metallurgy for non-metallurgists training by khalid dubai

Metallurgy for Non-Metallurgists

27 31 December 2009

Dhow Palace Hotel, Dubai

By

Walid Jouri Senior Consultant

Participants Name

This material is intended for the personal use of the delegate attending the programme presented by GLOMACS. No part of the material may be reproduced, stored electronically, or transmitted in any form or by any means without the prior written consent of GLOMACS.

Metallurgy for Non-Metallurgists 27 31 December 2009

FORWARD Welcome to Metallurgy for non Metallurgists. This four day program will provide an integrated practical overview of metals, starting from materials testing and physical/mechanical properties, through corrosion properties and strength/deformation principals, and to ferrous and non ferrous alloys and heat treatment. Each of the major topics will be presented as individual units, and in the context of the overall usage of metal components and structures and failure mechanisms, and mechanical integrity.

INSTRUCTOR: DR. WALID JOURI Walid Jouri obtained his B.Eng. and Ph.D. Degrees, in Mechanical Engineering, in the United Kingdom. He has extensive operational experience as an industrial consultant across a wide range of industries, focussing on engineering design in key infrastructure projects and the manufacturing sector. He specialises in advising on engineering design, factory process and layout, prototype testing and type qualification He was formally the head of School of Engineering and Manufacture. He is currently a Senior Lecturer, specialized in Metallurgy, Materials, Advanced Composites, Structures and Mechanical Engineering Design. His main research activities involved the testing and analyses of the mechanical properties of metals, polymers and advanced composites. He also specialized in the design and production of instrumented testing machines for determining the properties of materials. He has been involved with numerous materials research projects for the nuclear, automotive, railway, gas and chemical industries. He has conducted a variety of short courses including Metallurgy for non Specialists, Metallurgy, Corrosion and Failure Prevention, Materials Technology, Materials Properties and Selection, Metallurgy and Pipes, Materials of Construction for Process Equipment and Piping System, Corrosion for non Specialists, Cathodic Protection, Damage Analysis Investigation Techniques, Advanced Polymers and Composites, Mechanical Technology, Mechanical Equipment, Practical Pump Technology, Practical Valve Technology, Welding Technology, Engineering Design Drawing, Print Reading and Symbols.


PROGRAMME OVERVIEW Programme Objectives To provide participants with an integrated practical knowledge on the basic structure of metal alloys and relating it to the mechanical and physical characteristics of metals. The behaviour of metals under various loading conditions (static, dynamic, fatigue and creep) will be presented and related to design methodology and procedures; rules of thumb, standards, and best industry practices. Programme outcome Upon completion of this course, participants will have gained an understanding of the important principals of engineering involving properties and characteristics of metals and alloys, including fabrication and heat treatment of commercial steels and non-ferrous alloys. Participants will acquire sufficient knowledge and skills to independently evaluate possible metallurgical and design solutions, to recognise crucial metallurgical phenomena and intelligently discuss their metal problems with design engineers, metallurgists and fabricators. Training Methodology The course combines presentations and discussions of topics covered with relevant examples. It combines knowledge of fundamental principles related to the structure of metals sound engineering principles, methods, and applicable standards and best industry practices and enforces learning with Question & Answer sessions to maximize the benefits to the participants. Videos of relative manufacturing processes of metals will also be included. Participants will be provided with comprehensive course notes and copies of presentation material that will be very valuable for detailed study and future reference. INTENDED AUDIENCE This programme is intended for those who use or supervise activities requiring the use of metal parts or structures. Those with little or no prior formal background who function as managers, supervisors, engineers, planners, inspectors, designers, researchers, investors or procurers, and who seek a basic understanding of the practical aspects of metallurgy should find this course valuable.


1. TESTING AND MECHANICAL PROPERTIES OF ENGINEERING MATERIALS

Introduction The following topics will be covered in detail:

Tensile, impact and hardness tests

Fatigue and creep failure

Objective

By the end of this module, you will be able to:

Understand the fundamental bases of mechanical testing

Compare mechanical properties of metals for selection purposes 2. THE CRYSTALLINE STRUCTURE OF METALS Introduction The following topics will be covered in detail:

Bonding in metals

Solidification crystal growth and structures of metals

Defects in metals during solidification Objective By the end of this module, you will be able to:

Gain an understanding of the nature of metallic bonds and crystal structures of metals

Understand the solidification process of pure metals

Learn of examples of probable defects that may occur within the solidified structure of a metal


3. SPECIMEN PREPARATION

AND MICROSCOPIC EXAMINATION Introduction The following topics will be covered in detail:

The preparation (Mounting Grinding, Polishing and Etching) of metal specimens

Metallurgical and Electron Microscopes Objective By the end of this module, you will be able to:

Gain a basic understanding of the way metal specimens are prepared prior to microscopic examination

Understand the use of microscopes for metallurgical examination

4. DISLOCATIONS AND STRENGTHENING MECHANISMS IN METALS

Introduction The following topics will be covered in detail:

Edge Dislocation (line imperfections) in crystals

Strengthening of metals by Grain Size Reduction, Solid Solution and Strain Hardening

Softening of metals by annealing

Comparison of Cold and Hot working of Metals


Objective By the end of this module, you will be able to:

Understand the phenomenon of dislocations; one of the most fundamental properties, related to the strength of metals.

Gain knowledge in controlling the strength of metals by controlling the movement of

dislocations.

Gain initial knowledge of softening metals by the application of heat (annealing) 5. BINARY EQUILIBRIUM DIAGRAMS Introduction The following topics will be covered in detail:

Solubility and cooling curves

Thermal Equilibrium Diagrams (Eutectic Type, Solid Solution Type and Combination Type)


Understand temperature/time cooling curves of pure substances.

Gain knowledge of thermal equilibrium diagrams (phase diagrams) and related microstructures of selected types of metal alloys.


6. FERROUS ALLOYS Introduction The following topics will be covered in detail:

Definitions and classifications and some uses of ferrous alloys including; Carbon steels Alloy steels Stainless steels Cast irons


Gain an understanding of the nature, properties , classifications and uses of a variety of ferrous alloys

7. HEAT TREATMENT OF PLAIN CARBON STEEL Introduction The following topics will be covered in detail:

Hardening of carbon steel (by quenching)

Annealing, normalising and tempering

Surface treatments

Heat affected zone (HAZ) in welding



Understand fundamental hardening, softening and grain control processes related to steel

Understand additional processes such as surface hardening of steel

Gain knowledge of related topics related to the heat affected zone (HAZ) 8. NON-FERROUS ALLOYS Introduction The following topics will be covered in detail:

Nickel and cobalt

Titanium alloys Objective By the end of this module, you will be able to:

Gain knowledge of additional special application non-ferrous alloys such as nickel based, cobalt based and titanium based alloys.

9. FABRICATION OF METALS Introduction The following topics will be covered in detail:

A selection of metal fabrication methods, including;

Forming

Casting

Welding



Understand basic metal fabrication techniques

Learn about practical examples of welding and repair technique related to pipes 10. CORROSION IN METALS Introduction The following topics will be covered in detail:

The electrochemical cell

Types of electrochemical corrosion

Protection against electrochemical corrosion Objective By the end of this module, you will be able to:

Gain an fundamental understanding of the nature and types of corrosion

Learn about various methods of protection against corrosion

Gain knowledge of obtaining the remaining life of a metal component, from corrosion consideration


11. NON-DESTRUCTIVE TESTING Introduction The following topics will be covered in detail:

Introduction to a selection of Non Destructive Techniques (NDT), applications and standards, such as;

Dye penetrant

Magnetic methods

x-ray methods and - ray methods

Ultra-sonic methods

Eddy current testing


Gain an understanding in various NDT methods and standards

Be able to determine appropriate NDT techniques for a variety of applications


I

1. TESTING AND MECHANICAL

PROPERTIES OF ENGINEERING MATERIALS 1.1 1. 1 TENSILE TESTING 1.1

1.1. 1 The Tensile Test Piece 1.1 1.1. 2 The Tensile Test 1.2 1.1. 3 Force Extension Curve 1.2 1.1. 4 Engineering Stress & Strain Curve 1.3 1.1. 5 Properties Obtained From the Tensile Test 1.5 1.1.6 Ductility in metals 1.10 1.1.7 Fracture of Metals 1.10

1.2 IMPACT TESTS 1.11 1.2.1 Standard Impact Tests, Izod and Charpy 1.11 1.2.2 Impact Specimen 1.11 1.2.3 Izod Impact Test 1.11 1.2.4 Charpy Test 1.12 1.2.5 Transition Temperature 1.13

1.3 HARDNESS TESTS 1.14 1.3.1 Introduction 1.14 1.3.2 The Vickers Hardness Test 1.15 1.3.3 The Brinell Test 1.16 1.3.4 The Rockwell Test 1.17 1.3.5 The Shore Skleroscope Hardness Test 1.17 1.4 COMPARISON OF MECHANICAL PROPERTIES OF METALS 1.18 1.5 FATIGUE FAILURE 1.20

1.5.1 Nature of Fatigue Failure 1.20 1.5.2 The Mechanism of Fatigue Failure 1.21 1.5.3 Fatigue Testing 1.22 1.5.4 Improving Fatigue resistance 1.23

1.6 CREEP FAILURE 1.26 1.6.1 Nature of Creep 1.26 1.6.2 Creep Test 1.26

2. THE CRYSTALLINE STRUCTURE OF METALS 2.1 2.1 THE STATES OF MATER 21 2.2 SOLIDIFICATION AND STRUCTURES OF METALS 2.3

2.2.1 Solidification of pure metals 2.3 2.2.2 Basic Metallic structures 2.3 2.2.3 Polymorphic transformation of metals 2.5

2.3 CRYSTAL GROWTH AND OVERALL BULK SOLIDIFICATION 2.6 OF METALS

2.4 DEFECTS IN METALS DURING SOLIDIFICATION 2.8 APPENDIX 2A BONDING IN METALS 2.12

2A.1 SIMPLIFIED STRUCTURE OF AN ATOM 2.12 2A.2 METALIC BONDING 2.13 2A.3 BONDING ENERGY AND INTERATOMIC SPACING 2.14

APPENDIX 2.B THE VARIOUS POSSIBLE CRYSTAL STRUCTURES 2.16

CONTENTS


II

3. SPECIMEN PREPARATION 3.1 AND MICROSCOPIC EXAMINATION

3.1 INTRODUCTION 3.1 3.1.1 The preparation of metal specimens 3.1

3.1.1.1 General Process 3.1 3.1.1.2 Mounting of specimen 3.1 3.1.1.3 Grinding and polishing the specimen 3.2 3.1.1.4 Etching the Specimen 3.3

3.2 MICROSCOPES 3.5 3.2.1 The Metallurgical Microscope 3.5 3.2.2 The Electron Microscopes 3.6

4. DISLOCATIONS AND

STRENGTHENING MECHANISMS IN METALS 4.1 4.1 THEORETICAL AND OBSERVED MECHANICAL

PROPERTIES OF METALS 4.1 4.2 DISLOCATIONS IN CRYSTALS 4.3

4.2.1 Imperfections in crystals 4.3 4.2.2 Edge Dislocation 4.3 4.2.3 Summary of the general properties of dislocations 4.5

4.3 MECHANISMS OF STRENGTHENING IN METALS 4.5 4.3.1 Introduction 4.5 4.3.2 Strengthening by Grain Size Reduction 4.6 4.3.3 Solid Solution Strengthening 4.8 4.3.4 Strain Hardening and Annealing 4.10

4.3.4.1 Strain Hardening 4.10 4.3.4.2 Annealing 4.12 4.3.4.3 Summary and Comparison of Cold and

Hot working of Metals 4.15 5. BINARY EQUILIBRIUM DIAGRAMS 5.1 5.1 SOLUBILITY 5.1 5.2 ALLOYS 5.3

5.2.1 General Concepts 5.3 5.2.2 Solid Solutions 5.4

5.3 COOLING CURVES 5.6 5.3.1 Pure substance 5.6 5.3.2 Solutions 5.7

5.4 ALLOY TYPES 5.9 5.4.1 General definitions 5.9 5.4.2 Thermal Equilibrium Diagrams (Eutectic Type) 5.9 5.4.3 Thermal Equilibrium Diagram (Solid Solution Type) 5.13 5.4.4 Thermal Equilibrium Diagram (Combination Type) 5.14

APPENDIX 5A BINARY ALLOY TYPES 5.19


III

6. FERROUS ALLOYS 6.1 6.1 DEFFENITION OF FERROUS ALLOYS 6.1 6.2 CARBON STEELS 6.1

6.2.1 General classifications 6.1 6.2.2 Iron-Carbon (Fe3C) Phase Diagram 6.1 6.2.3 The Uses of Plain Carbon Steels 6.6 6.2.4 Classification of carbon steels 6.8

6.2.4.1 Low-Carbon Steels 6.8 6.2.4.2 Medium-Carbon Steels 6.9 6.2.4.3 High-Carbon and tool Steels 6.12

6.3 ALLOY STEELS 6.13 6.3.1 The Need for Alloying 6.13 6.3.2 Classification of alloying elements 6.13 6.3.3 Alloying Elements 6.14 6.3.4 The Classification of alloy Steels 6.16

6.4 STAINLESS STEELS 6.19 6.4.1 General properties and classification 6.19 6.4.2 The Iron-Chromium-Carbon Phase Diagram 6.19 6.4.3 Ferritic Stainless Steels 6.20 6.4.4 Martensitic Stainless Steels 6.21 6.4.5 Austenitic Stainless Steels 6.22 6.4.6 Precipitation Hardening (PH) Stainless Steels 6.24 6.6.7 Duplex Stainless Steel 6.25

6.5 CAST IRONS 6.29 7. HEAT TREATMENT OF PLAIN CARBON STEEL 7.1 7.1 GENERAL HEAT TREATMENT PROCESSES 7.1 7.2 HARDENING OF CARBON STEEL (BY QUENCHING) 7.1

7.2.1 Basic Process 7.1 7.2.2 Critical Cooling Rate and Mass Effect 7.2 7.2.3 Hardenability of steel; The Jominy (End-Quench) test 7.3

7.3 ANNEALING, NORMALISING AND TEMPERING 7.4 7.3.1 General Principals 7.4 7.3.2 Annealing 7.5

7.3.2.1 Basic process 7.5 7.3.2.2 Process Annealing (Stress Relief annealing) 7.7 7.3.2.3 Full Annealing 7.7 7.3.2.4 Spheroidising Annealing 7.8

7.3.3 Normalizing 7.9 7.3.4 Tempering 7.10

7.4 TIME-TEMPERATURE-TRANSFORMATION (TTT) DIAGRAMS 7.11 7.5 SURFACE TREATMENTS 7.13

7.5.1 Introduction 7.13 7.5.2 Selectively Heating the Surface 7.14 7.5.3 Carburising 7.15 7.5.4 Nitriding 7.16

7.6 HEAT AFFECTED ZONE (HAZ) IN WELDING 7.17 7.6.1 Deffinition 7.17 7.6.2 HAZ of Carbon and Alloy Steels 7.18 7.6.3 HAZ of Stainless Steel 7.20


IV

APPENDIX 7A DIFFUSION PROCESS IN METALS 7.22 8. NON-FERROUS ALLOYS 8.1 8.1 NICKEL AND COBALT 8.1

8.1.1 General Characteristics 8.1 8.1.2 Nickel and Monel 8.1 8.1.3 Super alloys 8.2

8.1.3.1 General characteristics 8.2 8.1.3.2 Solid Solution Strengthening 8.3 8.1.3.3 Carbide Dispersion 8.3

8.1.3.4 Precipitation Hardening 8.2 TITANIUM ALLOYS 8.5

8.2.1 Types of titanium alloys 8.5 8.2.2 Processing of Titanium Alloy 8.7 Appendix 8A Selection of Nickel Based Alloys 8.8 9 FABRICATION OF METALS 9.1 9.1 INTRODUCTION 9.1 9.2 FORMING OPERATIONS 9.1

9.2.1 General Classification 9.1 9.2.2 Forging 9.2 9.2.3 Rolling 9.2 9.2.4 Extrusion 9.2 9.2.5 Drawing 9.2 9.2.6 Bending 9.4 9.2.7 Sheet Forming 9.6 9.2.8 Shake - down (Auto frettage) 9.8

9.3 CASTING 9.9 9.3.1 Introduction 9.9 9.3.2 Sand Casting 9.9 9.3.3 Die Casting 9.10

9.3 WELDING 9.11 9.4.1 Basic Techniques 9.11 9.4.2 Gas Welding 9.13 9.4.3 Shielded Metal-Arc Welding 9.13 9.4.4 Gas Tungsten Arc Welding 9.14 9.4.5 Gas Metal-Arc Welding 9.14 9.4.6 Submerged Arc Welding 9.15 9.4.7 Resistance spot welding 9.15


V

10 CORROSION IN METALS 10.1 10.1 INTRODUCTION 10.1 10.2 THE ELECTROCHEMICAL CELL 10.1 10.3 TYPES OF ELECTROCHEMICAL CORROSION 10.3

10.3.1 General Corrosion (Uniform Corrosion) 10.3 10.3.2 Galvanic Corrosion (Composition Cell) 10.4 10.3.3 Crevice Corrosion (Concentration Cell) 10.5 10.3.4 Pitting Corrosion 10.6 10.3.5 Demetallification (Selective Leaching) 10.8 10.3.6 Impingement Attack Corrosion (Erosion Corrosion) 10.8 10.3.7 Cavitation Attack Corrosion 10.9 10.3.8 Stress Corrosion Cracking (SCC) 10.9 10.3.9 Inter-Granular Corrosion. 10.10

10.4 PROTECTION AGAINST ELECTROCHEMICAL CORROSION 10.13 10.4.1 Protection by Design 10.13 10.4.2 Protection Using Inhibitors and Coatings 10.15 10.4.3 Cathodic Protection 10.16

Appendix 10A Remaining life calculations (for corrosion) 10.17 11 NON-DESTRUCTIVE TESTING 11.1 11.1 CHARACTERISATION OF NDT METHODS 11.1 11.2 HISTORY OF NDT 11.1 11.3 DYE PENETRANT 11.2 11.4 MAGNETIC METHODS 11.4

11.4.1 Basic Principles 11.4 11.4.2 Types of particles 11.6 11.4.3 Variations 11.7 11.4.4 Advantages and limitations of magnetic methods 11.8

11.5 X-RAY METHODS 11.9 11.6 - RAY METHODS 11.12 11.7 ULTRA-SONIC METHODS 11.7 11.8 EDDY CURRENT TESTING 11.23 11.9 SUMMARY AND SELECTION OF N. D. T. METHODS 11.26 Appendix 11 A ISO Standards for NDT Inspection 11.31


1.1

1.1 Tensile Testing One of the most common mechanical stress-strain tests is performed in tension. The tension (or tensile) test can be used to ascertain several mechanical properties of materials that are important in design. A specimen is deformed, usually to fracture, with a gradually increasing tensile load that is applied uniaxially along the long axis of a specimen. 1.1.1 The Tensile Test Piece Standard tensile specimens are shown in Figures 1.1 and 1.2. Normally, the cross section is circular, but rectangular specimens are also used. During testing, deformation is confined to the parallel centre region, which has a uniform cross section along its length. In order to eliminate any variations in tensile test data due to differences in the shapes of test pieces, standard shapes are adopted. The following are the dimensions of some standard test pieces:

Thickness to

Gauge Length, Lo (Original Length)

Breadth, bo

Original Cross-Sectional Area, Ao = bo x to

Figure 1.1 Flat Tensile Specimen

Gauge Length, Lo (Original Length)

Diameter, do

Original Cross-Sectional Area, Ao = (do)2 4

Figure 1.2 Round Tensile Specimen

ro

roEnd

End

1 TESTING AND MECHANICAL PROPERTIES OF METALS


1.2

An important feature of the dimensions is the radius given for the shoulders of the test pieces. Variations in the radii can affect markedly the tensile test data. Very small radii can cause localised stress concentrations, which may result in the test piece failing prematurely. The surface finish of the test piece is also important for the same reason. The reason for the specification of a relationship between the gauge length and the cross-sectional area of the test piece is in order to give reproducible test results for the same test material when different size test specimens are used. 1.1.2 The Tensile Test The tensile specimen is mounted by its ends into the holding grips of the tensile testing machine (Figure 1.3). The tensile testing machine is designed to elongate the specimen at a constant rate, and to continuously and simultaneously measure the instantaneous applied load (with a load cell) and the resulting elongations (using an extensometer). A load (force) elongation (extension) test typically takes several minutes to perform and is destructive; that is, the test specimen is permanently deformed and usually fractured. Figure 1.3 Schematic representation of the apparatus used to conduct tensile stress-strain

tests. (The specimen is elongated by the moving crosshead; load cell and extensometer measure, respectively, the magnitude of the applied load and the elongation) 1.1.3 Force Extension Curve The output of such a tensile test is recorded on a strip chart (or by a computer) as a load (or force) versus elongation as shown in Figure 1.4 for a ductile metal specimen (e.g. low carbon steel). These load-deformation characteristics are dependent on the specimen size. For example, it will require twice the load to produce the same elongation if the cross-sectional area of the specimen is doubled. To minimise these geometrical factors, load and elongation are normalised to the respective parameters of engineering stress and engineering strain.

Jaws

Load Cell Extensometer Specimen Moving Cross-Head


1.3

0 Figure 1.4 Typical load-extension curve for a ductile metal (e.g. low carbon steel) 1.1.4 Engineering Stress & Strain Curve The results of a single tensile test may apply to all sizes and shapes of specimens for a given material if we convert the force to stress and the extension to strain. Engineering Stress and Engineering Strain are defined by the following equations: Engineering Stress = Load = F

Original cross-sectional area Ao Engineering Strain = Extension = e = L - Lo

Original length (gouge length) Lo Lo Where A0 is the original cross-section area of the specimen before the test begins, L0 is the original distance between the gauge marks, and L is the distance between the gauge marks after force F is applied.

Extension (e)

Load ( F )

Elastic

Maximum Load

Yield Load

Plastic

Elastic Extension

Plastic extension

Neck forming

Total Extension (Extension at Fracture)

Area at Fracture (Af)

Original (Gouge) Length lo Original Area (Ao)

Fracture

Fully Plastic


1.4

The conversions from load-gauge length to stress-strain are also included in Table 1.1. The stress-strain curve (Figure 1.5) is usually used to record the results of a tensile test. Table 1.1 Results of tensile test of a 12.5 mm diameter aluminium alloy, with a gouge length of 50 mm

Force, F (KN)

Stress, , (MN/m2)

Extension, e (mm)

Strain

0 0 0.00 0.0000 5 40.7 0.03 0.0006 10 81.5 0.06 0.0012 15 122.1 0.09 0.0018 20 162.8 0.12 0.0024 25 203.5 0.15 0.0030 30 244.2 0.18 0.0036

33.8 275 2.00 0.0400

34.4 (max)

280

3.00

0.0600 28.9 (fracture) 235 5.20 0.1400

Figure 1.5 The stress strain curve for an aluminium alloy from Table 1

0

50100

150200

250300

350

0 0.05 0.1 0.15

Strain

Stre

ss (M

Pa)

Yield Tensile Strength Strength Fracture

Strength


1.5

1.1.5 Properties Obtained from the Tensile Test Information concerning the strength, stiffness, and ductility of a material can be obtained from the tensile test. Elastic Versus Plastic Deformation In the initial part of the stress strain curve (Figure 1.5), the stretching of the specimen is said to be elastic. When a force is first applied to the specimen, the bonds between the atoms are stretched and the specimen elongates. When we remove the force, the bonds return to their original length and the specimen returns to its initial size. The stress is proportional to the strain (i.e. straight-line curve). Eventually, the material yields (with the increase in stress), after which the specimen will be permanently deformed (or set) upon the release of load. Hence, upon the onset of yield and thereafter, the deformation of the specimen is said to be plastic. Yield Strength The yield strength is the stress at which the metal becomes permanently deformed, or set, (Figure 1.5). It therefore is the stress that divides the elastic and plastic behaviour of the material. Practical applications of the Yield-Stress If we are designing a component that must support a force during use, we must be sure that the component does not plastically deform. We must therefore select a metal that has high yield strength, or we must make the component large so that the applied force produces a stress that is below the yield strength. On the other hand, if we are manufacturing shapes or components by some deformation process (such as bending or drawing etc.), the applied stress must exceed the yield strength to produce a permanent change in the shape of the metal.


1.6

Proof Stress In some materials, the stress at which the material changes from elastic to plastic behaviour is not easily detected. In this case, we may determine an offset yield strength or proof stress (Figure 1.6). We can decide that a small amount of permanent deformation, such as 0.2% or 0.002 strain might be allowable without damaging the performance of our component. We can construct a line parallel to the initial portion of the stress-strain curve but offset by 0.002 strain from the origin. The 0.2% proof strength is the stress at which our constructed line intersects the stress-strain curve. EXAMPLE 1.1 Determine the 0.2% proof strength for grey cast iron (Figure 6) Answer: By constructing a line starting at 0.002 strain (0.2/100), which is parallel to the elastic portion of the stress-strain curve, we find that the 0.2% proof strength is 275MPa. Stress (MPa) 0 Figure 1.6 Part of the stress-strain curve for grey cast iron

0.002 0.004 0.006 0.008 Strain

100

200

300 275

0.2 % Proof Stress


1.7

Tensile Strength The tensile strength is the stress obtained at the highest applied force and thus is the maximum stress on the engineering stress-strain curve (Figure 1.5). In many ductile metals, deformation does not remain uniform. At some point, one region deforms more than other areas and a large decrease in the cross-sectional area occurs (Figure 1.7). This locally deformed region is called a neck. Because the cross-sectional area becomes smaller at this point, a lower force is required to continue its deformation, and the engineering stress, calculated from the original area A0, will decrease. The tensile strength is the stress at which necking begins in ductile materials. Figure 1.7 Localised deformation (necking) of a ductile tensile specimen that been has deformed beyond the Tensile stress. Tensile strengths are often reported in handbooks because they are easy to measure; they are useful in comparing the behaviours of materials, and they permit us to estimate other properties, which are more difficult to measure. However, the tensile strength is relatively unimportant for materials selections or materials fabrication the yield strength determines whether the material will or will not deform. Modulus of Elasticity The modulus of elasticity, or Youngs modulus, E, is the slope of the stress-strain curve in the elastic region (Figure 5). This relationship obeys Hookes Law. Modulus of Elasticity E = Stress =

Strain

Neck


1.8

The modulus is a measure of the stiffness of the material. A stiff material, with a high modulus of elasticity, maintains its size and shape even under an elastic load. If we are designing a shaft and bearing, we may need very close tolerances. Figure 1.8 shows the elastic behaviour of steel and aluminium. If a stress of 210 MPa is applied to the shaft, steel deforms elastically 0.001mm/mm while, at the same stress, aluminium deforms 0.003mm/mm. Steel has a modulus of elasticity (around 210 GPa) three times greater than that of aluminium (around 70 GPa). Stress (MPa) 210 200 0 Figure 1.8 Comparison of the elastic behaviour of steel and aluminium

0.001 0.002 0.003 0.004 0.005 Strain

100

Steel Aluminium


1.9

1.1.6 Ductility in metals Ductility is a measure of the amount of deformation that a material can undergo without breaking. There are two ways of measuring the ductility, namely, Percentage Elongation and Percentage Reduction in Area. Percentage Elongation This describes the amount of stretching of the specimen at the point of fracture (Figure 1.9), using the equation; %Elongation = (Lf L0)100

Lo Where L0 = Gauge length Lf = Fracture length Load

Extension Lf

Percentage Reduction in Area This describes the amount of thinning a specimen undergoes during the test (as shown in Figure 1.9) and is represented by the equation; %Reduction in area = (A0 Af )100

Ao Where A0 = Original cross-sectional area Af = Area at fracture Ductility is important to both designers and manufactures. The designer of a component would require some ductility so that if the stress is increased and becomes too high, the component will deform plastically before it breaks. Fabricators will usually require ductile material so that they can perform complicated shapes without breaking the material.

A0

Lo

Lf

Af

Figure 1.9- Schematic representation of a ductile tensile specimen before and after fracture.

Construction line parallel to original elastic line


1.10

1.1.7 Fracture of Metals If a metal deforms for an appreciable amount without breaking (i.e. high %Elongation and high %Reduction in Area), it is said to be ductile. On the other hand, metals that break without, or with A small amount of plastic deformation breaking (i.e. low %Elongation and low %Reduction in Area) then it is said to be brittle. Ductile fracture Ductile fracture surfaces will have their own distinctive features on both macroscopic and microscopic levels. Figure 1.10 shows schematic representations for two characteristic macroscopic fracture profiles and their respective stress-strain curves. The configuration shown in Fig. 10 a) is found for soft metals. These, ductile, materials neck down to a point fracture, showing a certain amount of percentage area reduction (sometimes called, cup-and-cone fracture).

Stress Stress Strain Strain a) b) Figure 1.10 (a) Ductile fracture after some necking


1.11

(b) Brittle fracture without any plastic deformation brittle fracture Brittle fracture takes place without any appreciable deformation, and by rapid crack propagation. The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yields a relatively flat fracture surface, as indicated in Figure 1.10 b). Fracture surfaces of materials that failed in a brittle manner will have their own distinctive patterns, any signs of gross plastic deformation will be absent. For most brittle crystalline materials, crack propagation corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes; such a process is sometimes termed cleavage. 1.2 Impact Testing 1.2.1 Standard Impact Tests, Izod and Charpy Impact tests indicate the behaviour of a material under conditions of mechanical shock and to some extent measure its toughness. Brittleness and consequent lack of reliability resulting from incorrect heat-treatment or other caused may not be revealed during a tensile test but will usually be evident in an impact test. Two standard impact tests are usually utilised; the Izod and the Charpy tests. 1.2.2 Impact specimen Figure 1.11 a) shows standard notched test pieces for both the Izod and Charpy impact tests. To set up stress concentrations, which ensure that fracture does occur, test pieces are notched. It is essential that notches always be standard, for which reason a standard gauge is used to test the dimensional accuracy of the notch 1.2.3 The Izod Impact Test In this test, a standard notched specimen (Figure 1.11 a)) is held in a vice and a heavy pendulum striker (or hammer), mounted on ball bearings, is allowed to strike the specimen after swinging from a fixed height (Figure 1.11b). The striking energy, typically of 167J, is partially absorbed in breaking the specimen. Hence, as the pendulum swings past the test specimen, it carries a pointer to its highest point of swing, thus indicating the amount of energy (related to the difference in the height of the striker before (ho) and after (hf) impact) impact used in fracturing the test piece. 1.2.4 The Charpy Test The Charpy test procedure (Figure 1.11 c)) employs a test piece (mounted as a simply-supported beam instead of in the cantilever form used in the Izod Test. The striking energy is typically 300J.


1.12

10mm 10mm a) Notch (450) 8mm Centre of gravity of striker (hammer) Pointer Scale

ho (Initial height) (Final height) hf Point of impact Clamp b) Supports

Striker (Top View) c) Figure 1.11


1.13

a) Dimensions of standard test pieces for both Izod and Charpy Tests. b) Standard Izod and c) Charpy test set-ups. 1.2.5 Transition Temperature The results of a series of impact tests performed at various temperatures, for a particular metal, are shown in Figure 1.12. At high temperatures, a large absorbed energy is required to cause the specimen to fail, whereas at low temperatures even a relatively ductile material may fail with little absorbed energy. Absorbed Energy

Brittle Ductile

Transition Temperature

Test Temperature Figure 1.12 - Typical results from a series of impact tests on a particular metal At high temperatures, the material behaves in a ductile manner, with extensive deformation and stretching of the specimen prior to failure. At low temperatures, the material is brittle, and little deformation at the point of fracture is observed. The transition temperature is the temperature at which the material changes from ductile to brittle failure. A material that may be subjected to an impact blow during service must have a transition temperature below the temperature of the materials surroundings.


1.14

1.3 Hardness Tests 1.3.1 Introduction Classically, hardness could be defined as the resistance of a surface to abrasion and early attempts to measure surface hardness were based on this concept. Hardness may be measured by observing the resistance of the surface layers to plastic deformation under static pressure is measured rather than true hardness. In most of these methods, the static force used is divided by the numerical value of the surface area of the resulting impression to give the hardness index. A typical hardness test set up is illustrated in Figure 1.13, in which an indenter (e.g. diamond tip pyramid, cone or ball) is allowed, for a short pre-determined time to, locally and plastically, deform the surface of a specimen by a very small, but representative amount. Figure 1.13 Basic Principles of the force application system in the Hardness Testing

Machine Several standard hardness tests ( e.g. Vickers, and Rockwell) will be described. The test set-ups are essentially similar except for the type of indenters used and, to some extent, the method of measuring the indentations.

Automatic timing mechanism

Load, F (Force)

Fulcrum Rigid Beam

Indenter (e.g. Diamond, pyramid, cone or Ball)

Specimen


1.15

1.3.2 The Vickers Hardness Test or Diamond Pyramid Hardness Test This standard hardness test uses, as its indenter, a diamond square-based pyramid, with an included angle of 1360, (Figure 1.14), which will give geometrically similar impression (square) under different applied forces. Force, F

Specimen

a) b) Figure 1.14- Standard Vickers diamond pyramid a) Schematic of the diamond pyramid. b) The 136 diamond pyramid is pushed with constant force, F. into the surface of the specimen for a specified time. c) Top view of the resulting impression, square, on the surface of the specimen as seen

under the microscope. In this test, the diagonal length, d, of the square impression is measured by means of a microscope, which has a variable slit built into the eyepiece, (Figure 1.14c). The width of the slit is adjusted so that its edges coincide with the corners of the impression and the relative diagonal length of the impression then obtained from a small instrument attached to the slit, which works on the principle of a revolution counter. The ocular reading ( of the dimension, d) thus obtained is converted to Vickers Pyramid Hardness Number by reference to tables. The Vickers hardness, Hv, may also be calculated using the following relationship; Vickers hardness = Force = 2Fsin(136/2)

Surface Area of impression d2 Vickers harnesses for a selection of metals (and some ceramics and polymers, for comparison) are included in Table 1.2.

1360

d

Adjustable slits

c)

Square impression on surface of specimen


1.16

Material Hv (kgf /mm) Tin 5Aluminium 25Gold 35Copper 40Iron 80Mild steel 140Fully hardened steel 900Limestone 250MgO 500Window glass 550Fused silica 720Granite 850Quartz 1200Tungsten carbide 2500Polypropylene 7Polycarbonate 14PVC 16Polyacetal 18PMMA 20Polystyrene 21Epoxy 45 1.3.3 The Brinell Test Probably the best known of the hardness tests, was devised by a Swede, Dr Johan August Brinell in 1900. In this test, a hardened steel ball is pressed into the surface of the test piece using the appropriate specified force as shown in Figure 1.15. D d Force, F

d Specimen Top view of circular impression Figure 1.15 A typical Brinell-type Test set-up, using a ball indenter with diameter, D, and

leaving a circular impression with diameter, d, at the surface of the specimen.

The diameter of the impression, d, so produced is then measured and the Brinell Hardness Number, HB, derived from:

Table 1.2 Typical Vickers, Hv, values of a selection of materials


1.17

Brinell hardness = Force = 2F Surface Area of impression D(D (D2 d2)1/2)

And the units will be kgf/mm. To obviate tedious calculations, HB, is found by reference to the appropriate set of tables. 1.3.4 The Rockwell Test The Rockwell hardness test was devised in the USA and is particularly suitable for rapid routine testing of finished material since it indicates the final result direct on a dial which is calibrated with a series of scales. A number of different combinations of indenter and indenting force can be used in conjunction with the appropriate scale as shown in figure 1.16. SCALE INDENTER TOTAL FORCE (kgf)

A Diamond Cone 60 B 1/16" Steel Ball 100 C Diamond Cone 150 D Diamond Cone 100 E 1/8" Steel Ball 100 F 1/16" Steel Ball 60 G 1/16" Steel Ball 150 H 1/8" Steel Ball 60 K 1/8" Steel Ball 150

Of these, Scale C is probably the most popular for use with steels. Figure 1.16 The Rockwell table of scales and Diamond Cone Indenter

1.3.5 The Shore Scleroscope hardness test (Greek: skleros hard) In this hardness test, the instrument embodies a small diamond-tipped miniature tup which is allowed to fall from a standard height, inside a metal tube with a graduated glass window, and strike the surface of the specimen as shown in Figure 1.17. The height of rebound is taken as the hardness index. Since the shore Scleroscope is a small, portable instrument, it is very useful for the determination of hardness of large rolls, castings and gears, and other large components, which could not easily be placed on the testing tables of any of the more orthodox testing machines.

1200 0.2mm tip radius

Metal tube Miniature Tup (drop mass) Graduated glass window


1.18

Specimen

Figure 1.17 The Shore Scleroscope hardness instrument and set-up 1.4 Comparison of Mechanical Properties of Metals Based on the data obtained from tensile, impact and hardness tests, it should now be possible to compare various metals according to their mechanical properties. Table 1.3 include the mechanical properties of several metals and alloys. Hence, consider , for example the mechanical properties of mild steel and a Ni/Cr/Mo steel alloy. It can be clearly seen that the steel alloy has a superior strength when compared with mild steel (in terms of the proof stress, Tensile strength and hardness). However, mild steel is much tougher (i.e. higher impact value) and more ductile (%Elongation) that the steel alloy. As a general rule, the strength of a metal may be increased, but at the expense of ductility and toughness. Other factors such as prise must be considered. For example, the steel alloy will be much more expensive than mild steel. This problem may be overcome by specifying thicker sections of mild steel rather than thinner ones from the steel alloy. However, this means that the component made from mild steel will be much heavier than an equivalent component made from the steel alloy, hence, a weight penalty. The fabrication costs must also be considered. For example, the steel alloy will be more expensive to fabricate (e.g. bending, cutting machining etc) than mild steel. It is therefore important to specify the requirement of a particular design and then chose a material accordingly. Hence, the use of more expensive material must always be justified.


1.19

Table 1.3 Typical Mechanical Properties of Some Metals and Alloys

Metal or Alloy Condition

0.1% Proof Strength (N/mm)

Tensile Strength (N/mm)

Youngs Modulus (kN/mm)

Elongation (%)

Hardness (Brinell*)

Impact Value

(Izod) (J) Lead Soft Sheet --- 18 16 65 4 ---

Aluminium Wrought & Annealed 25 60 70 60 15 27

Duralum Extruded & fully heat treated

275 430 71 15 115 22

Magnesium 6A1/1Zn Extruded Bar 170 300 48 10 60 8

Copper Wrought & Annealed 46 216 130 60 42 59

Annealed 85 320 68 62 90 70/30 Brass

Deep drawn 370 465 100

19 132 --- Rolled & Annealed 120 340 66 72 ---

Phosphor Bronze (5% tin) Hard rolled 650 710

101 5 188 61

Mild Steel Hot Rolled Sheet 270 400 210 28 100 75

Normalised 420 665 27 152 44

0.45% Carbon Steel

Water quenched & tempered at 600C

540 780 200

25 200 65

4Ni/Cr/Mo Steel

Air hardened & tempered at 300C

1200 1550 225 12 444 22

18/8 Stainless Steel Softened 185 525 220 30 170 68

Grey Cast Iron As cast --- 300 150 0 250 1

Titanium - commercially pure

Annealed Sheet 370 450 120 30 --- 61

Titanium Alloy (4Sn/4A1/ 4Mo 0.5Si)

Precipitation Hardened 1200 1390 150 16 --- ---


1.20

1.5 Fatigue failure 1.5.1 Nature of Fatigue Failure In many applications, a component or structure (e.g. from a bridge, aircraft or machine) is subjected to the repeated (cyclic) application of a stress below the yield strength of the material. This repeated stress might occur as a result of tension and compression, rotation, bending, or even vibration as shown in Figure 1.18. a) Repeated b) Repeated bending c) Repeated torsion tension /compression Figure 1.18 Types of fatigue loading Although the stress may be below the yield strength, the material may fail after a large number of applications of stress. This mode of failure is known as fatigue. Final Fatigue failure is catastrophic, occurring very suddenly and without warning. It is brittle in nature, even in normally ductile materials. There is little, if any, gross plastic deformation. The fatigue process occurs by initiation, propagation and final fracture.


1.21

1.5.2 The Mechanism of Fatigue Failure Fatigue failure begins (nucleates) quite early in the service life of the member by the formation of a small crack, generally at some point on the external surface, as shown in Figure 1.19. A fatigue crack 'front' advances a very small amount during each stress cycle This crack develops slowly into the material in a direction roughly perpendicular to the main tensile axis. Ultimately the cross-sectional area of the member will have been so reduced that it can no longer withstand the applied load and ordinary tensile fracture will result. Initial crack

Figure 1.19 The progress of fatigue failure A fatigue fracture thus develops in three stages - nucleation, crack growth and final catastrophic failure. Since the crack propagates slowly from the source, the fractured surfaces rub together due to the pulsating nature of the stress and so the surfaces becomes burnished whilst still exhibiting the conchoidal marking (beach marks) representing the large ripples. Final fracture, when the residual cross section of the member is no longer able to carry the load, is typically crystalline in appearance. Fatigue failures in metals are therefore generally very easy to identify.

Nucleation Crack growth Fracture

Final fractured surface


1.22

1.5.3 Fatigue Testing A common method to measure the resistance to fatigue is the rotating cantilever beam test (Figure 1.20). Tension/compression

Load Figure 1.20 - The rotating cantilever beam fatigue tester One end of a machined, cylindrical specimen is mounted in a motor-driven chuck. A weight is suspended from the other end. The specimen initially has tensile force acting on the top surface, while the bottom surface is compressed. After the specimen turns 90, the locations that were originally in tension and compression have no stress acting on them. After a half revolution of 180, the material that was originally in tension is now in compression. Thus, the stress at any one point goes through a complete cycle from zero stress to maximum tensile stress to zero stress to maximum compressive stress. After a sufficient of cycles, the specimen may fail. Generally, a series of specimens are tested at different applied stresses and the stress (S) is plotted versus the number of cycles to failure (N), as shown in Figure 1.21.

105 Number of cycles to failure (N) Figure 1.21 - The stress-number of cycles to failure curve for an Tool steel and an

aluminium alloy. (Not to scale).

Motor Chuck Specimen Counter

Tool Steel Endurance Limit

Aluminum alloy

Stress, (or S) (MPa)

600 400


1.23

The fatigue test can tell us how long a part may survive or the maximum allowable loads that can be applied to prevent failure. Fatigue Life The fatigue life indicates how long a component survives when a stress, ,

is repeatedly applied to the material. For example, If we are designing a tool steel part that must undergo 100,000 cycles during its lifetime, the part must be designed so that the applied stress is lower than 600Mpa (Figure 1.21).

Fatigue Limit The fatigue limit, which is the stress below which failure by fatigue never

occurs, is our preferred design criterion. At the fatigue limit the S-N curve become horizontal. To prevent a tool steel part from failing, we must be sure that the applied stress is below 400 MPa (Figure 1.21).

Fatigue Strength Some materials, including many aluminium alloys, have no true fatigue

limit. For these materials, we may specify a minimum fatigue life; then the fatigue strength is the stress below which fatigue does not occur within this time period. In many aluminium alloys, the fatigue strength is based on 500 million cycles.

1.5.4 Improving Fatigue resistance Methods of improving the fatigue resistance of engineering components are discussed as follows. Surface Finish Fatigue cracks initiate at the surface of a stressed material, where the

stresses are at a maximum. Any design or manufacturing defect at the surface concentrates stresses and encourages the formation of a fatigue crack. This susceptibility may be measured using a notched fatigue specimen (Figure 1.22). Sometimes highly polished surfaces are prepared in order to minimise the likelihood of a fatigue failure.

Stress

Number of cycles Figure 1.22 - The effect of a notch on the fatigue properties of a metal.

Un-Notched specimen Notched specimen


1.24

Care should also be taken during the welding of engineering components. Bad welding (e.g. undercut and lack of penetration as shown in Figure 1.23)) could lead to the inclusion of stress raisers , which me, ultimately lead to catastrophic fatigue failure.

Lack of penetration

Bad Good Bad Good Figure 1.23 Examples of weld defects Material Strength The fatigue resistance is related to the strength of the material at the

surface. In many ferrous, or iron-base, alloys, the fatigue limit is approximately one-half the tensile strength of the material. This ratio of fatigue limit to tensile strength is the fatigue ratio.

Fatigue ratio = Fatigue limit 0.5

Tensile strength

If the tensile strength at the surface of the material increases, the resistance to fatigue also increases.

Environmental Temperature influences the fatigue resistance. As the temperature of the Effects material increases, the strength decreases and consequently both fatigue

life and fatigue limit decrease.

Corrosion may also cause a component to loose its surface smoothness and eventually lead to fatigue failure, in service.

Undercut


1.25

Design effects Proper design of engineering components can dramatically improve their

fatigue life. On of the most important design techniques is to reduce the effects of sharp corners, hence reduce the effects of so called stress raisers, as shown in Figure 1.24.

Figure 1.24 Typical design considerations for the improvement of the fatigue resistance

of engineering components

Improved by a Fillet Radius


1.26

1.6 Creep Failure 1.6.1 Nature of Creep If we apply a stress to a metal at a high temperature, the strength of the metal may drop as shown in Figure 1.25. The metal may also stretch and eventually fail, even though the applied stress is less than the yield strength at that temperature.

Figure 1.25

1.6.2 Creep Test To determine the creep characteristics of a material, a constant stress is applied to a cylindrical specimen placed in a furnace (Figure 1.26 a).

Constant Stress Furnace (constant Temperature)

Creep Specimen

b) Plot of modulus of elasticity versus temperature for Tungsten, Steel, and Aluminium

a) Engineering stress strain behaviour for iron at three temperatures.


1.27

Figure 1.26 a) A specimen is placed in a furnace at an elevated temperature under a

constant applied stress in the creep test. During the creep test, the strain or elongation is measured as a function of time and plotted to give the creep curve, as shown in Figure 1.26 b). Strain Figure 1.26 b) A typical creep curve showing the strain produced as a function of time for

a constant stress and temperature It can bee seen from figure 1.26 b), that creep will take place in three stages First stage As soon as stress is applied, the specimen stretches elastically a small amount 0 (Figure 1.26 b), lecture notes), depending on the applied stress and the modulus of elasticity of the material at the high temperature Second Stage During second-stage, or steady-state creep, the metal stretches (slips) at an almost steady rate, as long as the temperature and stress (loading conditions) are kept constant. The slope of the steady-state portion of the creep curve is the creep rate.

0 First Second Stage Third

Stage (Steady state) Stage Time

Creep rate = t

t

Constant Stress Fracture Constant Temperature


1.28

Third Stage Eventually, during third-stage creep, necking begins, the stress increases and the specimen deforms at an accelerated rate until failure occurs. This is partly due to the eventual sliding of parts of the metal at grain boundaries as shown in Figures 1.27a) and b). Figure 1.27 a) - Effect of third stage creep at grain boundary

Grain Grain Grain

Eventual opening and rupture at grain boundaries


1.29

Figure 1.27 b) Cracking between 3 grains of Ni (16%)-Cr (9%)-Fe alloy after 35%

elongation at 3600C and initial strain rate of 3x10-7sec-1.


2.1

2. THE CRYSTALLINE STRUCTURE OF METALS 2.1 The States of Mater All chemical elements can exist in either the solid, liquid or gaseous state depending on the prevailing conditions such as temperature and pressure. The atoms (or ions or molecules) in mater will also posses a certain amount of energy (potential energy) by virtue of their state as shown in Figure 2.1. Potential Energy

Inter-atomic distance Figure 1 Relative potential energy and atomic arrangements in the three states of matter. (In the gaseous and liquid states, these arrangements are disorderly, but in the solid state, the ions conform to some geometrical pattern.) In the gaseous state, the molecules will have a relatively large amount of potential energy (Figure 2.1) and move about in a disorderly fashion with relatively large intermolecular distances. On condensation to a liquid, the atoms come into contact with each other to form bonds (Figure 2.1), but there is still no orderly arrangement of the atoms, though a large amount of potential energy is given up in the form of latent heat. When solidification takes place (Figure 2.1), there is a further discharge of latent heat, and the potential energy falls even lower as the atoms take up orderly positions in some geometrical pattern, which constitutes a crystal structure. The rigidity and cohesion of the structure is then due to the operation of the metallic bond (see Appendix 2A, at the end of this chapter). Substances can be classified as either amorphous or crystalline.

GAS Condensation LIQUID Crystallisation SOLID


2.2

Crystalline structure The crystalline structure consists of atoms, or, more properly, ions, arranged according to some regular geometrical pattern as shown in Figure 2.2. This pattern varies, as we shall see, from one substance to another. All metals are crystalline in nature. If a metal, or other crystalline solid, is stressed below its elastic limit, any distortion produced is temporary and when the stress is removed, the solid will return to its original shape. Thus, removal of stress leads to removal of strain and we say that the substance is elastic. Amorphous state In the amorphous state, the elementary particles are mixed together in a disorderly manner, their positions bearing no fixed relationship to those of their neighbours. The amorphous structure is typical of all liquids in that the atoms or molecules of which they are composed can be moved easily with respect to each other, since they do not conform to any fixed pattern. In the case of liquids of simple chemical formulae in which the molecules are small, the forces of attraction between these molecules are not sufficient to prevent the liquid from flowing under its own weight; that is, it possesses high mobility. Many substances, generally regarded as being solids, are amorphous in nature and rely on the existence of long-chain molecules such as many polymers, as shown in Figure 2.3.

H H H H H

C-C-C-C-C------

H H H H H Basic Polyethylene chain (Carbon, C, and Hydrogen, H) Figure 2.3- Representation of polyethylene (in the amorphous condition)

Figure 2.2 Idealised space lattice, of a metal showing a basic unit (crystal) and repeat units. Hard-sphere unit cell representation of the simple cubic crystal structure.

Basic unit (crystal) Atom (or ion) Repeat units

Bond

Representation of a polyethylene sample

Chains


2.3

2.2 SOLIDIFICATION AND STRUCTURES OF METALS 2.2.1 Solidification of pure metals When a pure liquid solidifies into a crystalline solid, it does so at a fixed temperature called the freezing point. During the crystallisation process the atoms assume positions according to some geometrical pattern (Fig. 1), and whilst this is taking place, heat (the latent head of solidification) is given out in accordance with the laws of thermodynamics, without any fall in temperature taking place. A typical cooling curve for a pure metal is shown is Figure 2.4. Temperature Freezing begins Freezing ends Freezing point

Time Figure 2.4 Typical cooling curve of a pure metal 2.2.2 Basic Metallic structures There are several types of pattern or space lattice in which metallic atoms can bind together and arrange them selves on solidification (see Appendix 2.B at the end of this chapter). But the three most common structures, namely, body-centred cubic, face-centred cubic and hexagonal close-packed are shown in Figure 2.5. The hexagonal close-packed represents the closest packing, which is possible with atoms. It is the sort of arrangement obtained when one set of snooker balls is allowed to fall in position on top of a set already packed in the triangle. The face-centred cubic arrangement is also close packing of the atoms, but the body-centred cubic is relatively open. Typical metallic structures are included in Table 2.1.


2.4

Hard Sphere Model Simple Model Aggregate of many atoms

a)

Hard Sphere Model Simple Model Aggregate of many atoms

b)

Simple Model Aggregate of many atoms

c) Figure 2.5 The three principal types of structure in which metallic elements crystallise.

a) Body-centered cubic crystal, b) Face-centered cubic crystal and c) Hexagonal -close-packed crystal.


2.5

Table 2.1- Typical metallic crystalline structures Body-centered cubic

(BCC) Face-centered cubic

(FCC) Hexagonal-close-packed

(HCP) Vanadium Copper Beryllium

Molybdenum Silver Magnesium Tungsten Cobalt () Cobalt () Iron (, ) Iron () Cadmium

Chromium () Chromium () Zinc Gold Aluminium Lead Nickel Platinum

2.2.3 Polymorphic transformation of metals Some metals (e.g. Iron, Cobalt and Chromium, see table 2.1) may changes their crystalline form as the temperature is raised or lowered. This is also accompanied by a noticeable change in volume or the body of metal. An element, which can exist in more than one crystalline form in this way is said to be Polymorphic. Thus pure iron can exist in three separate crystalline forms, which are designated by letters of the Greek alphabet: alpha (), gamma () and delta (). -iron, which is body-centred cubic and exists at normal temperatures, changes to -iron, which is face-centred cubic, when heated to 910C. At 1400C the face-centred cubic structure reverts to body centred cubic -iron. (The essential difference between -iron and -iron, is only in the temperature range over which each exists.) These Polymorphic changes are accompanied by changes in volume-contraction and expansion respectively as shown in Figure 2.6. Volume 910 1400 Temperature 0C Figure 2.6 The effect of Polymorphic transformations on the expansion of pure iron. (The close packing of the phase causes a sudden decrease in volume of the unit cell at 910C and a corresponding increase at 1400C when the structure changes to )


2.6

2.3 Crystal growth an overall bulk solidification of metals When a pure metal solidifies, each crystal begins to form independently from a nucleus or centre of crystallisation. The nucleus will be a simple unit of the appropriate crystal lattice, and from this the crystal will grow. The crystal develops by the addition of atoms according to the lattice pattern it will follow, and rapidly begins to assume visible proportions in what is called a dendrite. This is a sort of crystal skeleton, rather like a backbone from which the arms begin to grow in other directions, depending upon the lattice pattern. From these secondary arms, tertiary arms begin to sprout, somewhat similar to the branches and twigs of a fir-tree. In the metallic dendrite, however, these branches and twigs conform to a rigid geometrical pattern. A metallic crystal grows in this way because heat is dissipated more quickly from a point, so that it will be there that the temperature falls most quickly leading to the information of a rather elongated skeleton (Figure 2.7).

The dendrite arms continue to grow and thicken at the same time, until ultimately the space between them will become filled with solid. Meanwhile the outer arms begin to make contact with those of neighbouring dendrites, which have been developing quite independently at the same time. All these neighbouring crystals will be orientated differently due to their independent formation; that is, their lattices will meet at odd angles. When contact has taken place between the outer arms of neighbouring crystals further growth outwards is impossible, and solidification will be complete when the remaining liquid is used up in thickening the existing dendrite arms. Hence, the independent formation of each crystal leads to the irregular overall shape of crystals. The dendritic growth of crystals during the bulk solidification of a metal is illustrated in Figure 2.8. In these diagrams, however, the major axes of the crystals are all shown in the same horizontal plane, i.e. the plane of the paper (for the sake of clarity), whereas in practice, grains will grow in a three-dimensional manner. Figure 2.8 The dendritic growth of metallic crystals (grains) from the liquid state.

Independent Grain (dendritic) Contact between Final structure nucleation growth dendrites of dendrites Grain

Grain boundary

Figure 2.7 The early stages in the growth of a metallic dendrite


2.7

Dendritic growth may be observed as shown in Figure 2.9, where an iron dendrite was allowed to form within a copper matrix.

Figure 2.9 Dendritic Growth This iron dendrite grew from a nucleus at n in a molten mixture of iron and copper. After all the available iron has been used up, the dendrite ceased to grown and the molten copper solidified as the matrix in which the iron dendrite remains embedded. (In fact, the iron dendrite will contain a little dissolved copper in solid solution whilst the copper matrix will contain a very small amount of the dissolved iron.) x 300 magnification. 2.4 Defects in Metal During Solidification Porosity in metals If the metal we have been considering is pure, we shall see no evidence whatever of dendritic growth once solidification is complete, since all atoms are identical. Dissolved impurities, however, will often tend to remain in the molten portion of the metal as long as possible, so that they are present in that part of the metal which ultimately solidifies in the spaces between the dendrite arms. Since their presence will often cause a slight alteration in the colour of the parent metal, the dendritic structure will be revealed on microscopic examination. The areas containing impurity will appear as patches between the dendrite arms. Inter-dendritic porosity may also reveal the original pattern of the dendrites to some extent. If the metal is cooled too rapidly during solidification, molten metal is often unable to feed effectively into the spaces, which form between the dendrites, due to the shrinkage (Figure 2.11 a)). which accompanies freezing. These spaces then remain as cavities following the outline of the solid dendrite. Such shrinkage cavities can usually be distinguished from blowholes formed by dissolved gas (gas porosity). The former are of distinctive shape and occur at the crystal

n


2.8

boundaries, whilst the latter are quite often irregular in form and occur at any point in the crystal structure (Figure 2.11 b)). a) b) Figure 2.10 Porosity in Cast Metals Shrinkage cavities (a) tend to follow the shape of the dendrite arms and occur at the crystal boundaries, whilst gas porosity (b) is usually of irregular shape and occurs at almost any point in the structure. Defects in metal castings The rate at which a molten metal is cooling when it reaches its freezing point affects the size of the crystals, which form. A slow fall in temperature, which leads to a small degree of undercooking at the onset of solidification, promotes the formation of relatively few nuclei, so that the resultant crystals will be large (they are easily seen without the aid of a microscope). Rapid cooling, on the other hand, leads to a high degree of undercooking being attained, and the onset of crystallisation results in the formation of a large shower of nuclei. This can only mean that the final crystals, being large in number, are small in size. In the language of the foundry, chilling causes fine-grain casting. (Throughout this study, the term grain and crystal are used synonymously). Thus the crystal size of a pressure die-casting will be very small compared with that of a sand-casting. Whilst the latter cools relatively slowly, due to the insulating properties of the sand mould, the former solidifies very quickly, due to the contact of the molten metal with the metal mould. Similarly, thin sections, whether in sand or die-casting, will lead to a relatively quicker rate of cooling, and consequently smaller crystals.

Shrinkage Gas porosity (Blow holes)


2.9

In a large ingot the crystal size may vary considerably from the outside surface to the centre (Figure 2.11). This is due to the variation, which exists in the temperature gradient as the ingot solidifies and heat is transferred from the metal to the mould. When metal first makes contact with the mould the latter is cold, and this has a chilling effect, which results in the formation of small crystals , Chill Crystals (grains), at the surface of the ingot. As the mould warms up, its chilling effect is reduced, so that the formation of nuclei will be retarded as solidification proceeds. Thus crystals towards the centre of the ingot will be larger, Large Equi-Axed Crystals. In an intermediate position, the rate of cooling is favourable to the formation of elongated columnar crystals, so that we are frequently able to distinguish three separate zones in the crystal structure of an ingot, as shown in (Figure 2.11). Planes of weakness, originating from the corners of the casting may also be apparent due to the irregular clashing of crystals from the sides of the mould.

Figure 2.11 The Crystal Structure in a section of a large cast ingot Grain Boundary Defects Grain boundary defect is an example of what is termed Interfacial Defects. In this case, a grain boundary separates two small grains having different crystallographic orientation in a polycrystalline metal. Within the boundary region, there is some atomic mismatch in the transition from the crystalline orientation of one grain and that of an adjacent one, as shown in Figure 2.12. Various degrees of crystallographic misalignment between adjacent grains are possible as shown in Figure 2.12. Fore example, when the orientation mismatch is slight (on the order of a few degrees), then the term Low-angle boundary is used. Similarly, the term High- angle boundary is used when the orientation mismatch is large (on the order of many degrees).


2.10

Figure 2.12 Schematic diagram showing High and Low grain angle boundaries and the

adjacent atom positions The atoms are bonded less regularly at grain boundaries. Consequently, there is an interfacial, or grain boundary energy (measured in J/m2), present between atoms at the boundaries, which in higher than the energy of atoms at the interior positions of the grain. The magnitude of the boundary energy is related to the degree of mismatch, being higher for high angle boundaries. Grain boundaries are more chemically reactive than grains because of grain boundary energy. Furthermore, impurity atoms (for example in metal alloys; section 4.3.3) tend to segregate at grain boundaries because of their high-energy state. In addition, the total inter-granular energy is lower in large coarse grain metals than ones in fine grained ones since there is less total boundary are in coarse grain metals. Hence, at elevated temperatures, fine grains tend to inter-connect and grow in order to reduce the total boundary energy (section 4.3.4.2). Grain boundaries are important factors in reducing (hindering) the movement of dislocation (section 4.3.2). In spite of this disordered arrangement of atoms and lack of regular bonding between atoms along grain boundaries, a polycrystalline metal is still strong: the cohesive forces within and across the boundaries are present. Furthermore, the density of a single crystal is virtually identical to that of a polycrystalline sample of the same metal.


2.11

Vacancies A vacancy (or a vacant lattice site) is the simplest example of what is termed a point defects. A vacancy denotes a lattice position (or site) from which an atom is missing, as shown in Figure 2.13. Atoms occupying regular Lattice Positions vacancy Figure 2.13 Two-dimensional representation of a vacancy All crystalline solids contain vacancies and, in fact, it is not possible to create such a material that is free of these defects. The number of vacancies increases with the increase in temperature of the crystalline solid. Vacancies play an important role in inter-atomic diffusion (Appendix 7.A). Other examples of point defects are Substitiotional and Interstitial defects, which ore related to the production of metal alloys (section 5.2.2). Line defects Line defects are related to what is termed Dislocations. Dislocations are extremely important and chapter 4 has been dedicated to discuss this subject.


2.12

APPENDIX 2A Bonding in Metals 2A.1 Simplified Structure of an Atom (EXAMPLE LITHIUM, LI) Nucleus Neutron Proton Shell (K, L, M, etc.) Electron L k Contents of an Atom Particle Charge (C,Coulomb) Relative Charge Mass (Kg) Relative MassProton + 1.602 x 10-19 +1 1.6727 x 10-27 1 Neutron 0 0 1.6747 x 10-27 1 Electron - 1.602 x 10-19 -1 0.00091085 x 10-27 0 Atom Characteristics Atomic Number = Number of Electrons = Number of Protons (e.g. 3 for Li) For a Neutral atom, Number of Electrons = Number of Protons Atomic Weight, or Atomic Mass, (relative) Number of Neutrons + Number of Protons (e.g. 6 for Li) Ions An atom, which has acquired an electrical charge as a result of a gain (-ve charge) or a loss (+ve charge) of electrons is called an ion. Cation is an ion possessing a +ve charge (e.g. Na+ or Fe+ + etc). Anion is an ion possessing a -ve charge (e.g. Cl- or S - - etc).


2.13

2A.2 Metallic Bonding There are several mechanisms that atoms (or ions) will adopt in order to bind (make bonds) and, hence, create a material. Metals for example create metallic bonds as shown in Figure 2A.1. In this mechanism, the metal atoms become positively charged ions by giving up some up their valence electrons (outer electrons), which, in tern, forms an electron sea (negatively charged). Hence the positively charged ion cores are bonded by the mutual attraction of the negatively charged electrons. _ __ __ __ + __ + __ + + __ __ __ __ __ __ __ __ __ __ + __ + __ + __

__ __ __ __ __ __ __ __ __ __ + __ + __ + __ + Figure 2A.1- Model of metallic bonding in metals

Positively charged metallic ions Sea of negatively charged electron

Metallic Bonds


2.14

2A. 3 Bonding energy and Inter-Atomic Spacing Inter-atomic spacing is the distance between atoms and is caused by a balance between attractive and repulsive forces acting on the atoms (or ions). In a metallic Bond, for example, the attraction between the electrons and the atom core is balanced by the repulsion between the atom cores as shown in Figure 2A.2. Equilibrium separation, ro, occurs when the total energy between a pair of atoms (or ions) is at a minimum and when no net force is acting to either attract or repel atoms. The minimum energy is said to be the Bonding Energy. Atomic Radius __ __ __ __ __ __ __ __ __ __ __ __ Figure 2A.2 Variation of Energy and Force with inter-atomic Distance

Positive core Negative electrons Outer shell

Repulsion ( + ) Energy 0 Attraction ( - ) (minimum) Attraction ( + ) Force 0 Repulsion ( - )

Equilibrium Separation, r0

Interatomic Distance ( r ) Bonding Energy ( U0 ) Inter-atomic Distance ( r )


2.15

As mentioned above, the minimum energy in Figure 2A.2 is the Bonding Energy, U0, or the energy required to create or break the bond. Consequently, materials having high bonding energies also have high values of strength and melting temperatures. The Bonding Energy of the metallic bond varies between 100 800 (kJ/mol). The Modulus of Elasticity (Youngs Modulus) of a metal which is the amount a material will stretch when a force is applied (i.e. a measure of stiffness) is related to the Force-Extension curve of Figure 2A.2. A steep slope (which correlates to a higher binding energy and melting temperature) means that a greater force is required to stretch the bond; thus, the metal has a higher Modulus of Elasticity (and, hence, a higher stiffness).


2.16

APPENDIX 2B The Various Possible crystal structures (14 in total), or Bravais Lattices


3.1

3. SPECIMEN PREPARATION AND MICROSCOPIC EXAMINATION

3.1 Introduction On accessions it is necessary to examine the structural elements, e.g. grain size and defects that influence the properties of metals. Sometimes, the grains are macroscopic and may be viewed by the naked eye, e.g. zinc grains on the surfaces of galvanised steel components. However, in most metals, the constituent grains are of microscopic dimensions and their details must be investigated using some kind of microscopy. Grain size, shape and defects are only few of the many features in metals of what is termed microstructure. In order to view the microstructure at the of a metal, first the surface of the metal must be prepared and then viewed by a microscope. 3.1.1 The Preparation of Metal Specimens 3.1.1.1 General Process In preparing a metal specimen for microscopic examination it is first necessary to produce in it a surface which appears perfectly flat and scratch free when viewed with the aid of a microscope. This involves first grinding the surface flat, and then polishing it to remove the marks left by grinding (it is necessary to mount small specimens to facilitate the grinding process). The polishing process causes a very thin layer of amorphous metal to be burnished over the surface of the specimen, thus hiding the crystal structure. In order to reveal its crystal structure, the specimen is etched in a suitable reagent. This etching reagent dissolves the flowed or amorphous layer of metal and preferentially attacks the grain boundaries, thus making them visible and distinguishable. 3.1.1.2 Mounting of the specimens It is difficult to handle small specimens in order to grind them. It is therefore necessary to mount the specimen, usually using some plastic material, in order to be able to better handle the specimen and facilitate the grinding and polishing process. A typical set-up for mounting the specimen is shown in Figure 3.1. This involves placing the specimen in a small mould and surrounding it with a plastic powder. Force (using a plunger) and heat (using an electric heater) is applied for a short time. This compacts the powder and binds it together. The specimen will be securely embedded in the mount and may be more easily handled, thus, enabling its surface be ground and polished.


3.2

(a) (b) Figure 3.1 Mould for mounting specimens in plastic materials when pressure is

necessary (a) Moulding the mount (b) Specimen securely embedded in Finished mount 3.1.1.3 Grinding and polishing the specimen Grinding creates an evenly flat surface specimen. This is achieved by subjecting the surface of the specimen to coarse, intermediate and fine hand grinding which is carried out on emery papers (e.g. silicon carbide paper) of progressively finer grade, as shown in Figure 3.2 a). These must be of the very best quality, particularly in respect of uniformity of particle size. Usually, four grades of paper are necessary (e.g. 220, 320, 400 and 600 from coarse to fine), since by using a paper with a waterproof base, wet grinding can be employed. Grinding Motion

Plunger Outer Sleeve Heater

Plastic Powder Specimen

Coarse Fine

Water Grinding Paper Specimen Rotary wheel Motor

Figure 3.2 Hand-grinding and b) Rotary-wheel grinding set-ups


3.3

Since it has been recognised that the dust of many heavy metals is dangerously toxic. Rotary grinding wheels are available on to which discs of grinding paper are clamped. These are driven by two-speed motors and are fitted with water drip-feeds and suitable drains as shown in Figure 3.2 b). Most metallographic specimens are then polished using one of the proprietary diamond-dust polishing compound, which is smeared on a polishing pad (which in turn is mounted on a rotary wheel, similar to the set up in Figure 3.2 b)). In these materials, the graded diamond particles are carried in a cream base, which is soluble in both water and the special polishing fluid, a few spots of which are applied to the polishing pad, in order to lubricate the work and promote even spreading of the compound. These compounds are graded and colour-coded according to particle size (in micrometers, m). For polishing irons and steels it is generally convenient to use a two-stage technique necessitating two polishing wheels. Preliminary polishing is carried out using, for example, a 6m particle size. The specimen is then washed and finished on the second wheel using a 1m particle size. To summarise, the most important factors affecting a success

metallurgy for non-metallurgists training by khalid dubai

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