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8/11/2019 Diat Htt Lect 1 2 3
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HeatTreatment
Dr.SantoshS.Hosmani
1
Brief Intro. about Myself
uca ona ac groun :
1997 2001: B.E. (Metallurgy), V. Regional College of Engineering (currently NIT), Nagpur
2001 2003: M.Tech. (Process Meta.), Indian Institute of Technology, Bombay
2003 2006: Ph.D. (Physical Meta.), Max-Planck Institute for Metals Research, Stuttgart, Germany
Assistant Professor,Dept. Metallurgy & Materials Science, College of Engineering, Pune, India.
Assistant Professor,Dept. of Applied Mechanics,Indian Institute of Technology, Delhi, India.
Lecturer,Dept. of Metallurgical & Materials Engineering, National Institute of Technology
Karnataka, Surathkal, India.
Postdoctoral researcher, Dept. of Materials Science and Engineering, Case Western Reserve
, . . ..
Postdoctoral researcher,Max-Planck-Institute for Metals Research, Stuttgart, Germany.
2
HEAT TREATMENT
LECTURES: 3 hrs/week
FACULTY: Dr. Santosh S. Hosmani
Dept. of Metallurgy & Materials Science,
COE, Pune
Email: ssh.meta@coep.ac.in
Phone (use only in case of emergency plz): 9762316594
OFFICE HOURS: Open
3
GRADING / EXAMINATION SCHEME:
e g age
Quiz**- 1 & 2 20%
Mid-Sem Exam 30%
End Sem Exam 50%
**Note: Quiz can be surprise quiz OR Quizzes can be held on shortnotice. Therefore, continuous stud and attendance is desired.
Minimum passing marksfor this course is 40%.
Total attendancerequirement is as per the insti tu te rules. Any kind of
proxy is strictly prohibited.
Quiz/Exam wil l not be conducted againif anyone is absent without any
.
Depending upon the requirement of the course, Assignments/Home-work
could be given. However, there will be no weightage for the assignmentsin this course.
4
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Introduction of the course
In many engineering applications, steels are the most preferred material. There
are various types of steels which are evolved from the requirements of the
engineering components. Requi rement s is di rect ly l inked t o desi red
properties .
Properties can be manipulated by altering the chemistry of the alloyand/or bymechanical treatmentsand/or by giving appropriate heat-treatments.
As a materials engineer, literacy about the heat-treatment technology
processing & fundamental concepts is very essential. This knowledge teaches
the intelligent use of the existing grade of steel for a particular application.
Heat-treatment technology touches many important applications in automobile
and aerospace sectors.
-
treatment and metallurgy of the some important iron-based alloys.
Note: There are some topics in the course which does not require class-room
-
5
, - .
SYLLABUS:Available on your department website.
Heat Treatment of Metals, Vijendra Singh, 2007, Standard Publishers andDistributors, New Delhi
R.A. Hi ins En ineerin Metallur Part I A . Ph sical Met ELBS 5th
. . , , , . , ,
ed., 1983
REFERENCE BOOKS:
Steel and its Heat Treatment -K.E Thelning, Butterworth, London Handbook of Heat Treatment of Steels Prabhudev-Tata Mc Graw Hill.
New Delhi, 1988
, , .,1979
=>You can read any book available in your library.
=>Whenever lectures are in power-point-presentations, pdf-files of the
slides will be provided to you by email.
6
Friendly suggestion: Please learn to make good class-notes.
Some Basics revision of the concepts
7
Familytrees:organizingmaterialsandprocesses
likenessuseful to know when deciding which family to use for a given
design.
Choosing a material is only half the story. The other half is the choice of a
process route to shape, join and finish it.
processed in some ways but not others, and a given process can be applied to
some materials but not to others.
8
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Thematerialstree Classificationofmaterials
It is conventional to classify the materials of engineering into the six
broad families:
There is sense in this classification: the members of a family have certain features in
common: similar properties, similar processing routes and, often, similar applications.
9
, ,
each of which is characterized by a set ofattributes:its properties..
10
Metals:
They have relativelyhigh stiffness(modulus, E).
Most,when pure, are softand easily deformed, meaning that yis low.
Theycan be made strongby alloying and by mechanical and heat treatment,
y, ,
deformation processes.
And, broadly speaking, they aretough, with a usefully high fracture toughnessK1c. They are good electrical and thermal conductors.
But metals have weaknesses too: they are reactive; mostcorrode rapidlyif not
protected.
11
Stiffness and strength are central to mechanical design, often in
combination with thedensity,.
What is the stiffness?
,
meaning that the material returns to its original shape
when the stress is removed. Stiffness is measured bythe elastic modulusE.
E reflects stiffness, S, of the bonds that hold them
together.But, remember thatES
What is the strength?
failure. Strength is measured by the elastic limit y or
tensile strength ts.
Note: Stress and strain are not material properties.12
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It is theplasticityof iron and steel that made them thestructural materials
on which theIndustrial Revolutionwas built
18061859
British
Engineer
(17571834)
ThomasTelford He was perhaps the greatest engineer of the
Industrial Revolution in terms of design
co s eng neer, ,
theplasticity of iron and steel.
13
18061859
British
Engineer
IsambardKingdomBrunel
GreatEastern
Launched:
31
Jan.
1858
greatthingsarenot
donebythosewho
simplycountthecost14
engineering, derives from their ability
o e ro e , orge , rawn an
stamped.
15
Strength,plasticworkandductility
y ,
onset of plasticity is not always
distinct so we identify y with the
0.2% roo stressthat is the stress
forMetals
,
at which the stressstrain curve for
axial loading deviates by a strain of
0.2% from the linear elastic line. It is
the same in tension and
compression.
,
most metals work harden, causing
the rising part of the curve, until a
maximum, the tensile strength, is
reached.
This is followed in tension by non-
fracture.
16
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Strength,plasticworkandductility
forMetals
Howstressstraincurvelookslikeformetalsincompression? 17
Stress-strain dia rams for
Strength,plasticworkandductility
compression have different shapes
from those for tension. forMetals
limits in compression very close tothose in tension. However, when
ieldin be ins the behavior is uite,
different.
When a small specimen of ductile
,
bulge outward on the sides and
become barrel shaped. With
,
flattened out, thus offering increased
resistance to further shortening
which means the stress-strain
Ref.:DepartmentofCivilEngineeringattheUniversityofMemphis
curve goes upward).
18
TheoriginsofstrengthandductilityFundamentals
Perfection:theidealstrength
Thebondsbetweenatoms,likeanyotherspring,haveabreakingpoint.
Figure:Stressstrain curve for a
single bond.
Here an atom is assumed to occu a cube of sidea so that a force F corres ondsoto astress =F/ao
2.
The force stretches the bond from its initial length ao to a new length a, giving a
strain = a a a .o o.
In case of the modulus we focused on the initial, linear part of this curve, with a
slope equal to the modulus, E. Stretched further, the curve passes through a
.
bond strengthif you pull harder than this it will break.
19
The distance over which interatomic forces act is smalla bond is broken if it is
Perfection:theidealstrength
.
So the force needed to break a bond is roughly:
F
t nesson =,
10%10
%10
oaaa
lengthbondorigionalof
===
=
10100oo
10/oa
FS=
On this basis the ideal strength of a solid
should therefore be roughly:
10
oaSF =Q
SF
ES
=
=
:andbetweenRelation
2
o
oo
a
S=
&etweenrelationlinearAssume
oa
SE=Q
Note: This relationship doesnt allow for the
curvatureof the forcedistance curve;more refined
calculations give a ratio of 1/15. 20
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Perfection:theidealstrength
Surprisingly,
None of the metals, polymers and ceramics achieve the ideal value of1/10; most dont even come close.
21
CriticalResolvedShearStress
eory
(GPa)
xper men
(MPa)
a o
Theory/Exp
Fe (BCC) 12 15 800
Cu (FCC) 7 0.5 14,000
Zn (HCP) 5 0.3 17,000
22
Perfection:theidealstrength
Surprisingly,
None of the metals, polymers and ceramics achieve the ideal value of
1/10; most dont even come close.
Whynot?
Nothing is perfect in this world.!
Existence of Imperfections / Defects in materials.!
23
Dislocation
Thedislocationis the key player in explaining important mechanical properties, like
strength and ductility.
Dislocated means out of joint and this is not a bad description of what is
happening here. The upper part of the crystal has extra halflayer of atoms than
the lower part.
It is dislocations that make metals soft and ductile.
because of this they have elastic energy associated with them.
24
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Inventorsofdislocationconcepts
an ideal strength aroundE/15 (whereEis the modulus).
In reality the strengths of engineering materials are nothing like this big;
o ten t ey are are y 1% o it.
Hungarian/US
physicist and
British
mathematician,
metallurgistphysicist and
expert on fluid
dynamics and
wave theor
Sir Geoffrey Ingram Taylor Egon Orowan
ese two persona t es rea ze t at a s ocate crysta
could deform at stresses far below the ideal.25 Ref.: Book by W.D. Callister
Ref.: Book by W.D. Callister Ref.: Book by W.D. Callister
8/11/2019 Diat Htt Lect 1 2 3
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Ref.: Book by W.D. Callister Ref.: Book by W.D. Callister 30
Thelatticeresistance
Dislocation Motion Plastic Deformation
asy s oca on o on asy as c e orma on
Weak Cr stal
Difficult
Dislocation Motion
Difficult
Plastic Deformation
Strong Crystal
31
Where does the resistance to
. . , ,
come from?
32
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The lattice resistance
There are several contributions to this resistance.
I. Lattice resistance, fi:
s s e n r ns c res s ance o e crys a s ruc ure o p as c s ear.
Plastic shear, as we have seen, involves the motion of dislocations.
Pure metals are soft because the non-localized metallic bond does little
to obstruct dislocation motion, whereas ceramics are hard because their
more localized covalent and ionic bonds (which must be broken and
reformed when the structure is sheared) lock the dislocations in place.
The electrons and +ve ions
are all in a fixed position.
The electrons are in a fixed
position
The electrons are not
fixed and free to move
throughout the lattice. 33
The lattice resistance
When the lattice resistance is hi h, as in ceramics, further hardenin is
more than sufficientthe problem becomes that of suppressing fracture: i.e.yield strength is much larger than fracture strength of ceramics.
On the other hand, when the lattice resistance fiis low, as in metals, the
material can be strengthened by introducing obstacles to slip.
II. other dislocations giving what is called Work Hardening (fwh),
III. grain boundaries introducing Grain-size Hardening (fgb),
IV. precipitates or dispersed particles giving Precipitation Hardening (fppt),
V. by adding alloying elements to give Solid Solution Hardening (fss).
ese ec n ques or man pu a ng s reng are cen ra o a oy es gn.
34
II. Work hardening / Strain hardening, fwh:
ur ng p ast c e ormat on s ocat on ens ty
of a crystal should go down
But,
Experimental Result
Well-annealed crystal: 1010 m-2
Lightly cold-worked: 1012 m-2
eav y co -wor e : m-
35
II. Work hardening / Strain hardening, fwh:
Workhardeningor
StrainHardening
y
Strain,
see Book by V. Raghvan36
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II. Work hardening / Strain hardening, fwh:
37
II. Work hardening / Strain hardening, fwh:
During plastic deformation dislocation density increases.
Plastic deformation increases the yield strength of the
crystal: strain hardening or work hardening
Why deformation increases strength?
What exactly is the Strain Hardening?
38
II. Work hardening / Strain hardening, fwh:
Strain Hardening:
Dislocation against Dislocation
A dislocation in the path of other
dislocation can act as an obstacle to the
motion of the latter
39
Sessile dislocation in an FCC crystal:
< + a
2
2
1bE =
g,
fwh:
1
222aaa
+ is the shear stress required to move a single dislocation in theabsence of any other dislocation
42
II. Work hardening / Strain hardening, fwh:
metals and alloys, e.g. Al-based alloys
43
III. Grain-size / Grain-boundary hardening, fgb:
ran
Grain1
Grain boundary
44
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III. Grain-size / Grain-boundary hardening, fgb:
Discontinuity of a slip plane across a grain boundary
Slip plane
Dislocation
Grain Boundary
45
III. Grain-size / Grain-boundary hardening, fgb:
A dislocation cannot glide across a grain
Higher stresses required for deformation
Finer the grains, greater the strength
Ref.: Book by V. Raghavan46
III. Grain-size / Grain-boundary hardening, fgb:
Role of Grain Size in Strengthening
Hall-Petch Relation
k y
D= +0
y y e strengt
D: average grain diameter
0,k: constants
Coarse Grains Fine Grains0 => yield strength of a single
crystal
47
III. Grain-size / Grain-boundary hardening, fgb:
Role of Grain Size in Strengthening
k y
D= +0
Coarse Grains Fine Grains
This concept is relevant to annealing,-
alloy design, e.g. HSLA steel 48
Recommended