Physical Metallurgy English M1

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Physical Metallurgy

Course Introduction

Module 1: Introduction to Steel Structures

• Introduction

• Crystalline Structures

• Phase Diagrams

• Iron Carbon Diagram

Module 2: Phase Transformations

• Introduction

• Diffusion

• Solid-Liquid Transformations

• Solid Phase Transformations

• Solid Phase Transformation in steels

• Heat Treatments

Module 3: Deformation

• Introduction

• Plastic Deformation

• Softening Mechanisms of Cold Deformed Materials

• Deformation Resistance at High Temperatures

Module 4: Mechanical Characteristics

• Introduction

• Stress and Strain

• Fracture

• Mechanical Tests

Module 5: Structure-Properties Relationship

• Introduction

• Structure-Properties Relationship

• Structure-Properties Relationship Examples

Glossary

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LEARNING OBJECTIVES

Objective Objective Objective Objective oooof fff tttthehehehe Course Course Course Course This course presents the basic concepts related to the transformations that steels undergo during different thermomechanical and heat treatments. These processes allow TENARIS to manufacture products with specific properties for each particular application. Such requirements incluse, among others:

� Mecanical Strenght � Toughness � Resistance to corrosive enviroments

Metallurgy is a branch of Engineering and Materials Science of high importance due to the wide variety of applications that metals have. At the same time, Metallurgy could be considered a sub-division of Physical Chemistry.

Physical MetallurgyPhysical MetallurgyPhysical MetallurgyPhysical Metallurgy

EngineeringEngineeringEngineeringEngineering Engineering contributes to practical metallurgy. For example: - Mechanical Engineering with Manufacturing and Testing. - Chemical Engineering with Extractive Metallurgy.

Physics Physics Physics Physics Physics contributes with metallurgy, since every metallic behavior is associated with the individual nature of the atoms and their interactions

Materials Science Materials Science Materials Science Materials Science It deals with all materials

Metallurgy gives us the basis to understand the relationships between the process variables with microstructure, and with properties, thus allowing an optimum design.

Understanding these Understanding these Understanding these Understanding these relationships we can relationships we can relationships we can relationships we can obtain:obtain:obtain:obtain:

Optimum designOptimum designOptimum designOptimum design

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M1: INTRODUCTION TO STEEL

STRUCTURES

INTRODUCTION

The first objective of this module is to present the basic concepts regarding crystalline structure in solid and mainly in steels. As a second item solid solution as well as binary phase diagrams are described. Finally the iron-carbon phase diagram is explained in detail.

� Crystalline Structures � Phase Diagrams � Iron Carbon Diagram

CRYSTALLINE STRUCTURES

Blinding in SolidsBlinding in SolidsBlinding in SolidsBlinding in Solids Atoms in solids are arranged forming a three-dimensional regular pattern called crystalline structure, that can be formed with different types of atomic binding. The type of binding directly influences the properties of solids. There are 4 types of atomic binding:

Metalic BindingMetalic BindingMetalic BindingMetalic Binding This type of binding occurs when each atom shares its valency electron thus forming a cloud of electrons that extends on the metallic solid. The forces of attraction (between the cloud of electrons and the positive ions) keep the metal together, resulting in highly symmetrical crystalline structures.

The freedom of movement of the electrons of this type of solids is responsible for electrical and thermal conductivity.

This type of binding occurs when atoms do not have valency electrons available for metallic binding, as in the case of rare gases and methane. The molecules polarize, this means that the center of the positive electric charge mismatches the center of the negative electric charge. The polarization is transmitted to the neighboring molecules generating a force of electric attraction between the dipoles which is called Van der Waals force. This bind easily breaks due to an increase of the thermal agitation when the temperature increases. For this reason, this type of binding is weak and the fusion point of these solids is usually below 0 °C.

Van der Waals BindingVan der Waals BindingVan der Waals BindingVan der Waals Binding

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Ionic BindingIonic BindingIonic BindingIonic Binding This type of binding is caused by the electric attraction between positive and negative ions that are alternatively located in the crystalline lattice. A sodium chloride crystal is an example of this type of binding. The electric conductivity is not as the one that can be found in metals, instead there is a weak conductivity due to the mobility of individual ions.

Elements that have three or more valency electrons, bind forming crystalline structures by means of the forces generated when sharing electrons. To reach the atomic stability the octet of electrons should be completed. To achieve this, each atom should share its electrons with 8-N neighboring atoms, where N is the number of valency electrons of the given element. A typical example of a covalent crystal is the diamond. This type of solids is usually of high hardness and electric low conductivity.

Crystalline LatticeCrystalline LatticeCrystalline LatticeCrystalline Lattice A crystal is an orderly arrangement of atoms in space. There are many types of crystalline structures, but metals crystallize in four main types of structures:

BodyBodyBodyBody----Centered Cubic (bcc)Centered Cubic (bcc)Centered Cubic (bcc)Centered Cubic (bcc)

FaceFaceFaceFace----Centered Cubic (fcc)Centered Cubic (fcc)Centered Cubic (fcc)Centered Cubic (fcc)

BodyBodyBodyBody----Centered Tetragonal (bct)Centered Tetragonal (bct)Centered Tetragonal (bct)Centered Tetragonal (bct) MET

ALS

MET

ALS

MET

ALS

MET

ALS

Hexagonal CloseHexagonal CloseHexagonal CloseHexagonal Close----Packed (hcpPacked (hcpPacked (hcpPacked (hcp))))

Steels only crystallize in fcc, bcc or bct structures.

BODYBODYBODYBODY----CENTERED CUBIC (CENTERED CUBIC (CENTERED CUBIC (CENTERED CUBIC (bccbccbccbcc))))The cell of a bccbccbccbcc lattice has an atom in the center of the cube and one in each corner. Notice that the atom “a" is part of 8 different cells; therefore we can say that 1/8 of each corner atom belongs to another cell. Also, as each cell has a central atom, the bcc lattice has 2 atoms per cell. On the other hand we can observe that any atom can be indistinctly chosen as the center of the cell.

FACEFACEFACEFACE----CENTERED CUBIC (CENTERED CUBIC (CENTERED CUBIC (CENTERED CUBIC (fcfcfcfccccc))))The cell of fcc lattice has an atom in the center of each face and one on each corner. The number of atoms per cell can be calculated as in the previous case. This way, 1/8 of an atom at each corner contribute with an atom, while the halves of the atoms in each face contribute with 3. Therefore the fcc lattice has 4 atoms per cell.

CCCCovalentovalentovalentovalent Binding Binding Binding Binding

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A cell of fcc lattice has twice the atoms per cell than a bcc cell.

BODYBODYBODYBODY----CENTERED TETRAGONAL (CENTERED TETRAGONAL (CENTERED TETRAGONAL (CENTERED TETRAGONAL (bctbctbctbct))))The figure shows a cell of a bct lattice. It has an atom in the center of the tetragon and one in each corner. In the same way that in the bcc lattice, this structure has 2 atoms per cell.

HEXAGONAL CLOSEHEXAGONAL CLOSEHEXAGONAL CLOSEHEXAGONAL CLOSE----PACKED PACKED PACKED PACKED (hcp(hcp(hcp(hcp))))The hcp lattice has 6 atoms per cell. (1/6 of an atom per each corner, ½ of an atom per each hexagonal face, and 3 atoms in the middle of the unitary cell).

UNITARY CELLUNITARY CELLUNITARY CELLUNITARY CELL It is the smallest group of atoms that form a crystalline lattice, when they repeat themselves in all directions.

NUMBER OF COORDINATIONNUMBER OF COORDINATIONNUMBER OF COORDINATIONNUMBER OF COORDINATION It is the number of close neighbors that an atom has in the crystalline lattice.

Crystalline LatticeCrystalline LatticeCrystalline LatticeCrystalline Lattice It is convenient to represent the crystalline structure using the solid spheres in contact model, where their radios are taken as half the distance between the center of the nearest atoms. The close-packed directions are those in which the spheres in contact are aligned. BODYBODYBODYBODY----CENTERED CUBIC (bcc) CENTERED CUBIC (bcc) CENTERED CUBIC (bcc) CENTERED CUBIC (bcc)

The four diagonals of the cube form the close-packed directions of a bcc crystal. Each atom is “touched” by 8 neighboring atoms, therefore the number of coordination of the bcc lattice is 8.

FACEFACEFACEFACE----CENTERED CUBIC (CENTERED CUBIC (CENTERED CUBIC (CENTERED CUBIC (fccfccfccfcc) )))

This is the most compact crystal structure. The diagonals of the cube faces form the six close-packed directions of the fcc crystal.

The diagonals on the rear faces are not considered since the directions are crystallographic equivalent. Each atom is “touched” by 12 neighboring atoms, therefore the number of coordination of the bcc lattice is 12.

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The fcc lattice is the only one that has four close packed planes (*), where each plane has 3 close-packed directions. This is fundamental since it gives rise to different physical properties to the fcc metals, compared to metals with other structures. One of these properties is the ability to withstand ability to withstand ability to withstand ability to withstand important plastic deformationsimportant plastic deformationsimportant plastic deformationsimportant plastic deformations. (*) four close packed planes

Crystalline Imperfections (Lattice Defects)Crystalline Imperfections (Lattice Defects)Crystalline Imperfections (Lattice Defects)Crystalline Imperfections (Lattice Defects) Real crystals present deviations from the perfect arrangement previously assumed.

What is a defect or imperfection?What is a defect or imperfection?What is a defect or imperfection?What is a defect or imperfection? A defect or an imperfection is generally described as any deviation in the orderly structure of the lattice. Why is the study of the imperfections important?Why is the study of the imperfections important?Why is the study of the imperfections important?Why is the study of the imperfections important?

Defects explain structureDefects explain structureDefects explain structureDefects explain structure----sensitive propertiessensitive propertiessensitive propertiessensitive properties. Practically almost all the mechanical properties are sensitive to the structure, this finding contributed to make important advances in the study of the mechanical behavior of metals. For example, electrical conductivity, yield stress, fracture strength and strength creep are sensitive to structure properties. Classification of DefectsClassification of DefectsClassification of DefectsClassification of Defects

1.1 Vacancy 1.2 Interstitial atom

Defec

tsDe

fects

Defec

tsDe

fects 1. 1. 1.1. Point Point Point Point

defectsdefectsdefectsdefects 1.3 Substitutional atom

Line defects Dislocations

2. 2. 2.2. Lattice Lattice Lattice Lattice imperfectionsimperfectionsimperfectionsimperfections Surface

/ plane defects

Grain Boundaries

DefectsDefectsDefectsDefects 1. 1. 1.1. Point defects:Point defects:Point defects:Point defects: (the defect is located close to some atoms). 1.11.11.11.1 VacancyVacancyVacancyVacancy An atom is missing in a position of the lattice 1.21.21.21.2 Interstitial atom Interstitial atom Interstitial atom Interstitial atom An atom of another element occupies an interstice of the lattice. 1.31.31.31.3 Substitutional Substitutional Substitutional Substitutional

atomatomatomatom An atom of another element replaces an atom of the lattice. 2. 2. 2.2. Lattice imperfectionsLattice imperfectionsLattice imperfectionsLattice imperfections: ::: (the defect extends microscopically along the crystal).

Lattice imperfections Lattice imperfections Lattice imperfections Lattice imperfections –––– Line defects Line defects Line defects Line defects

A dislocation may be defined as the distorted sector of the lattice that separates a displaced from a non displaced region of a crystal.

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Which phenomena do dislocations explWhich phenomena do dislocations explWhich phenomena do dislocations explWhich phenomena do dislocations explain?ain?ain?ain? Dislocation is the two-dimensional most important defect since it explains plastic deformation of metals. Dislocations are important to explain: � Slips of the planes in the crystals, � Strain hardening, � Yield point, � Creep, � Fatigue, � Brittle fracture.

DISTORTED SECTOR OF THE LATTICEDISTORTED SECTOR OF THE LATTICEDISTORTED SECTOR OF THE LATTICEDISTORTED SECTOR OF THE LATTICE

In the absence of obstacles, a dislocation can easily move with the application of a small force; explaining why real crystals deform much more real crystals deform much more real crystals deform much more real crystals deform much more easily than what would be expected from a crystal easily than what would be expected from a crystal easily than what would be expected from a crystal easily than what would be expected from a crystal with a perfwith a perfwith a perfwith a perfect lattice.ect lattice.ect lattice.ect lattice.

TYPICAL DISLOCATIONTYPICAL DISLOCATIONTYPICAL DISLOCATIONTYPICAL DISLOCATION

The simplest type of dislocation is the edge dislocation, as the figure shows. The displacement occurs in the direction of the slip vector on the area ABCD. Line AD is the limit between the displaced (right) section and the one that has not been displaced yet (left). The shaded area indicates how much crystal has been displaced under the slip plane. All the points of the lattice were equally displaced, the measure of this displacement is equal to the Burger vector b of the dislocation.

PHASE DIAGRAMS

PhasesPhasesPhasesPhases The concept of phase is very important in the field of metallurgy.

PHASE PHASE PHASE PHASE is defined as a macroscopically is defined as a macroscopically is defined as a macroscopically is defined as a macroscopically homogeneous state of a material body.homogeneous state of a material body.homogeneous state of a material body.homogeneous state of a material body.

This is the precise thermodynamic definition, although the term is often used to speak of a solid body or any other solution that can have a variable composition inside the body. For the moment we will not take this fact into account; it will be studied in detail in future chapters. System may have one or more elements:

SIMSIMSIMSIMPLE SYSTEM PLE SYSTEM PLE SYSTEM PLE SYSTEM –––– ONE ELEMENT ONE ELEMENT ONE ELEMENT ONE ELEMENT A simple system is a one component system. Iron, for example, is a single metallic element. Iron, like many other metals, is polymorphic and crystallizes in several structures, each one stable in a different range of temperature.

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Each crystalline structure defines a separate phase, so that polymorphic metals can exist in more than one solid phase. Iron PressureIron PressureIron PressureIron Pressure----Temperature DiagramTemperature DiagramTemperature DiagramTemperature Diagram

β----FeFeFeFe It is an allotrope of iron that is the same as α-FeFeFeFe except that it is nonmagnetic; stable between 768ºC and 906ºC.

COMPLEX SYSTEMCOMPLEX SYSTEMCOMPLEX SYSTEMCOMPLEX SYSTEM Let us now consider alloys instead of pure metals

� Binary alloys: (two-component systems) they are mixtures of two metallic elements.

� Ternary alloys: (three-components systems) they are mixtures of three metallic elements.

The alloys actually used in metallurgy are generally formed by more than three components. However two or three are generally the fundamental ones and the system is analyzed considering simplified binary or ternary systems. These systems, composed ofThese systems, composed ofThese systems, composed ofThese systems, composed of several elements, are several elements, are several elements, are several elements, are thermodynamically known as “solutions”.thermodynamically known as “solutions”.thermodynamically known as “solutions”.thermodynamically known as “solutions”.

SolutionsSolutionsSolutionsSolutions SOLUTION is a homogeneous mixture of two or more kinds of atoms. Each type of atom that forms a solution is called a component.

It is possible to have: � Gaseous solutions: such as the atmosphere

that surrounds the earth. � Liquid solutions: such as NaCl in water,

molten steel or any other molten alloy. � Solid Solutions: such as all the alloys usually

found in metallurgy. For example:::: Steel is a solid solution of iron alloyed with carbon, manganese and many other elements depending on the required properties.

The relative proportions of several components of which a solution is composed affect the properties of the solutions.

Concentration UnitsConcentration UnitsConcentration UnitsConcentration Units Atom fraction and weight percent are generally used to express concentration.

Atom Fraction /Molar FractionAtom Fraction /Molar FractionAtom Fraction /Molar FractionAtom Fraction /Molar Fraction In a three- component system, for example, the atom (or molar) fraction is given by: XA= nA /(nA+nB+nC) XB= nB /(nA+nB+nC) XC= nC /(nA+nB+nC) Where XA, XB and XC are the atom (or molar) fractions and nA, nB and nC are the actual numbers of atoms (or moles) of A, B and C.

Weight PercentageWeight PercentageWeight PercentageWeight Percentage In a three-component system, for example, weight percentages are given by: CA(%)= mA.100/(mA+mB+mC) CB(%) = mB.100/(mA+mB+mC) CC (%)= mC.100/(mA+mB+mC) Where CA(%),CB(%), and CC (%)are the weight percentages and mA, mB and mC are the mass of A, B and C.

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Solid SolutionsSolid SolutionsSolid SolutionsSolid Solutions Solid solutions occur in either of two different ways

SUSBTITUTIONAL SOLID SOLUTIONSUSBTITUTIONAL SOLID SOLUTIONSUSBTITUTIONAL SOLID SOLUTIONSUSBTITUTIONAL SOLID SOLUTION In this case a direct substitution of one type of atom for another occurs when solute atoms enter the crystal to take a position normally occupied by solvent atoms.

CONDITIONS FOR SUBSTITUTIONAL CONDITIONS FOR SUBSTITUTIONAL CONDITIONS FOR SUBSTITUTIONAL CONDITIONS FOR SUBSTITUTIONAL SOLUBILITYSOLUBILITYSOLUBILITYSOLUBILITY Several factors determine the extension of substitutional solid solubility:

� Size factor:Size factor:Size factor:Size factor: An extensive solid solubility of one metal in another is only possible when the diameters of the atoms differ by less than 15 %. This factor is directly related to the strain produced in the lattice of the solvent by the solute atoms.

� Relative position of the eRelative position of the eRelative position of the eRelative position of the elements in the lements in the lements in the lements in the electromotive series:electromotive series:electromotive series:electromotive series: If the elements lie far apart in this series, the more electropositive element yields its valence electrons to the more electronegative element, with the result that a crystal with ionic bonding is formed ( for example the Na Cl crystal). When the metals lie close to each other in the electromotive series, they tend to act as if they were chemically the same, which leads to metallic bonding instead of ionic.

For a complete soluble system two other factors are required:

� Both components have the same valence.

� Both components crystallize in the same lattice form

INTERSTITIAL SOLID SOLUTIONINTERSTITIAL SOLID SOLUTIONINTERSTITIAL SOLID SOLUTIONINTERSTITIAL SOLID SOLUTION

The atoms of solute enter the holes or interstices between the solvent atoms.

CONDITIONS FOR INTERSTITIAL CONDITIONS FOR INTERSTITIAL CONDITIONS FOR INTERSTITIAL CONDITIONS FOR INTERSTITIAL SOLUBILITYSOLUBILITYSOLUBILITYSOLUBILITY The solute atoms in interstitial alloys must be small in size. The conditions that determine the solubilities in both interstitial and susbtitutional alloy systems have been studied by Hume- Rothery. According to his results extensive interstitial solid solutions can be formed if the interstitial atom’s diameter is smaller than 0.59 of the solvent atom’s diameter. The four more important interstitial solute atoms are carbon, nitrogen, oxygen and hydrogen, all of which are small in size. The solvent atom characteristics are also important. Small interstitial solute atoms dissolve much more readily in transition metals. Some of the commercially important transition metals are: Iron; Titanium; Zirconium; Nickel; Vanadium; Chromium; Manganese; Molybdenum; Tungsten; Thorium; Uranium.

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READ MORE The ability of transition elements to dissolve interstitial atoms is believed to be due to their electronic structure. All transition elements have an incomplete electronic shell inside the outer or valency shell. The extent to which interstitial atoms can dissolve in transition metals depends on the metal in question, but it is usually very small. Interstitial atoms can diffuse easily through the lattice of the solvent; therefore, that they have a large effect on the properties of the metal. Diffusion, in this case, occurs by solute atoms jumping from one interstitial position to another.

Equilibrium between two phasesEquilibrium between two phasesEquilibrium between two phasesEquilibrium between two phases The figure shows a binary system with two phases and two components in equilibrium.

The equilibrium condition can be expressed as:

The partial molar free energy of each component is the same in both phases. How many variables (temperature, pressure, How many variables (temperature, pressure, How many variables (temperature, pressure, How many variables (temperature, pressure, composition) have to be specified to fix the composition) have to be specified to fix the composition) have to be specified to fix the composition) have to be specified to fix the thermodynamic state of a system?thermodynamic state of a system?thermodynamic state of a system?thermodynamic state of a system?

The number of independent variables or degrees of freedom (L) is given by the Phase Rule of Gibbs. If we have a system of one component and one phase, the degree of freedom is 2. This is the case of a pure gas, whose thermodynamic state can be defined by fixing the temperature and the pressure. For a system with M components and µ phases, the Gibbs phase rule is: L= M-µ+2. The last number follows from the thermodynamic state. However in most metallurgical processes the pressure remains constant (P=1atm), therefore the Gibbs phase rule is L= ML= ML= ML= M----µ+1+1+1+1.

Phase DiagramsPhase DiagramsPhase DiagramsPhase Diagrams Phase diagrams are also called equilibrium diagrams or constitution diagrams. They are a very important tool in the study of alloys. They define the regions of stability of the phases that can occur in an alloy system under constant pressure.

READ MORE The phase diagrams for different alloy systems are obtained using:

� the thermodynamic equilibrium conditions, � models that allow describing the interaction

between the atoms that form the solution, and

� experimental results.

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What does the TemperatureWhat does the TemperatureWhat does the TemperatureWhat does the Temperature----Composition Composition Composition Composition equilibriumequilibriumequilibriumequilibrium diagram indicate at constant pressure? diagram indicate at constant pressure? diagram indicate at constant pressure? diagram indicate at constant pressure?

0000---- A solution with a temperature T0 and a composition X0 is completely liquid.

1111---- We decrease temperature to T1. The first solid crystal appears with a composition S1.

2222---- If temperature is decreased to T2, the liquid composition changes following the liquidus curve.

We now have in equilibrium: Liquid Comp: L2 Solid Comp: S2

3333---- If we continue cooling to T3 we will have a system with two phases with the composition: Liquid Comp: L3 Solid Comp: S3

4444---- If we continue cooling to T4 we will finally have a system with 1 solid phase and a composition X0

Note that there is not a single melting point like in the case of pure components. MP = melting point

Why these composition changes during Why these composition changes during Why these composition changes during Why these composition changes during solidification are important?solidification are important?solidification are important?solidification are important? In this example, cooling from Xo, initially solidifies a solid with a lower B concentration S1.

As a consequence, the resulting liquid becomes richer in this component. During further solidification, the concentration of B in the liquid and solid phases will increase, although always the concentration of B in the solid phase will be lower than in the remaining liquid phase. This fact is responsible for the phenomenon of micro and macrosegregation, which happen when any alloy solidifies. Macro and microsegregations will be explained later.

IMPORTANT REMARKSIMPORTANT REMARKSIMPORTANT REMARKSIMPORTANT REMARKS 1111---- The diagram is valid at a constant pressure. 2222---- It assumes that in each point of the liquid and solid curve, phases are in thermodynamic equilibrium. It does not take into account the melting and solidification time. In this diagram A and B components are completely miscible in liquid and solid phase.

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Isomorphous Diagram with Two Components Two component systems have much more Two component systems have much more Two component systems have much more Two component systems have much more complex diagrams when there is partial solid complex diagrams when there is partial solid complex diagrams when there is partial solid complex diagrams when there is partial solid miscibilitymiscibilitymiscibilitymiscibility These systems have reactions that involve three independent phases

Binary Diagrams: Lever RuleBinary Diagrams: Lever RuleBinary Diagrams: Lever RuleBinary Diagrams: Lever Rule When two phases are at equilibrium, it is posible to determine the fraction of each of them.

1.1.1.1. Let's consider a simple phase diagram with two components (A and B).

2.2.2.2. Identify the liquidus and solidus lines. 3.3.3.3. Consider a liquid with composition X0.4.4.4.4. Draw the temperature isotherm at which we

want to know the % of each phase (T1).

5.5.5.5. A lever with a support point is made at the crossover of both lines.

6.6.6.6. We measure the segments D-E D-C C-E

With this information, we are able to calculate the proportions of each phase in equilibrium at a constant temperature, starting from a liquid with composition X0.

Lever RuleLever RuleLever RuleLever Rule The Lever Rule can be stated as follows: The arm of the lever, opposite to the phase whose amount we wish to calculate, divided by the total length of the lever, will give the amount of this phase (% of phase).

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The Lever Rule can be used in any two - phase region of a binary phase diagram. Phase Diagrams Phase Diagrams Phase Diagrams Phase Diagrams –––– Eutectic Syste Eutectic Syste Eutectic Syste Eutectic SystemmmmIn the eutectic system there are three monophasic zones: two solid solutions ( α and β) and a liquid solution. Three two phase zones also exist: α +L; β+L; α + β

Pure Lead solidifies at 327,5º C Starting with pure lead and adding increasing amounts of tin, the liquidus temperature begins to decrease. Up to the moment a liquid solution with a minimum liquidus temperature is formed.

From that point, an increase of tin, increases the liquidus temperature up to the pure tin melting point.

Phase Diagrams Phase Diagrams Phase Diagrams Phase Diagrams –––– Eutectic Reaction Eutectic Reaction Eutectic Reaction Eutectic Reaction In the eutectic reaction liquidIn the eutectic reaction liquidIn the eutectic reaction liquidIn the eutectic reaction liquid solidifies at a single temperature to form a mixture of two solid solutions (α and β)

Liquid Liquid Liquid Liquid → α ++++ βAssuming constant pressure, three phases can only Assuming constant pressure, three phases can only Assuming constant pressure, three phases can only Assuming constant pressure, three phases can only be in equilibrium at an invariant point: at constant be in equilibrium at an invariant point: at constant be in equilibrium at an invariant point: at constant be in equilibrium at an invariant point: at constant temperature (eutectic temptemperature (eutectic temptemperature (eutectic temptemperature (eutectic temperatureratureraturerature) and e) and e) and e) and composition (eutectic composition).composition (eutectic composition).composition (eutectic composition).composition (eutectic composition).

The number of degrees of freedom is 1, but once The number of degrees of freedom is 1, but once The number of degrees of freedom is 1, but once The number of degrees of freedom is 1, but once the the the the pressure is set there is no degree of freedom, pressure is set there is no degree of freedom, pressure is set there is no degree of freedom, pressure is set there is no degree of freedom, composition and temperature are fixed. Using the composition and temperature are fixed. Using the composition and temperature are fixed. Using the composition and temperature are fixed. Using the phase rule: L=Mphase rule: L=Mphase rule: L=Mphase rule: L=M----m+1=2m+1=2m+1=2m+1=2----3+1=03+1=03+1=03+1=0

When there is a reaction similar to the eutectic one but with three solid phases, α → β+ µ, the reaction is called eutectoideutectoideutectoideutectoid. A key steel phase transformation includes a eutectoid reaction. We will analyze this in the next chapter.

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PHASE DIAGRAMS PHASE DIAGRAMS PHASE DIAGRAMS PHASE DIAGRAMS ---- EUTECTIC SYSTEM EUTECTIC SYSTEM EUTECTIC SYSTEM EUTECTIC SYSTEM Eutectic CompositionEutectic CompositionEutectic CompositionEutectic Composition

Hypoperitectic CompositionHypoperitectic CompositionHypoperitectic CompositionHypoperitectic Composition

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Phase Diagrams Phase Diagrams Phase Diagrams Phase Diagrams –––– Peritectic System Peritectic System Peritectic System Peritectic System Another type of solidification is the peritectic reaction. This is another phase transformation that we will find in the iron carbon diagram. At 1525ºC (2)(2)(2)(2) the first solid appears (δ phase) It is shown that as temperature decreases, the amount of δ phase increases. Just above 1493ºC (3)(3)(3)(3), the system is formed by L (0,53% C) + δ phase (0,09% C). The relative amounts of phases are given by the lever rule: % L / % δ= AB / BC At the peritectic temperature, 1493ºC (4)(4)(4)(4), the peritectic reaction occurs. L + δ →γ

As in the eutectic case the number of degrees of freedom is 1, and once pressure is set the peritectic reaction occurs at constant T and composition.

READ MOREREAD MOREREAD MOREREAD MORE Peritectic Peritectic Peritectic Peritectic CompositionCompositionCompositionComposition

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Hypoperitectic CompositionHypoperitectic CompositionHypoperitectic CompositionHypoperitectic Composition

PPPPhase Diagrams hase Diagrams hase Diagrams hase Diagrams –––– Intermediate PhasesIntermediate PhasesIntermediate PhasesIntermediate Phases Most phase diagrams do not consist merely of one eutectic reaction or one peritectic reaction, but rather of a combination of various fundamental reactions. In most cases the appearance of several reactions in a single binary diagram is the result of the presence of intermediate phases. The chemical compositions of these phases are an intermediate between two pure metals. Their crystalline

structure can be different from the primary solid solutions (the one that corresponds to pure metals). Some intermediate phases can accurately be called intermetallic compounds when, (like Fe3Ccementite), they have a fixed simple ratio of the two kinds of atoms.

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However, many intermediate phases exist over a range of compositions and are considered intermediate or secondary solid solutions. INTERMETALLIC SOLID SOLUTION

INTERMETALLIC COMPUNDS

IRON CARBON DIAGRAMS

Iron carbon alloys are usually divided into three categories based on their composition:

� Irons:Irons:Irons:Irons: carbon content is very low and has negligible effect on their properties.

� Steels:Steels:Steels:Steels: carbon content is usually in the range of 0.1 to 2 %. The term plain carbon describes steels in which other alloying elements have little influence in determining steel properties. Alloyed steels

are those in which alloying elements both in solid solution and forming compounds (usually carbides), have an important effect on the final properties.

� Cast irons:Cast irons:Cast irons:Cast irons: carbon content is usually in the range of 2 to 4,5%

We will focus on low and medium carbon alloyed steels, which cover TENARIS production.

The diagram shows the relationships of equilibrium, not only between iron and carbon (graphite) but also between iron and cementite (Fe3 C). Graphite is more stable than carbide. However, the decomposition of cementite does not occur easily and, from a practical point of view, one may consider that, in carbon steels, the breakdown of carbide to form iron and graphite does not occur. Furthermore, when steel is cooled from the liquid to the solid state, cementite is the easiest to nucleate. According to the above considerations the iron-Fe3 C is the diagram used in the study of steels.

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Cementite is an intermetallic compound with negligible solubility limits and a carbon content of 6,69%. Because of this negligible solubility the diagram can be divided into two independent parts just at its composition. That part of the diagram with carbon concentrations higher than 6,69% has little commercial significance and is usually ignored.

The diagram is characterized by three invariant points:

A peritectic point at 0,17 % C and 1495 ºC.A eutectic point at 4,30% C and 1154 ºC. A eutectoid point at 0.77 % of C and 727 ºC.

Most carbon steels contain less than 1% carbon. Most Tenaris OCGT steels have carbon contents lower than 0,45%.

If a liquid with less than 0,09 % of carbon is frozen a bcc phase is obtained. This phase is identified with name delta ferrite (“ δ”). In these steels there is no peritectic reaction. As seen in the previous submodule, the peritectic reaction takes place between 0,09%C and 0,53%C.

All compositions that, as they freeze, pass through the peritectic transformation region, enter the single-phase-centered cubic field. This phase is given the identifying name of gamma “γ” or austenite. All compositions containing less than

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2.11%C pass through the austenite region on cooling from liquid state to room temperature.

The A3 line shows the lowest temperatures at which fully austenite is stable. This temperature decreases as carbon content increases to a minimum value A1 at the eutectoid composition. A3 – It is the boundary line between γ and α+γ.ACM – It is the boundary line between γ and γ+Fe3CA1 – It is the eutectoid temperature line. α= Ferrite = Ferrite = Ferrite = Ferrite γ= Austenite Fe= Austenite Fe= Austenite Fe= Austenite Fe3333C =CementiteC =CementiteC =CementiteC =Cementite

COOLING AND HEATING TRANSFORMATION TEMPERATURES

Due to the thermal inertia of phase transformations, sometimes it is necessary to distinguish between the cooling (AR) and heating

(AC) transition temperatures. For example, AR3 represents the transition temperature from γ to α+γ, while AC3 from α+γ to γ.

The phase called α-ferrite is a solid solution of carbon in bcc Fe-α. The maximum solubility of this phase is 0,022%C at 727°C. Due to this limited solubility this region is not always shown in the Fe-C diagram.

The microstructures obtained when steel is slowly cooled from the austenitic region also depend on the original carbon concentration. Proeutectoid ferrite nucleates preferentially and heterogeneously on the austenitic grain boundaries. There are two main reasons for this: firstly because the boundaries are energetically favorable sites for nucleation and secondly because there are regions with higher diffusion rates.

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If the carbon content is 0,77%, when the eutectoid temperature is reached on cooling the austenite, two solid phases are formed side by side in a given region of the austenite to produce a nodule of pearlite. Pearlite consists of alternate plates of ferrite and cementite. If the carbon content is less than the eutectoid composition (0.77 %), the microstructure will contain proeutectoid ferrite and pearlite. If the carbon content is higher than 0,77% the microstructure will contain proeutectoid cementite and pearlite.

The phase transformations associated with the eutectic point at 4,3% C are useful in the study of cast irons, and they are not going to be considered here. We are going to focus on the carbon content less We are going to focus on the carbon content less We are going to focus on the carbon content less We are going to focus on the carbon content less than 0,77%.than 0,77%.than 0,77%.than 0,77%.