Compare and contrast Annealing heat treatment process with Normalizing heat treatment

1. MME 291
Bangladesh University of Engineering and Technology
Subject: Compare and contrast annealing heat treatment process with normalizing heat
treatment process.
Preparedby:MohammadMinhajul Anwaremail:resgef@gmail.com
Figure: Annealing in a typical furnace

2. Annealing
Figure: Annealing at a glance
Annealingisaheattreatment processthataltersthe physical andsometimeschemical propertiesof
a material toincrease itsductilityandreduce itshardness,makingitmore workable.Itinvolves
heatinga material above itsre-crystallizationtemperature,maintainingasuitable temperature for
an appropriate amountof time andthencooling.
In annealing,atomsmigrate inthe crystal lattice andthe numberof dislocationsdecreases,leading
to a change in ductilityandhardness.Asthe material coolsitre-crystallizes.Formanyalloys,
includingcarbonsteel,the crystal grainsize andphase composition,whichultimatelydetermine the
material properties,are dependentonthe heatingrate andcoolingrate.Hot workingor cold
workingafterthe annealingprocessaltersthe metal structure,sofurtherheattreatmentsmaybe
usedto achieve the propertiesrequired.Withknowledgeof the compositionandphase diagram,
heattreatmentcan be usedto adjustfromharderand more brittle tosofterand more ductile.
In the case of ferrousmetals,such assteel,annealingisperformedbyheatingthe material (generally
until glowing) forawhile andthenslowlylettingitcool toroom temperature instill air.Copper,
silverandbrasscan be eithercooledslowlyinair,orquicklybyquenchinginwater.[1] Inthis
fashion,the metal issoftenedandpreparedforfurtherworksuchas shaping,stamping,orforming.
Alloysare annealedattemperaturesof between300-410°C, dependingonthe alloy,withheating
timesrangingfrom0.5 to 3 hours,dependingonthe size of the workpiece andthe type of alloy.
Alloysneedtobe cooledata maximumrate of 20°C per houruntil the temperature isreducedto
290°C, afterwhichthe coolingrate is notimportant.
The annealingprocessisnotsame for all the metals;the difference liesinthe coolingmechanism.

3. For example,steel isheatedtoredhot1000 ᵒC (1900 ᵒF) and cooledslowlyforachievingdesired
propertiesof the metal.The combinationof heatingandcoolingisusedtoobtaindesired
mechanical propertiesof metal.
Annealingworksinthree stages –the recoverystage,re-crystallizationstage andthe graingrowth
stage.These workas follows:
1. RecoveryStage
Thisstage is where the furnace orother heatingdevice isusedtoraise the temperatureof the
material tosuch a pointthat the internal stressesare relieved.
Figure:Stored energy of cold workand fraction of the totalworkof deformation remaining as stored
energy for high-purity copper plotted as functions of tensile elongation.

4. Figure: Residual strain hardening vs. recovery time at three constant annealing temperatures
2. Re-crystallizationStage
Heatingthe material above itsre-crystallizationtemperature butbelow itsmeltingpointcausesnew
grainsto form withoutanyresidual stresses.
Figure: A typical re-crystallization curve at constant temperature
Figure: Effect of prior deformation on the temperature for the start of re-crystallization of copper

5. Figure: Effect of time and temperature on annealing
3. GrainGrowth Stage
Coolingthe material ata specificrate causesnew grainstodevelop.Afterwhichthe material will be
more workable.Subsequentoperationstoaltermechanical propertiescanbe carriedout following
annealing.
Figure: Effect of temperature on re-crystallized grain size
Types ofannealing:
Full Annealing In full annealingthe carbonsteel isslowlyheatedtoatemperature of 50 C (122 F)
above the austenitictemperature (Liesbetween750-900 °C / 1320-1652 °F) also
knownas “holdingtemperature,”andthencooleddownslowlytothe room
temperature.The coolingrate recommendedis20 °C (68 °F) per hour.
The long annealingtime producesidealsoftening.Full annealingisdone inside the
furnace.Afterreachingthe holdingtemperaturethe furnace isturned off,and

6. metal annealsinsidethe furnace.
Once the metal is reachedat 50 °C (122 °F) it can furtherbe cooledto room
temperature usingairwithnatural draft.The basicheattreatmentprocessesfor
carbon steel involve the decompositionorconversionof austenite.
The appearance of these conversionproductsdeterminesthe mechanical and
physical propertiesof anymetal.
SoftAnnealing The soft annealingheattreatmentcanbe performedonsteel andalloysof copper
and brass.Steel withhighcarbon contentistypicallytreatedwithsoftannealing
processwhichgivesitsofterandeasiertoworkstructure.
The processtakesabout 12 to 48 hoursand can be performedincontinuousor
batch-wise inthe oven.The loadisheatedtothe temperature of 800 °C (1472 °F).
The metal is heldatthistemperature for2-4 hoursso that the structure fully
convertsintoaustenite.
The temperature of the metal isquicklybroughtdownto790 C (1454 °F).Further
coolingof thissteel isperformedatacontrolledand steadyrate of 10 °C (50 °F) per
hour until the temperature reaches690 °C (1274 °F).The metal isthencooledto
ambienttemperature.The structural changesinthe steel make itsoft.
The coolingconditiondefinesthe degree of softnessattained.The advantage of
thisprocessisthat a softand ductile carbonsteel isobtainedthathasa good ability
to form.
StressRelief
Annealing
The workedpiecesof metalstendtohave stressesdue toworkhardeningor
thermal cycling.The large castingssuchas cold formed,weldedparts,etc.are
heatedupto the temperature of 600 to 650 C (1112 to 1202 F) andkeptin this
conditionforhouror more.
The metal is allowedtocool inthe furnace till the temperature dropsto426 °C
(800 °F) thencooledtoambienttemperatureslowlyinthe still air.
Process
Annealing
Processannealingissimilartostressrelief annealing.The processisusedinwire
and sheetindustriestosoftenthe steelbyre-crystallizationforfurtherworking
withoutfracture.Itisalso usedintreatingthe hardenedpartsof low carbon steel.
The processinvolvesthe heatingof steel tothe temperature of 700 °C (1292 °F).
The time is givenforre-crystallizationandre-structuringof the ferrite phase.The
steel isthencooledslowly.
Effectsand advantages ofannealing:
Annealingisusedtoreverse the effectsof workhardening,whichcanoccur duringprocessessuchas
bending,coldformingordrawing.If the material becomestoohardit can make workingimpossible
or resultin cracking.

7. By heatingthe material above the re-crystallizationtemperature,itismade more ductile and
therefore readytobe workedonce more.Annealingalsoremovesstressesthatcanoccur when
weldssolidify.Hotrolledsteelisalsoshapedandformedbyheatingitabove the re-crystallization
temperature.Whilesteelandalloysteelannealingiscommon,othermetalscanalsobenefitfrom
the process,suchas aluminum,brass,andcopper.
Metal fabricatorsuse annealingtohelpcreate complex parts,keepingthe material workable by
returningthemclose totheirpre-workedstate.The processisimportantinmaintainingductilityand
reducinghardnessaftercoldworking.Inaddition,some metalsare annealedtoincrease their
electrical conductivity.
The main advantagesof annealingare inhow the processimprovesthe workabilityof amaterial,
increasingtoughness,reducinghardnessandincreasingthe ductilityandmachineabilityof ametal.
The heatingandcoolingprocessalsoreducesthe brittlenessof metalswhile enhancingtheir
magneticpropertiesandelectrical conductivity.
Figure: Effect of Cold work-anneal cycle on strength, hardness, ductility and microstructure

8. Figure: Annealing of 70-30 brass after 50 percent cold reduction with time constant at 30 minute
Figure: Schematic representation of the changes in microstructure during the annealing of a 0.20%
carbon steel. (a) Original structure, coarse-grained ferrite and pearlite. (b) Just above the A, line;
pearlite has transformed to small grains of austenite, ferrite unchanged. (c) Above the A, line; only
fine-grained austenite.(d) Aftercooling to room temperature; fine-grained ferrite and small pearlite
area

9. Figure: Proportions of the constituents present in the microstructure of the annealed steels as a
function of the carbon content
Normalizing in contrast with Annealing Heat Treatment
Figure: Normalizing at a glance
Normalizingof steelsiscarriedoutbyheatingapproximately100degree above the uppercritical
temperature A3or ACMline followedbycoolinginstill airtoroom temperature.

10. Figure:Annealing and Normalizing rangeforplain carbon steels
The purpose of normalizingistoproduce a harderand strongersteel thanfull annealingsothatfor
some applicationsnormalizingmaybe a final heattreatment. Therefore forhyper-eutectoid steelsit
isnecessarytoheat above the ACMline inorderto dissolve the cementite network.
Normalizingmayalsobe usedtoimprove machineability,modifyandrefine castdendriticstructure
and refine the grainandhomogenizethe microstructure inordertoimprove the responsein
hardeningoperations.The increase incoolingrate due toair coolingas comparedwithfurnace
coolingaffectsthe transformationof austenite andthe resultantmicrostructure inseveral ways.
Since we are nolongercoolingunder equilibriumconditionsthe iron-ironcarbide diagramcannotbe
usedto predictthe propertiesof pro-eutectoidferrite and pearliteorpro-eutectoid cementite and
pearlite thatwill existatroomtemperature.
There is lesstime forthe formationof the pro-eutectoidconstituentconsequentlytherewillbe less
pro-eutectoid ferriteinnormalizedhypo-eutectoidsteelsandlesspro-eutectoidcementitein
hypereutectoidsteelsascomparedwithannealedones.

11. Figure: Normalized .5 percent Carbon steel heated to 1800 degree F and air cooled, 100X,
Pro-eutectoid ferrite surrounding pearlite areas
Figure above showsthe microstructure of normalized 0.50percentcarbon steel.Inthe annealed
conditionthissteel wouldhave approximately62 percentpearlite and38 percentpro-eutectoid
ferrite.Due toair cooling,thissample has onlyabout10 percentpro-eutectoidferrite,whichisthe
white networksurroundingthe darkpearlite areas.
For hypereutectoidsteels,normalizingwillreduce the continuityof the pro-eutectoidcementite
network,andinsome casesit may be suppressedentirely.Sinceitwasthe presence of the
cementite networkwhichreducedthe strength of annealedhypereutectoidsteels,normalizedsteels
shouldshowanincrease instrength.Thisisillustratedbythe strengthvaluesgiveninTable below

12. Figure: mechanical properties of annealed vs normalized steel
Aside frominfluencingthe amountof pro-eutectoidconstituentthatwill form, the fastercooling
rate innormalizingwill alsoaffectthe temperature of austenite transformation.andthe finenessof
the pearlite.Ingeneral,the fasterthe coolingrate,the lowerthe temperature of austenite trans-
formationandthe finerthe pearlite.The difference inspacingof the cementite platesinthe pearlite
betweenannealingandnormalizingisshownschematicallyinFigure below:
Figure: Schematic picture of the difference in pearlitic structure due to annealing and normalizing

13. Ferrite isverysoft,while cementite isveryhard.Withthe cementite platesclosertogetherinthe
case of normalizedmedium pearlite;theytendtostiffenthe ferrite soitwill notyieldaseasily,thus
increasinghardness.If the annealedcoarse pearlite hasahardness of aboutRockwell C10,thenthe
normalizedmediumpearlitewillbe aboutRockwell C20.Non-equilibriumcoolingalsoshiftsthe
eutectoidpointtowardlowercarboncontentinhypo-eutectoidsteelsandtowardhighercarbon
contentinhypereutectoidsteels.The neteffectisthatnormalizingproducesafinerandmore
abundantpearlite structure thanisobtainedbyannealing,whichresultsinaharderand stronger
steel.While annealing,spheroidizing,andnormalizingmaybe employedtoimprove machineability,
the processto be usedwill dependuponcarboncontent.Basedonmanystudies,the optimum
microstructuresformachiningsteelsof differentcarboncontentsare usuallyasfollows:
Fig: the optimum microstructures for machining steels of different carbon contents

14. More Microstructure examples:
Figure: 1% C steel spheroidized-annealed (Etched 2% nital, 750X)
Figure: Steel (a) as-received and steel normalized at (b) 1313 K, (c) 1333 K, (d) 1353 K

15. Figure: Microstructure of an HR Steel strip at the strip surface,
Normalized at (a) 860°C (b) 900°C (c) 940°C (d) 960 °C at various soaking time
Case study: 0.31% C steel Annealed vs Normalized
Fig: Subject steel before heat treatment

16. Fig: Annealed at 950 degree Celsius for 2 hrs
Fig: Normalized at 850+60 degree Celsius for 72 minutes

17. Comparing generalized Stress-strain, Elongation and Tensile strength through graph
Fig: Nominal stress-Strain graph for Annealed vs Normalized steel

18. Fig: Elongation-Carbon content graph for Annealed vs Normalized steel
Fig: Tensile strength-Carbon content graph, Annealed vs Normalized steel

19. Final Words
Normalizingdiffersfromannealinginthatthe metal isheatedtoa highertemperature andthen
removedfromthe furnace forair coolingratherthanfurnace cooling.Formany manufacturing
engineersthereisoftenagreatdeal of confusionastowhento specifynormalizingandwhentocall
out annealing.There isalogical reasonforthisbecause,inmanyinstances,the procedurefor
normalizingandthatof annealingare one andthe same.Forexample,very-low-carbonsteel canbe
almostfullyannealedbyheatingabove the transformationrange andcoolinginair.
Normalizingisaprocessthatimprovespartqualityandplaysan importantrole incontrolling
dimensional variationinhardeningandcase hardening.Itshouldbe done wheneverdimensional
stability isimportantorwhenmanufacturingoperationsare expectedtoimpartsignificantamounts
of stressintothe material.Ithelpsavoidmanyheat-treatingproblems.
Thank You

20. Source:
[1] Book:IntroductiontoPhysical Metallurgy bySidney HAvner
[2] WikipediaArticle:Annealing
[3] https://gearsolutions.com/departments/hot-seat/understanding-different-types-of-heat-
treatment-annealing/
[4] https://www.inspection-for-industry.com/annealing-heat-treatment.html
[5] https://www.twi-global.com/technical-knowledge/faqs/what-is-annealing
[6]https://www.researchgate.net/publication/282318918_Effect_of_normalizing_and_tempering_te
mperatures_on_microstructure_and_mechanical_properties_of_P92_steel
[7] https://www.semanticscholar.org/paper/Effect-of-Normalizing-Temperature-and-Time-on-and-
Pitakkorraras-Tangroekwarasakul/c8b7bc311eca5cb12bb4e9bf13875b56655954cb/figure/3
[8] https://www.slideshare.net/MusavvirMahmud/design-of-annealing-normalizing-and-hardening-
heat-treatment-of-steel
[9] https://indeeco.com/news/2015/01/12/importance-normalizing
[10] https://slideplayer.com/slide/10731387/