CWB Level 2 Module 20 Structure and Properties of Metals PDF

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C ( C | | | ( - \" C ( C ( ' 8260 Parkhill Drive, Milton ON L9T 5V7 ( Tel: 1-800-844-6790 Fax: 905-542-1318 C www.cwbgroup.org C C. C p S (‘ © Copyright 2014 The CWB Group Industry Services Revised June 2015 | All rights reserved - (\j The works of authorship contained in this document, including but not limited to all design, text and images, are the property, ) except as otherwise expressly stated, of the CWB Group-Industry Services or one of its affiliates, the CWB (Canadian Welding. Bureau) and QUASAR. This document, whether in current form or electronic, may not be copied, reproduced, transmitted, C displayed, performed, distributed, rented, sublicensed, altered, stored for subsequent use, or otherwise used in whole or in (° part in any manner without the prior written consent of the CWB Group industry Services, except to the extent that such use {‘W constitutes “fair use” under the Copyright Act of 1976 as amended, and except for a temporary copy in a single computer’s \ memory and one unaltered permanent copy to be used by the viewer or student for personal and non-commercial use only... - ( p - { (+ Ve, = 1% X r \'L :\\: ‘\, L (S 7 ‘I Nome. (‘ 4. Crystals 8 ( 41 Crystal Structures 8 C 4.2 Allotropy 10 (. ' 4.3 Polycrystalline Materials ' ' N ( 5. Dislocations 11 ( 51 Slip 11 , 5.2 Dislocation Multiplication 13 (‘, 5.3 Dislocation Locking 14 ( 6. Solutions and Alloying 14 ( 6.1 Types of Solutions 14 (‘ 6.2 Phases 16 C 7. Strengthening Methods 23 ( 71 Work Hardening 23 V4 by 8. Solid Solution Strengthening 26 L 8.1 Dislocation Locking 28 ( 8.2 Strain Aging 28 (“ 8.3 Precipitation Hardening 30 - 8.4 Grain Size Strengthening 31 s C 9. Solidification 33 C 9.1 Constitutional Supercoolin P ¢ 34 - 9.2 Weld Metal Solidification 37 ( (O 10. Chemical Effects 40 10.1 Porosity 41 - 10.2 Fluxes 43 \ - - © Copyright 2014 CWB Group - Industry Services ' ( 11 Objectives 'S After successfully completing this module you will be able to: (,\,f » describe the crystal structures of metals (: ‘ « explain what dislocations are and how they affect plastic properties ' « list the types of solid solutions \ » use phase diagrams to determine phases and their compositions : » describe various strengthening mechanisms in alloys (.- « explain how a welding procedure affects the solidification structure (, « predict the general welding behaviour of a metal from its basic chemical and physical properties CJ ( 2. Metals [f The group of materials known as metals have certain properties that make them useful as engineering /\' ‘ materials and allow them to be joined by fusion welding processes. Compared with non-metals they have: (f » high electrical conductivity - » high thermal conductivity p C * high specific gravity. « are often ductile and can be formed (s » a metal’s physical properties can be greatly influenced by it's alloy content O ( 21 Conductivity Q Table 2.1 lists the electrical and thermal conductivity of some common metals. A few non-metals are ’ included for comparison. In the metals the electrical and thermal conductivity parallel each other, - which is to be expected since both involve electrons in the transport of energy. It is clear from the table - why copper and aluminum are used for electrical conductors in electrical distribution systems, and ) why gold and silver are used as electrical contacts in electronic equipment. The electrical conductivity ( allows many metals to be joined by electric welding methods such as arc welding or resistance - -. © Copyright 2014 CWB Group - Industry Services Page 1  Table 2.2: Relation between physical properties and resistance welding behaviour. Thermal conductivity Electrical resistivity Resitance welding behaviour Wi(m.°C) BTU/hr/sq ft/°FIft X10%Q.m Pure copper 394 230 1.72 Cannot be spot, seam or projection welded Can be spot, seam welded in thin sections. Aluminum 222 130 2.83 High welding currents required. Easily welded by all resistance welding Carbon steel 46 26.8 16.0 methods. , 2.2 Stability of Metal Oxides Unfortunately, most metals are chemically unstable as pure elements, and exist in their natural state as oxide or sulphide ores. They are, therefore, extracted from these ores by smelting or other means to separate the pure metal from the oxygen or sulphur. Iron, for example, exists in the natural state in a variety of forms: typical ores being magnetite (Fe,O,) and hematite (Fe,O,). Iron smelting in a blast furnace involves blowing hot air over coke to produce carbon monoxide, which removes oxygen from the ore, leaving the iron as a liquid metal. Aluminum also occurs naturally as an oxide Al,O, in ) the bauxite ore. This oxide, however, is so stable that heating methods cannot be used to extract the aluminum and electrical means are employed. 2.21 Free Energy of Formation of Oxides The natural instability of metals means that most of them will tend to revert to their natural oxide state when exposed to the air, a process known as oxidation and a cause of corrosion. : The stability of metals will be illustrated by plotting the free energy of formation of the oxide ‘ against temperature. A large negative free energy indicates a very stable oxide. This plot is known as the Ellingham diagram and a simplified version is shown in Figure 2.1. Page 2 © Copyright 2014 CWB Group - Industry Services _  , negative free energy indicates a very stable oxide. ( ( The diagram shows that gold is stable in the elemental form at room temperature and does not oxidize ‘ ( or corrode. Most other elements are only stable at very high temperatures and then only when a ( reducing agent is present to lower the oxygen potential. This illustrates why high temperatures are ( usually needed to smelt metals from their ores. ( 2.2.2 Rate of Oxidation C Because the rate of oxidation increases as the temperature rises, a metal will tend to form. an oxide more easily at high temperatures despite the stability of the oxide being lower as ( f predicted by the Ellingham diagram. Figure 2.2 illustrates the effect of increasing temperature ( on the oxidation of some steels. (\ 2.5 g ‘}- ;j‘ Q' : E Carbon steel arbon steels, Lol CRrdis” /y C £ 2 low alloy, @%}"}l 4 9Cr-1Mo C), o]5 2.25Cr- ® 1Mo yi s //[— o !f}g“% ( E 1.5 e i.&‘, y /4 - 2 V. ot 3 1 ‘ ,;i‘;i#-;fi:‘hf & @ £ el 18 - 8 stainless ( , C © B N g /4 R B y g : 0 Mfi’ 4/ ‘ = C. : ) 500 600 700 800 900 ( \ Temperature °C (L Figure 2.2: Although the stability of an oxide decreases as the temperature is raised, the rate of oxide _ formation increases. (J | © Copyright 2014 CWB Group - Industry Services Page 3 -  Copper 1083 1981 -35 Flux or inert gas. Aluminum 660 1220 -215 Inert gas is preferred. Iron (Steel) 1350 2462 -75 Flux, inert gas or CO, Titanium 1704 3099 - -125 A Inert gas with special precautions. 3. Atomic Structure Many of the properties of metals, as well as their behaviour during welding, can be appreciated through an ‘ understanding of their microscopic structure. ; Consider first the simple model of an atom illustrated in Figure 3.1, in which a number of electrons orbit a central nucleus. Although somewhat outdated, the orbital model of the atom is easy to visualize and is adequate for our present purpose to explain basic metal properties. In this model, electrons are normally confined to well defined shells around the nucleus with a set number of electrons in each shell. The outer shells are not normally filled with the maximum possible number of electrons. Most shells can accommodate up to eight electrons. If the outer shell has fewer than eight, the atoms may donate or share electrons with atoms of other elements causing the atoms to bond together as a compound molecule. The “excess” number or “shortage” of electrons in the outer shell is known as the valence, and determines many of the chemical properties of the element. ' A e mnt A e T i, I »H—— inwel deined shells, ' Figure 3.1: Simple orbital model of an atom. 3 Page 4 © Copyright 2014 CWB Group - Industry Services ( ‘ This atom has two This atom of another Donating two electrons from one atom to the ( electrons in the outer element has six electrons other completes the shells and binds the two ( shell. in the outer shell. atoms together forming a molecule. () Figure 3.2: lllustration of the origin of ionic bonds between atoms. % , 100 ( C - » 80 ( = K , S ~ N/ © et @ 40 Electrical resistance drops when C = slag becomes fluid and ions are ( = mobile C o 20 C. Q,' 1000 1100 1200 1300 1400 1500 - C Temperature °C it Figure 3.3: When mobile, ions become effective conductors of electricity. The graph shows the effect of ( temperature on the electrical resistance of a typical welding flux. o. - ———————————————— e e C © Copyright 2014 CWB Group - Industry Services Page 5  spending part of the time in each orbit. _ ' 3.3 Free Electrons ' ' ' In metals the outer electrons are neither confined to one atom nor shared with a neighbouring atom, but are free to move throughout the bulk of the material. These “free”, or conduction, electrons are responsible for the electrical and thermal conductivity of the metal. There are, therefore, two ways to conduct electricity: the movement of electrons and the movement of ions. 3.4 lonization Potential To remove an electron from an atom, creating an ion, requires a certain amount of energy known as the ionization potential. The amount of energy depends on the type of atom, and Table 3.1 lists the ionization potentials of several elements. Note that the inert gases used in welding have much higher ionization potentials than the metals. Some metals, notably potassium, ionize easily. Table 3.1: First ionization potential of some elements (volts). Aluminum 5.96 Argon 15.76 Copper 7.68 Hydrogen 13.6 Iron 7.83 Helium 24.59 Nickel 7.61 Nitrogen 14.53 Potassium 4.3 Oxygen 13.62 : Sodium 512 Tungsten 8.1 page 6 © Copyright 2014 CWB Group - Industry Services (» tungsten (EWTh-2) electrode operates at a much lower temperature than a pure tungsten electrode = and, consequently, has a longer life. : ' Table 3.2: Work functions of some materials (eV). ' ' ) Aluminum 4.0 Potassium | 2.2 C Copper 4.3 Sodium 23 - Iron 45 Tungsten 4.5 ( Nickel 5.0 Barium 21 ( Thoria 2.7 Calcium 2.2 C S ( 20 ' E ) S 16 § > 2% Thoriated Pure C 2 12 tungsten tungsten ( @ < £ p (V)]. = 8 ~ o C 5 C °o4 ( p - 1500 2000 2500 3000 3500 £ i Temperature °C N C Figure 3.5: Effect of temperature on thermionic emission from tungsten and 2% thoriated tungsten. - \ O P————E———————_——E—————— S ———————r e i C © Copyright 2014 CWB Group - Industry Services Page 7  Figure 4.1: Atomic arrangement in amorphous and crystalline solids. |. The crystalline structure of a metal may sometimes be seen when it is broken and the fracture occurs along specific planes of atoms through the crystal. The shiny appearance of the tiny crystal faces reveals the crystalline character of the metal (Figure 4.2). r?:\ ,_ ’[—‘#»"‘.a.'. : ‘ 1 S=x bty P U £ s W B B % Q G } A".‘ \ A!. AL "?‘:}s B LR e Figure 4.2: A sample of metal fractured at a low temperature reveals the crystalline character of the metal. 41 Crystal Structures Not all metals have the same regular pattern of atoms or crystalline structure, although most will have one of three basic types: * body-centred cubic + face-centred cubic ' * hexagonal close-packed These structures are illustrated in Figure 4.3, where the circles represent the location of individual atoms. The pattern of atoms in each structure cell would be repeated throughout the crystal. Page 8 © Copyright 2014 CWB Group - Industry Services (" Figure 4.3: Three basic crystal structures. C ( | 411 BCC Structure | ( The body-centred cubic (BCC) structure comprises an atom at each corner of a cube and one o in the centre. If the atoms are considered as hard spheres pressed together they would touch ('- along a diagonal. A small gap, however, would be left between atoms along the edges of the cube as is apparent from Figure 4.4. This is important when considering the idea of solubility, &, since these spaces provide room for other atoms. The presence of spaces also implies the (_-‘ structure is not close-packed, where each atom would be in contact with all of its neighbours. ¢. : v c A = ( mall spaces left TR - ’ between atoms e C I'e. p Figure 4.4: Body-centred cubic structure where atoms are assumed to be hard spheres in contact. S C ( 41.2 FCC Structure C In fact, the FCC structure simply comprises layers of close packed planes — the most of the three common crystallographic arrangements. The presence of these planes gives bt FCC metals special properties, among them being the ability to undergo extensive plastic g deformation. Most FCC metals are very ductile due to these close packed planes and their (~' ability to “slip” independently of one another. Slip will be discussed in Section 5. -. - L © Copyright 2014 CWB Group - Industry Services Page 9  The hexagonal close-packed structure also consists of layers of close-packed planes but the ‘ stacking sequence is different. The close-packed planes are the base planes of the structure cell and these can give directional properties for plastic deformation with these metals. 4.2 Allotropy Some metals, notably iron, can exist in different crystal forms depending on the temperature. These are termed allotropic forms. Up to 910°C iron has a BCC structure, but from 910°C to 1390°C it assumes a FCC structure. From 1390°C up to the melting point iron reverts back to the BCC structure. Since the BCC form is not close-packed while the FCC structure is close-packed and therefore more dense, iron will actually contract when heated above 910°C and transformation of the structure takes place (Figure 4.6). liquid Expansion BLG s FCC _ BCC o 910 1390 1534 Temperature °C Figure 4.6: When iron is heated above 910°C it transforms to the denser FCC structure, causing a small ' contraction in the metal. page 10 © Copyright 2014 CWB Group - Industry Services ( (»‘ Grain boundary between two grains having different Polygonal grain structure in which ~ crystallographic orientations showing the region of each grain has a regular pattern of C , disorder. , atoms but with different orientations , ( Figure 4.7: Metals are usually composed of many grains separated by a region of atomic disorder. C c ( 5. Dislocations ( A crystal is described as a solid with a regularly repeating pattern of atoms, but this structure is rarely perfect. An atom may be missing here or a group of atoms displaced there. The crystal may be full of i p imperfections of one kind or another. A detailed discussion of the dislocation is important because it t'l provides an explanation for many of the plastic properties of metals. /(;7. (- 51 Slip C, Let us start by observing what happens when a metal bar is loaded in tension as in Figure 5.1. Initially ( it will deform elastically but, at a sufficiently high stress it will yield then continue to deform plastically. g Some metals may be ductile enough to stretch several times their original length. Examination of >" a deformed bar will reveal many bands on the surface, providing evidence that deformation has -~ occurred by a process of slip. Slip is illustrated in Figure 5.2, where extension of the bar takes place - by sections of the metal slipping relative to adjacent sections along well defined slip planes. These slip C planes correspond to specific crystallographic planes in the individual grains or crystals of the metal. e - U ’\, - B - e e e e e l\/ © Copyright 2014 CWB Group - Industry Services Page 11  deformation. Figure 5.2: Plastic deformation occurs by slip on preferred crystallographic planes. 5.1.1 Dislocations Facilitate Slip ‘ Although plastic deformation can be explained by slip, the stress at which slip occurs is ’ much lower than the theoretical stress to slide one layer of atoms across another. The theoretical strength of pure iron is about 6,000 MPa, but in practice it yields at only 10 MPa. ' This is because real crystals contain many imperfections—dislocations—that facilitate slip. Dislocations are places where the regular crystal pattern of atoms breaks down and there is a local distortion as the surrounding atoms try to fit the proper pattern. Figure 5.3 illustrates one type of dislocation, a simple edge dislocation, represented by an extra plane of atoms partially inserted into the structure. The dislocation can be represented by a line running through the crystal marking the bottom edge of the extra half plane of atoms. Page 12 © Copyright 2014 CWB Group - Industry Services - \NNEIS L NG 7/ N ANS 1 b= TR [ L- L P(- L LI IhBEas) e ; jamess) jaEes29 REESY) (‘ ) In the unstressed crystal the When subject to a shear stress The dislocation has now moved o dislocation is location at position 3. the original bond breaks and a to position 4. L The arrow shows the direction of new bond is formed with the » bonding between two atoms next lowest atom of the dislocation \ to the dislocation. half plane. " N ( Figure 5.3: Edge dislocation comprised of an extra half plane of atoms. p A C A stress on the crystal causes the dislocation to move and the passage of one dislocation C across the crystal will contribute a slip of one atomic spacing. To produce macroscopic slip a ( very large number of dislocations is required. Typically a metal has 108 to 10° dislocation lines ( per square centimetre. ' - 5.2 Dislocation Multiplication C It does not take a large stress to move a dislocation, which accounts for the relatively low yield stress C of most metals. Once in motion, dislocations can multiply themselves rapidly, so after reaching the C yield point and the initial movement of a few dislocations, there is a rapid rise in their number and , a corresponding increase in plastic strain. As the number of dislocations increases, however, their density can become so high that they start impeding each other, thus requiring even higher stresses C to move them or create new ones. As plastic deformation proceeds the stress increases and the metal (_, becomes work hardened. There is a limit to this process and finally the metal reaches its ultimate C tensile strength, where plastic deformation continues without increasing stress (this only applies to ( engineering stress/strain where the stress is based on the original cross-sectional area). Figure C 5.4 shows a typical engineering stress-strain curve with each part explained in terms of dislocation ] behaviour. C C - C © Copyright 2014 CWB Group - Industry Services Page 13  Figure 5.4: Typical load/extension curve. The load and extension are proportional to the enginegring : stress and strain respectively. 5.3 Dislocation Locking The presence of foreign atoms at the dislocations may “lock” them, requiring higher stresses before they break free and become mobile. Carbon and nitrogen in iron do this and lead to the well-known yield point phenomenon. There are many other ways in which dislocations can be impeded, and each is used as a basis of strengthening metals. 6. Solutions and Alloying : Most metals are not used in their pure form. It is one of the fortunate characteristics of metals that their properties can be changed by alloying them with other elements; it is one of the responsibilities of metallurgists to understand how this happens. Any discussion of alloying starts with an explanation of solid solutions. 6.1 Types of Solutions ' Solid solutions are homogenous mixtures of two or more different atoms in the solid state. The components of a solution do not naturally separate out, and a solution is considered a single phase. : There are two types of solid solution: ' * substitutional * interstitial Page 14 © Copyright 2014 CWB Group - Industry Services P O 00 O 0 the atoms in the regular atomic array are \ @ 00 00 00 @) Q@ O replaced by atoms of another element. (¢ Q000 @9 9000 000000000009 4 C - 330080000000009 | ) Q9000000 ( ( Figure 6.1: Substitutional solid solution. C B 6.1.2 Interstitial Solutions p- Interstitial solutions are formed when the solute atoms fit into the spaces or interstices of - the hose metal as Figure 6.2 illustrates. Naturally this only occurs when the solute atom is C very small, for example, hydrogen, carbon or nitrogen. These three elements are particularly C important in welding because they are soluble in many metals and can be the cause of - cracking, embrittlement and other problems. ( - C QP99 C Q @ Interstitial solid solution where small P solute atoms of one element locate in N OQ O Q Q Q the spaces between the atoms of the C © host element. Only very small atoms, ) @ @ * D Q C, H, B, and N can form interstitial | - solutions. - 3 Q999009 - CC.. Figure 6.2: Interstitial solid solution. , - - e C © Copyright 2014 CWB Group - Industry Services Page 15  Site for carbon atoms when dissolved In the BCC Structure of iron the space A in body—centergd cubic iron _ occupied by carbon atoms is very small _ Figure 6.3: The site occupied by carbon dissolved in BCC iron. Table 6.1: Size of interstitial sites compared with solubility of carbon in iron. : |o eeec [ Fc Radius of space between 0.36 0.52 Fe atoms (A) Radius of carbon atom (A) 07 0.7 Maximum solubility of 0.02 1.7 : carbon (wt%) 6.2 Phases Material that is homogeneous on a macroscopic scale is termed a phase. For example, ice, liquid. water and steam are all different phases of the same material, each one being homogeneous. Solid solutions of metals are single phases since they are homogeneous even though they contain two ' or more different elements. If two metals exhibit complete solid solubility over the entire range of composition then only a single phase exists in the solid regardless of composition. If, however, an alloy of two metals does not exhibit complete solid solubility then there will be a range of composition in “ which two phases will exist in the solid. The two separate phases may be visible under a microscope and may be present in a variety of morphologies (shapes). _ 6.2.1 Phase Diagrams It is important to know which phases are present at various temperatures for the complete range of composition of an alloy and this information can be conveniently represented in the page 16 © Copyright 2014 CWB Group - Industry Services : ( © C g ) g ooe Solidus line b : - B Solid : : (; 5 [ ( 200 ( ( 0 40 60 80 100 ( Ni Cu '/‘ Weight per cent copper :’ Figure 6.4: Equilibrium phase diagram for binary alloys of copper and nickel. N ; 6.2.1.1 No Solubility in the Solid ,\ In some cases the two metals have no solubility for each other in the solid :‘ state, such as lead and antimony. The phase diagram for lead/antimony - is illustrated in Figure 6.5. At low temperatures two phases, the two C separate pure metals, exist together with the proportion of each depending E on the average composition of the alloy. The solid is simply a mixture of C the two metals, which would be seen separately under a microscope. At C high temperatures the singe phase, liquid, will be present. At intermediate C temperatures two phase regions are evident. At the antimony-rich end of C the composition range this two phase region is a mixture of liquid and pure p antimony. At the other end it is liquid and pure lead. In between these two ,L regions the freezing point of the liquid is lowered to a point below that of either - pure antimony or pure lead. The composition of the alloy with the lowest. melting point (about 12% Sb) is called a eutectic. - - k‘ © Copyright 2014 CWB Group - Industry Services Page 17  6.2.1.2 Limited Solubility The more general form of a eutectic phase diagram shows some limited : - solubility of each metal in each other. Additional single phase regions, : - therefore, appear at each end of the diagram for the two regions of solid solubility as shown in Figure 6.6. It is common practice among metallurgists to designate the various phases in alloy systems by Greek letters a, B, Y , etc. 1100 , 1000 o i Liquid - 800 © i g 800 ?, T Eutectic temperature 779°C & 700 % Eutectic. = composition 600 o+ 28.1% Cu B + Eutectic l Eutectic ‘ 500 0 10 20 30 40 50 60 70 80 90 100 - Ag Weight per cent of copper Cu. Figure 6.6: Phase diagram for the copper-silver system showing limited solid solubility and a eutectic. ‘ page 18 © Copyright 2014 CWB Group - Industry Services ( 1300 ( ‘ _ » | 1250 A ( a b ' 1200 | Solid 57% Cu |Liquid 75% Cu 0£ 1150 © (: , b g % Amount of solid = 77 ) Amount of liquid = fi) 1100 \ 1050 C 40 50 60 70 80 90 100 C Weight per cent of copper C Figure 6.7: Copper-rich end of the copper-nickel phase diagram illustrating the lever rule. C (. 6.2.1.4 Use of Phase Diagrams C To illustrate how to use a phase diagram, consider the eutectic system C copper-silver. The silver-rich end of the diagram shown in Figure 6.6 is C repeated in detail in Figure 6.8. Suppose an alloy of silver and 15% copper - cools from the liquid state. At about 855°C the first solid appears (alpha 'L.. phase). According to the diagram, it has a composition of 5% copper even » though the liquid from which it was formed has a composition of 15% C copper. On further cooling more solid is formed but it has an increasing C content of copper. The remaining liquid in equilibrium with the solid will also - k © Copyright 2014 CWB Group - Industry Services Page 19  2 ' Eutectic o add : First solid formed 600 600 T V has 5% Cu 500 500 500 : _ ' 10 20 ‘ 10 20 ' 10 20 28% Weight per cent of copper Figure 6.8: Silver-rich end of the copper-silver phase diagram illustrating how the diagram can be used to determine the composition of phase present at any temperature. 6.2.2 Structure of Eutectics B Eutectic mixtures can be very fine and a microscope is needed to resolve the separate phases. Notice that the alpha phase is present in two forms: the original phase solidifying from the liquid (known as primary alpha) and as a component of the eutectic mixture. The &8 photomicrograph in Figure 6.9 illustrates the appearance of a eutectic. L — R N X Y R R S R R ppasns v el oo =] e= =~ = = = o TR g e—— e = ] S T T R N A S S A= KRy e e e e S I S e, B et = T 2o vs e g oo e — T A e e L B W e ST TR R N A T m. ’ M'- w e ———————— e e ' Figure 6.9: Appearance of a typical eutectic. Page 20 © Copyright 2014 CWB Group - Industry Services ( phase) is present in two forms: as primary ferrite and as a component of the ( eutectoid structure. ( | ‘ | , ( 910°C Austenite (solid) Austenite (solid) ( * l Austenite (solid) : ( 723°C RN ; : \ E:L i, @ ~ - Ferrite @ As austtianite of T ( 0.5% C cools to (2) Remaining p 800°C new phase austenite gets N ferrite is formed richer in @ Eutectoid reaction. Austentite P i carbon of 8% C transforms to fine p : mixture of ferrite and cementite / b 0.5 0.8 ,\ wt % carbon wt % carbon e 0.8 C wt % carbon /\- Figure 6.10: Portion of the iron—carbon phase diagram illustrating the formation of the eutectoid pearlite. ( (e :. : 6.2.2.2 Eutectics at Grain Boundaries ( The depression of melting points that are observed in eutectic diagrams may C occur in local regions of a weld, for example at a grain boundary. Although - the average concentration of a certain element may be very small, a local - concentration at a grain boundary could be high enough to significantly lower _ the melting temperature in that region. In welding this could lead to cracking C since the grain boundary may still be liquid when the rest of the metal is solid, C causing the grains to tear apart. (\. e —EEEEEEEE———————————————————————— et e C © Copyright 2014 CWB Group - Industry Services Page 21  structural steels because the sulphur levels are so low—commonly less than 0.01% today—but problems may be encountered if an older structure made of : a high-sulphur steel needs to be welded. : 1600 1537°C & o § 1200 ; \~\\\\ | 3 F 2 P o = 1000 988°C i \ - Fe S e 10 20 30 40 50 _ Weight percent of sulphur , Figure 6.11: Phase diagram for iron and sulphur showing a eutectic of low melting point.. 6.2.3 Non-equilibrium Conditions Remember that phase diagrams depict equilibrium conditions and are only applicable to very. slow cooling rates. If the alloy is cooled more rapidly, the diagram becomes distorted and the 3 transformation temperatures usually decrease. The eutectic and eutectoid points may shift to different compositions under conditions of non-equilibrium cooling. A few of these effects are illustrated in Figure 6.12. Page 22 © Copyright 2014 CWB Group - Industry Services ( eutectoid compositions shift to lower values. ( ( 7. Strengthening Methods ' ' ) Metallurgists have at their disposal a variety of means for increasing the strength of metals. The methods \f they select depend on the intended application for the material and the exact balance of properties being ( sought. Modern alloys employ many different strengthening methods capable of achieving a wide range ( of properties, but when an alloy is subjected to the heating and cooling that occurs during welding, these ( properties will usually change. To understand the changes and devise appropriate welding procedures to ( minimize any detrimental effects, the welding engineer must know what strengthening methods were used ( in the alloy and how the properties will be affected by heat. ( Since plastic flow is the result of dislocation motion, the yield stress of a metal—that is, the stress at ( which dislocation motion becomes appreciable—can be raised by somehow impeding the dislocations. ( Strengthening methods, therefore, depend on restricting the motion of dislocations. /\” 71 Work Hardening - The simplest way of impeding dislocations is by increasing their density so that they interfere with - each other. This is achieved by plastically deforming the metal and results in work hardening. Let us. consider work hardening in terms of the shape of the material’s stress-strain curve. The stress-strain ( relationship for most metals follows a simple law: ( a.=Keg" ' ( where ( o, = true stress (based on instantaneous cross-sectional area) oud £ = true strain K = a constant - n = the work hardening coefficient C The true strain, ¢, is related to the engineering strain, e, by the equation:. €=1In(1 +¢) - K, © Copyright 2014 CWB Group - Industry Services Page 23 _ High n. @9(, 3. Highg n 4 " 17 2 g £» B [0) Low n £ 2 is) c Low n = w True strain Engineering strain Figure 7.1: Effect of the value of the work hardening coefficient on the shape of the stress-strain curve. 71.2 Practical Work Hardening In practice, work hardening must be accomplished at a temperature below that at which the process known as recovery takes place. Recovery causes a subsequent reduction in dislocation density and removes any work hardening. Cold rolling causes work hardening but \ hot rolling will not. Table 7.2 shows the strength levels for a variety of commercial alloys that have been work hardened. page 24 © Copyright 2014 CWB Group - Industry Services ( The amount of work hardening is designated by the temper, which ( corresponds roughly to the percentage reduction in thickness relative to the ( final thickness during cold rolling. For example, aluminum sheet for beverage (* cans is supplied in the full hard condition. ' ( Substantial increases in strength are achieved by work hardening during wire ( drawing, the extent of reduction being termed the “plough”. Table 7.2 shows ( that increases in strength of over 300% may be realized, and dislocation (‘ densities can reach over 10®m2. Further advantages of cold forming are a ‘ good finish and close dimensional tolerances. [4 o ( A disadvantage of this method of strengthening is that strength can be lost o on heating, which may severely limit the weldability of the alloys. As the cold \' worked metal is heated a temperature is reached where the dislocations ¢ N rearrange themselves into lower energy configurations called cells. Some of ( the material properties change (e.g., electrical resistivity), and the strength - may drop in some metals (e.g., aluminum, copper and iron). This process is C called recovery. C 71.3 Recovery and Recrystallization - At still higher temperatures new grain boundaries form and move, creating new grains. This C is recrystallization. The new grains have a low dislocation density, and the metal will exhibit a C substantial loss of strength and hardness. The changes during recovery and recrystallization - are shown in Figure 7.2. It should be noted that recrystallization as referred to here does not _ involve any ¥ phase changes. The new grains formed are the same phase as the original grains. L If, of course, the temperature is raised enough to cause a phase change, for example steel ( heated above 723°C, then recrystallization takes place because a new phase is formed. But ( steel will recrystallize below 723°C without any phase change if it has been cold work hardened ),S—-— C before (although no phase change occurs) C © Copyright 2014 CWB Group - Industry Services Page 25  Heating to cause recovery and recrystallization after cold work may be employed in practice to: ‘ ' * restore ductility ' ' ' + soften the material, allowing further working or forming * control the grain size * improve specific properties such as corrosion resistance Brass, for example, may be subject to a stress relief heat treatment at 230°C-260°C to improve the corrosion resistance. A high-carbon steel wire may be subject to a short heat treatment (typically 15 seconds at 375°C) to provide enough ductility for bending. A final example would be a thick-walled pressure vessel, cold formed in stages with a stress relief, typically at 625°C, between stages. : 8. Solid Solution Strengthening A second method of strengthening is solid solution hardening. Both substitutional and interstitial solutions can increase the strength, with the latter providing very large increases for very small concentrations of the alloying element. Figure 8.1 shows the increase in yield strength of a brass, which is a substitutional solid solution, with increasing zinc content. The rate is quite gradual and this method is less effective than cold = work in this alloy. In comparison, interstitial elements such as carbon and nitrogen in iron have very large effects on the yield strength.. Page 26 © Copyright 2014 CWB Group - Industry Services 3 ( the increase is much less than that achieved by work hardening. ( C £ Molybdenum ()] C g’ 0 Nickel C S L Chromium -100 \. 0.5 1.0 1.5 2.0 2.5 3.0 Yo Weight percent of alloy element - - Figure 8.2: Effect of various alloy elements on the strength of ferrite. C - C © Copyright 2014 CWB Group - Industry Services Page 27  1 2 3 Percentage elongation Figure 8.3: Discontinuous yielding behaviour resulting from dislocation locking by carbon and nitrogen. iniron. 8.2 Strain Aging If a specimen is unloaded after attaining the yield point and reloaded immediately it would fail to show a yield point because dislocations are not now locked. If, however, the unloaded specimen is allowed to sit for a period of time before reloading then the yield point will be observed (Figure 8.4). The yield ; point returns because there is time for the nitrogen atoms to diffuse back to the dislocations and lock them once again. This process is called strain aging, and it occurs more rapidly at moderately elevated temperatures. : Stress Stress i Specimen unloaded then tSh%?‘c:r;ggduen(:oaded reloaded after a delay of immediately several months at room temperature “ Strain Strain Figure 8.4: Strain aging behaviour in steel and the effect on the stress-strain curve. ; Page 28 © Copyright 2014 CWB Group - Industry Services ( ( Figure 8.5: Stretcher marks on surface of deformed sheet steel resulting from discontinuous yielding. (. \ 8.2.2 Blue Brittleness - ( It is possible for strain aging to occur while the steel is actually being deformed if deformation ( takes place within a certain temperature range, a process known as dynamic strain aging. ( Carbon and nitrogen atoms diffuse sufficiently fast at moderately elevated temperatures to (. catch up with moving dislocations and immobilize them temporarily. The result is an increase in ( strength with a stress-strain curve showing a series of yield points. The increase in strength of ( the 1020 carbon steel in the range 150°C—-400°C (Figure 8.6) is due to dynamic strain aging. (V Accompanying the strength increase is a reduction in ductility known as “blue brittleness”. The ( temperature range of “blue brittle” behaviour is normally avoided when forming steels. C 600 ( p s 500 C s ( £ 400 % C S 300 C 2 = o 200 1020 steel e & Typical results for a ( 'g 100 strain rate of 10%/s G C 200 400 600 ( Temperature °C. - Figure 8.6: Increase in strength of steel at moderately elevated temperatures resulting from dynamic strain C aging. The temperature range is affected by the strain rate. _ © Copyright 2014 CWB Group - Industry Services Page 29  600 500 C ' o o ' @ Heat alloy to 540°C to dissolve S2 400 all I copper. A § @ Quench to produce a super- 2 300 saturated solution. £ a + CuAly = @ Heat to aging temperature. Aging 200 allows precipitates to form which will tend towards the equilibrium @ phase (CuAl,). 100 T © Al 2 4 6 8 Weight percent of Copper Figure 8.7: Part of the aluminum-copper phase diagram showing basic steps in precipitation hardening. 8.31 Aging P Aging progresses through various stages as the second phase is precipitated, accompanied | by a change in the hardness of the alloy (Figure 8.8). There is an optimum point at which the ) maximum hardening effect is achieved. Page 30 © Copyright 2014 CWB Group - Industry Services ' C ( Over-aging occurs if the aging temperature is too high or the time too long. This results in a n - reduction in strength (coarse particles a large distance apart). Maximum strength is achieved ( when the precipitates are extremely fine and closely spaced. This is accomplished by a lower ( aging temperature (limiting diffusion distances) and a long holding time (to allow sufficient ( diffusion to occur), as illustrated in Figure 8.8. (7 Second phases may provide increases in strength even though they are not precipitated in the \ foregoing manner. Carbon steels, for example, gain their strength from the presence of iron- ( carbide. This may exist in a variety of forms, such as plates, layers, small spheres or rods. C Each form provides a specific combination of properties. ( - : (‘ 8.4 Grain Size Strengthening ‘ One of the most useful methods of strengthening and one used increasingly in modern steels is grain C refinement. Grain boundaries offer an obvious obstacle to dislocation motion and slip because there C is a change in the orientation of the crystal pattern across the boundary. Increasing the number of s\’» boundaries by reducing the size of the grains has a corresponding effect on strength. Experiments C show that the yield strength is inversely proportional to the square root of the grain diameter as C illustrated in Figure 8.9. The lower yield stress, o, is given by the following equation known as the Hall-Petch relationship: (_, o = o, + ka-172 (LV where. g, = constant which relates to the friction stress to cause slip in a grain (L k = a constant related to the stress required to initiate slip in an adjacent grain o d = the average grain diameter. ( - L B ————— C © Copyright 2014 CWB Group - Industry Services Page 31  Figure 8.9: Effect of grain size on the yield strength of aluminum and an aluminum alloy. Note that a Co small grain size leads to a higher strength. ' The relationship is valid for FCC and HCP metals as well as BCC metals. Figure 8.10 indicates the validity of the equation for several types of steel. 350 : steel manganese /)On - N 250 Annealed mild steel a. f 200 2) = 150. - Annealed Swedish iron £ 100 | 50 0 2 4 6 8 10. d—1l2 (mm-1/2) Figure 8.10: Effect of grain size on yield strength for iron and steel showing the validity of the Hall-Petch ' relationship. 8.4.1 Advantage of Fine Grain Size | The reason grain refinement is so valuable is that of all the strengthening methods available it is the only one that simultaneously improves strength and toughness. All other methods of increasing strength cause a reduction in the toughness of the metal. This has made grain size Page 32 © Copyright 2014 CWB Group - Industry Services ( Dislocations ( 20 ' ‘ o ‘ Grain refinement ' ( Precipitation ( 10 ( ( #30 +20 +10 0 -10 20 -30 ( ( Change in impact transition temperature °C (‘ Figure 8.11: Vectors showing the effect of various strengthening methods on strength and toughness. C C 9. Solidification ( A pure metal will solidify at a specific temperature whereas alloys, in general, solidify over a range. This is ( represented in the phase diagrams where the solidification range of the alloy is the two-phase region with C solid and liquid co-existing as indicated in Figure 9.1. As a pure metal cools from the liquid, nuclei form C upon reaching the solidification temperature. Atoms from the liquid will continue to attach themselves to o these nuclei, creating crystals of the solid metal. Each crystal grows until it impinges on a neighbour, and k,‘ solidification is complete. Growth of the crystal may y take place on a stable plane interface between the X solid and liquid or in the form of spikes as illustrated in Figure 9.2. Growth of spikes can result in a tree-like C structure and is termed dendritic growth. C - - [\.. : e L © Copyright 2014 CWB Group - Industry Services Page 33  p : ' Planar growth Jr— , / Dendritic growth - A N R e _ " @. Liquid : @ Y. Liquid Growth direction il ; Growth direction o Figure 9.2: Two types of growth patterns: planar and dendritic growth. ( 9.1 Constitutional Supercooling - In an alloy dendritic freezing is common, and the following example will help to explain how it , occurs. Consider part of a phase diagram as shown in Figure 9.3. Under steady state conditions the. advancing solid interface will have the same composition as the average of the original alloy, but just | ahead of the interface the liquid must be enriched in alloy element as shown in the phase diagram. There is, therefore, a composition gradient ahead of the interface, and when this is plotted the : _ effective freezing point of the liquid in that region may be determined by comparison with the phase (. diagram. Figure 9.4 illustrates this. If the effective freezing temperature at each location is compared ( with the actual temperature gradient in the liquid shown in Figure 9.5, the temperature in the alloy ‘ just ahead of the interface could be lower than the freezing point of the alloy. This effect is known as. constitutional supercooling. The result is that the interface will form spikes as the solid tries to grow ) into the supercooled region and a dendritic structure ensues. = Page 34 © Copyright 2014 CWB Group - Industry Services i ( Solid Liquid C , Composition of liquid ( _ Average composition A ‘ ( ( Liquidus temperature ( Freezing temperature of liquid ( Solidus temperature C ; Interface C ( Figure 9.4: The freezing temperature of liquid just ahead of the solid interface depends on its ( composition. ( ( Actual temperature (s C This liquid is below its ( freezing point Liquid ( ) — C Solid C C ( Interface \x«‘ ( Figure 9.5: Liquid ahead of the solid interface may be below its freezing point. This effect contributes to - dendritic growth. - EEEEEEEE———————— L © Copyright 2014 CWB Group - Industry Services Page 35  Co /k Cells and Co average composition , , dendrites _ , k distribution coefficient, the ratio of solid to liquid Cells composition / G temperature gradient Platelets R growth rate G /R 12 Figure 9.6: Diagram summarizing the effect of solidification parameters on solidification structure. 9.1.2 Hot Cracking When there is a high degree of constitutional supercooling the enriched liquid can get trapped between the arms of dendrites and result in local regions of high alloy concentration. Other modes of solidification may also result in local regions of the solid having higher alloy concentration. Segregation of this type is particularly important in welding since it can lead to a number of practical problems such as hot cracking. The shape of the phase diagram can often « give a clue as to whether a segregation problem can be anticipated. As an example consider the phase diagram shown in Figure 9.7 for an aluminum-silicon alloy. With increasing silicon content the solidification range begins to widen to some maximum value then decreases. Under conditions of continuous cooling this occurs at lower alloy ' compositions. Thus a certain range of silicon could give rise to segregation and cracking problems. Indeed this is true; when this alloy is welded it is susceptible to the phenomenon of hot shortness. Results of cracking tests shown in Figure 9.8 reveal a critical range of silicon Page 36 © Copyright 2014 CWB Group - Industry Services ( ( % Composition ( Figure 9.7: The shape of the phase diagram determines the composition at which the solidification range % © is a maximum. : : i ' C ( ( Zz E ( 3 C 2 3 : ’ © C 5 C ( % Silicon C Figure 9.8: Effect of silicon content on the hot cracking susceptibility of an aluminum-silicon alloy. r s A The critical ranges g may be dependent on the presence of other alloy elements and some (- combinations are particularly undesirable. A high magnesium-aluminum g alloy Y filler should not C be used to weld a high silicon—aluminum alloy because the combination of magnesium and ( silicon produces a high susceptibility to hot shortness. C 9.2 Weld Metal Solidification - These general principles explain many of the phenomena observed in the solidification of weld metal. - As the weld progresses, metal ahead of the pool is constantly being melted, new liquid metal is added L, from the electrode, and the liquid pool solidifies behind (Figure 9.9). This dynamic process reaches a C steady state where the pool remains a constant size and is stationary relative to the moving arc. - P S ————————S C © Copyright 2014 CWB Group - Industry Services Page 37  9.2.3 Weld Bead Shape ( The welding process also has a major influence on the shape of bead. The effect of the weld ( ' pool shape on risk of cracking suggests a simple criterion for avoiding cracks. A depth-to-width ( ratio not exceeding one is often suggested as a guide for crack-free welds. This may be an “ oversimplification, since other factors such as the ratio of heat input to the base plate to that | of the electrode are also important. It is possible to reduce the risk of cracking in submerged arc welding by using electrode negative polarity instead of positive polarity. This increases the | amount of added filler metal and reduces the risk of cracking even when there is no change in ‘ the depth-to-width ratio of the weld. : 10. Chemical Effects - The natural instability of most metals in their elemental form causes them to react with oxygen in the air ‘ when heated during welding. Therefore most welding operations require some form of shielding, usually a gas or flux, to exclude the air from the molten weld metal. Oxygen in the air may also react with alloy | elements in the material when it melts, producing oxides which could contaminate the weld. Chromium in stainless steels, for example, is highly reactive, and additional shielding over and above that normally \ required for carbon steels must be employed when welding stainless steels. : In addition to reacting with the metal the components of the air, oxygen and nitrogen, may dissolve in the : molten weld pool. Another gas, hydrogen, although not a normal component of the air, may be present in : the atmosphere around the arc as shown in Figure 10.1. because the very high temperatures can break ‘ down moisture, grease and other hydrogen containing materials. Hydrogen readily dissolves in the liquid ( weld metal. L Page 40 © Copyright 2014 CWB Group - Industry Services. (’ =) () 10.1 Porosity ( Dissolved gases in the liquid may not be very soluble in the solid and must therefore escape as the ' liquid solidifies. Figure 10.2 illustrates how the rejection of gas at the solidifying interface may lead to ¢ porosity in the weld. The tendency for porosity would be expected to depend on the relative solubility ( of the gas in the liquid at its maximum compared to that at the freezing point. Some examples will ( illustrate this. Figure 10.3 shows the solubility of hydrogen in several liquid metals. Aluminum, for U example, has a high maximum solubility for hydrogen in the liquid and a low solubility at the freezing ( point, the solubility being strongly temperature dependent. If there is appreciable hydrogen dissolved C in the liquid weld metal it will tend to escape as the metal freezes and porosity is likely to occur. ( C Escaping gas forms ( bubbles C C L Continuous bubble Bubble gets trapped ( trapped forming worm forming round pore - hole porosity C - - C Figure 10.2: Formation of porosity in weld metal from escaping gases. e { (\__ - — e C © Copyright 2014 CWB Group - Industry Services Page 41  1000 2000 3000 : Temperature °C. : Figure 10.3: Solubility of hydrogen in three liquid metals. ] :. 10.1.1 Relative Solubility A similar behaviour may be expected with nitrogen in nickel. Even small traces of nitrogen ' in the weld pool can cause porosity when nickel is welded. However, neither hydrogen nor \ nitrogen is likely to cause porosity in steel when welded under normal conditions, and the , relative solubility shown in Figure 10.3 supports this. A summary of the effects of gases on ' porosity is given in Table 10.1. To successfully weld those metals very prone to porosity, a - number of precautions must be taken. Table 10.1: Summary of effects of gases in weld metal. : Aluminum Hydrogen Moisture. Avoid moisture, grease and other contaminants. - Nickel Nitrogen Air. Requires good shielding and addition of titanium from electrode