrasonics Cambridge February 2015

–  –  –

The study of the erosion of metallic surfaces by solid particles has been an area of dispute recently (1980) especially concerning the importance of target melting as a mechanism for the removal of material. In addition, erosion by particles at a normal angle of impingement has remained unexplained and there has been no satisfactory theory of erosion which has taken into account the statistical nature of erosion, that is, the continual bombardment of a surface by a large number of eroding particles.

This work concentrates on the foregoing aspects of erosion. Apparatus is described which is capable of producing erosion by single impact and continual bombardment. Conditions conducive to target melting are discussed and under equivalent experimental conditions target melting is deduced to

have occurred. The statistical nature of erosion has been approached from two directions:

1. The importance of the shapes of eroding particles.

2. Considering the influence of the erosive flux on the temperature of the target and resulting erosion rate.

Material removal by single impacts at normal impingement has been observed using high-speed photography.

–  –  –

Contents The Erosion of Metals

Memorandum I

Memorandum II



Importance of Erosion

Extract from The Times 28th April 1980.

Historical context

Chapter 1 Review

1.1 Introduction

1.2 Erosion Prediction

1.3 Mechanism (1): Erosion by Cutting

1.4 Fragmentation and Scouring

1.5 Mechanism (2): Fatigue of the Impact Surface

1.6 Mechanism (3): Target Melting

1.7 The Effects of Temperature and Strain-rate

1.8 Aerodynamic and Geometric Effects

1.9 Prediction of Erosion Resistance

Chapter 2 Aims of This Work and Theory

2.1 General Aims

2.2 (a) Statistical Nature of Erosion - I

2.3 (b) The Importance of Target Melting

2.4 (a) Statistical Nature of Erosion - II

2.5 Forces Experienced by a Sphere during Impact

2.6 Erosion at Normal Impingement

2.7 Improving the Erosion Resistance

2.8 Apparatus

Chapter 3 Single Particle Erosion Rig

3.1 Design Specification

3.2 Gas gun

3.3 Projectile Velocity Measurement

The Erosion of Metals Timer Circuit Operation

3.4 Specimen Heating

3.5 Specimen Chamber

3.6 Safety Considerations

3.7 Performance

Chapter 4 Quantitative Observations of Single Impacts

4. 1 Introduction

4.2 Moiré Topography

4.3 Specimens

4.4 Results

4.5 Change of Mass as a Result of Impact

4.6 Conclusion

Chapter 5 Qualitative Observations of Single Impacts

5. 1 Introduction

5.2 Strain Fields

5.3 Features of Copper Deformation

5.4 Features of Nimonic 105T Deformation

5.5 Features of the Deformation of Mild Steel (ENIA)

5.6 Features of Titanium Deformation

5.7 Features of Bismuth Deformation

5.8 Conclusion

Chapter 6 Statistics of Real Erosive Particles: Towards a Geometric Classification

6.1 Types of Erosive Particles

6.2 Correlation between Shape Index and Impact

6.3 Conclusion and Discussion

6.4 Summary

Chapter 7 Erosion at Normal Impingement Angles

7.1 High Speed Photography

7.2 Theoretical considerations

7.3 Conclusion

Chapter 8 Multiple Particle Erosion Rig

8.1 Introduction

8.2 Acceleration Tube

The Erosion of Metals

8.3 Grit Ingestion Controller

8.4 Specimen Chamber

8.5 Velocity Measurement

Time of flight: a deduction by cross-correlation

Velocity Measurement: Rapid Assessment

8.6 Safety Considerations

8.7 Performance of the Erosion Rig

Chapter 9 Conclusion and Discussion

9.1 Major Themes of This Work

9.2 Importance of Target Melting

9.3 Statistics of Impact

9.4 Normal Impact

9.5 Discussion

Appendix A

Correct adjustment of the velocity measuring circuit (Single impact)

Appendix B

Curve fitting to impact craters

Computer Listing

Appendix C

Computer aided cross-correlation

PLS method of C-C: Flow diagram

PLS method of C-C: computer program (HP-L)

PLS and SM method of C-C: Test data

Appendix D

Properties of the Cross-Correlation Function

Velocity Distributions


–  –  –

Extract from The Times 28th April 1980.

“The flight was about 500 miles, the limit of the helicopters’ range. Three of them flew into a severe sandstorm.

The other two helicopters caught in the sandstorm landed and waited for conditions to improve.

They then went on to the rendezvous.

One of the seven remaining helicopters suffered a severe mechanical failure and had to land. It is not clear whether this was one of those affected by the sandstorm. Its crew was picked up and six helicopters, therefore, reached the base, known as Desert One where the six C130s were waiting.

Everyone said the flight from the Nimitz to Desert One was the most difficult part of the operation.

The helicopters had to fly very low to avoid Iranian radar and found the weather conditions worse than expected.” Historical context There was a revolution in Iran in the years 1978 and 1979. The Shah of Persia went into exile in 1979, resulting in the formation of an Islamic state in Iran. The new regime did not respect the sovereignty of the embassy buildings of the United States of America in Teheran and on 4th November 1979 the Embassy staff and occupants were taken as prisoners and hostages for 444 days. The then US President, Jimmy Carter, authorised the armed forces of the USA to mount a mission to rescue the American hostages on 24th April 1980. The mission was unsuccessful due to mechanical failure of the helicopters, which were forced to fly close to the ground through a sandstorm. Almost certainly erosion of the leading edges of turbine blades (probably made of Nimonic alloy – see later in this study) in the helicopter engines contributed to the failure of the mission, caused by the engines ingesting large quantities of sand. In Iran, the failure of the American military mission was hailed as an act of god.

–  –  –

Erosion is a wear process. In the work described here only one class of erosion will be discussed in detail: the erosion of ductile metal surfaces by the impingement of solid particles.

Erosion is commonly measured in terms of a parameter W which is equal to the mass of material removed from the surface divided by the mass of the eroding material. Occasionally it is more convenient to refer the parameter to the volume loss divided by the volume of eroding material. In either case the parameter is dimensionless.

In most cases W 0, a condition which indicates that material is removed during erosion but under certain circumstances W 0.

Within the last twenty years erosive wear has become of increasing interest and a considerable research effort has been directed towards elucidating the mechanisms of erosion. This intensive study has been made usually in order to minimize the undesirable effects of erosion, however, erosion can have beneficial effects, for example: the shot-peening and peen-forming processes (Meguid, Johnson, & Al-Hassani, 1976).

It seems likely that a heuristic approach will still remain more useful for predicting erosion under a given set of industrial conditions. The reason for this is because there are a large number of parameters which influence the rate of erosion and these can be interrelated (for example: eroding particle velocity, site and angle of impingement are not independent due to the aerodynamic flow field around the target).

The examples of industrial problems attributable to erosion are exhaustive but some of the more

common and well-documented cases are listed briefly here:

(a) The erosion of turbine blades in gas-turbine aero-engines, especially helicopter engines which suffer from the adverse effects of ingesting sand or salt grains whilst hovering.

(b) Damage to pressure vessels and pipes in the catalyst cracking of petroleum oils (Finnie A., 1960).

(c) Erosion of components in fluidized-beds (Wood & Woodford, 1979).

(d) Erosion of ducts and pipes carrying pulverised coal in coal-fired power stations (Raask, 1979).

(e) Plant erosion, especially in the cyclone stages of the so-called coal-gassification process to produce methane gas directly from coal (Dapkunas, 1979).

(f) Erosion of jet nozzles and nozzle guide vanes.

–  –  –

In many industrial cases erosion occurs along with chemical attack of the metallic surface. This cooperative effect can be particularly damaging and is termed erosion-corrosion.

The range of particle sizes commonly encountered in industrial erosion is roughly 1 µm — 1 mm, the larger sizes being flakes of friable material that may disintegrate forming smaller sized particles.

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