heat treatment of metals-smr

8/19/2019 Heat Treatment of Metals-Smr

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Heat Treatment ofMetals


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Heat Treatment

Metallic materials consist of a microstructure of

small crystals called “grains" or crystallites

Grain size and composition is one of the most

effective factors that can determine the overallmechanical behavior of the metal

Heat treatment provides an efficient way to

manipulate the properties of the metal bycontrolling rate of diffusion, and the rate of

cooling within the microstructure


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Heat Treatment

Involves controlled heating and cooling of themetal or alloy

This imparts desirable physical characteristicsdue to change in microstructure

Improvementso Toughness

o Hardness

o esistance to shoc!

o atigue resistance Two broad categories with respect to materials

o Heat treatment of ferrous metals

o Heat treatment of non ferrous metals


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#road $ategories of Heat TreamentThe various heat treatment processes may be broadly classified as:

Hardening process -This process is intended to produce hardenedstructure by %uench&hardening' Hardening increases wear resistance and

strength of material' However, hardening often results in turning the

structure of the wor! brittle' #esides, internal stress increases

tremendously while machinability and ductility of the metal decrease'

Softening processes & These processes are intended primarily to softenthe material, such as Annealing( also those intended primarily to remove

stresses either inherent or conse%uent upon prior operations'

Toughening process- This process is intended to produce a structure

possessing good strength and ductility by means of Normalizing ' esults

into improved machinability, grain structure refinement, homogenization and

modification of residual stresses'

Case and Surface-hardening process & This process is employed to

produce a )case) or surface layer substantially harder than the interior or

core of the wor!&piece' They include carburizing , nitriding , induction

hardening , selective laser hardening .'


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Hardening *rocess

*art is heated to pre determined temperature& $riticalTemperatureo Temperature at which steel will harden is called its critical

temperature

o $ritical temperature depends on the type of alloy and carboncontent +-..&/-..01

2fter heating part is %uenched in brine, water, oil or airblast

o 3ater or brine is used to %uench plain carbon steel

o 4il used to %uench alloy steels

o $old air blast is used for high alloy steel 5uenching leaves the steel hard and brittle, this

brittleness needs to be reduced by Tempering or6rawing


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Hardening *rocesses7

In this process steels which

contain sufficient carbon, and

perhaps other alloying elements,

are cooled +%uenched1 sufficiently

rapidly from above thetransformation temperature to

produce Martensite, the hard

phase already described'

There is a range of %uenchingmedia of varying severity, water or

brine being the most severe,

through oil and synthetic products

to air which is the least severe'


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8oftening *rocess

8tress elieving elieves the stresses developed in parts

due to cold wor!, machining or welding

*arts heated to below critical temperatureie ..&/..9

Held at this temperature for hr per inch

of thic!ness

$ooled slowly in still air at room

temperature or in furnace


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8oftening *rocess&2nnealing

2nnealing reduces the hardness of the part toma!e it easy to machine or wor!

Metal is heated to :.&..0 above its criticaltemperature +normal hardening temperature1

Holding time depends upon the shape andthic!ness of the piece

8low cooling is performed in some insulatingmaterial such as ashes or a furnace

*rimarily used for ferrous metals, but nonferrous metals can also be annealed after theybecome wor! hardened


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8oftening *rocesses& Annealing…


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Toughening *rocessesTempering Process

2fter %uenching the steel is hard,

brittle and internally stressed' #efore

use, it is usually necessary to reduce

these stresses'

Tempering is the process of heating amartensitic steel at a temperature

below the eutectoid transformation

temperature' This ma!es it “softer;

and more “ductile;'

Tempering increases the toughnessof the wor! piece' There will also be a

reduction in hardness and the

selection of tempering temperature

dictates the final properties'


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Toughening *rocesses&Normalising


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$ase Hardening

=ow carbon steel cannot be effectively hardened

by conventional heat treatment

*art is heated to red heat and small %uantity of

carbon or nitrogen is introduced in its surface This produces a hard shell on the surface

ollowing three methods

o *ac! method or carburizingo =i%uid salt method

o Gas method


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*ac! Method or $arburizing

*art is buried in a carbonaceous material

in a container

$ontainer is placed in a furnace for :&>.

minutes

Time controls the depth of the case

2fter removal from the furnace part is%uenched


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=i%uid 8alt Method

*art is heated in molten cyanide salt bath

up to an hour

$yanide is introduced in surface and

immersion time determines the thic!ness

of the hard case

2fter holding for desired time part is

%uenched


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Gas Method&


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*art #eing emoved from

$yanide #ath


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*adloc! 8hac!les #eing =oaded

in


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8urface Hardening

8urface hardening is the process that permitsthe surface of high carbon and alloy steels to behardened without affecting the internal structureof the metal

Three techni%ueso lame Hardening@ 8urface is heated rapidly by flame

of gas torch and then %uenched

o Induction Hardening@ Heating is performed by high

fre%uency electrical current and then %uenchedo =aser Hardening@ A'/ to :'B mm =aser beamfocused on area to be hardened' 8mall area gets self%uenched within few seconds' *art does not getwarped or distorted


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8teel Heat Treatment *rocesses It should be noted that not all steels will respond to all heat

treatment processes, Table below summaries the response,

or otherwise, to the different processes Anneal Normalise Harden Temper

Low Carbon

!"#$

yes yes no no

%edium Carbon!"#-!"&$

yes yes yes yes

High Carbon

'!"&$

yes yes yes yes

Low Alloy yes yes yes yes

%edium Alloy yes yes yes yes

High Alloy yes maybe yes yes

Tool Steels yes no yes yes

Stainless Steel

(Austenitic eg

#!)* #!+,

yes no no no

Stainless Steels

(erritic eg )!&*

)#! )).,

yes no no no

Stainless Steels

(%artensitic eg

)/!* ))!,

yes no yes yes


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2luminum 2lloys Heat Treatment

*reheating or homogenizing, to reducechemical segregation of cast structures and to improvetheir wor!ability

2nnealing, to soften strain&hardened +wor!&hardened1

and heat treated alloy structures, to relieve stresses, andto stabilize properties and dimensions

8olution heat treatments, to effect solid solutionof alloying constituents and improve mechanical

properties

*recipitation heat treatments, to providehardening by precipitation of constituents from solid

solution'


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*reheating or Homogenization

This thermal operation applied to ingots prior to

hot wor!ing is referred to as "ingot preheating;

*urposes depending upon the alloy, product,

and fabricating process involved areo *rincipal obCectives is to improve wor!ability

o The microstructure of most alloys in the as&cast

condition is %uite heterogeneous' Microstructure is

homogenized


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2nnealing

The distorted, dislocated structure resulting from cold

wor!ing of aluminum is less stable than the strain&free,

annealed state, to which it tends to revert

2ccompanying the structural reversion are changes in

the various properties affected by cold wor!ing

=ower&purity aluminum and commercial aluminum alloys

undergo these structural changes only with annealing at

elevated temperatures

These changes occur in several stages, according to

temperature or time, and have led to the concept of

different annealing mechanisms or processes'


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2luminum Heat Treatment to

Increase 8trength 2 three&step process

o 8olution heat treatment' 6issolution of solublephases

o 5uenching' 6evelopment of supersaturationo 2ge hardening' *recipitation of solute atoms either

at room temperature +natural aging1 or elevated

temperature +artificial aging1


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5uenching

The most critical step in the se%uence of

heat treating operations

The obCective of %uenching is to preserve

as nearly intact as possible the solid

solution formed at the solution heat

treating temperature, by rapidly cooling to

some lower temperature, usually nearroom temperature'


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2luminum $opper 2lloy


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2luminum Heat

Treatment 6esignations As abricated &


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Heat Treatment Temper $odes T/ & $ooled from an elevated temperature shaping process and naturally

aged to a substantially stable condition'

T. & $ooled from an elevated temperature shaping process, cold wor!ed,and naturally aged to a substantially stable condition'

T# & 8olution heat treated, cold wor!ed, and naturally aged to asubstantially stable condition'

T) & 8olution heat treated, and naturally aged to a substantially stablecondition'

T& & $ooled from an elevated temperature shaping process then artificiallyaged'

T+ & 8olution heat treated then artificially aged'

T2 & 8olution heat treated then overagedEstabilized' T3 & 8olution heat treated, cold wor!ed, then artificially aged'

T4 & 8olution heat treated, artificially aged, then cold wor!ed'

T/! & $ooled from an elevated temperature shaping process, cold wor!ed,then artificially aged'


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8train Hardening $odes

H/ & 8train hardened only

H. & 8train hardened and partially annealed

H# & 8train hardened and stabilized

H) & 8train hardened and lac%uered or painted'


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8ummary

Heat treatment basic concepts

8teel heat treatment *rocesses

2luminum Heat Treatment *rocesses


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5uestionsF


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Toughness and 8trength

Toughness, in materials science and metallurgy, is the resistance to

fracture of a material when stressed' It is defined as the amount of energy

per volume that a material can absorb before rupturing'

Tests can be done by using a pendulum and some basic physics to

measure how much energy it will hold when released from a particular

height' #y having a sample at the bottom of its swing a measure oftoughness can be found, as in the $harpy and Izod impact tests'

Toughness is measured in units of Coules per cubic metre +EmA1 in the 8I

system and inch&pound&force per cubic inch +inlbfEinA1 in D8 customary

units'

8trength and toughness are related' 2 material may be strong and tough if it

ruptures under high forces, e?hibiting high strains( on the other hand, brittle

materials may be strong but with limited strain values, so that they are not

tough' Generally spea!ing, strength indicates how much force the material

can support, while toughness indicates how much energy a material can

absorb before rupture'

http://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Metallurgyhttp://en.wikipedia.org/wiki/Metallurgyhttp://en.wikipedia.org/wiki/Materials_science


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8hoc! resistance is the property by virtue

of which material will withstand impact or

thermal shoc! without failure

atigue resistance is related to number of

stress cycle a material can ta!e before

failure


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heat treatment of metals-smr

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