An_Introduction_to_the_Metallurgy_of_Ste

Prévia do material em texto

Robert A. Francis 
 
 
 
 
 
AN INTRODUCTION TO THE 
METALLURGY OF STEEL AND ITS 
ALLOYS 
 
 
 
 
 
 
 
Robert A. Francis 
 
Ashburton, Victoria, Australia 
 
 
 
 
 
 
 
 
 
Version 1.0 
 ii 
 
 
 
 
 
 
 
 
 
Copyright  2017 
 
 
 
 
 
Product names mentioned in this document may be trademarks or registered 
trademarks of their respective companies and are hereby acknowledged. 
 
DISCLAIMER 
The information contained in these notes is derived from various sources and 
was believed to be correct when published. The information is advisory only, 
provided in good faith and not claimed to be an exhaustive treatment of the 
relevant subject. The use of materials and methods described is solely at the 
risk of the user. Further professional advice might be needed to be obtained 
before taking any action based on the matter contained in this publication. 
 
 
 iii 
 
CONTENTS 
Foreword ....................................................................................................................... vi 
Chapter 1: Iron and Steel Manufacture 
Production of Iron ................................................................................... 1-1 
Basic Oxygen Steel Making .................................................................... 1-2 
Electric Arc Steel Making ....................................................................... 1-4 
Ladle and Secondary Refining ................................................................ 1-4 
Casting and Rolling ................................................................................. 1-6 
Pipe Manufacture .................................................................................... 1-7 
Chapter 2: Introduction to the Structure of Metals 
The Structure of Pure Metals .................................................................. 2-1 
Plastic Deformation of Metals ................................................................. 2-2 
The Structure of Polycrystalline Metals .................................................. 2-4 
Phases and Phase Diagrams .................................................................... 2-6 
Phase Diagrams of Alloys ....................................................................... 2-8 
Other Important Alloys ......................................................................... 2-12 
Chapter 3: Tensile Properties of Metals 
The Tensile Test ..................................................................................... 3-1 
Stresses in Three Dimensions ................................................................. 3-2 
The Elastic Region .................................................................................. 3-3 
The Yield Strength .................................................................................. 3-3 
The Yield Point ....................................................................................... 3-4 
The Plastic Region .................................................................................. 3-5 
Comparison of Tensile Properties ........................................................... 3-6 
Tensile Testing of Pipe and Plate ............................................................ 3-7 
Hardness Testing ..................................................................................... 3-8 
Summary ............................................................................................... 3-10 
Chapter 4: Fracture and Fracture Testing 
Ductile Fracture ....................................................................................... 4-1 
Brittle Fracture ........................................................................................ 4-2 
The Ductile to Brittle Transition ............................................................. 4-3 
Impact Testing ......................................................................................... 4-4 
Introduction to Fracture Mechanics ........................................................ 4-8 
Case Studies in Fracture Mechanics ...................................................... 4-11 
Fracture Toughness of Low Strength Materials .................................... 4-12 
Notes: Additional Information on Fracture Mechanics ......................... 4-14 
 
 iv 
 
Chapter 5: Fatigue, Creep and Other Metallurgical Failures 
Fatigue ..................................................................................................... 5-1 
Hydrogen Damage ................................................................................... 5-6 
Stress Corrosion Cracking ..................................................................... 5-13 
Creep ..................................................................................................... 5-15 
Metallurgical Embrittlement ................................................................. 5-18 
Chapter 6: The Structure of Steels and Alloys 
The Iron-Carbon Equilibrium Diagram ................................................... 6-1 
Equilibrium Steel Microstructures .......................................................... 6-2 
Other Steel Microstructures .................................................................... 6-4 
Deoxidation and Non-metallic Inclusions ............................................... 6-5 
Chapter 7: Heat Treatment and Hardening of Steels and Alloys 
Normalising and Annealing of Steels ...................................................... 7-1 
Quenching and Tempering of Carbon Steels .......................................... 7-2 
Predicting Heat Treatment Behaviour ..................................................... 7-4 
Changes on Continuous Cooling ............................................................. 7-5 
Precipitation Hardening ........................................................................... 7-8 
Surface Hardening of Steel ..................................................................... 7-9 
Chapter 8: Rolling and Working of Metals 
Effect of Rolling on Properties ................................................................ 8-1 
Recrystallisation and Grain Growth ........................................................ 8-2 
Cold and Hot Working ............................................................................ 8-3 
Thermo-Mechanical Processing of Steel ................................................. 8-5 
Chapter 9: Classification and Selection of Carbon and Alloy Steels 
Steel Classification .................................................................................. 9-1 
Standards for Steel Products ................................................................... 9-2 
Plain Carbon Steels ................................................................................. 9-3 
Effects of Alloying Elements .................................................................. 9-4 
The AISI-SAE Classification .................................................................. 9-6 
Alloy Steels ............................................................................................. 9-7 
High Strength Low Alloy Steels ............................................................. 9-8 
Structural Steels ..................................................................................... 9-10 
Steels for Hot Dip Galvanizing ............................................................. 9-11 
Weathering Steels .................................................................................. 9-13 
Pipeline Steels ....................................................................................... 9-14 
Shipbuilding Steels ................................................................................ 9-15 
Pressure Vessel and High Temperature Steels ...................................... 9-15 
High Strength Quench and Tempered Steels ........................................ 9-16 
Steels for LowTemperature Applications ............................................ 9-17 
An Example Steel Standard ................................................................... 9-19 
Mill Certificate ...................................................................................... 9-21 
 
 v 
 
Chapter 10: Cast irons and Stainless Steels 
Cast Irons ............................................................................................... 10-1 
Corrosion Properties of Stainless Steels ................................................ 10-3 
Structure and Types of Stainless Steels ................................................. 10-4 
The Schaeffler Diagram ........................................................................ 10-6 
Mechanical Properties of Stainless Steels ............................................. 10-7 
Cast Alloys and Fasteners ..................................................................... 10-8 
Fabrication and Finishing ...................................................................... 10-9 
Chapter 11: Welding 
Welding Methods .................................................................................. 11-1 
Arc Welding Processes .......................................................................... 11-1 
Other Welding Processes ...................................................................... 11-3 
The Metallurgy of Welding ................................................................... 11-5 
Welding Faults ...................................................................................... 11-9 
Welding of Stainless Steels ................................................................. 11-11 
Welding of Dissimilar Metals ............................................................. 11-15 
Weld Metal Selection .......................................................................... 11-16 
Welding Procedures ............................................................................ 11-18 
Chapter 12: Non Destructive Testing 
Visual Inspection ................................................................................... 12-1 
Magnetic Particle Inspection ................................................................. 12-1 
Liquid Penetrant Inspection .................................................................. 12-2 
Eddy Current Inspection ........................................................................ 12-3 
Ultrasonic Inspection ............................................................................. 12-4 
Presentation of Ultrasonic Testing Results ........................................... 12-7 
Radiography .......................................................................................... 12-8 
Presentation of Radiographic Results ................................................. 12-10 
Acoustic Emission ............................................................................... 12-10 
Magnetic Flux Leakage ....................................................................... 12-11 
Method Selection ................................................................................. 12-12 
Thermal Inspection .............................................................................. 12-12 
Hydrostatic and Pressure Testing ........................................................ 12-12 
Rapid Identification of Metals ............................................................. 12-13 
Appendix: Introduction to Mechanical Properties 
References & Further Reading 
Glossary of Metallurgical Terms 
Index 
 
 vi 
 
FOREWORD 
 
The basic properties of an engineering material are directly related to the structure of the 
material. Some properties, such as strength, are sensitive to very small variations in the 
structure, some of which are microscopic, some of which are on an atomic scale. In order to 
control the properties of a material, and to use them in an optimum manner, the engineer must 
have a working knowledge of the material structure. Furthermore, to modify the properties, 
changes have to be made to the internal structure of the material. Finally, if processing or 
service conditions alter the internal structure, then the properties will be affected. Clearly, the 
properties of the material depend on the internal structure, and modification of the internal 
structure can alter the properties. This circular relationship is the key principle that underlies 
the use of all materials. Throughout this publication, we will keep coming back to this 
relationship. 
A study of all engineering materials, their properties and structures would fill many books and 
is beyond the requirements of most engineers. The first criterion for this publication is that only 
the barest coverage of fundamentals is given, enough to ensure that the engineer or 
technologist will be able to distinguish between the properties of the main materials that are 
used. Secondly, we cover mainly ferrous metallurgy, with coverage of non-ferrous metals 
where essential for explaining specific properties and concepts. Even so, there are probably 
over 1000 steels on the market, and selection can be daunting. These notes mainly deal with 
steels used in heavy engineering such as construction, pipelines, tanks, etc. However, many of 
the principles apply to steels used in automobiles, aerospace, marine, and many other 
industries. 
Even within these limitations, the publication can only touch on the subject of steels and their 
properties. The books in the Further Reading should be consulted for more information. In 
addition, the internet these days provides a wealth of useful technical and commercial 
information, and entering a few key words into a search engine such as Google will provide a 
wealth of up-to-date information, although technical knowledge will be necessary to sort out 
the good from the bad. 
The information in this book is largely taken from a number of text books, from journals, web 
sites, trade literature and from the notes to courses written or developed by the author for 
Monash University, the Australasian Corrosion Association and private course providers. 
However, any errors are mine and I would be grateful to readers if they could point them out. 
Any other comments would be welcome. 
 
Rob Francis 
Melbourne, July 2017. 
1-1 
Chapter 1 
 
IRON AND STEEL MANUFACTURE 
 
 
 
The structure and mechanical properties of steel and alloy products depend to some degree on 
their method of manufacture, as well as processing after manufacture. This chapter looks 
briefly at the manufacture of steel and steel products, including: 
 Production of iron in the blast furnace and from direct reduction 
 Steel making using the basic oxygen furnace 
 Steel making using the electric arc furnace 
 Ladle and secondary refining of the steel 
 Casting and rolling of the steel 
 Methods of manufacture of pipe 
 
PRODUCTION OF IRON 
Smelting of iron ore takes place in the blast furnace (see Figure 1-1). A modern blast furnace 
is something like 60 metres high and 7.5 metres in diameter at the base and may produce from 
2,000 to 10,000 tonnes of iron a day. Processed ore, coke and limestone are charged through 
the double-bell gas-trap system at the top. A blast of heated air is blown in through the tuyeres 
near the hearth of the furnace. At the bottom is a slag notch and a tap hole to run off the slag 
and molten iron at regular intervals. 
 
Figure 1-1: The iron blast furnace 
 
The smelting operation involves a number of reactions. The coke burns when it comes into 
contact with the heated air blast producing carbon monoxide gas and a large amount of heat. 
IRON AND STEEL MANUFACTURE 
1-2 
The gases rise up through the stack, reducing the iron oxide to iron. The limestone begins to 
decompose at about 800ºC, forming calcium oxide, which reacts with the silica in the ore to 
form a liquid slag.As the iron falls down the stack, it absorbs carbon and melts at about 
1300ºC. It is tapped at the bottom and usually transferred, still molten, to the steel-making 
plant. Alternatively, it may be cast into 'pigs' for subsequent use in an iron foundry. 
 
The molten iron, known as hot metal, contains the following approximate levels of impurities: 
 3 to 5 per cent carbon, 
 up to 2 per cent manganese, 
 1 to 4 per cent silicon, 
 up to 0.12 per cent sulphur and 
 up to 2.5 per cent phosphorus 
depending on the original ore and the furnace conditions. The hot furnace gases contain a high 
proportion of carbon monoxide and are burnt in the coke ovens and the Cowper stoves used for 
preheating the air. 
 
While the blast furnace is the usual means of producing the raw material for steel making, a 
more-recent process known as direct reduction has a number of advantages. It does not 
require access to coking coal, there are fewer pollution problems with coke ovens and the size 
and capital investment can be significantly smaller. Against this, it can be more complex and 
therefore more costly, and the product contains unreacted ore (gangue) which must be removed 
in the steel making process. Direct reduction usually involves the production of iron by 
reducing the ore with carbon monoxide or hydrogen or a mixture of the two. These reductants 
are usually produced by reacting natural gas, or less commonly non-coking coal, with steam. 
Direct reduction produces a product called direct reduced iron (DRI) or sponge iron. It is 
usually much lower in impurities such as carbon or sulphur than the product from a blast 
furnace. DRI is usually refined in an electric arc furnace. 
 
BASIC OXYGEN STEEL MAKING 
The aim of steel making is to refine the pig iron so that the impurities are at an acceptable 
level. Unlike the blast furnace, steel making is largely a batch rather than continuous process. 
Each batch of steel is known as a heat, and will have a ‘heat number’ which is usually stamped 
on subsequent products as part of the steel mill’s quality assurance process. 
 
Basic oxygen steelmaking (BOS, also known as the LD, BOP or BOF process) is the most 
common form of steel making with high production rates and a product with low impurity 
levels. Impurities are removed in the BOS by blowing oxygen through the molten iron. The 
impurities are oxidised, releasing heat, which raises the temperature of the metal. No external 
fuel is required. 
 
The furnace consists of a refractory-lined, barrel-shaped vessel supported on a trunnion ring 
which allows it to be rotated through 360 degrees (see Figure 1-2). A taphole is located on one 
side of the vessel below the mouth. Scrap, lime and hot metal are loaded into the converter 
first. Oxygen is then blown at the surface of the molten charge from a water-cooled lance 
which is lowered through the mouth to within 0.5 m of the surface. 
IRON AND STEEL MANUFACTURE 
1-3 
 
Figure 1-2: Basic oxygen furnace. 
 
Silicon and manganese readily oxidise because of their strong affinity for oxygen (see Figure 
1-3) and these enter the slag. Phosphorus is also oxidised but may not be removed to a 
sufficient level unless the chemical composition of the slag is just right. Some sulphur is 
removed in the slag, but for complete removal it requires reducing conditions, not the oxidising 
conditions which exist in the BOS. Carbon is oxidised to carbon monoxide which burns at the 
mouth of the converter. When this flame drops, the reactions are complete and the vessel is 
tilted, first to remove the slag. Then the charge is transferred to a ladle in an operation known 
as tapping. 
 
Figure 1-3: Refining progress in the BOS 
 
Later developments to basic oxygen steel making include the use of bottom blowing of oxygen 
(known as Q-BOP or OBM) which has better slag/metal contact to reduce the phosphorus and 
sulphur levels. It also has less slopping so improves the iron yield. At some plants, normal top 
blowing is used but with an inert gas blown in the bottom (the LBE process). This gives similar 
advantages to bottom blowing but can be achieved by transforming an existing converter. 
Other changes in recent years have been improvements in lance technology which reduce slag 
splashing and improve refractory life. 
IRON AND STEEL MANUFACTURE 
1-4 
ELECTRIC ARC STEEL MAKING 
The electric arc furnace (EAF) has historically been used for high-grade steels and scrap 
melting, but it is growing in use for ordinary grades. It is an integral part of the ‘mini-mill’ 
steel making process consisting of an EAF along with a continuous caster to provide a small, 
low capital cost steel mill utilising abundant, inexpensive steel scrap. Today, mini-mills can 
produce over 80 per cent of all steel products. The electric arc furnace is also usually used to 
refine high alloy steels, such as stainless steels. 
 
The electric arc furnace is illustrated in Figure 1-4 and is from 25 to more than 150 tonnes 
capacity. The charge can be of scrap of the required final composition although carbon is 
usually lost during the carbon boil. The carbon electrodes in the roof strike an arc directly with 
the metal to melt it. Reducing conditions allow for removal of sulphur in the slag, and alloying 
elements such as nickel, chromium, manganese, vanadium etc. can be added and will not be 
lost through oxidation. Oxygen can be blown into the furnace to purify the steel, and lime and 
fluorspar added to combine with impurities to form slag. At the end of the process, the furnace 
is tilted, first to pour off the slag, and then in the opposite direction where the molten steel is 
tapped into a ladle. 
 
Figure 1-4: Electric arc furnace. 
 
The efficiency of electric arc steel making has been substantially improved in recent years. As 
well as adopting oxygen injection, oxy fuel burners, coal powder injection, high-power 
transformers, preheating scrap and new systems of cooling and protecting furnace walls have 
been introduced, enabling production efficiency increases from 80 to 120 tonnes per hour. 
 
LADLE AND SECONDARY REFINING 
When the steel making process is finished, it is tapped into a ladle and adjustments made to 
obtain the required chemical composition of the steel. Scrap may be added to minimise ladle 
refractory wear if the temperature is too high. Anthracite or char may be added to raise the 
carbon level if required. Alloying elements are added in the form of their ferro-alloys, such as 
ferromanganese, ferrovanadium, etc. Deoxidation, however, is the most important of these 
preliminary operations. It is achieved by adding elements such as silicon, aluminium or 
manganese which react with the oxygen in the melt. A fully killed steel has no gas evolution 
during solidification and produces a clean, sound structure. With modern continuous casting, 
almost all steel is fully killed these days. Generally, the oxygen content is kept as low as 
possible to produce an internally ‘clean steel’, free from silicate and aluminate non-metallic 
inclusions, with improved ductility, toughness and fatigue properties. Deoxidation and its 
effects on properties are discussed in chapter 6. 
 
IRON AND STEEL MANUFACTURE 
1-5 
The trend is to make increased used of ladle metallurgy or secondary steelmaking to improve 
the quality of the steel. There are a large number of processes available and those used depend 
on the final use of the product. Inert gas injection such as argon helps blend contents, equalise 
temperatures and assist in removal of non-metallic impurities. Vacuum degassing subjects the 
ladle to a vacuum to reduce the oxygen and hydrogen content and to stir the melt. Both these 
processes, along with the alloy and other additions that are necessary, lead to a large drop in 
temperature of the melt. This has been overcome using a separate or secondary process whichcombines a furnace with a ladle degassing process. The best known of these is the Argon 
Oxygen Decarburisation (AOD) process which has revolutionised production of stainless 
steels by greatly increasing quality and production. This process blows the melt in a converter 
with an argon-oxygen mixture, gradually increasing the argon content so that the melt is first 
deoxidised and then desulphurised. Figure 1-4 illustrates such a converter, along with a 
diagram showing typical changes in carbon content and temperature behaviour during refining. 
The low levels of carbon necessary for the L grades of stainless steel can be easily achieved, 
composition is more uniform, low levels of sulphur and other contaminants can be achieved 
and nitrogen can be added or removed as required. 
 
(a) (b) 
Figure 1-4: Argon oxygen decarburisation (a) Vessel, (b) Refining process. 
 
Other processes are available for melt refining. Vacuum arc degassing (VAD) uses a small-
scale electric arc furnace under a vacuum on the ladle. Vacuum oxygen decarburisation 
(VOD) uses a consumable oxygen lance under a vacuum on the ladle to refine the melt. Like 
AOD, oxidation of the carbon provides heat, and the process is used to refine high alloy steels, 
such as stainless steels. Electroslag refining (ESR) is used for high quality steels which 
require a very high degree of cleanliness, such as for ball bearings. In this process, the ingot is 
lowered into a mould, slag is poured in and a current is struck between the ingot and an 
electrode. The slag absorbs any oxides, silicates or sulphides from the original steel as it melts. 
 
An example of the need for additional treatments in steel making is shown by the demand for 
increasingly lower sulphur contents in structural and pipeline steels. In the 1960s, levels of 
0.025 to 0.05 per cent sulphur were common minimums, but modern products often require 
sulphur levels of 0.005 per cent or less. As already mentioned, removal of sulphur requires 
reducing rather than oxidising conditions so it cannot be removed in the BOS converter. One 
way of lowering the sulphur level is through hot metal desulphurisation which is carried out 
before the hot metal from the blast furnace is charged to the converter. This involves injection 
of lime which reacts with the sulphur to form a slag. Carbon, along with fluorspar, is also 
added to produce carbon monoxide to stir the melt. Ladle desulphurisation uses similar 
procedures after tapping. For sulphur levels to about 0.01 per cent, lime and fluorspar can be 
IRON AND STEEL MANUFACTURE 
1-6 
added on tapping. For levels less than 0.005 per cent, a special slag and argon bubbling must be 
used. Injecting powdered agents such as lime powder (with fluorspar to improve fluidity) into 
the ladle help reduce sulphur levels while injecting calcium carbide or silicide (the TN 
process) helps alter the shape of non-metallic inclusions which reduces their deleterious effect 
on mechanical properties. This process will lower the sulphur level as well. 
 
CASTING AND ROLLING 
Traditionally, steel has been cast or teemed into ingots of 5 to 15 tonnes. Ingots had many 
problems, including poor yield, blowholes which lead to seams on the surface when rolled and 
segregation of elements leading to variations in chemical composition, and thus properties, 
across the final product. While ingots provide the steelmaker with much flexibility, most steel 
plate and sections are made by continuous casting. Continuous casting overcomes many of the 
problems associated with ingots and also bypasses the ingot teeming, ingot stripping, soaking 
pit and primary mill. Figure 1-5 shows a common continuous casting procedure wherein the 
liquid metal flows from the ladle to a tundish to control flow rate and then into a bottomless, 
water-cooled mould, usually made of copper. Cooling is controlled so that the outside has 
solidified before the metal exits the mould. The metal is further cooled by direct water sprays 
to assure complete solidification. The resulting bloom or slab is then bent horizontally and cut 
to the desired length. Continuous casting provides greater productivity, marked improvements 
in yield, reduction in segregation, lower energy consumption and a better surface finish over 
the ingot route. 
 
Figure 1-5: A continuous casting process for producing slabs 
 
The product from the continuous caster is reheated to about 1200ºC and further reduced in 
rolling mills. For strip or plate, slabs are reheated and passed repeatedly back and forth though 
rolls. The first stage, called roughing, reduces the thickness to an intermediate size of about 30 
to 50 millimetres, and a second stage, called finishing, produces the final gauge, normally of 
the order of 1 to 5 millimetres. The number of passes depends upon the input material and the 
size of the finished product but can be up to seventy passes. 
 
IRON AND STEEL MANUFACTURE 
1-7 
The basic rolling unit is called a stand and a typical rolling mill comprises a group of stands, 
complete with auxiliary facilities such as drive motors, roller tables for entering and removing 
the metal, shears, etc. As the steel passes through each stand, the thickness is progressively 
reduced and the length increased. Strip is coiled after it leaves the last stand of the mill. Plain 
barrel rolls are used for flat products such as plate, strip and sheet. Structural sections, rails, 
beams, sheet piles, etc are manufactured in a similar manner, but a bloom or billet is usually 
the starting shape and grooved rolls are used to produce the final product. Large sections can 
also be made by welding plate of the required dimensions. Merchant bar is a traditional term 
for small cross-sections such as rounds, squares, hexagons, flats, etc. 
 
Hot rolled products may contain defects which affect properties, appearance or both, although 
these are far less common with modern steelmaking techniques. Cracks may arise due to 
cooling stresses developing in the feed. Scabs are flattened protrusions on the surface which 
arise before rolling as a result of splashes from teeming or casting. Laps are longitudinal folds 
resulting from rolling of surface projections. Seams are longitudinal grooves or lines from 
surface defects such as porosity. Such surface defects are usually ground out, or welded if 
deep. Laminations are internal planar defects which occur parallel to the surface in plate or 
sheet. They are a result of flattening internal discontinuities such as large inclusions or porosity 
and normally require NDT such as ultrasonics for detection. 
 
PIPE MANUFACTURE 
Pipe and tubing can be manufactured using a number of different methods and necessitate extra 
processes not required for other products. Tubing can be either welded or seamless. Welded 
pipe utilises steel in the form of long, narrow strips (sometimes called skelp) of the desired 
thickness. This is unwound from a continuous coil and fed through shaping rolls which bend it 
into a tubular shape either longitudinally or in a spiral. 
 
The resulting seam is joined together by one of a variety of welding processes. Electric 
resistance welding (ERW) locally heats the edges of the plate to a suitable forging temperature 
by an electric current (Figure 1-6). Constricting shaping rolls transform the flat steel to a pipe 
section. A high-frequency induction coil induces a current in the pipe and rollers force the 
heated edges together forming a fusion weld. ERW pipe can also be manufactured using 
contacting electrodes. 
 
Figure 1-6: Sequence of operations for making electric resistance welded pipe. 
 
Submerged arc welding (SAW) can also be used for pipe manufacture. In this method, heat is 
supplied by an arc which is shielded from atmospheric contamination by a layer of flux. 
Usually, two passes are made: one on the inside and one on the outside(hence they are often 
termed Double SAW or DSAW). The strip can be welded spirally or longitudinally. Other 
welding methods used for pipe include continuous butt welding (CW) where the heated edges 
IRON AND STEEL MANUFACTURE 
1-8 
of the strip are forged together by the pressure of rolls. CW pipe is used for low strength pipe 
for low pressure applications. The UOE process subjects plates to two separate cold forming 
stages: the “U” press followed by the forming in the “O” press. Then the formed plate is 
welded inside and out using the SAW process to produce a pipe. The pipe is finally expanded 
(“E”) to provide circularity. This process is used for high strength, thick-walled, high pressure 
pipe. 
 
Steel tubular products made by seamless processes are made in diameters up to 0.66 metres by 
the rotary piercing method and up to 1.22 metres by hot extrusion. In the rotary piercing 
method or Mannesmann process as shown in Figure 1-7(a), inclined rollers pull a heated, round 
bar by their rotary action onto a mandrel which forms a cavity in the bar as it moves through 
the rolls. 
 
(a) (b) 
Figure 1-7: Forming seamless tube using (a) Mannesmann and (b) hot extrusion processes 
 
Hot extrusion forces hot, pre-pierced billets through a suitably shaped orifice formed by an 
external die and internal mandrel as shown in Figure 1-7(b). 
 
ERW is mainly used for low to medium pressure applications such as transportation of water or 
oil. SAW can be used for large diameter pipe and is used for transportation of large volumes of 
liquid or gases. Seamless pipe is mainly used for smaller diameter, high-pressure applications 
such as oil and gas exploration drilling, boilers, automobiles etc and is generally more 
expensive. It is perceived to be a stronger, more reliable product due to the absence of a weld, 
but there is little hard data to back this belief up. Seamless pipe may show a wider variation in 
wall thickness than welded pipe. 
 
Pipe made by the above operations is usually subjected to a number of finishing operations. 
Large DSAW pipes may be cold expanded progressively on an expanding mandrel which 
increases its diameter. This increases its yield strength slightly and straightens and rounds the 
pipe. Other operations include straightening, end cropping and chamfering for welding or 
threading the ends as required. The pipe is usually non-destructively tested for defects. 
Ultrasonic testing is normally used, checking for lack of fusion in welded pipe and seams in 
seamless pipe. Electromagnetic techniques or radiography can also be carried out although 
ERW is not amenable to radiography. Pipe lines are also hydrostatically tested before being 
put into service, as described in chapter 12. This ensures the pipeline, at the time it is put into 
service, contains neither leaks nor defects that might induce failure at its operating stress level. 
2 - 1 
Chapter 2 
 
INTRODUCTION TO THE STRUCTURE OF METALS 
 
 
 
The behaviour of a metal in service depends on features of the structure of the metal, from the 
atomic to the visible. This chapter briefly looks at the structural factors which influence the 
behaviour of metals. These include the metallic bond, crystal structure, imperfections and 
defects and effect of alloying elements. The chapter also introduces the concept of phase or 
equilibrium diagrams, which help in describing the structure of alloys. 
 
THE STRUCTURE OF PURE METALS 
Metals have relatively high melting points and boiling points and are good conductors of heat 
and electricity. They also possess ductility, the property which permits permanent deformation 
without fracture occurring. These properties are related to the nature of the metallic bond. 
Metals consist of positive ions arranged in a regular repeating array with their valency 
electrons forming a cloud of free electrons (see Figure 2-1). The attraction between the 
positively-charged ions and the electrons is responsible for these properties. 
 
 
Figure 2-1: Simplified picture of the metallic bond. 
 
The arrangement of the atoms, or the way they are packed in a solid, varies depending on the 
metal. There are three main types of structures, known as crystal structures or lattices, 
depending on the relationship of the atoms to one another. The basis of these structures is 
called a unit cell, as shown in Figure 2-2, which is the smallest group of atoms which exhibit 
the overall symmetry of the crystal. The crystalline nature of metals is not normally apparent 
because a metal conforms to the shape to which it has been cast or formed, but has a 
considerable influence on the physical properties of the metal. 
 
The face centred cubic (FCC) structure is exhibited by metals such as copper, silver, gold, 
aluminium, nickel, lead and a form of iron known as -iron (gamma iron) or austenite. FCC 
metals are generally the most ductile metals. The body centred cubic structure (BCC) is 
exhibited by metals such as chromium, tungsten, molybdenum, vanadium and a form of iron 
known as -iron (alpha iron) or ferrite. BCC metals are less ductile than FCC metals. 
Hexagonal close packed (HCP) metals, such as magnesium, zinc and cadmium are also less 
ductile than FCC metals. 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 2 
 
 (a) (b) (c) 
Figure 2-2: Unit cells for the (a) face centred cubic, (b) body centred cubic and (c) hexagonal 
closed packed structures. 
 
The perfectly-regular crystal structures are idealised concepts and, in reality, the crystal 
structures of metals and alloys contain many imperfections. Point defects, shown in Figure 2-
3, include vacancies caused by missing atoms in the structure, impurity atoms replacing one of 
the atoms (known as substitutional atoms) and interstitials where an extra atom is positioned in 
an interstitial site (between atoms). Point defects can also influence physical properties of the 
metal. For example, the presence of vacancies is important as it influences diffusion of atoms 
through the lattice, and play an important role in creep. Substitutional and interstitial impurities 
increase the strength of the metal and their effect on structure is discussed later in this chapter. 
Line defects, more commonly known as dislocations, directly influence the strength of a metal 
and are discussed in the next section. 
 
 
Figure 2-3: Point defects in the crystal structure of a metal. 
 
PLASTIC DEFORMATION OF METALS 
Dislocations are line imperfections which form a continuous path of misalignment through a 
crystal lattice. The simplest form of dislocation is called an edge dislocation as shown in Figure 
2-4(a) and consists of an extra half plane of atoms in the lattice. A screw dislocation, shown in 
Figure 2-4(b) corresponds to partial tearing or shear of the crystal planes. In real metals, 
dislocations are of mixed type: edge dislocations in some regions and screw dislocations in 
others. Dislocations can be moved about with relatively low applied forces, and it is the 
movement of dislocations that produces observed deformation of metals. 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 3 
 
 (a) (b) 
Figure 2-4: (a) Edge dislocation and (b) screw dislocation. 
 
The mechanism by which dislocations move causing plastic deformation is known as slip. A 
zone of elastic distortion of the lattice forms around the dislocation so it can easily move along 
the plane of atoms normal to the extra plane (see Figure 2-5). When the slip process is 
completed, the crystal is again perfect but with the atoms above the slip plane displaced one 
unit to the right relative to the atoms below the slip plane. Instead of every bond on the slip 
plane having to rupture at one instant, the presence of a dislocation means that only one row of 
bonds at a time have to be ruptured for slip to occur. 
 
Figure 2-5: Movement of an edge dislocationcausing slip. 
 
Dislocations are important defects within crystals. Their density and interaction with each other 
and various microstructural features affect the strength and ductility of metals. If dislocations 
are free to move, the metal can be deformed with only a small applied force so is relatively 
weak. However, the metal will be ductile, meaning it can be deformed considerably before 
breaking. On the other hand, if dislocations are blocked, the metal is more resistant to slip and 
is stronger. For example, a metal containing foreign atoms is usually much stronger than the 
pure metal, and this can be explained in terms of blocks to dislocation movement. A 
dislocation moving through a lattice encounters foreign atoms, impurities or precipitates and 
has to distort, requiring extra force to move past the impurities (see Figure 2-6). Furthermore, 
the looping around the impurities generates new dislocation loops in a process known as 
Orowan looping, effectively increasing the size of barrier. The next dislocation will require 
even greater force to move past the impurities, and will generate further loops. However, if 
dislocation movement is restricted too much, the metal will become brittle. Finding means of 
preventing dislocation movement to strengthen a metal without it becoming too brittle is an 
important subject so covered in later Chapters. 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 4 
 
Figure 2-6: Dislocation movement though a lattice impeded by impurities. 
 
After undergoing some deformation, metals possess the unique property that they become 
stronger. This phenomenon is known as strain hardening or, more commonly, work 
hardening. This behaviour can again be explained in terms of dislocations and barriers to their 
movement. As dislocations move, they encounter and interact with other dislocations in exactly 
the same way that they interact with impurities which impedes further motion. Moreover, the 
same mechanism causes a marked increase the number of dislocations in a metal undergoing 
deformation. As a result, the probability of dislocation interaction, and therefore the strength, 
increases with the amount of plastic deformation. The dislocation density, the total length of 
dislocations per unit volume, in a metal increases from around 10
5
 centimetres per cubic 
centimetre in an undeformed metal to around 10
12
 centimetres per cubic centimetre after 
deformation. 
 
THE STRUCTURE OF POLYCRYSTALLINE METALS 
So far, metals have been considered as single crystals consisting of a uniform lattice with point 
and line defects. However, most metals are, in fact, aggregations of many crystals, more 
commonly called grains. Grains are formed because, when molten metal solidifies, 
solidification does not take place all at once but rather small tree-like nuclei of solid material 
known as dendrites (Figure 2-7) form each having their characteristic lattice structure. These 
particles act as nuclei onto which other atoms tend to attach themselves, producing growth of 
the solid. These grow until they interfere with their neighbours. At the places where they 
interfere, further growth ceases and the boundaries between them are known as grain 
boundaries. A diagrammatic representation of the process of solidification is shown in Figure 
2-8, showing growth of solid dendrites in the melt until grains are finally formed. 
 
Grains are the smallest structural units of a metal that are observable with a light microscope. 
They can be various sizes and shapes within a metal. Typical sizes vary from approximately 
0.5 millimetres for coarse grained metals to 0.005 millimetres for fine grained metals. They 
may be ‘equiaxed’, with all grains having approximately the same diameter, or elongated, 
where one dimension is many times the other. Rolling and working metals usually results in 
grain elongation, but heat treatment may result in formation of new equiaxed grains. The grain 
size and shape and their relationship to fabrication and heat treatment are discussed in chapters 
7 and 8. 
 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 5 
 
 
Figure 2-7: Growth of a metal 
dendrite. 
Figure 2-8: Stages in the process of solidification of a 
metals 
 
The grain size will depend on the rate at which a molten metal cools. Slow cooling promotes 
the formation of few nuclei, and the resultant grains will be large. On the other hand, rapid 
cooling results in large numbers of nuclei and smaller grains. Thick sections, sand casting and 
other methods of reducing cooling rates lead to large grains, while thin sections, metal moulds 
and other means of rapid cooling result in fine grains. In a large ingot, the grain size and shape 
will vary from the outside to the centre, because of the temperature variation as heat is 
transferred from the metal to the mould. The chill effect as the molten metal makes contact 
with the mould results in small crystals at the surface. As the mould warms up, the chilling 
effect is reduced so formation of new nuclei diminishes as solidification proceeds. Therefore, 
grains toward the centre of the ingot will be larger. In the intermediate position, elongated 
columnar grains tend to form. As a result, three separate zones can often be distinguished in the 
crystal structure of an ingot, as shown in Figure 2-9. 
 
 
Figure 2-9: The grain structure of a large ingot. 
 
When a metal solidifies, there is a tendency for impurities to remain in the portion of metal that 
solidifies last. When steel solidifies, the first crystals to form are relatively pure iron, and the 
liquid becomes enriched in elements such as carbon, manganese and sulphur present in steel. 
Consequently, the last liquid to solidify is enriched in these elements and has a different 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 6 
chemical composition, microstructure and may have different mechanical properties. This 
effect of variation in chemical analysis across a section is known as segregation. If severe, 
segregation can result in loss of properties and brittleness, as well as problems such as 
crumbling during working. Segregation can be expressed as local departure from average 
chemical composition of the complete heat; positive segregation when the concentration of the 
element is greater than the average and negative segregation when the concentration is less 
than the average. Sulphur tends to segregate most, with carbon, phosphorus and manganese 
segregating to a lesser degree. Continuous casting results in less segregation and more 
uniformity than the product made from ingots. 
 
Grains and grain boundaries are very important features of metals and have considerable 
influence on their properties. Grain boundaries are regions of high energy and may be attacked 
more rapidly than grains when exposed to a certain corrosives. Metallurgical etching of metals 
to observe microstructure depends on the ability of the etchant to highlight grain boundaries 
and other features of the structure of the metal. During solidification, impurities tend to 
accumulate at the grain boundaries. Also, because of the defective, open structure, diffusion 
tends to occur much more rapidly along grain boundaries. Grain boundaries hinder slip by 
preventing dislocation movement so the smaller the grains, the greater the grain boundary 
density and the stronger the metal. 
 
When a metal is subject to plastic deformation, as well as increasing the dislocation density, 
the grains will be deformed and the grain structure completely disrupted. There is a limit to the 
amount of deformation a metal can undergo and it will fracture if this limit is exceeded. 
Therefore, heavily cold worked metals are often annealed which means heating to a relatively 
low temperature. This softens the metals by removing internal stresses, decreasing the 
dislocation density and by causing new grains to form.Depending on the annealing 
temperature, these new grains may be very large, which gives a soft ductile metal or very small 
giving a strong metal. Annealing is an important way of changing mechanical properties of a 
metal and discussed in more detail in chapters 7 and 8. 
 
PHASES AND PHASE DIAGRAMS 
Metals are rarely used in their pure form, but rather two or more metals (or metals and non-
metals) are mixed to produce an alloy with desirable mechanical properties. Metallic and non-
metallic elements can combine in many different ways; they may completely dissolve in one 
another in a similar manner to water and alcohol, they may remain separate (similar to water 
and oil), they may form chemical compounds or act in some combination of these. To 
understand these various forms and combinations, we need to understand the concept of 
phases. A phase is a form of material forming a single characteristic structure with 
characteristic properties, and appears as a separate entity under the microscope if not apparent 
to the naked eye. A phase can be solid, liquid or gas; it can be a pure substance or it can be a 
solution, provided the composition is uniform throughout. A phase consisting of two 
components has essentially the same atomic arrangement and similar physical properties 
regardless of its composition. Therefore, an alcohol and water mixture is a single phase but oil 
and water form two distinct phases. Some other examples of phase are shown in Figure 2-10, 
showing the differences between components and phases. Water and ice are separate phases, 
even though they are made of the same material, because they have different physical 
properties. Similarly, water and steam are separate phases. Salt water is a single phase made of 
two components, water and sodium chloride. It has a single characteristic structure. If ice is 
added to the solution, two phases are now present, salt water and ice. If excess salt is now 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 7 
added so that the solution cannot dissolve any more (the solution is supersaturated) there is an 
additional phase, salt, although the number of components have not changed. 
 
Figure 2-10: Simple examples of phases and components. 
 
Two metals usually form a single phase in the liquid state, but when the liquid solidifies one of 
the following will occur: 
 The solubility in the liquid state is retained in the solid state and forms what is known as a 
solid solution. This has the properties of a liquid solution in that one component is 
dissolved in the other, but exists as a solid rather than a liquid. This is a single solid phase. 
 Two different solid solutions may form. Each solid solution is a separate single phase and 
these appear as distinct components under the microscope. These two phases may be of two 
almost pure metals, phases of similar chemical composition or phases of completely 
different chemical compositions. 
 The two metals react as they solidify forming what is known as an intermetallic 
compound. 
 
The phases that are present depend not only on the amounts of the various components but also 
the temperatures. To determine what state a particular combination of components is in at any 
given temperature, we use a phase diagram or, more correctly, an equilibrium diagram. A 
phase diagram shows what phases are stable as composition and temperature change. By 
convention, composition is plotted horizontally and temperature vertically. Consider what 
happens when sodium chloride is added to water. Figure 2-11 shows part of the phase diagram 
for this system. If a small amount salt is added to water at room temperature and stirred, it will 
dissolve, remaining as a single phase. Salt can be continually added until at a certain point, no 
more will dissolve and solid salt will remain at the bottom of the container. There are now two 
phases, a saturated solution and some solid salt. The point where the system changes from one 
to two phases depends on temperature. If our solution is heated, then the salt will dissolve and 
only solution remains and we go back to a single phase. If the solution is allowed to cool, 
particles of salt will precipitate from the solution. The line between the two phases is known as 
the solvus and shows as a rising curve on the phase diagram, as solubility generally increases 
as temperature increases. If we know the composition and temperature of our system, by 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 8 
referring to the phase diagram we can determine what phase or phases will be present at that 
temperature and composition. 
 
Figure 2-11: Part of the salt and water phase diagram. 
 
Solids such as alloys behave in exactly the same manner. In the case of a solid solution, the 
process of solution and precipitation is far slower since the atoms in the metal cannot move as 
fast as the water molecules. Furthermore, the process of precipitation and solution is not 
normally visible to the naked eye and the system must be examined under a microscope. Also, 
changes in crystalline arrangement from, for example, a FCC to a BCC structure are considered 
a phase change, known as an allotropic phase transformation. We use phase diagrams of solid 
metals in exactly the same way as for liquids, although they are usually far more complicated. 
The lines on the phase diagrams are usually solubility limits of one component in the other like 
the line on the above diagram. 
 
PHASE DIAGRAMS OF ALLOYS 
Metals in their pure form are seldom used in engineering applications, mainly because of their 
low strength. Most metallic materials used in engineering are combinations of metals known as 
alloys. Even small amounts of a second element added to a pure metal can significantly change 
the properties. For example, pure iron is a very weak metal, but addition of 0.1 per cent of 
carbon increases its strength to make it a very useful material. Small amounts of chromium, 
nickel and other elements can increase the strength, and improve the heat treating properties. 
Adding 12 per cent of chromium to iron dramatically improves its corrosion resistance by 
forming a stainless steel. Therefore a study of alloying, and the different ways elements can 
combine, is important in understanding metal properties and can be understood through a study 
of metallic phase diagrams. 
Solid Solutions 
When elements completely dissolve in one another they form a solid solution. This can take 
place in two ways, shown in Figure 2-12. In a substitutional solid solution, the solute atoms 
take the place of some of the solvent atoms. For example, copper will dissolve in nickel in all 
proportions forming a solid solution. Such properties are only possible if the atomic diameters 
are approximately the same. Most metals have limited solubility in one another and will 
dissolve only a small proportion of a second metal before a solubility limit is reached. The 
second form of solid solution is an interstitial solid solution. In such cases, the solute is much 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 9 
smaller than the solvent atoms and, as the name suggests, these fit in the gaps between the 
larger solvent atoms. Examples are carbon, nitrogen, hydrogen, etc. in iron. Since mixing of 
elements is on the scale of the atomic lattice, solid solutions appear homogeneous on the 
microscopic scale, with every grain having the same composition and structure. Adding a 
second element to a metal makes dislocation movement harder and provides ‘pins’ to lock 
dislocations, so the strength of a solid solution is much greater than the pure solvent. 
 
(a) (b) 
Figure 2-12: (a) Substitutional solid solution and (b) interstitial solid solution. 
 
The copper-nickel system provides perhaps the simplest phase diagram of any two metals (see 
Figure 2-13). There are only two lines on the diagram: 
 an upper curve knownas the liquidus, above which any point represents a completely 
molten, single phase alloy. For example, point A representing 20% Ni/ 80% Cu at 1250C 
would be in this state. 
 a lower curve, known as the solidus, below which any point represents a completely solid, 
single phase alloy. For example, point B representing 70% Ni/ 30% Cu at 1000C would be 
in this state. 
Any point between the two lines represents a composition that is a part solid/part liquid or 
‘mushy’ state, and consists of two phases, liquid and solid. 
 
Figure 2-13: The copper-nickel phase diagram 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 10 
 
The diagram can tell us what happens when an alloy solidifies. Consider, for example, an alloy 
of 40% Ni/ 60% Cu cooling slowly from the liquid state along line X. When the temperature 
reaches the liquidus (T1), solid crystals start forming in the liquid. The composition of these is 
not 40Ni/60Cu, but rather the composition of the solidus at that temperature, given at point Y. 
That is, the crystals have composition Y or about 70%Ni/30%Cu, richer in nickel than the 
liquid. As the temperature falls slowly, the crystals grow following the solidus line in 
composition, still nickel-rich but increasingly less so. At this same time, the composition of the 
liquid becomes copper rich, following the liquidus line at the given temperature. As the 
proportions of solid and liquid change, the compositions must change to keep the overall 
composition of the alloy at 40%Ni/60%Cu. By the time the temperature reaches T2, all the 
liquid has solidified and the solid alloy has the uniform original composition. 
 
Between the liquidus and solidus is the area where two phases are present in equilibrium; a 
liquid and a solid in this case. Since they are in equilibrium, the composition of the two phases 
at any temperature can be read of by drawing a horizontal line (known as a tie line) through the 
point representing the alloy composition and the temperature. Figure 2-14 shows a portion of 
the phase diagram with a tie line through points a and b. At temperature T and composition x, 
the liquid has composition xa and the solid has composition xb. The relative amounts of solid 
and liquid can also be obtained from the diagram using the lever law. As with levers, if the 
fulcrum is at the alloy composition (point o), the weight of liquid times length ao equals the 
weight of solid times length ob. In mathematical terms, the amounts of the two phases are 
given by: 
 length ob 
 % liquid = ––––––– 
 length ab 
 
 length oa 
 % solid = –––––––– 
 length ab 
 
 
Figure 2-14: Detail of nickel-copper phase diagram with a tie line. 
 
In the above application, we have assumed that cooling takes place very slowly and 
completely, so that the final composition of the solid is uniform throughout. Under actual 
conditions of relatively rapid temperature change this is rarely possible. The first crystals to 
form are richer in nickel and later solid is less rich, but these atoms do not get a chance to 
equalise before the alloy completely solidifies. As a result, there are regions in the centre of the 
grains which have a greater proportion of nickel than the alloy at the edge of the grains. This 
phenomenon is known as coring or microsegregation, and produces a solid that is less 
homogeneous than the phase diagram would predict. Figure 2-15 shows the development of 
coring during rapid cooling of a 50 per cent nickel – 50 per cent copper alloy. Cored structures 
can affect mechanical properties and corrosion characteristics. Slower rates of solidification 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 11 
will reduce the degree of coring, and it can be dissipated by heating the alloy to a temperature 
just below the solidus line. 
 
Figure 2-15: Development of coring in a 50% nickel-50% copper alloy 
 
The information learned from this simple phase diagram can now be used in reading any other 
phase diagram, no matter how complex. The important points are: 
1. If you know the composition and temperature, you can plot the point on the diagram. 
2. Read the label on the region of the diagram to determine what phase or phases are present. 
3. If the point falls in a single phase region, there is just one phase which must have the same 
composition as the material as a whole. 
4. If the point falls in a two phase region, draw a horizontal line through the point and read off 
the phases at either end of the line. The composition of each phase at that temperature is 
given by the compositions at these intersecting points. 
5. Use the lever law to determine the amounts of the two phases. 
Alloys with limited solubility 
While many alloy systems will show solubility in one another when small amounts of solute 
are added to the solvent, at higher proportions new phases will appear. For example, lead and 
tin have only limited solubility in one another. A maximum of about 20 per cent of tin will 
dissolve in lead forming a lead-rich phase. Similarly, lead will dissolve a maximum of about 3 
per cent tin forming a tin-rich phase. These are shown as the  and ß phases in the equilibrium 
diagram shown in Figure 2-16(a). If more than 20 per cent lead is added to tin, a mixture of the 
tin-rich phase and the lead-rich phase will form. In addition, as solute is added to solvent, the 
melting point of the alloy is decreased. An alloy of 62 per cent tin is the composition which 
shows a minimum freezing point – known as a eutectic point. Tin-lead solder, which requires 
a low melting point, has a composition close to the eutectic. If a molten alloy of this 
composition is allowed to cool, it will remain completely liquid until the temperature falls to 
183°C, when it will solidify forming alternating layers of the tin-rich and lead-rich phases 
Figure 2-16(b)) until solidification is complete. Eutectic structures, known as a lamellar 
structure, often have good mechanical properties. If one layer is hard and strong while the other 
is soft and ductile, a combination of strength and toughness can be achieved. 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 12 
 
 (a) (b) 
Figure 2-16: (a) The lead-tin phase diagram and (b) eutectic microstructure of  and . 
 
An alloy of, say 30 per cent tin (a hypoeutectic composition), on cooling from the melt will 
start to solidify when the temperature reaches the liquidus at approximately 250°C. Lead-rich 
alpha phase will precipitate from the liquid with composition given by the solidus line on 
cooling and amounts given by the lever law. When the temperature reaches the eutectic 
temperature, 183C, the remaining liquid changes to the eutectic mixture of  and . No 
further significant changes occur on cooling and the final microstructure consists of a mixture 
of the -phase and eutectic. An alloy containing more tin than the eutectic (a hypereutectic 
composition) will cool in much the same way but the final microstructure consists of a mixture 
of ß phase and the eutectic. 
 
Other examples of eutectic reactions, a liquid changing to two solid phases in a layer structure, 
are lead-antimony, copper-silver and aluminium-silicon. There are a number of other 
solidification mechanisms. The eutectoid reaction is closely related to the eutectic but forms 
from the breakdown of a solid (rather than liquid) solution to form two different solid phases in 
a layered structure. The phase diagram is interpreted in exactly the same way and the 
microstructures of alloys formed are similar. The iron-carbon system shows a eutectoid 
reaction and is critical in understanding the structure of steels. Other solidification 
mechanisms, such as the peritectic, are outside the scope of this discussion. 
Chemical Compounds 
Rather than phases of varying composition forming, one or several chemical compounds may 
form in many alloy systemswhere bonding attractions of the components are sufficiently 
strong. These are often of fixed chemical composition, such as cementite (Fe3C) in the iron-
carbon system. In fact, a number of carbides can form with various metals, such as chromium 
and titanium carbides in alloy steels and tungsten carbides in high speed cutting tools. They are 
single phase solids shown as a vertical line on the phase diagram, rather than an area with 
phases of varying composition. The compounds tend to break the equilibrium diagram up into 
recognisable sub areas. These are known as intermetallic compounds and are usually very 
hard and brittle and often in the form of small, isolated particles in the matrix. In such a form, 
they will provide excellent barriers to dislocation movement so will greatly strengthen a metal. 
This process is known as precipitation hardening and discussed in chapter 7. However, if 
present in larger amounts, the metal will be brittle and of little use for engineering purposes. 
 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 13 
OTHER IMPORTANT ALLOYS 
Steel and its alloys are by far the most important engineering material and discussed in detail in 
this publication. There are other important engineering metals and some of these and their 
properties of these are outlined below. 
 
Aluminium is alloyed with a range of additions and can be heat treated to improve its 
properties. Manganese and magnesium are added to form a solid solution with aluminium 
which has little effect on properties, but such alloys can be hardened by work hardening. 
Aluminium alloys with small amounts of zinc, lithium or copper can be heat treated to form an 
alloy whose strength is achieved by precipitation hardening. Aluminium alloys containing 
copper are strong but unfortunately very susceptible to corrosion. 
 
Copper is often alloyed with metals such as zinc, tin, nickel, aluminium and others to improve 
its strength and other properties. Many of these alloys are single phase and strength is 
improved due to solid solution hardening. Brasses (copper-zinc) are single -phase up to 37 
per cent zinc, single -phase above about 45 per cent zinc, and two phase (+) between these. 
The -brasses are tough and can be easily worked whereas the + brasses are stronger but 
lack ductility. Tin bronzes are single -phase up to about 7 per cent tin and contain two phases 
with more tin. The single phase alloys are more ductile and usually rolled or drawn, and the 
higher tin alloys are usually used in the cast condition. As mentioned earlier, alloys of copper 
and nickel are single phase in all proportions. 
 
Nickel has extensive solid solubility for many elements so a range of alloys are available. It 
has complete solubility with copper and almost complete solubility with iron. It will dissolve 
up to 35 per cent chromium and 20 per cent of molybdenum and tungsten. Alloys with 
approximately 33 per cent copper are known as Monels, and alloys with chromium and 
molybdenum are Inconels and Hastelloys respectively. These alloys are usually single phase 
alloys although elements such as aluminium or titanium form intermetallic phases and can 
provide precipitation hardening. Single phase materials are usually strong, tough, corrosion-
resistant and can be used from cryogenic to temperatures above 1000°C. They are relatively 
easy to fabricate. 
 
Titanium undergoes a phase change in the pure metal at 882°C. Below this temperature, pure 
titanium has a hexagonal close packed structure known as alpha (); above it, the structure is 
body centred cubic and termed beta (). Alloying elements influence the stability of either of 
the two phases by either raising or lowering the transformation temperature. Alpha titanium 
alloys have low to medium strength and are easily fabricated. Beta alloys can be heat treated 
similar to steels, and provide higher strength. Alpha-beta alloys have intermediate properties. 
INTRODUCTION TO THE STRUCTURE OF METALS 
2 - 14 
 
3-1 
Chapter 3 
 
TENSILE PROPERTIES OF METALS 
 
 
 
The strength of a material is probably its most important single property. Almost all machinery, 
structures, vessels, pipelines, etc are subject to stresses and strains and it is important to specify 
materials which will endure the stresses encountered in service (see the Appendix for an 
introduction to stresses and strains). Therefore materials must be tested so that their properties, 
especially strength, can be assessed and compared. Furthermore, mechanical tests are often 
carried out as an inspection procedure to ensure that material quality is maintained. In this 
chapter, tensile and related strength testing of materials, specifically metals, is described, 
including the methods used and information obtained from such tests. 
 
THE TENSILE TEST 
Of all the tests used to evaluate the mechanical properties of metals, the tensile test is probably 
the most useful. In this test, shown in Figure 3-1(a), a sample is pulled to failure in a relatively 
short time. The sample, Figure 3-1(b), with dimensions usually specified in the required 
standard, is elongated in uniaxial tension and the load necessary to produce a given elongation 
is measured as the dependent variable. Accurate measurement of the extension requires the use 
of an extensometer which is clipped to the specimen in the early stages of the test. AS 1391 
and ISO 6892 cover tensile testing. Mechanical testing, including definitions, hardness testing 
and impact testing, of steel products is covered by ASTM A370. 
 
 
 
 
 
 
(a) (b) 
Figure 3-1: Schematic drawing of (a) a tensile-testing apparatus, (b) test specimen. 
 
Knowing the dimensions of the sample means the load can be converted to stress and the 
elongation can be converted to strain so the resulting curve is then independent of geometry. 
 
Engineering or nominal stress, , is defined as the ratio of the load on the sample, F, to the 
original cross sectional area, Ao. 
 F 
  = –– 
 Ao 
 
TENSILE PROPERTIES OF METALS 
3-2 
Engineering or nominal strain, , is defined as the ratio of the change of length of the sample, 
L-Lo, to its original length, Lo. 
 L-Lo 
  = –––– 
 Lo 
 
Figure 3-2 shows a stress-strain curve for a hypothetical metal with the various quantities that 
can be obtained from such a curve marked on it. The various stages which occur during the test 
and the metallurgical factors which influence the properties are described in the following 
sections. 
 
Figure 3-2: Stress-strain behaviour of a metal. 
 
STRESSES IN THREE DIMENSIONS 
The above discussion considered only tensile, or uniaxial, stresses where the stress only acts 
along one of the principle axes. In practice, for an engineering body there will be other types of 
stresses, such as shear stress, and stresses in other directions. A material which is subject to 
stresses in three directions is said to be subjected to a triaxial stress state. For example, a 
block of material immersed in water is subjected to compressive stresses which are equal in all 
three directions and known as a hydrostatic stress. Dealing with such situations can involve 
very complex mathematics. Fortunately, in many situations, only two significant stresses 
operate and condition is easier to understand. 
 
If a thin sheet is loaded, the stresses are largely concentrated along the length and width, with 
virtually no tensile stress in the thickness. When one of the principal stresses is zero, the stress 
state is called plane stress. Conversely, if one dimension is very large compared to the other 
two, such as a long roller subjected to pressure across its diameter, the strain in the longest 
dimension can be considered zero. This condition is called plane strain. In simple terms, plane 
stress applies to thin specimens and plane strain appliesto thick specimens. 
 
For simplicity, and because uniaxial tensile stresses are by far the most important, these are the 
only ones considered. The principles described below apply to other types of stresses, and 
those in other directions. 
 
TENSILE PROPERTIES OF METALS 
3-3 
THE ELASTIC REGION 
At the beginning of the test, (region OA in Figure 3-2) the specimen extends elastically; if the 
load was released the sample would return to its original length. Elastic behaviour results when 
atoms or molecules are subjected to only small displacements, and readily return to their initial 
stable position after removal of the load. The relation between stress and strain in this region is 
linear and described by Hooke’s Law: 
  = E  
 
where E is a constant, Young's modulus. The Young’s modulus is a measure of the 
interatomic bonding forces and is a vital property as it affects the stiffness or rigidity of a 
material. Some typical values are as follows: 
 Steel 200,000 MPa 
 Aluminium and its alloys 70,000 MPa 
 Glass fibre reinforced plastics 10-50,000 MPa 
 Concrete 50,000 MPa 
 Rubbers 10-100 MPa 
 
Interestingly, the Young’s modulus is not greatly affected by alloying, heat treatment, or other 
processes which change a metals structure and have a significant effect on its other mechanical 
properties. While it relates to the rigidity of engineering designs so is of great importance to the 
engineer, the fact that it cannot be controlled to any degree by metallurgical factors means it 
will not be discussed further. 
 
THE YIELD STRENGTH 
At higher strains (above point A on Figure 3-2), permanent deformation occurs so that much of 
the strain is not recovered when applied stresses are removed; the material is now undergoing 
plastic extension. The point at which deformation is no longer elastic, but plastic, is that stress 
at which the slope of the stress-strain curve deviates from elastic behaviour. This stress is 
known as the elastic limit or, more commonly, the yield strength (y) and is a measure of the 
ability of the material to resist plastic deformation. Yield strength is probably the single most 
important mechanical property of an engineering material. Because of the difficulty in 
determining this point precisely, various approximations are used. The most common is the 
0.2% proof stress which is the stress at 0.2% plastic strain. In some materials (e.g. low carbon 
steels) there is a definite yield point (see the following section) and the yield strength is then 
clearly defined. Steels are often graded according to their yield strength. For example, the 
common constructional grade AS 3678 Grade 350 has a nominal yield strength of 350 MPa 
and the pipeline grade API 5L X65 has a nominal yield strength of 65,000 psi (448 MPa). The 
Specified Minimum Yield Strength (SMYS) is the value used for design purposes, while 
Actual Yield Strength (AYS) is the true, measured value and may be significantly greater 
than the SMYS. Some typical yield strengths of a range of different materials are: 
 Low density polyethylene 5–20 MPa 
 Lead and its alloys 10–55 MPa 
 Aluminium and its alloys 50–650 MPa 
 Copper and its alloys 60–1000 MPa 
 Glass Fibre Reinforced Plastics 100–300 MPa 
 Carbon & low alloy steels 250–600 MPa 
 Low alloy steels (quenched & tempered) 500–2000 MPa 
 Soda glass 3,600 MPa 
 Silicon carbide 10,000 MPa 
 
TENSILE PROPERTIES OF METALS 
3-4 
THE YIELD POINT 
In simple terms, the yield strength is the stress required to move dislocations through the 
material taking into account all obstacles to their motion. However, if the stress required to first 
operate the dislocation source is higher than the stress required to move them then we get a 
yield point. This operating stress can be that required to create dislocations. For example, in a 
pure copper whisker free from dislocations, the stress-strain curve (Figure 3-3) shows a high 
stress is required to nucleate the dislocations which will then multiply rapidly and the stress 
required to move them through the lattice is much lower. If sufficient dislocations already exist 
in a single crystal then they do not need creating and the yield drop is not observed. 
 
Figure 3-3: Stress-strain curve of a copper whisker 
 
A yield point is also observed when a higher stress is required to ‘unlock’ existing dislocations. 
The most common occurrence of a yield point is in low carbon steel (see Figure 3-4 curve 1). 
Here the carbon and nitrogen atoms in the iron solid solution pin the dislocations and the upper 
yield point is the stress required to break dislocations free of this pinning. The lower yield 
point is then the stress required to move the dislocations. Very pure iron (<0.001% carbon or 
nitrogen) shows no upper yield point, nor does medium to high carbon steel. The former is 
because the dislocations are free to move and the latter because the stress required to move the 
dislocations through the much stronger lattice is greater than that required to free the 
dislocations in the first place. 
 
Figure 3-4: Yielding of soft steel. 
 
The stress required to unpin the dislocations is dependent on different factors from that 
required to move them. Alignment of the specimen, strain rate, grain size and temperature all 
influence the upper yield point to a much greater degree than the lower yield point. The lower 
yield point then is much more consistent for a given material and is normally taken as the yield 
strength of a material with yield point phenomena. 
 
TENSILE PROPERTIES OF METALS 
3-5 
During the tensile test, such a metal starts yielding at one point in the specimen, usually the 
fillets as these are stress concentrators. This yielding takes the form of a band at 45° to the 
stress axis known as a Lüders band. This deformed region then spreads to an adjoining region 
because of local stress concentration and the Lüders bands advance into the unyielded region 
(see Figure 3-5). The lower yield stress remains roughly constant during yield elongation but 
may jump as new Lüders bands form. When the entire sample has yielded, the stress-strain 
curve begins to rise through work hardening. Lüders bands can be a problem as they disfigure 
stamped or drawn steel sheet. 
 
Figure 3-5: A simple Lüders band 
 
If a steel specimen under test is unloaded at a position after yield elongation and immediately 
retested, the yield point will not occur (see Figure 3-4 curve 2). This is because dislocations are 
free to move and do not have to be unpinned. If, however, the specimen is heated to a low 
temperature (approximately 100°C) for about half an hour, or allowed to sit at room 
temperature for some days, the interstitials will migrate back to the dislocations and the yield 
point will reappear (curve 3). This means a steel undergoing plastic deformation will show an 
increase in strength or hardness as a result of ageing. This increase in hardness is known as 
strain ageing. This also results in a loss of ductility and can cause fracture after a structure has 
been built. This problem, and the disfigurement of drawn sheet caused by Lüders bands, can be 
minimised by lowering the carbon and nitrogen in solution by adding carbide and nitride 
formers such as aluminium, vanadium, niobium, titanium, etc. The usual solution with sheet 
steel is to deform the steel immediately before use by giving it a light rolling (‘skin rolling’). 
As a result, the steel does not show a yield point and Lüders bands do not form when the steel 
is pressed. 
 
THE PLASTIC REGION 
Beyond the yield strength, the stress necessary for additional elongation increases with plastic 
deformation. That is, the stress-strain curve rises due to work hardening or strain hardening 
which is related to an increase in the dislocation density. As dislocations move through the 
material they will create many new dislocations which interact