Mat ii chapter 1

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Wollega University
Collage of Engineering and Technology
Department of Mechanical Engineering
Chapter One – STEEL
Materials Engineering II
INS. Asegid Tadesse

Objectives
Mechanical Engineering
This chapter aims in understanding of:
Part 1:
 Classification of ferrous metals
 Steel and alloy steels
Part 2:
 Heat treatment of plain carbon steels and
Part 3:
 Production of steel
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Introduction
• A material is a substance or mixture of substances that constitutes an
object. Materials can be pure or impure, living or non-living matter.
• Materials can be classified based on their physical and chemical
properties, or on their geological origin or biological function.
• Materials science is the study of materials and their applications.
• Raw materials can be processed in different ways to influence their
properties, by purification, shaping or the introduction of other
materials.
• New materials can be produced from raw materials by synthesis.
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• Materials can be broadly categorized in terms of their use, for example:
• Building materials are used for construction
• Building insulation materials are used to retain heat within buildings
• Refractory materials are used for high-temperature applications
• Nuclear materials are used for nuclear power and weapons
• Aerospace materials are used in aircraft and other aerospace applications
• Biomaterials are used for applications interacting with living systems
• Material selection is a process to determine which material should be
used for a given application.
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Intro…….
Classification by structure
• The relevant structure of materials has a different length scale depending on the material.
The structure and composition of a material can be determined
by microscopy or spectroscopy.
• Microstructure In engineering, materials can be categorized according to their
microscopic structure:
Ceramics: non-metal, inorganic solids
Glasses: amorphous solids
Metals: pure or combined chemical elements with specific chemical bonding behavior
Polymers: materials based on long carbon or silicon chains
Hybrids: combinations of multiple materials, for example composites.
• Larger-scale structure In foams and textiles, the chemical structure is less relevant to
immediately observable properties than larger-scale material features: the holes in foams,
and the weave in materials.
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Engineering materials
Ferrous metals Non-ferrous metals
Synthetic materials Natural materials
Non-metallic materials
Metallic materials
Classification
Introduction
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High-duty cast iron
Malleable cast iron
White cast iron
Grey cast iron
Alloy cast iron
Alloy steel
High carbon steel
Low carbon steel
Medium carbon steel
Steel Cast iron Wrought iron
Ferrous metals
Classification
Introduction
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Metals Alloys
Aluminum
Chromium
Copper
Lead
Manganese
Nickel
Silver
Titanium
Vanadium
Cadmium
Cobalt Gold
Magnesium
Molybdenum
Platinum
Tin
Tungsten
zinc
• Brass (copper + zinc)
• Tin bronze (copper + tin)
• Aluminum bronze ( copper + aluminum)
• Cupro-nickel alloys ( copper + nickel)
• Aluminum alloys
• Magnesium alloys
• Zinc-based die casting alloys
• Tin-lead alloys
Non-ferrous metals
Classification
Introduction
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Carbon fiber
Concrete
Glassreinforced plastic
Thermoplastic
Ac
rylic
P
T
F
E
Polythene
PVC
Nylon
Polystyrene
Glass
Porcelain
Cemented carbides
Thermosetting
Epoxides
Alkyds
Bakelite
Melamine-formaldehyde
Urea-formaldehyde
Plastics Ceramics Composites
Synthetic materials
Classification
Introduction
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Ferrous metals and alloys
 Ferrous metals and alloys are based upon the metallic element iron
 Iron is a soft, grey metal and it is rarely found in the pure state outside the laboratory.
 Engineers usually find it associated with the non-metal carbon, with which it forms
solid solutions and the compound iron carbide.
 The carbon content is carried over from smelting process during which the iron is
extracted from its ore.
Introduction
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Introduction
 Since all the ferrous materials used by engineers contain iron in association with carbon, it could be
argued that all such materials are ferrous alloys.
 The term ferrous alloy is reserved for those ferrous materials containing additional metallic alloying
elements in sufficient quantities substantially to modify the properties of the material.
 Those 'alloys' containing only carbon as the main alloying element are referred to as:
• wrought iron,
• plain carbon steels and
• plain cast irons
 Classification is depending upon the amount of carbon present and the way in which it is
associated with the iron content.
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Introduction
 Factors that let ferrous alloys to be widely used:
 Iron containing compounds exist in abundant quantities within the
earth’s crust.
 Metallic iron and steel alloys may be produced using relatively
economical extraction, refining, alloying, and fabrication techniques.
 Ferrous alloys are extremely versatile: may be tailored to have a
wide range of mechanical and physical properties.
 Principal disadvantage:
 Susceptible to corrosion.
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 Ferrous metals contain a high proportion of the element iron.
 They are the strongest materials available and are used for applications where:
• Where high strength is required at relatively low cost and
• Where weight is not of primary importance.
 Examples:
• bridge building
• the structure of large buildings
• railway lines
• locomotives & rolling stock and
• bodies & highly stressed engine parts of road vehicles.
• However, light weight materials such as aluminum alloys and plastics (polymers) are
being increasingly used in railway and road vehicles to reduce their weight and make them
more efficient in the use of energy.
Introduction
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Steel And Alloy Steels
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High-duty cast iron
Malleable cast iron
White cast iron
Grey cast iron
Alloy cast iron
Alloy steel
High carbon steel
Low carbon steel
Medium carbon steel
Steel Cast iron Wrought iron
Ferrous metals
Steel and alloy steels
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Steel
• Steel is an alloy of iron with typically a few percent of carbon to improve
its strength and fracture resistance compared to iron.
• Because of its high tensile strength and low cost, steel is used
in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances,
and weapons.
• Iron is the base metal of steel.
• Depending on the temperature, it can take two crystalline forms (allotropic
forms): body-centred cubic and face-centred cubic.
• The interaction of the allotropes of iron with the alloying elements, primarily carbon,
gives steel and cast iron their range of unique properties.
• So, if you are here wondering, what type of steel to buy for your particular needs, you
must understand the chemical structure of the physical steel properties, which are
broken down into four foundational types.
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• The Four Main Types of Steel
Carbon steel
Alloy steel
Tool Steel
Stainless steel
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Plain carbon steels:
 Carbon steel or plain-carbon steel is a combination of two elements, iron and carbon. Other
elements are present in quantities too small to affect its properties.
 Alloys of iron and carbon in which the carbon is chemically combined with the iron.
 Plain carbon steels are ferrous materials containing between 0.1 and 1.7% carbon as the
main alloying element.
 Impurities (such as sulfur and phosphorus) from the extraction process and a small amount
of the metal manganese offset the toxic effects of the impurities.
 None of the plain carbon steels can be alloying elements.
 In addition, plain carbon steels contain the following elements.
 Manganese: up to 1%
 Phosphorous: up to 0.05%
 Silicon: up to 0.3%
 Sulfur: up to 0.05%
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Low carbon steels Property Application
• Contain less than about 0.25
wt% C.
• Unresponsive to heat treatments
intended to form Martensite.
• Strength is accomplished by cold
work.
• Microstructures consist of ferrite
and pearlite constituents.
• Relatively soft and weak.
• Have excellent ductility and
toughness.
• Highly machinable and
weldable.
• Easy to produce than all of
steels.
• Automobile body
components
• Structural shapes (I-
beams, channel and
angle iron).
• Sheets that are used in
pipelines, buildings,
bridges and
Typical values:
o Yield strength: 275MPa
o Tensile strength: 415-550MPa
o Ductility: 25 %EL
Low carbon steels:
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• Martensite is a very hard form of steel crystalline structure. It is
named after German metallurgist Adolf Martens. By analogy the term
can also refer to any crystal structure that is formed by diffusionless
transformation
• A diffusionless transformation is a phase change that occurs without
the long-range diffusion of atoms but rather by some form of
cooperative, homogeneous movement of many atoms that results in a
change in crystal structure.
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High-strength, low alloy
(HSLA) steels
Property Application
• Other group of low carbon
steels.
• Highstrengththan plain low- carbon
steels.
• Bridges
• Towers
• Support
columns
• In high rise
buildings
• Pressure
vessels.
• Contain other alloying
elements such as copper,
vanadium, nickel, and
molybdenum in combined
concentrations as high as 10
wt%.
• Most can be strengthened by heat
treatment, giving tensile strengths in
excess of 480 MPa.
• Are ductile, formable and machinable.
• More resistant to corrosion than the
plain carbon steels, which they have
replaced in many applications where
structural strength is critical.
Low carbon steels:
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Medium carbon steels Property Application
• Carbon
between
wt%.
concentration
0.25 and 0.6
• May be heated by austenitizing,
quenching, and then tempering to
improve mechanical properties.
• Railway wheels, and
tracks, gears, crankshafts,
and other machine parts.
• Have low hardenabilities.
• Can be heat treated only in very
thin sections and with very rapid
quenching rates.
• High-strength structural
components calling for a
combination of high
strength, wear resistance
and toughness.
• Addition of Cr, Ni and Mo
improve the capacity of these alloy
to be heat treated.
Medium carbon steels:
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High carbon steels Property Application
• Carbon concentrations
between 0.6 and 1.4 wt%.
• Almost always used in a
hardened and tempered
condition.
• Wear resistance and capable of
holding a sharp cutting edge. • Cutting tools and dies
for forming and shaping
materials.
• Knives, razors, hacksaw
blades, springs, and high
strength wire.
High carbon steels:
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High carbon steels Property Application
• The predominantly
alloying elements is Cr.
• A concentration of 11
wt% Cr is required.
• Divided into three on the
basis of the predominant
phase constituent of the
microstructure.
 Martensitic
 Ferritic
 Austenitic
• Highly resistance to corrosion
(rusting).
• Corrosion resistance may be enhanced
by Ni and Mo additions.
• Martensite stainless steels are capable of
being heat treated in such a way that
martensite is the prime
microconstituents.
• Austenitic and ferritic stainless steels
are hardened and strengthened by cold
work because they are not heat treatable.
• Austenitic stainless steels are most
corrosion resistance due to high Cr
content and Ni additions.
• Martensite and Ferrite are magnetic.
• Frequently used at elevated
temperatures, and in sever
environments because they
resist oxidation, and maintain
their mechanical integrity
under such conditions.
• The upper temperature limit in
oxidizing atmospheres is about
10000C.
• Equipment employing these
steels includes gas turbines,
high-temperature steam boilers,
heat-treating furnaces, aircraft,
missiles, and nuclear power
generating units.
Stainless a steels:
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Compositions of five plain low-carbon steels and three high-strength, low-alloy steels. Unless indicated,
composition contains 0.04 wt% P, 0.05 wt% S, 0.3 wt%Si
o AISI – American Iron and Steel Institute
o SAE – Society of Automotive Engineers
o ASTM – American Society for Testing and Materials
o UNS – Uniform Numbering System
Plain low-carbon and high strength, low-alloy (HSLA) steels:
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Mechanical characteristics of hot-rolled material and typical applications of various plain low-carbon
steels, high strength, low-alloy steels.
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AISI/SAE and UNS designation Systems and Composition Ranges for Plain Carbon steel and Various Low-alloy steels.
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Typical application and Mechanical property ranges for oil-quenched and tempered Plain Carbon and
Alloy Steels. a-classified as high-carbon steels
Plain carbon and alloy steels: Oil quenched and tempered
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Designation, compositions, and applications for six tool steels.
Tool steels:
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Designation, compositions, mechanical properties and typical applications for
austenitic, ferritic, martensitic, and precipitation-hardenable stainless steels.
Stainless steels: annealed, quenched, and tempered
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Designation, compositions, mechanical properties and typical applications for austenitic, ferritic,
martensitic, and precipitation-hardenable stainless steels.
17-7PH (ultrahigh-strength stainless steel): is unusually strong and corrosion resistant.
Strengthening is accomplished by precipitation-hardening heat treatment.
Stainless steels: annealed, quenched, and tempered
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Steel standards
SAE, AISI, and ASTM:
• Are responsible for the classification and specification of steels as well as other alloys.
• The AISI/SAE designation for these steels is a four-digit number: the first two digits indicate the alloy
content; the last two give the carbon concentration.
• For plain carbon steels, the first two digits are 1 and 0;
• Alloy steels are designated by other initial two-digit combinations (e.g., 13, 41, 43).
• The third and fourth digits represent the weight percent carbon multiplied by 100.
• For example, a 1060 steel is a plain carbon steel containing 0.60 wt% C.
UNS:
• Is used for uniformly indexing both ferrous and nonferrous alloys.
• Each UNS number consists of a single-letter prefix followed by a five-digit number.
• The letter is indicative of the family of metals to which an alloy belongs.
• The UNS designation for these alloys begins with a G, followed by the AISI/SAE
number; the fifth digit is a zero.
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• Society of Automotive
Engineers (SAE),
• The American Iron and
Steel Institute (AISI),
• The American Society
for Testing and
Materials (ASTM):
• Unified Numbering
System (UNS):

XX - XX
• The first two digits represent the type of material selected.
• The last two digits represent amount of carbon present in the steel.
Type of steel 1st digit 2nd digit
C steels “1” (10xx, 11xx. 12xx) Describe processing:
“1” resulfurized
“2” resulfurized & rephosphorized
Mn steel “1” (13xx) Always 3
Ni steel “2” (23xx, 25xx) % of Ni in the steel
Ni-Cr steel “3” (31xx, 32xx) % of Ni and Cr in the steel
Mo steel “4” (40xx, 44xx) % of Mo in the steel
Cr steel “5” (51xx, 52xx) % of Cr in the steel
Cr-V steel “6” (61xx) % of Cr and V in the steel
W-Cr steel “7” (72xx) % of W and Cr in the steel
Ni – Cr – Mo steel “8” (81xx, 86xx, 87xx, 88xx) % of Ni, Cr and Mo in the steel
Si-Mn steel “9” (92xx) % of Si and Mn in the steel
Triple alloy steels
(contains 3 alloys)
“4”, “8” or “9” depending on the
predominate alloy
% of the remaining two alloys.
AISI / SAEsteel designation system
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Effect of alloying Elements
• Chromium (Cr) makes steel hard whereas Nickel (Ni) and Manganese (Mn) make
it tough.
• Note that:
• 2% C, 12% Cr tool steel grade - very hard and hard-wearing
• 0,10% C and 12% Cr - Modest hardening
• 13% manganese steel, so-called Hadfield steel - increases steel toughness
• Mn between l% and 5%, however - toughness may either increase or decrease
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Manganese Phosphorous
• Increases the strength and toughness of the steel.
• It also increases the tendency of the steel to crack
and distort when quench hardened and, for this
reason, the content should be kept below 0.5% in
medium- and high-carbon steels.
• An impurity carried over from the iron ore.
• It forms compounds which make the steel brittle and,
therefore, should be removed as far as possible during the
refinement processes.
• It should not be present in excess of 0.05%.
Silicon Sulfur
• An impurity from the iron ore.
• Its presence should be limited to between 0.1
and 0.3% in the steels otherwise it can cause
breakdown of the cementite which would result
in weakness.
• Silicon has little direct effect upon the
mechanical properties of plain carbon steels
providing the amount present is limited to the
%age quoted above.
• Silicon improves the magnetic properties of the
'soft' ferro-magnetic materials.
• An impurity carried over from the fuel used in the blast
furnace to extract the iron from its ore.
• With iron, it forms iron sulfide which greatly weakens the
steel. Therefore, its content must be kept below 0.05% and
there should always be at least 5 times as much manganese
present as there is sulfur.
• It has a greater affinity for Mn than it has for steel and will
combine with the Mn in preference to the iron. Unlike iron
sulfide which weakens the steel, manganese sulfide has no
such adverse effect.
• Some free-cutting steels contain up to 0.2% sulfur to
improve their machinability.
Principal alloying elements:
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 Fig, shows the effect of the carbon content upon the properties of plain carbon steels which have been cooled
slowly enough to enable them to achieve phase equilibrium.
 It can be seen from Fig that low-carbon steels, consisting mainly of ferrite, are soft and ductile and relatively
weak, reflecting the properties of the ferrite itself.
Effect of carbon on the properties of plain carbon steels:
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Effect of carbon on the properties of plain carbon steels:
 The increased amount of carbon in medium-carbon steels promotes:
 The formation of cementite.
 This results in an increased presence of pearlite, making such steels stronger, tougher and
harder, but not so ductile.
 When the carbon content reaches approximately 0.83%:
 The steel consists entirely of pearlite.
 This is the eutectoid composition previously described
 It produces plain carbon steel of maximum toughness and strength.
 Increasing the carbon content still further increases the amount of cementite (Fe3C) present in the
steel.
 Since the maximum amount of combined cementite occurred at 0.83% carbon content, where the
composition of the steel is totally pearlitic, any increase in the carbon content results in the formation
of excess cementite appearing around the crystal boundaries. This increases the hardness and wear
resistance of the steel, but at the expense of reduced toughness, strength and ductility.
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Alloy steels:
Alloy:
 Is an intimate association of two or more component materials which form a
single metallic liquid or solid.
 The component materials may be metal elements, or metal elements and non- metal elements, or
metal elements and chemical compounds.
 Useful alloys can only be produced from component materials which are soluble in each other in the
molten state.
 The alloying elements must be completely miscible.
 So design of alloying should be considering and knowing the property of the
alloying elements especially during heating and cooling.
Example:
 If one tries to make alloy from the immiscible alloys of zinc and lead, the molten zinc will float on top of
the molten lead and up on cooling, they would form separate layers in the solid state with only tenuous
bonding at the interface.
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Alloy steels:
Alloying elements:
 Are component materials of an alloy which are added in controlled quantities to modify
the properties of a material to match a particular specification.
Impurities:
 Are undesirable elements which are usually carried over from some previous processes
such as smelting or casting.
 They should be removed or reduced to a level where their effects should become
insignificant.
 Impurities must not be confused with alloying element.
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Alloy steels:
 Good electrical conductivity, thermal conductivity and good corrosion
resistance or combinations of theses properties are required.
 But strength is also another mechanical property which is highly required for structural materials.
 Pure metals have all the property except they lack the strength required for most application.
 Alloys are mainly used for structural materials since they can be formulated to give superior
mechanical properties.
 Properties such as tensile strength, yield strength, and hardness are improved by alloying. It is
also known that the property of ductility of the alloy is reduced. So alloying is designed for a
particular application.
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Alloy steels:
 Are similar to low-carbon and medium-carbon steels with the addition of
other metals such as manganese, nickel, chromium, molybdenum and
vanadium, in sufficient quantities to materially alter and enhance the
properties of the metal.
 May be present singly or in combination for an increased strength,
improved corrosion resistance, improved heat resistance
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Quiz - 1
1. Classify the nature of materials.
2. How to classify ferrous metals?
3. Name the following
SAE, AISI, and ASTM
4. List some of the containing of Plain carbon steels.
5. List application of low carbon steel.
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Effect of Alloying Elements
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Aluminum
Ferrite hardener
Graphite former
Deoxidizer
Chromium
Mild ferrite hardener
Moderate effect on hardenability
Graphite former
Resists corrosion
Resists abrasion
Cobalt
High effect on ferrite as a hardener
High red hardness
Molybdenum
Strong effect on hardenability
Strong carbide former
High red hardness
Increases abrasion resistance

Effect of Alloying Elements…cont
Manganese
Strong ferrite hardener
Nickel
Ferrite strengthener
Increases toughness of the hypoeutectoid steel
With chromium, retains austenite
Graphite former
Copper
Austenite stabilizer
Improves resistance to corrosion
Silicon
Ferrite hardener
Increases magnetic properties in steel
Phosphorus
Ferrite hardener
Improves machinability
Increases hardenability

Effect of alloying elements
2XXX Nickel steels
5 % Nickel increases the tensile strength without reducing ductility
8 to 12 % Nickel increases the resistance to low temperature impact
15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic
properties. (Alnicometals)
25 to 35 % Nickel creates resistance to corrosion at elevated temperatures.
3XXX Nickel-chromium steels
These steels are tough and ductile and exhibit high wear resistance
, hardenability and high resistance to corrosion.
4XXX Molybdenum steels
Molybdenum is strong carbide former. It has a strong effect on
hardenability and high temperature hardness. Molybdenum also
increases the tensile strength of low carbon steels.
5XXX Chromium steels
Chromium is a ferrite strengthener in low carbon steels. It
increases the core toughness and the wear resistance of the case in
carburized steels.

Part two - Heat treatment of plain carbon steels
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Heat-treatment processes of plain carbon steels:
 Heat treatment is a controlled heating and cooling of metals to change their properties to
improve their performance or to facilitate processing.
 Plain carbon steels and alloy steels are among the relatively few engineering materials which
can be usefully heat treated in order to vary their mechanical properties.
 Steels can be heat treated because of the structural changes that can take place within solid
iron-carbon alloys.
 The various heat-treatment processes appropriate to plain carbon steels are:
 Annealing
 Normalizing
 Hardening &
 Tempering
Heat treatment
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Heat-treatment processes of plain carbon steels:
 In all these processes the steel is heated slowly to the appropriate temperature for its carbon content and
then cooled.
 It is the rate of cooling which determines the ultimate structure and properties that the steel will have,
providing that the initial heating has been slow enough for the steel to have reached phase equilibrium
at its process temperature.
 Strength in steels correlates how much MARTENSITE remains in the final structure.
 Martensite:
 BCT (body centered tetragonal)
 Hardest and strongest among microstructures
 Most brittle and negligible ductility.
 Tempering heat treatment allows formation of tempered martensite (composed of
stable ferrite and cementite phases).
 Hardenability is often considered as the ability of a structure to transform to
martensite.
Heat treatment
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Heat-treatment processes:
 During cooling:
 The surface cools more rapidly than the interior producing a range of microstructures
throughout.
 The martensitic content and the hardness will drop from a high value at the surface
to a low value in the interior of the specimen.
 Therefore, production of uniform martensitic structure depends on:
• Composition
• Quenching conditions
• Size and shape of the specimen
 Heat treatment is associated with
 Increasing the strength of materials.
 Alter manufacturability: improve machinability, formability, restore ductility after cold working.
 Refine grain structure, remove internal stresses, and improve wear resistance.
Heat treatment
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Portion of Fe-Fe3C phase diagram in the
vicinity of the eutectoid.
 Shows heat treatment temperature ranges for
plain carbon steels.
Labeling of phase boundaries:
 Lower critical temperature:
• Labeled by A1, at the eutectoid temperature
• Below A1, all austenite will have transformed
into ferrite and cementite phases
 Upper critical temperature line for hypoeutectoid steels
, <0.76 wt% C :
• Labeled by the phase boundary A3
• For temperatures and compositions above A3, only the
austenite phase will prevail.
 Upper critical temperature line for
hypereutectoid steels, 0.76-2.14 wt% C:
• Labeled by the phase boundary Acm.
• For temperatures and compositions above Acm,
only the austenite phase will prevail.
Heat treatment
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Basic principles of heat-treatment processes:
Recrystallization:
 During cold-working processes, the grain of the metal becomes distorted and internal stresses are
introduced into the metal.
 If the temperature of the cold-worked metal is now raised sufficiently, nucleation occurs and 'seed'
crystals form at the grain boundaries at points of maximum internal stress. This is called
nucleation.
 The more sever the cold working and the greater the internal stress, the lower will be the
temperature at which nucleation occurs for a given metal. The principle of nucleation is shown in
Fig.
 The transformation of cold-worked grains to an undistorted shape is called recrystallization.
 Recrystallization temperature is the minimum temperature at which the reformation of the grain
occurs.
Heat treatment
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a) Before cold working
b) After cold working
c) Nucleation commences at recrystallization
temperature
d) Crystals commence to grow. Atoms migrate from
the original crystals and attach themselves to the
nuclei.
e) After annealing is complete the grain structure is
restored
Process of recrystallization:
Heat treatment
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Stress relieving:
 During cold-working processes, the grain of the metal becomes distorted and
internal stresses are introduced into the metal.
 One reason for heat treatment is to remove internal stresses from a metal that has been
subjected to cold-working or welding.
 Stress relievingis a heat treatment used to remove internal strains without significantly
lowering the strength.
 It is used where close dimensional control is needed on welded parts, forgings, castings, etc.
Heat treatment
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Forming operations:
 Operations in which the shape of a metal is changed by plastic deformation.
 The deformation must be induced by an external force or stress, the magnitude of
which must exceed the yield strength of the material.
 These operations include:
• Forging
• Rolling,
• Extrusion and
• Drawing.
Heat treatment
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 Deforming a single piece of a normally hot metal.
 It is classified as open or closed die.
• Closed die: a force is brought to bear on two or more die halves having
the finished shape such that the metal is deformed in the cavity between
them.
• Open die: two dies having simple geometric shapes (e.g. parallel flat, or
semicircular) are employed, normally on large workpiece.
 Forged pieces have outstanding grain structure and best combinations of
mechanical properties.
Crankshaft
Connecting rod
Wrenches
Basicprinciples of heat-treatment processes:
Forging:
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 Is the most widely used deformation process, consisting
of a piece of metal between two rollers.
 In this process, a reduction in thickness results from
compressive stresses exerted by the rolls.
 Used in the production of sheet, strip, and foil with high
quality surface finish.
 Circular shapes, as well as I beams and railroad rails can be
fabricated using grooved rolls.
Rolling:
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 A bar of metal is forced through a die orifice by a
compressive force that is applied to a ram.
 The extruded piece that emerged has the desired shape and a
reduced cross-sectional area.
 Extrusion products include:
• Rod and tubing that have complicated cross-sectional
geometries.
• Seamless tubes.
Extrusion:
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Drawing:
 The pulling of a metal piece through a die having a tapered bore
by means of a tensile force that is applied on the exit side.
 A reduction in cross-section results with a corresponding increase
in length.
 The total drawing operation may consist of a number of dies in a
series sequence.
 Typical examples: rod, wire, and tubing products.
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Cold working:
 This a process where a metal is bent, squeezed or stretched to shape below the
temperature of recrystallization.
 Cold working results in distortion of the grain of the metal and, eventually, the metal
becomes so stiff and brittle that it breaks.
 Metal that has become harder and stiffer as a result of cold working is said to be work
hardened.
 The metal must not be allowed to become excessively work hardened or it will be prone
to fracture.
 Once work hardened, it needs to be annealed to restore its grain structure before further
cold working is performed upon it.
 Cold working processes include: pressing out car body panels, cold drawing rods, wires
and tubes, cold-heading rivets, and cold-rolling strip and sheet metal.
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Hot working:
 This is a process where a metal is bent, squeezed or stretched to shape above the temperature of
recrystallization.
 Forging, hot rolling and extrusion are among hot working processes.
 Since the process temperature is above the temperature of recrystallization, the grains reform as
fast as they are distorted by the processing.
 If the metal could be retained at this temperature, there would be no limit to the amount of hot
working to which the metal could be subjected.
 For hot working operations, large deformations are possible.
 Deformation energy requirements are less than for cold working.
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Strict limitations to be considered during hot working:
 The initial temperature has to be limited so that:
 Overheating and 'burning' of the metal
 It won’t be excessively weakened
 The melting point is not reached
 If the finishing temperature is too high, subsequent grain growth occurs which leads to:
 A reduction in strength.
 Poor machining properties
 It is not too low so that surface cracking occurs.
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Annealing processes:
 Annealing:
• is a heat treatment in which a material is exposed to an elevated temperature for an extended time
period and then slowly cooled.
• Each annealing process is characterized by the changes induced in
microstructure and alterations in mechanical properties.
 It is carried out to:
• To relieve stresses
• To increase softness, ductility and toughness
• To produce a specific microstructure.
• To create best conditions for flow forming. (Grains will be too coarser for it to machine to a good
finish.)
 Annealing processes vary by the changes they result in microstructure which results changes in
mechanical property.
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 Any annealing process consist of three stages where time is an important parameter:
• Heating to the desired temperature
• Holding or “soaking” at that temperature
• Cooling, usually to room temperature
 Time and Temperature are the two important annealing parameters.
• Annealing time must be long enough to allow for any necessary transformation reactions.
• The magnitude of temperature gradient depends on the size and geometry of piece.
• Too high rate of temperature change may warping or cracking.
• Annealing may be accelerated by increasing the temperature, since diffusional processes are normally involved.
• By preventing annealing at a relatively low temperature (but above the recrystallization temperature) or in a non-
oxidizing atmosphere.
 A heat treatment that is used to negate the effects of cold work. That is it softens, and increases the ductility of a previously strain-
hardened metal.
 It is used in fabrication procedures that require extensive plastic deformation. This allows a continued deformation without
fracture or excessive energy consumption.
 Recovery and recrystallization processes are allowed to occur.
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 The three basic annealing processes
as shown in the figure:
 Process(Stress- relief) annealing
 Spheroidized annealing and
 Full annealing
Annealing temperatures for plain carbon steels.
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Process (stress relief) annealing:
 It is a process where a metal piece is heated to recommended temperatures, held long enough to
attain a uniform temperature, and finally slowly cooled to room temperature in air.
 This process is reserved for steels below 0.4 % C content. They are relatively ductile, they are
frequently cold worked and become work hardened.
 Annealing temperatures are usually low such that the effects resulting from cold working and other
heat treatments are not affected.
 Internal residual stresses develop in response to:
 Plastic deformation processes such as machining and grinding.
 Non-uniform cooling of a piece that was processed or fabricated at an elevated temperature,
such as welding and casting.
 A phase transformation that is induced up on cooling wherein parent and product phases have
different densities.
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Process (stress relief) annealing:
 Critical temperatures are temperatures at which change of state (phase changes) occur on
phase equilibrium diagrams. Heat treatment pressures are related to these temperatures.
 Since the grain structure will have been severely distorted by the cold working,
recrystallization can commence at 5000C.
 In practice, annealing is usually carried out between 630 and 7000C to speed up the process
and limit grain growth.
 The rate of cooling and the length of time for which the steel is heated depend upon the
subsequent processing and use to which the material is going to be put.
 If further cold working is to take place then increased ductility and malleability will be
required. This is achieved by prolonging the heating and slowing the cooling to encourage
grain growth.
 However, if grain refinement and strength and toughness are of more importance, then heating
and cooling should be more rapid.
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Spheroidized annealing:
 When steels contain more than 0.4%
carbon are heated to just below the
critical temperature (650-7000C) the
cementite in the crystals tends to 'ball up'.
This is referred to as the aspheroidization
of pearlitic cementite, Fig.
 The rate at which spheroidite forms
depends on:
• Prior microstructure. For example, it is
slowest for pearlite and the finer the
pearlite, the more rapid the rate.
• Prior cold work. This also increases the
spheroidizing reaction rate.
Spheroidized annealing
a. Lamellar pearlite
b. Pearlite commences
to “ball up”
c. Aspheroidization of the
pearlitic cementite (balling
up) complete.
a b
c
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Spheroidized annealing:
Spheroidized annealing can take place by:
 Heating to a temperature just above the
eutectoid temperature, and then either cooling
very slowly in the furnace, or holding at a
temperature just below the eutectoid
temperature.
 Heating and cooling alternately within about +/-
500C of the A1 line.
 Heating the alloy at a temperature below the
eutectoid, A1 (≈ 7000C) in the α+Fe3C.
 If the precursor microstructure contains
pearlite, spheroidizing times range between 15
to 25 hours.
 Spheroidized steels have a maximum softness and
ductility and are easily machined or deformed.
 Steel which has been subjected to spheroidizing annealing
will re-harden more uniformly and with less chance of
cracking.
 After spheroidizing annealing the steel can be cold worked
and it will be machined freely to a good surface finish.
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Full annealing:
 A heat treatment often utilized in low and
medium carbon steels that will be machined or
will experience extensive plastic deformation
during a forming operation.
 After the alloy is treated, as in Fig, it is then
furnace cooled. That is the heating furnace is
turned off and both furnace and steel cool to
room temperature at the same rate. This takes
several hours.
 Microstructure product is coarse pearlite that is
relatively soft and ductile.
Generally, the alloy is treated by heating to a temperature of
above 500C above the A3 line (to form austenite) for
compositions less than the eutectoid and above the A1 line
(to form austenite and Fe3C phases) for compositions in
excess of the eutectoid, respectively
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 The full annealing cooling procedure is
time consuming.
 Fig, shows moderately rapid and slow
cooling curves superimposed on a
continuous cooling transformation
diagram for a eutectoid iron-carbon alloy.
Full annealing:
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2. Normalizing:
 It is an annealing heat treatment used to refine the grains (decrease the average grain size) and
produce a more uniform and desirable size distributions.
 Normalizing treatment is heating the metal again to dull red heat but cooling less slowly than for
full annealing.
 It results in finer grains than annealing. This improves metal’s machining properties and strength.
 Normalizing should be done:
• To prepare the metal for machining after annealing
• Since flow forming or working metals at room temperature will cause the grains to be
distorted, normalizing should be done before working.
 Normalizing is accomplished by heating at least 550C above the upper critical temperature. That is:
• Above A3 for compositions less than the eutectoid.
• Above Acm for compositions greater than the eutectoid.
 After sufficient time has been allowed for the alloy to completely transform to austenite (the
procedure is called austenitizing), the treatment is terminated by cooling in air.
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Normalizing:
 Figure shows normalizing curves superimposed
on the continuous cooling transformation
diagram:
 Moderately rapid and slow cooling curves
superimposed on a continuous cooling
transformation diagram for a eutectoid iron-
carbon alloy.
 More rapid cooling associated with the
normalizing process results in the transformation
of the fine grain austenite into fine grain ferrite
and pearlite.
 The fine grain structure resulting from the more
rapid cooling associated with normalizing gives
improved strength and toughness to the steel but
reduces its ductility and malleability.
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Hardening:
 One criteria for hardening of steels is to have sufficient carbon content.
 Low carbon steels can be hardened by heating at an elevated temperature in an atmosphere
containing an alloying element that will diffuse into the steel and allow surface hardening on
quenching.
 Carbon is frequently diffused into the surface of soft steels for surface hardening.
 Similarly, elements such as Cr, B, N and Si can be diffused into the surface of steel for special
purposes.
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Reasons for hardening:
 The orderly arrangement of atoms in
the crystals allows individual layers
of atoms (slip planes) to slide over
each other as shown in a.
 The metal becomes hard and brittle
if slip movement becomes difficult
due to:
• Distortion of the lattice
occurs, Fig b. or
• If particlesof another material
are introduced, Fig c
Slip and hardness:
(a) Indentation is easy in a ductile material as slip occurs. This
indicates that the material is soft;
(b) Distortion of the slip planes makes slip extremely difficult. This
reduced amount of indentation indicates that the material is hard.
(c) Particle hardening is the introduction of particles to distort the
slip planes and makes slip difficult
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 Quenching is a process of very rapid
cooling.
 The temperature band from which plain
carbon steels are cooled when they are
quench hardened is shown, Fig.
 The band is not continued below 0.4
wt%C, although some grain refinement
and toughening occurs, no appreciable
hardening takes place.
Hardening temperatures for plain carbon steels.
Quench hardening:
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Quench hardening:
 If a steel with a carbon content
above 0.4 % is quenched from an
appropriate temperature for its
carbon content, there is not
sufficient time for the equilibrium
transformations to take place and
the steel becomes appreciably
harder.
 The final hardness will depend
solely upon the carbon content
and the rate of cooling.
Hardening temperatures for plain carbon steels.
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Tempering:
 It is a process or reheating a quench-hardened steel (which is hard, brittle
and hardening stresses are present) to relieve the stresses and reduce
brittleness.
 Tempering is important because quench-hardened steel is of little practical
use.
 Tempering causes the transformation of martensite into less brittle
structures. Unfortunately, any increase in toughness is accompanied by
some decrease in hardness.
 Tempering always tends to transform the unstable martensite back into the
stable pearlite of the equilibrium transformations.
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Tempering:
 Tempering temperatures below 2000C only relieve the hardening stresses, but above 2200C the
hard, brittle martensite starts to transform into a fine pearlitic structure called Troostite.
 Troostite:
• It is much tougher
• But it is less hard than martensite
• It is the structure found in most carbon-steel cutting tools.
 Tempering above 4000C causes any cementite particles present to 'ball-up' giving a structure
called Sorbite.
 Sorbite:
• It is tougher and more ductile than Troostite
• It is used in components subjected to shock loads and where a lower order of
hardness can be tolerated, for example springs.
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Case hardening:
 Often components require a hard case to resist wear and a tough core to resist
shock loads.
 These two properties do not exist in a single steel since, for toughness, the
core should not exceed 0.3-0.4 %C whilst, to give adequate hardness, the
surface of the component should have a carbon content of approximately 1%.
 The usual solution to this problem is case hardening.
 It is a process by which carbon is added to the surface layers of a low-carbon
plain or low-alloy steel component to a carefully regulated depth, after which
the component goes through successive heat-treatment processes to harden
the case and refine the core.
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The two distinct processes of case hardening: carburizing and heat treatment
(a) Carburizing
(b) After carburizing
(c) After quenching components from a temperature above 7800C
Case hardening:
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Case hardening:
Carburizing:
 Carburizing makes use of the fact that low-carbon steels (approximately 0.1 %C) absorb carbon
when heated to the austenitic condition.
 Various carbonaceous materials are used in the carburizing process.
• Solid media
• Fused salts
• Gaseous media
 Carburizing merely adds carbon to the outer layers and leaves the steel in a fully annealed
condition with a coarse grain structure. But it is a fallacy to suppose that carburizing hardens teel.
 Therefore, additional heat-treatment processes are required to harden and refine the case and to
refine and toughen the core.
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Solid media Fused salts Gaseous media
 Solid media such as bone
charcoal or charred leather,
together with an energizer
such as sodium and/or
barium carbonate.
 The energizer makes up to
40% of the total
composition.
 Fused salts such as molten
sodium cyanide, together
with sodium carbonate and
varying amounts of sodium
and barium chloride are used
in special salt bath furnaces.
 Since cyanide is a deadly
poison and represents from
20-50 % of the total content
of the molten salts, stringent
safety precautions must be
taken in its use.
 The components to be
carburized are immersed in
the molten salts.
 Gaseous media are increasingly
used now that 'natural' gas
(methane) is widely available.
 Methane is a hydrocarbon gas
containing organic compounds
of carbon which are readily
absorbed into steel.
 The methane gas is often
enriched by the vapors given off
when oil is 'cracked' by heating
it in contact with platinum which
acts as a catalyst.
Case hardening:
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Case hardening temperatures:
Case hardening:
Heat treatment processes after carburizing:
Heat treatment
Descriptions of the heat-treatment processes that follow carburizing.
2/11/2021

Refining the core Refining & hardening the case Tempering
 Since the core has a content of
less than 0.3 %C, the correct
annealing temperature is
approximately 8700C which is
well below the carburizing
temperature of 9500C which
caused the grain growth.
 After raising the component to
8700C it is water quenched to
ensure a fine grain.
 Although the temperature of
8700C is correct for the low-
carbon core of the component (A),
it is excessively high for the high-
carbon case of the component (B).
 Since the case has a carbon content of
approximately 1 %C its correct
hardening temperature is 7600C.
 Therefore the component is reheated to
this temperature (C) and again quenched.
 This hardens the case and ensures that
it has a fine grain.
 The temperature of 7600C is too low to
cause grain growth in the hyper- eutectic
core providing the component is heated
rapidly through the range 650-7600C
during the reheating and quenched
without soaking at the hardening
temperature.
 Ideally it is advisable to
relieve any quenching
stresses present in the
component by
tempering it at about
200-2200C.
Case hardening: Heat treatment processes after carburizing:
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Case hardening:
Heat treatment processes after carburizing:
 The above heat-treatment sequence is used to give ideal results in stressed
components.
 However, in the interests of speed and economy the process is often
simplified where components are lightly stressed or where alloy steels are
used having less critical grain growth and quenching characteristics.
 In such circumstances the tempering process is left out. Sometimes heat
treatment is limited to simply quenching immediately after carburizing whilst
the components are still at the carburizing temperature.
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 Various means are available for avoiding the local infusion of carbon during the carburizing process.
• Heavily copper plating those areas to be left soft.
• Encasing the areas to be left soft in fire clay.
• Leaving surplus metal on the component.
Case hardening:
Localized case hardening:
 It is often not desirable to harden a component all over.
 For example, it is undesirable to case harden screw threads. Not only would they be extremely brittle,
but any distortion occurring during carburizing and hardening could only be corrected by expensive
thread grinding operations.
 Surplus metal is left on the blank during carburizing. Additional carbon is then removed during screw
cutting so that the thread remains soft after heat treatment
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Heavily copper plating to those
areas to be left soft
Encasing the areas to be
left soft in fire clay
Leaving surplus metal on the
component
 The layer of copper prevents
intimate contact between the
component and the carbon, thus
preventing carburization.
 Note that copper plating cannot
be used for salt-bath treatment,
as cyanide dissolves the copper
from the component.
 This is mostly used
when pack-carburizing.
 The layer of copper prevents
intimate contact between the
component and the carbon,
thus preventing carburization.
 Note that copper plating
cannot be used for salt- bath
treatment, as cyanide
dissolves the copper from the
Casehardening: Localized case hardening:
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Surface hardening:
Figures show the two processes of surface hardening:
(a) Flame hardening (shorter process)
(b) Induction hardening
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88

Flame hardening Induction hardening
 Localized surface hardening can also be
achieved in medium and high carbon steels
and some cast irons by rapid local heating
and quenching.
 Fig (a) shows the principle of flame
hardening. A carriage moves over the work
piece so that the surface is rapidly heated by
an oxy-acetylene or an oxy-propane flame.
 The same carriage carries the water
quenching spray. Thus the surface of the
work piece is heated and quenched before its
core can rise to the hardening temperature.
 This process is often used for hardening the
slide ways of machine tools, for example,
lathe bed-ways.
 Fig (b) shows how the same surface hardening effect can be produced by high
frequency electromagnetic induction.
 The induction coil surrounding the component is connected to a high-
frequency alternating current generator.
 This induces high-frequency eddy currents in the component causing it to
become hot.
 When the hardening temperature has been reached, the current is switched
off and a water spray quenches the component.
 The induction coil can be made from copper tube which also carries the
quenching water or oil. This technique is often used for hardening gear teeth.
 The induction coil can be tailored to suit the profile of the component. The
depth of the case can be controlled by the frequency of the alternating
current.
 The higher the frequency, the nearer to the surface of the component will be
the eddy currents resulting in a shallower depth of heating and, therefore, a
shallower depth of hardening.
Surface hardening:
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89

Nitriding
 This process is used to put a hard, wear-resistant coating on components made from special alloy steels, for example, drill
bushes.
 The alloy steels used for this process contain either 1% aluminum, or traces of molybdenum, chromium and vanadium.
 Nitrogen gas is absorbed into the surface of the metal to form very hard nitrides.
 The process consists of heating the components in ammonia gas at between 500 and 6000C for upwards of 40 hours.
 At this temperature the ammonia gas breaks down and the atomic nitrogen is readily absorbed into the surface of the steel. The
case is applied to the finished component.
 No subsequent grinding is possible since the case is only a few micrometers thick. However, this is no disadvantage since the
process does not affect the surface finish of the component and the process temperature is too low to cause distortion.
 Some advantages of nitriding:
• Cracking and distortion are eliminated since the processing temperature is relatively low and there is no subsequent
quenching.
• Surface hardness as high as 1150 HD are obtainable with 'Nitralloy' steels.
• Corrosion resistance of the steel is improved.
• The treated components retain their hardness up to 5000C compared with the 2200C for case- hardened plain
carbon and low-alloy steels.
Surface hardening:
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90

Quenching media:
 As soon as the heated work piece is plunged into the quenching bath, it becomes surrounded by a
blanket of vaporized quenching media. Since this vapor has a low thermal conductivity it slows the
cooling process.
 The choice of quenching bath depends upon the type of steel being treated and the resultant
properties required.
 The most commonly used quenching media in order of severity are: Compressed air blast - least
severe, oil, water and brine (10 % solution) - most severe.
 “Severity of quench” is often a term used to indicate the rate of cooling; the more rapid the quench,
the more sever the quench.
 Of water, oil and air, water produces the most sever quench, followed by oil, which is more effective
than air.
 The degree of agitation of each medium also influences the rate of heat removal
 Increasing the velocity of the quenching medium across the specimen surface enhances the quenching
effectiveness.
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91

Quenching media:
 Air blast quenching:
• It is usually reserved for high-speed steel tools and components of small section.
• The alloy content of such steels is sufficiently high to reduce the critical cooling rate to the very low level
required for air blast quenching.
• Air cooling of austenitized plain carbon steels ordinary produces an almost totally pearlitic structure.
 Water:
• It provides a quenching rate approximately 3 times as great as oil and is usually used for plain carbon steel.
• For higher-carbon steels, a water quench is too sever because cracking and warping may be produced.
 Oil:
• It is usually used with very high-carbon (1.2-1.4 %) steels and alloy steels.
 Brine (solution of salt and water):
• It is only occasionally used to provide very rapid cooling for plain carbon tool steels and case-hardening steels
where maximum hardness is required.
• To avoid cracking and distortion, the quenching rate should be no greater than that needed to give the required
properties in the workpiece.
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Quenching media:
 Brine:
• It is a solution of common salt and water.
• It is only occasionally used to provide very rapid cooling for plain carbon tool steels and
case-hardening steels where maximum hardness is required.
• To avoid cracking and distortion, the quenching rate should be no greater than that
needed to give the required properties in the workpiece.
 Aqueous polymer quenchants:
• Media composed of water and a polymer (normally poly alkaline glycol or PGA).
• It provides quenching rates between those of water and oil.
• The quenching rate can be tailored to specific requirements by changing polymer
concentration and quench bath temperature.
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Quenching media:
 During quenching of steel specimen, heat energy must be transported to the surface before it is
dissipated into the quenching medium.
 Cooling rate within and throughout the interior of a steel structure varies with position and depends on
the geometry and size of the specimen.
 Figures (a) and (b):
• Shows quenching rate at 7000C as a function of diameter for cylindrical bars at four radial
positions.
• Quenching is in mildly agitated water (a) and oil (b).
• Cooling rate is also expressed as equivalent Jominy distance.
 Such diagrams can be used to predict the hardness traverse along the cross- section of a specimen.
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(a) water
(b) Oil
Quenching media:
Heat treatment
Cooling rate as a function of diameter at surface, three-quarters radius, mid-radius and center positions
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95

(a) (b)
Radial hardness profiles for:
(a) A 50 mm diameter cylindrical 1040 and 4140 steel specimens
quenched in mildly agitated water.
(b) A 50 and 75mm diameter cylindrical specimens of 4140 steel quenched
in mildly agitated water.
Mass effect on hardenability:
 Figures (a) and (b) show the radial hardness
distribution for plain carbon (1040) and
alloy steel (4140) specimen.
 As heat energy is dissipated to the
quenching medium at the specimen surface,
the rate of cooling for a particular quenching
treatment depends on the ratio of surface
area to the mass of the specimen.
 The larger this ratio, the more rapid the
cooling is and the deeper the hardening
effect.
 Irregular shapes with edges and corners
have large surface-to-mass ratios than
regular and rounded shapes. Thus, are more
amenable to hardening by quenching.
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96

(e)
Mass effect on hardenability:
 Surface hardness depends not only upon alloy composition
and quenching medium but also upon specimen diameter.
 Figures (e), (f) and (g) show the tensile strength, yield
strength and ductility of an oil-quenched 4140 steel versus
tempering temperatures for diameters of 12.5, 25, 50 and
100mm.
Heat treatment
(g)
(f)
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The Jominy End-quench test:
 This test is used to determine the hardenability of steels.
 It involves heating a specimen to the hardening
temperature appropriate for its carbon content so that it is
fully austenitic, and then quenching it by spraying a jet of
water against its lower end.
 The specimen cools very rapidly at the quenched end and
progressively less rapidly towards the opposite
(shouldered) end.
 A flat is ground along the side of the cold specimen and
its hardness is tested every 3mm from the quenched end.
Jominy end-quench test
(a) Mounting during quenching
Heat treatment
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 Conventional heat treatment procedures for producing martensite steels ordinarily involve continuous and
cooling of an austenitized specimen in some type of quenching media.
 During quenching treatment, the surface always cools more rapidly than interior regions. Therefore,
austenite will transform over a range of temperatures, yielding a possible variation of microstructure and
properties with position within a specimen.
 Successful heat treating of steels to produce a predominantly martensite microstructure throughout the
cross-section depends on three factors:
1. The composition of the alloy
2. The type and character of the quenching medium
3. The size and shape of the specimen
 Hardenability is the term that is used to describe the ability of an alloy to be hardened by the formation
of martensite as a result of a given heat treatment.
 It is a qualitative measure of the rate at which hardness drops off with distance into the interior of the
specimen as a result of diminished martensite content.
Heat treatment
2/11/2021

 It is a standard procedure widely utilized to
determine hardenability.
 With this procedure, except for an alloy
composition, all factors that may influence the
depth to which a piece hardens (specimen size,
shape, and quenching treatment) are maintained
constant.
 A cylindrical specimen of diameter 25.4mm (1
in) and 100 mm (4 in) long is austenitized at a
prescribed temperature for a prescribed time.
 After removal from the furnace, it is quickly
mounted in a fixture as shown, Fig. (a).
(a)
Jominy end-quench test
(a) Mounting during quenching
Heat treatment
2/11/2021

 The lower end is quenched by a jet of water of specified
flow rate and temperature.
 The cooling rate is a maximum at the quenched end and
diminishes with position from this point along the length of the
specimen.
 After the piece has been cooled to room temperature, shallow
flats 0.4 mm (0.015 in) deep are ground along the specimen
length and RC hardness measurements are made for the first
50mm (2 in) length along each flat. Shown, Fig (b).
 For the first 12.8mm (½ in), hardness readings are taken at
1.6mm (1/16 in) intervals and for the remaining 38.4mm (1
½ ), every 3.2mm (1/8 in).
 A hardenability curve is produced when hardness is plotted as a
function of position from the quenched end.
(b)
Jominy end-quench test (b) After hardness
testing from the quenched end along a
ground flat.
Heat treatment
2/11/2021

 A typicalhardenability curve has a profileas
represented, Fig.
 Each steel alloy has its own hardenability curve.
 The quenched end is cooled more rapidly and exhibits
the maximum hardness (100% martensite is the
product at this position for most steels).
 Cooling rate and hardness decreases with distance from
the quenched end.
Typical hardenability plot of Rockwell
hardness as a function of distance from the
quenched end.
 With diminishing the cooling rate more time is allowed for carbon diffusion and the formation of a
greater proportion of the softer pearlite, which may be mixed with martensite and bainite.
 Thus, a steel that is hardenable will retain large hardness values for relatively long distances.
Heat treatment
2/11/2021

 It is sometimes convenient to relate hardness to a
cooling rate rather than to the location from the
quenched end of a standard Jominy specimen.
 Cooling rate or position from the quenched end
is specified in terms of Jominy distance, one
Jominy distance = 1.6 mm.
 A correlation may be drawn between position
along the Jominy specimen and continuous
cooling transformations.
 Fig. A continuous cooling transformation diagram
for a eutectoid iron-carbon alloy onto which are
superimposed the cooling curves at four different
Jominy positions, and the corresponding
microstructures that result for each. The
hardenability curve for this alloy is also included.
Heat treatment
2/11/2021

 Fig, shows the hardenability curve for five
different steel alloys all having 0.4 wt %C,
yet different amounts of other alloying
elements.
 All the five alloys have identical hardness at
the quenched end (57 HRC). This hardness
is a function of carbon content only, which is
the same for all these alloys.
 The most significant feature of these curves
is shape, which relates to hardenability.
The hardenability of the plain carbon 1040
steel is low because the hardness drops off
precipitously (to about 30 HRC) after a
relatively short Jominy distance (6.4 mm or ¼ ).
Fig. Plain carbon steel (1040) and alloy steels
(4140, 4340, 5140, and 8640)
Heat treatment
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 For the other four alloys, the decreases in hardness
are distinctly more gradual.
 Example: a Jominy distance of 50mm (2 in), the
hardness of the 4340 and 8640 alloys are
approximately 50 and 32 HRC, respectively. This
implies the 4340 is more hardenable.
 At the quenched end (where the quenching rate
is ≈ 6000C/s), 100% martensite is present for all five
alloys.
 For cooling rates less than 700C/s or the Jominy
distance greater than 6.4 mm (1/4 in), the
microstructure of the 1040 steel is predominantly
pearlitic.
 The microstructure of the four alloy steels consist
primarily of a mixture of martensite and bainite;
bainite content increases with decreasing cooling
rate.
Fig. Plain carbon steel (1040) and alloy steels
(4140, 4340, 5140, and 8640)
Heat treatment
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 Hardenability curve also depend on
carbon content.
 The hardness at any Jominy position
increases with the concentration of
carbon.
 Also, during industrial production of
steel, there is always a slight, unavoidable
variation in composition and average
grain size from one batch to another.
Hardenability curves for four 8600 series alloys of
Heat treatment
indicated carbon content.
2/11/2021

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