Modern construction materials

Module I

Job Thomas

Atomic structure-arrangement of atoms within the metals

All matter is made up of atoms containing a nucleus of protons and neutrons and surrounding clouds, or orbits, of electrons.

Atoms can transfer or share electrons; in doing so, multiple atoms combine to form molecules. Molecules are held together by attractive forces called bonds

FIGURE 1.1 An outline of the topics described in Chapter 1.

Types of Atomic Bonds1. Ionic bonds-one or more electrons from an outer orbit

are transferred from one material to another (example Na+ and Cl- form salt)

2. Covalent bonds- electrons in outer orbits are shared by atoms to form molecules (H20 water). Typically low conductivity and high hardness

3. Metallic bonds-available electrons are shared by all atoms in contact. The resultant electron cloud provides attractive forces to hold the atoms together and results in generally high thermal and electrical conductivity.

4. Van Der Waals forces are weak attractions occurring between molecules.

CRYSTAL STRUCTURE

The crystal structure of metals- when metals solidify from a molten state, the atoms arrange themselves into various orderly configurations called CRYSTALS.

1.Body-centered cubic (BCC) least dense2.Face-centered cubic (FCC) more dense3.Hexagonal close-packet (HCP) most dense

FIGURE 1.2 The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.

FIGURE 1.3 The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.

FIGURE 1.4 The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells.

The reason that metals form different crystal structures is to minimize the energy required to fill space

At different temperatures the same metal may form different structures

Allotropism or polymorphism (MEANING MANY SHAPES)

the appearance of more than one type of crystal structure

Deformation & Strength of Single Crystals

Elastic deformation- a single crystal is subject to an external force, but returns to its original shape when the force is removed

Plastic deformation-a permanent deformation when the crystal does not return to its original shape

Two Basic Mechanisms for Plastic Deformations

Slipping of one plane of atoms over another adjacent plane (slip plane) under shear stress

Twinning- the second and less common mechanism of plastic deformation where a portion of the crystal forms a mirror image of itself across the plane of twinning

Definition: Anisotropy-a single crystal exhibits different properties when tested in different directions (ex. Woven cloth, plywood)

FIGURE 1.5 Permanent deformation of a single crystal under a tensile load. The highlighted grid of atoms emphasizes the motion that occurs within the lattice. (a) Deformation by slip. The b/a ratio influences the magnitude of the shear stress required to cause slip. (b) Deformation by twinning, involving the generation of a “twin” around a line of symmetry subjected to shear. Note that the tensile load results in a shear stress in the plane illustrated.

Imperfections in the crystal structure of metals explains why actual strength levels are one or two orders of magnitude lower than the theoretical calculations

Point defects-vacancy, missing atoms, interstitial atom extra atom in the lattice or impurity foreign atom that has replaced the atom of pure metal

Linear defections called dislocations Planar imperfections such as grain boundaries and phase

boundaries Volume or bulk imperfections-voids, inclusions, other

phases, cracks

FIGURE 1.7 Schematic illustration of types of defects in a single-crystal lattice: selfinterstitial, vacancy, interstitial, and substitutional.

Dislocations-defects in the orderly arrangement of a metal’s atomic structure. Because a slip plane containing a dislocation requires less shear stress to allow slip than does a plane in a perfect lattice, dislocations are the most significant defects that explain the discrepancy between the actual and theoretical strengths of metals.

FIGURE 1.8 Types of dislocations in a single crystal: (a) edge dislocation; and (b) screw dislocation.

FIGURE 1.9 Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals is much lower than that predicted by theory.

Work Hardening (Strain Hardening)

Dislocations can become entangled and interfere with each other and be impeded by barriers such as grain boundaries, impurities, and inclusions in the material. The increased shear stress required to overcome entanglements and impediments results in an increase in overall strength and hardness of the metal and is known as work hardening or strain hardening. (Ex. Cold rolling, forging, drawing)

Grains and Grain Boundaries

When molten metal solidifies, crystals begin for form independently of each other. They have random and unrelated orientations. Each of these crystals grows into a crystalline structure or GRAIN.

The number and size of the grains developed in a unit volume of the metal depends on the rate at which NUCLEATION (the initial stage of crystal formation) takes place

Is this what I mean by grain?

FIGURE 1.10 Schematic illustration of the stages during the solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.

• Rapid cooling – smaller grains

• Slow cooling – larger grains

• Grain boundaries – the surfaces that separate individual grains

• Grain size- at room temperature a large grain size is generally associated with low strength, low hardness, and low ductility (ductility is a solid material's ability to deform under tensile stress)

• Grain size is measured by counting the number of grains in a given area or by counting the number of grains that intersect a length of line randomly drawn on an enlarged photograph of the grains

TABLE 1.1 Grain Sizes

Plastic deformation of polycrystalline metalsCold working – a polycrystalline metal with uniform equiaxed

grains is subject to plastic deformation at room temperature. The grains become deformed and elongated. The deformed metal exhibits higher strength because of the

entanglement of dislocations with grain boundaries and with each other.

The higher the deformation, the stronger the metal becomes. Strength is higher for metals with small grains because they

have larger grain-boundary surface area per unit volume of metal hence more entanglements of dislocations

FIGURE 1.11 Plastic deformation of idealized (equiaxed) grains in a specimen subjected to compression (such as occurs in the forging or rolling of metals): (a) before deformation; and (b) after deformation. Note the alignment of grain boundaries along a horizontal direction; this effect is known as preferred orientation.

ANISOTROPY (texture)

Metal properties are different in the vertical direction from those in the horizontal direction

It influences both mechanical and physical properties of metals

FIGURE 1.12 (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging (caused, for example, by pushing a steel ball against the sheet). Note the orientation of the crack with respect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a crack (vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was vertical. Courtesy: J.S. Kallend, Illinois Institute of Technology.

Strengthening Ordinarily ductility is sacrificed when an

alloy is strengthened. The relationship between dislocation

motion and mechanical behavior of metals is significance to the understanding of strengthening mechanisms.

The ability of a metal to plastically deform depends on the ability of dislocations to move.

Virtually all strengthening techniques rely on this simple principle: Restricting or Hindering dislocation motion renders a material harder and stronger.

We will consider strengthening single phase metals by: grain size reduction, solid-solution alloying, and strain hardening

Strategies for Strengthening: 1: Reduce Grain Size

• Grain boundaries are barriers to slip.• Barrier "strength" increases with Increasing angle of misorientation.• Smaller grain size:

more barriers to slip.

• Hall-Petch Equation:21 /

yoyield dk

Adapted from Fig. 7.14, Callister 7e.(Fig. 7.14 is from A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)

Hall-Petch equation:

Grain Size Reduction Techniques:

•Increase Rate of solidification from the liquid phase.

•Perform Plastic deformation followed by an appropriate heat treatment.

Notes:

Grain size reduction also improves toughness of many alloys.

Small-angle grain boundaries are not effective in interfering with the slip process because of the small crystallographic misalignment across the boundary.

Boundaries between two different phases are also impediments to movements of dislocations.

Impurity atoms distort the lattice & generate stress. Stress can produce a barrier to dislocation motion.

Strategies for Strengthening: 2: Solid Solutions

• Smaller substitutional impurity

Impurity generates local stress at A and B that opposes dislocation motion to the right.

A

B

• Larger substitutional impurity

Impurity generates local stress at C and D that opposes dislocation motion to the right.

C

D

Stress Concentration at Dislocations

Adapted from Fig. 7.4, Callister 7e.

Strengthening by Alloying small impurities tend to concentrate at dislocations

on the “Compressive stress side” reduce mobility of dislocation increase strength

Adapted from Fig. 7.17, Callister 7e.

Strengthening by alloying

Large impurities concentrate at dislocations on “Tensile Stress” side – pinning dislocation

Adapted from Fig. 7.18, Callister 7e.

Ex: Solid Solution Strengthening in Copper

• Tensile strength & yield strength increase with wt% Ni.

• Empirical relation:

• Alloying increases sy and TS.

21 /y C~

Adapted from Fig. 7.16 (a) and (b), Callister 7e.

Tensi

le s

tren

gth

(M

Pa)

wt.% Ni, (Concentration C)

200

300

400

0 10 20 30 40 50

Yield

str

en

gth

(M

Pa)

wt.%Ni, (Concentration C)

60

120

180

0 10 20 30 40 50

• Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum).

• Result:S

~y

1

Strategies for Strengthening: 3. Precipitation Strengthening

Slipped part of slip plane

Side View

precipitate

Top ViewUnslipped part of slip plane

S

Large shear stress needed to move dislocation toward precipitate and shear it.

Dislocation “advances” but precipitates act as “pinning” sites with spacing S. which “multiplies” Dislocation density

• Internal wing structure on Boeing 767

• Aluminum is strengthened with precipitates formed by alloying & H.T.

Adapted from Fig. 11.26, Callister 7e. (Fig. 11.26 is courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

1.5mm

Application: Precipitation Strengthening

Adapted from chapter-opening photograph, Chapter 11, Callister 5e. (courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

Strategies for Strengthening: 4. Cold Work (%CW)

• Room temperature deformation.• Common forming operations change the cross sectional area:

Adapted from Fig. 11.8, Callister 7e.

-Forging

Ao Ad

force

dieblank

force-Drawing

tensile force

AoAddie

die

-Extrusion

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

100 x %o

do

A

AACW

-Rolling

roll

AoAd

roll

Impact of Cold Work

Lo-Carbon Steel!Adapted from Fig. 7.20, Callister 7e.

• Yield strength (sy) increases.• Tensile strength (TS) increases.• Ductility (%EL or %AR) decreases.

As cold work is increased

Dispersion Strengthening– Two Phase, Non-Coherent– Point and Surface Defect– Medium Strengthening Effect

Defects and Strengthening Mechanisms

Solid Solution Strengthening– Single Phase– Point Defects– Low Strengthening Effect

Precipitation Hardening– Two Phase, Coherent– Point and Surface Defects +– High Strengthening Effect

Strain Hardening– Introduces Line Defects– Varied Strengths

Grain Size Refining– Surface Defects– Varied Strengths

Exceed Solubility Limit

ISSUES TO ADDRESS...• When we combine two elements... what equilibrium state do we get?• In particular, if we specify... --a composition (e.g., wt%Cu - wt%Ni), and --a temperature (T)

1

then... How many phases do we get? What is the composition of each phase? How much of each phase do we get?

Phase BPhase A

Nickel atomCopper atom

PHASE DIAGRAMS

2

• Solubility Limit: Max concentration for which only a solution occurs.

• Ex: Phase Diagram: Water-Sugar System

Question: What is the solubility limit at 20C?Answer: 65wt% sugar.

If Co < 65wt% sugar: sugar

If Co > 65wt% sugar: syrup + sugar.• Solubility limit increases with T:

e.g., if T = 100C, solubility limit = 80wt% sugar.

Pure

Sugar

Tem

pera

ture

(°C

)

0 20 40 60 80 100Co=Composition (wt% sugar)

L (liquid solution

i.e., syrup)

Solubility Limit L

(liquid)

+ S

(solid sugar)

65

20

40

60

80

100

Pure

W

ate

rAdapted from Fig. 9.1, Callister 6e.

THE SOLUBILITY LIMIT

33

• Components: The elements or compounds which are mixed initially (e.g., Al and Cu)• Phases: The physically and chemically distinct material regions that result (e.g., a and b).

Aluminum-CopperAlloy

(darker phase)

(lighter phase)

Adapted from Fig. 9.0, Callister 3e.

COMPONENTS AND PHASES

4

• Changing T can change # of phases: path A to B.

• Changing Co can change # of phases: path B to D.

• water- sugar system

70 80 1006040200

Tem

pera

ture

(°C

)

Co=Composition (wt% sugar)

L (liquid solution

i.e., syrup)

A(70,20) 2 phases

B(100,70) 1 phase

20

100

D(100,90) 2 phases

40

60

80

0

L (liquid)

+ S

(solid sugar)

Adapted from Fig. 9.1, Callister 6e.

EFFECT OF T & COMPOSITION (Co)

5

• Tell us about phases as function of T, Co, P. • For this course: --binary systems: just 2 components.

--independent variables: T and Co (P = 1atm is always used).

• PhaseDiagramfor Cu-Nisystem

• 2 phases: L (liquid) (FCC solid solution)

• 3 phase fields: L L +

wt% Ni20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

(FCC solid solution)

L + liq

uidus

solid

us

Adapted from Fig. 9.2(a), Callister 6e.(Fig. 9.2(a) is adapted from Phase Diagrams of Binary Nickel Alloys, P. Nash (Ed.), ASM International, Materials Park, OH (1991).

PHASE DIAGRAMS

6

• Rule 1: If we know T and Co, then we know: --the # and types of phases present.

• Examples:

wt% Ni20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

(FCC solid solution)

L +

liquidus

solid

us

A(1100,60)B

(1250,3

5) Cu-Ni

phasediagram

A(1100, 60): 1 phase:

B(1250, 35): 2 phases: L +

Adapted from Fig. 9.2(a), Callister 6e.(Fig. 9.2(a) is adapted from Phase Diagrams of Binary Nickel Alloys, P. Nash (Ed.), ASM International, Materials Park, OH, 1991).

PHASE DIAGRAMS: # and types of phases

47

Strain Hardening

New yield strength yi higher than initial yield strength, y0.

Reason strain hardening.

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Strain Hardening (III)

Yield strength + hardness increased due to strain hardening,

but ductility decreased (material becomes more brittle).

So….cheap, convenient way to strengthen mat'ls

Work hardening

On the other hand, work hardening can be problemExample: cold rolling plate to sheet

platesheet

too hard to continue rolling

harder harder!!

extremely hard - need backup rollsto keep work rolls from deforming

fairly soft

50

Recovery, Recrystallization, and Grain Growth

Restoration to state before cold-work by heat-

treatment:

Recovery and Recrystallization,

followed by grain growth.

51

RecoveryHeating increased diffusion

enhanced dislocation motion

decrease in dislocation density by

annihilation, formation of low-energy dislocation

configurations

relieves internal strain energy Some of the mechanisms of dislocation annihilation:

vacancies

slip plane

Edge dislocation

52

Recrystallization (I)

After recovery grains can still be strained.

Strained grains replaced upon heating by

strain-free grains with low density of

dislocations.

Recrystallization: nucleation and growth of

new grains

Driving force: difference in internal energy

between strained and unstrained

Grain growth short-range diffusion Extent of

recrystallization depends on temperature and

time. Recrystallization is slower in alloys

53

Recrystallization (II)

Recrystallization temperature: temperature at which process is complete in one hour. Typically 1/3 to 1/2 of melting temperature (can be as high as 0.7 Tm in some alloys).

Recrystallization decreases as %CW is increased. Below "critical deformation", recrystallization does not occur.

54

Recrystallization (III)

55

Grain Growth

Deformed polycrystalline material maintained at annealing temperature following recrystallization further grain growth occurs

Driving force: reduction of grain boundary area and energy: Big grains grow at the expense of small ones

Grain growth during annealing occurs in all polycrystalline materials (i.e. they do not have to be deformed first).

Boundary motion occurs by short range diffusion of atoms across the grain boundary strong temperature dependence of the grain growth.

Solid Solution Strengthening Single Phase Point Defects Strengthening Effect

Depends on: Difference in size

between solute and solvent atoms

Amount of solute added

Effect of Solid Solution Strengthening on Properties

Strength Hardness Ductility

Electrical Conductivity Creep Resistance

Eutectics :Theory of Melting Point Indication of purity Physical property used for identification Pure compounds melt within a 1-2 degree

range

Impurities lower melting point:takes less energy to disrupt crystal lattice when impurities are presentmelting point will be lowermelting point will be broader

Theory of Melting Point

Mixed Melting Point - used to determine identity of compound:

UreaMP 120-121

Cinnamic acidMP 120-121

Have unknown compoundthat melts at 120-121. What is the unknown?

How could you tell?

Mixed Melting Point Mix unknown compound with a little urea

and measure melting point

+ unknown sample

If melting point is still 120-121 degrees, unknown compound was urea:

Mixed Melting Point Mix unknown compound with a little urea

and measure melting point

+ unknown sample

If melting point is lower and broader, i.e. 110-116, unknown compound was NOT urea:

Eutectic MixtureIt should be noted, however, that there is one unique mixture of two compounds, A and B, that has a lower melting point than any other mixture of the two compounds. This particular mixture is called the eutectic mixture. The melting point of the eutectic mixture is called the eutectic point. A mixture whose composition corresponds exactly to its eutectic mixture will have a relatively sharp melting point..

63

How to calculate the total amount of phase (both eutectic and primary)?

Fraction of phase determined by application of the lever rule across the entire + phase field:

W = (Q+R) / (P+Q+R) ( phase)

W = P / (P+Q+R) ( phase)

64

The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram

Steels: alloys of Iron (Fe) and Carbon (C). Fe-C phase diagram is complex. Will only consider the steel part of the diagram, up to around 7% Carbon.

65

Phases in Fe–Fe3C Phase Diagram a-ferrite - solid solution of C in BCC Fe

• Stable form of iron at room temperature. • The maximum solubility of C is 0.022 wt%• Transforms to FCC g-austenite at 912 C

g-austenite - solid solution of C in FCC Fe• The maximum solubility of C is 2.14 wt %. • Transforms to BCC d -ferrite at 1395 C • Is not stable below the eutectic temperature

(727 C) unless cooled rapidly (Chapter 10)

66

Phases in Fe–Fe3C Phase Diagram

d-ferrite solid solution of C in BCC Fe• The same structure as -ferrite• Stable only at high T, above 1394 C• Melts at 1538 C

Fe3C (iron carbide or cementite) • This intermetallic compound is metastable, it remains as

a compound indefinitely at room T, but decomposes (very slowly, within several years) into -Fe and C (graphite) at 650 - 700 C

Fe-C liquid solution

67

Comments on Fe–Fe3C system

C is an interstitial impurity in Fe. It forms a solid solution with , , a g d phases of ironMaximum solubility in BCC a-ferrite is 0.022 wt% at727 C. BCC:relatively small interstitial positionsMaximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger interstitial positionsMechanical properties: Cementite (Fe3C is hard and brittle: strengthens steels. Mechanical properties also depend on microstructure: how ferrite and cementite are mixed.Magnetic properties: -ferrite is magnetic below 768 C, austenite is non-magneticClassification. Three types of ferrous alloys: Iron: < 0.008 wt % C in -a ferrite at room T

Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ) a-ferrite + Fe3C at room T (Chapter 12)

Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)

Precipitate or Dispersion Hardening

Dispersion Strengthening- Strengthening by the introduction of a second phase.

Precipitation Strengthening- Through the formation of extremely small uniformly dispersed particles of a second phase within the matrix

Precipitate or Dispersion Hardening-Small particles

Particle Cutting

Particles should be small enough to be cut.

Large size particles offer more resistance to dislocation motion.

Heat Treatment of Steels for Strength: Steel = 0.06% to 1.0% carbon Must have a carbon content of at least .6%

(ideally) to heat treat. Must heat to austenitic temperature range. Must rapid quench to prevent formation of

equilibrium products. Basically crystal structure changes from BCC to

FCC at high Temp. The FCC can hold more carbon in solution and on

rapid cooling the crystal structure wants to return to its BCC structure. It cannot due to trapped carbon atoms. The net result is a distorted crystal structure called body centered tetragonal called martensite.

Almost always followed by tempering.

Final step: Temper!

Heat treating for strength!

10.4 Direct Hardening – Austenitizing and quench: Austenitizing – again taking a steel with .6%

carbon or greater and heating to the austenite region.

Rapid quench to trap the carbon in the crystal structure – called martensite (BCT)

Quench requirements determined from isothermal transformation diagram (IT diagram).

Get “Through” Hardness!!!

Quenching: Depending on how fast steel must be

quenched (from IT diagram), the heat treater will determine type of quenching required: Water (most severe) Oil Molten Salt Gas/ Air (least severe) Many phases in between!!! Ex: add

water/polymer to water reduces quench time! Adding 10% sodium hydroxide or salt will have twice the cooling rate!

Same requirements as austenitizing: Must have sufficient carbon levels (>0.4%) Heat to austenite region and quench

Why do? When only desire a select region to be

hardened: Knives, gears, etc. Object to big to heat in furnace! Large casting

w/ wear surface Types:

Flame hardening, induction hardening, laser beam hardening

13.4 Direct Hardening - Selective Hardening :

Flame Hardening:

Induction Hardening

Diffusion Hardening (aka Case Hardening): Why do?

Carbon content to low to through harden with previous processes.

Desire hardness only in select area More controlled versus flame hardening and

induction hardening. Can get VERY hard local areas (i.e. HRC of 60 or

greater) Interstitial diffusion when tiny solute atoms

diffuce into spaces of host atoms Substitiutional diffusion when diffusion atoms to

big to occupy interstitial sites – then must occupy vacancies

Diffusion Hardening: Requirements:

High temp (> 900 F) Host metal must have low concentration of the

diffusing species Must be atomic suitability between diffusing

species and host metal

Diffusion Hardening: Most Common Types:

Carburizing Nitriding Carbonitriding Cyaniding

Diffusion Hardening - Carburizing: Pack carburizing most common:

Part surrounded by charcoal treated with activating chemical – then heated to austenite temperature.

Charcoal forms CO2 gas which reacts with excess carbon in charcoal to form CO.

CO reacts with low-carbon steel surface to form atomic carbon

The atomic carbon diffuses into the surface Must then be quenched to get hardness!

Diffusion Hardening - Nitriding: Nitrogen diffused into surface being treated.

Nitrogen reacts with steel to form very hard iron and alloy nitrogen compounds.

Process does not require quenching – big advantage.

The case can include a white layer which can be brittle – disadvantage

More expensive than carburizing

Reduction process: 2NH3 2N + 3H2

Source of nitrogen

13.6 Softening and Conditioning - Recrystallization Annealing

Process anneal Stress relief anneal Normalizing

Tempering

13.6 Softening and Conditioning - Recrystallization Done often with cold working processes Limit to how much steel can be cold worked

before it becomes too brittle. This process heats steel up so grains return

to their original size prior to subsequent cold working processes.

Also done to refine coarse grains

13.6 Softening and Conditioning - Annealing Annealing – primary purpose is to soften the

steel and prepare it for additional processing such as cold forming or machining.

If already cold worked - allows recrystallization.

13.6 Softening and Conditioning - Annealing What does it do?

1. Reduce hardness2. Remove residual stress (stress relief)3. Improve toughness4. Restore ductility5. Refine grain size

13.6 Softening and Conditioning - Annealing Process Steps:

1. Heat material into the asutenite region (i.e. above 1600F) – rule of thumb: hold steel for one hour for each one inch of thickness

2. Slowly furnace cool the steel – DO NOT QUENCH

3. Key slow cooling allows the C to precipitate out so resulting structure is coarse pearlite with excess ferrite

4. After annealing steel is quite soft and ductile

Annealing versus Austenitizing: End result: One softens and the other

hardens! Both involve heating steel to austenite

region. Only difference is cooling time:

If fast (quenched) C is looked into the structure = martensite (BCT) = HARD

If slow C precipates out leading to coarse pearlite (with excess cementite of ferrite) = SOFT

13.6 Softening and Conditioning – Other forms of Annealing Normalizing – use when max softness not

required and cost savings desired (faster than anneal). Air cooled vs. furnace cooled.

Process Anneal – not heated as high as full anneal.

Stress Relief Anneal – lower temp (1,000F), slow cooled. Large castings, weldments

13.6 Softening and Conditioning - Temper Almost always done following heat treat as

part of the austenitizing process! Because of lack of adequate toughness and

ductility after heat treat, high carbon martensite is not a useful material despite its great strength (too brittle).

Tempering imparts a desired amount of toughness and ductility (at the expense of strength)

13.3 Softening and Conditioning - Temper Typical HT steps (Summarized Again):

Austenize: Heat into stable single phase region and HOLD for uniform chemistry single phase austenite.

Quench: Rapid cool – crystal changes from Austenite FCC to Martensite BCT which is hard but brittle.

Temper: A controlled reheat (BELOW AUSTENITE REGION). The material moves toward the formation of a stable two phase structure – tougher but weaker.

Quench: The properties are then frozen in by dropping temperature to stop further diffusion

The Heat Treat Processes

End of presentation