modern construction materials mod i
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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.
48
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
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