ENGINEERING MATERIALS AND METALLURGY

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ByProf.S.Rajendiran

Mechanical Department Ashoka Institute of Engineering and Technology

Constitution of alloys – Solid solutions, substitutional and

interstitial – Phase diagrams, isomorphous, eutectoid, eutectic,

peritectic, and peritectroid reactions – Iron – Iron carbide

equilibrium diagram – Classification of steel and cast iron,

microstructure, properties and applications.

UNIT – I CONSTITUTION OF ALLOYS

AND PHASE DIAGRAMS

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CRYSTALLIZATION1. Crystallization is the transition from the liquid to the solid state or

transformation of liquid phase to solid crystalline phase.

2. It occurs in two stages,

1. Nucleus formation - Nucleation is a process of formation of

stable crystallization centers of a new phase.

2. Crystal growth - atoms attaching themselves in certain preferred

directions, usually along the axes of a crystal.

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1. Nucleation may occur by either homogeneous or heterogeneous mechanism.

2. Presence of foreign particles or other foreign substance in the liquid alloy allows to initiate crystallization at minor value of under cooling (few degrees below the freezing point). This is heterogeneous nucleation.

3. If there is no solid substance present, under cooling of a hundred degrees is required in order to form stable nuclei or “seeds” crystals, providing homogeneous nucleation.

NUCLEATION

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CRYSTAL GROWTH1. Number of stable nuclei per unit volume of crystallizing alloy

determines the grain size.

2. The difference in potential energy between the liquid and solid states is

known as the latent heat of fusion.

3. When the temperature of the liquid metal has dropped sufficiently

below its freezing point, stable aggregates or nuclei appear

spontaneously at various points in the liquid.

4. These nuclei, which have now solidified, act as centers for further

crystallization. 5

CRYSTAL GROWTH5. As cooling continues, more atoms tend to freeze, and attach themselves

to already existing nuclei or form new nuclei.

6. Each nucleus grows by the attraction of atoms from the liquid into its

space lattice.

7. Crystal growth continues in three dimensions, the atoms attaching

themselves in certain preferred directions, usually along the axes of a

crystal.

8. This gives rise to a characteristic treelike structure which is called

DENDRITE. 6

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MECHANISM OF SOLIDIFICATION

NUCLEUS

FORMATION

GROWTH OF CRYSTALLITES

GRAIN

BOUNDARIES8

PROCESS OF CRYSTALLIZATION AND

DENDRITIC GROWTH

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1. Since each nucleus is formed by chance, the crystal axes are pointed at

random and the dendrites will grow in different directions in each

crystal.

2. Finally, as the amount of liquid decreases, the gaps between the arms

of the dendrite will be filled and the growth of the dendrite will be

mutually obstructed by that of its neighbors.

3. This leads to a very irregular external shape called as grains.

4. The area along which crystals meet, known as the grain boundary, is

a region of mismatch.

GRAIN AND GRAIN

BOUNDARY

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GRAIN BOUNDARY

FORMATION OF DENDRITES

IN MOLTEN METAL

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POLYMORPHISM AND ALLOTROPY OF

METALS

1. Polymorphism is a physical phenomenon where a material may

have more than one crystal structure.

2. A material that shows polymorphism exists in more than one type of

space lattice in the solid state.

3. If the change in structure is reversible, then the polymorphic change

is known as allotropy. The prevailing crystal structure depends on

both the temperature and the external pressure.

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4. Polymorphism example is found in carbon: Graphite is the stable

polymorph at ambient conditions, whereas Diamond is formed at

extremely high pressures.

5. The best known example for allotropy is iron. When iron crystallizes

at 2800 oF it is B.C.C. (δ -iron), at 2554 oF the structure changes to

F.C.C. (γ -iron or austenite), and at 1670 oF it again becomes

B.C.C. (α -iron or ferrite).

POLYMORPHISM AND ALLOTROPY OF

METALS

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ALLOTROPIC BEHAVIOR OF

PURE IRON

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CRYSTAL IMPERFECTIONS

(DEFECTS)1. The perfectly regular crystal structure are called as IDEAL

CRYSTALS in which atoms are arranged in regular way.

2. In actual crystals, imperfections or defects are always present, which

affect the properties of crystals.

3. The crystallographic defects are classified as,

1. Point defects or Zero dimensional defects.

2. Line defects or One dimensional defects.

3. Surface defects or Plane defects or Two dimensional defects. 16

POINT DEFECTS1. Vacancy – missing atom at a certain crystal lattice position.

2. Interstitial impurity atom – extra impurity atom in an interstitial

position.

3. Self-interstitial atom – extra atom in an interstitial position

4. Substitution impurity atom – impurity atom, substituting an atom

in crystal lattice.

5. Frenkel defect – extra self-interstitial atom, responsible for the

vacancy nearby 17

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LINE DEFECTSLinear crystal defects are edge and screw dislocations.

1. Edge dislocation is an extra half plane of atoms “inserted” into the

crystal lattice. Due to the edge dislocations metals possess high

plasticity characteristics: ductility and malleability.

2. Screw dislocation forms when one part of crystal lattice is shifted

(through shear) relative to the other crystal part. It is called screw as

atomic planes form a spiral surface around the dislocation line.

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LINE DEFECTS

EDGE DISLOCATION SCREW DISLOCATION

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1. Planar defect is an imperfection in form of a plane between uniform

parts of the material.

2. Important planar defect is a Grain boundary. Formation of a

boundary between two grains may be imagined as a result of rotation

of crystal lattice of one of them about a specific axis.

3. Tilt boundary – rotation axis is parallel to the boundary plane;

4. Twist boundary - rotation axis is perpendicular to the boundary

plane.

SURFACE DEFECTS

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5. Diffusion along grain boundaries is much faster, than throughout the

grains.

6. Grain boundaries accumulate crystal lattice defects (vacancies,

dislocations) and other imperfections, therefore they effect on the

metallurgical processes, occurring in alloys and their properties.

SURFACE DEFECTS

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1. A solid solution is simply a solution in the solid state and consists of

two kinds of atoms combined in one type of space lattice.

2. Any solution is composed of two parts a solute and a solvent.

3. The solute is the minor part which is dissolved and the solvent is the

major portion of the solution.

4. The amount of solute that may be dissolved by the solvent is

generally a function of temperature, which usually increases with

increasing temperature.

SOLID SOLUTIONS

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There are three possible conditions of a solution,

1. Unsaturated – The solvent is dissolving less of the solute than it

could dissolve at a given temperature and pressure.

2. Saturated – The solvent is dissolving the limiting amount of solute.

3. Supersaturated – The solvent dissolves more than the solute than it

should under equilibrium conditions.

SOLID SOLUTIONS

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SUBSTITUTIONAL SOLID SOLUTION

If the atoms of the solvent metal and solute element are of

similar sizes (not more, than 15% difference), they form substitution

solid solution, where part of the solvent atoms are substituted by

atoms of the alloying. Example – Cu-Ni

TYPES

1.Ordered

2. DisorderedCu

Ni

25NSK - AAMEC

INTERSTITIAL SOLID SOLUTION1. If the atoms of the alloying elements are considerably smaller, than the

atoms of the matrix metal, interstitial solid solution forms, where the

matrix solute atoms are located in the spaces between large solvent

atoms.

Smaller Atoms

Hydrogen, Carbon,

Boron and Nitrogen

INTERSTITIAL SOLID SOLUTION

2. The interstitial solution of carbon in iron constitutes the basis of

steel hardening.

3. Very small amount of hydrogen introduced into steels during acid

picking (cleaning), plating or welding operations cause a sharp

decrease in ductility known as Hydrogen embrittlement.

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PHASE DIAGRAM1. Phase Diagram or Equilibrium Diagram or Constitutional Diagrams

indicate the structural changes due to variation of temperature and

composition.

2. The diagram is essentially a graphical representation of an alloy

system.

3. The phase diagram will show the phase relationships under equilibrium

conditions.

4. Phase diagrams are plotted with temperature in ordinate and alloy

composition in weight % as the abscissa. 28

F = C F = C P + 2 P + 2For a system in equilibrium

F F C + P = 2 C + P = 2or

F – Degrees of FreedomC – Number of ComponentsP – Number of Phases

= What you can control What the system controls

F = C + 2 P

Can control the no. of components added and P

& T

System decided how many phases to produce given

the conditions

Degrees of Freedom

GIBBS PHASE RULE

2 = Temperature and Pressure

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CLASSIFICATION OF PHASE

DIAGRAM1. UNARY - One component phase diagram.

2. BINARY - Two component phase diagram.

3. TERNARY - Three component phase diagram.

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UNARY PHASE DIAGRAMThe simplest case-Water.

Also known as a P-T diagram

Sign of [dP/dT] for:

Solid-Liquid

Liquid-Gas

Gas-Solid equilibria

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BINARY PHASE DIAGRAMCopper-Nickel equilibrium diagram

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TERNARY PHASE DIAGRAM

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PHASE DIAGRAM1. SYSTEM – A system is a substance so isolated from its surroundings

that it is unaffected by these and is subjected to changes in overall

composition, temperature, pressure.

2. COMPONENT – A component is a unit of the composition variable

of the system. A system that has one component (Unary), two

(Binary), three (Ternary) and four (Quaternary).

3. PHASE – A phase is a physically and chemically homogeneous

portion of the system, separated from the other portions by a surface,

the interface. 34

CLASSIFICATION OF PHASE DIAGRAMPhase diagrams are classified according to the relation of the components

in the liquid and solid states.

1. Components completely soluble in the liquid state,

1. And also completely soluble in solid state. (Isomorphous reaction)

2. But partly soluble in the solid state. (Eutectic reaction).

3. But insoluble in the solid state. (Eutectic reaction)

2. Components completely partially soluble in liquid state,

1. But completely soluble in the solid state.

2. And partly soluble in the solid state.

3. Components completely insoluble in the liquid state and completely insoluble in

the solid state. 35

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IRON – IRON CARBIDE EQUILIBRIUM DIAGRAM

1. The following phases are involved in the transformation, occurring

with iron-carbon alloys:

1. L - Liquid solution of carbon in iron;

2. δ-ferrite – Solid solution of carbon in iron.

2. Maximum concentration of carbon in δ-ferrite is 0.09% at

2719 ºF (1493ºC) – temperature of the peritectic transformation.

3. The crystal structure of δ-ferrite is BCC (cubic body centered).

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4. Austenite – interstitial solid solution of carbon in γ-iron.

5. Austenite has FCC crystal structure, permitting high solubility of

carbon – up to 2.06% at 2097 ºF (1147 ºC).

6. Austenite does not exist below 1333 ºF (733ºC) and maximum

carbon concentration at this temperature is 0.83%.

7. α-ferrite – solid solution of carbon in α-iron.

8. α-ferrite has BCC crystal structure and low solubility of carbon – up

to 0.25% at 1333 ºF (733ºC).

9. α-ferrite exists at room temperature.

10. Cementite – iron carbide, intermetallic compound, having fixed

composition Fe3C. It is hard and brittle. 38

The following phase transformations occur with iron-carbon

alloys:

1. Alloys, containing up to 0.51% of carbon, start solidification with

formation of crystals of δ-ferrite. Carbon content in δ-ferrite

increases up to 0.09% in course solidification, and at 2719 ºF

(1493ºC) remaining liquid phase and δ-ferrite perform peritectic

transformation, resulting in formation of austenite.

2. Alloys, containing carbon more than 0.51%, but less than 2.06%,

form primary austenite crystals in the beginning of solidification and

when the temperature reaches the curve ACM primary cementite

stars to form. 39

3. Iron-carbon alloys, containing up to 2.06% of carbon, are called

steels.

4. Alloys, containing from 2.06 to 6.67% of carbon, experience eutectic

transformation at 2097 ºF (1147 ºC). The eutectic concentration of

carbon is 4.3%.

5. In practice only hypoeutectic alloys are used. These alloys (carbon

content from 2.06% to 4.3%) are called cast irons. When

temperature of an alloy from this range reaches 2097 ºF (1147 ºC), it

contains primary austenite crystals and some amount of the liquid

phase. The latter decomposes by eutectic mechanism to a fine

mixture of austenite and cementite, called ledeburite. 40

6. All iron-carbon alloys (steels and cast irons) experience eutectoid

transformation at 1333 ºF (733ºC). The eutectoid concentration of

carbon is 0.83%.

7. When the temperature of an alloy reaches 1333 ºF (733ºC),

austenite transforms to pearlite (fine ferrite-cementite structure,

forming as a result of decomposition of austenite at slow cooling

conditions).

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CRITICAL TEMPERATURES

1. Upper critical temperature (point) A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys.

2. Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.

3. Lower critical temperature (point) A1 is the temperature ofthe austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist.

4. Magnetic transformation temperature A2 is the temperature below which α-ferrite is ferromagnetic. 42

Phase compositions of the iron-carbon alloys

at room temperature1. Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of

primary (proeutectoid) ferrite and pearlite.

2. Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.

3. Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (proeutectoid)cementite and pearlite.

4. Cast irons (carbon content from 2.06% to 4.3%) consist of

proeutectoid cementite C2 ejected from austenite according to the

curve ACM , pearlite and transformed ledeburite (ledeburite in which

austenite transformed to pearlite).43

CLASSIFICATION OF STEEL

Classification of steels by composition

Carbon steels

1. Low carbon steels (C < 0.25%);

2. Medium carbon steels (C =0.25% to 0.55%);

3. High carbon steels (C > 0.55%).

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DESIGNATION OF STEELAmerican Iron and Steel Institute (AISI) together with Society

of Automotive Engineers (SAE) have established four-digit (with additional letter prefixes) designation system:

SAE 1XXX 1. First digit 1 indicates carbon steel (2-9 are used for alloy steels); 2. Second digit indicates modification of the steel. 3. 0 - Plain carbon, non-modified 4. 1 - Resulfurized 5. 2 - Resulfurized and rephosphorized 6. 5 - Non-resulfurized, Mn over 1.0% 7. Last two digits indicate carbon concentration in 0.01%.

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A letter prefix before the four-digit number indicates the steel

making technology:

1. A - Alloy, basic open hearth

2. B - Carbon, acid Bessemer

3. C - Carbon, basic open hearth

4. D - Carbon, acid open hearth

5. E - Electric furnace

Example: AISI B1020 means non modified carbon steel,

produced in acid Bessemer and containing 0.20% of carbon.

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1. Low alloy steels (alloying elements < 8%);

2. High alloy steels (alloying elements > 8%).

3. According to the four-digit classification SAE-AISI system:

4. First digit indicates the class of the alloy steel:

5. 2- Nickel steels;

6. 3- Nickel-chromium steels;

7. 4- Molybdenum steels;

8. 5- Chromium steels;

9. 6- Chromium-vanadium steels;

10. 7- Tungsten-chromium steels;

11. 9- Silicon-manganese steels.

12. Second digit indicates concentration of the major element in percents (1 means 1%).

13. Last two digits indicate carbon concentration in 0,01%.

Example: SAE 5130 means alloy chromium steel, containing 1% of chromium and 0.30% of carbon.

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CLASSIFICATION OF STEELS BY APPLICATION

Stainless steels

AISI has established three-digit system for the stainless steels:

1. 2XX series – chromium-nickel-manganese austenitic stainless steels;

2. 3XX series – chromium-nickel austenitic stainless steels;

3. 4XX series – chromium martensitic stainless steels or ferritic stainless

steels;

4. 5XX series – low chromium martensitic stainless steels.

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TOOL AND DIE STEELSDesignation system of one-letter in combination with a number is accepted for tool

steels. The letter means:

1. W - Water hardened plain carbon tool steels;

2. O - Oil hardening cold work alloy steels;

3. A - Air hardening cold work alloy steels;

4. D -Diffused hardening cold work alloy steels;

5. S – Shock resistant low carbon tool steels;

6. T – High speed tungsten tool steels;

7. M - High speed molybdenum tool steels;

8. H – Hot work tool steels; 49

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