heaty treatment

8/3/2019 Heaty Treatment

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HeatTreatment ofSteelUnder supervision of:Dr/adel badawy el-shabasyDr/hala abdel hakeem


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Heat-Treatment} Heat treatment is a method used to alter the

physical, and sometimes chemical propertiesof a material. The most common application

is metallurgical} It involves the use of heating or chilling,

normally to extreme temperatures, to achievea desired result such as hardening or softening of a material.


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Types of Heat-Treatment (Steel)

}

Annealing / Normalizing,} Case hardening,} Tempering, and Quenching


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Time-Temperature-Transformation (TTT)Curve} TTT diagram is a plot of temperature versus the

logarithm of time for a steel alloy of definitecomposition.

} TTT diagram indicates when a specifictransformation starts and ends and it also showswhat percentage of transformation of austenite

at a particular temperature is achieved.


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Time-Temperature-Transformation (TTT)Curve

The TTT diagram

for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at900 C


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Annealing} It is a heat treatment wherein a material is

altered, causing changes in its propertiessuch as s trength and hardne ss

} It the process of heating solid metal tohigh temperatures and cooling it slowly sothat its particles arrange into a definedlattice


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Annealing stages} 1-stress relief or recovery

} 2-recrystallization} 3-grain growth


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1-stress relief(recovery)} Tension and compression stresses(bad

stresses distribution) in part after coldworking processes(rolling&wire drawing)

} Results in part corrosion ,so part is heatedand no decrease in strength occures.


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2-recrystallization} Unstressed new crystals are born at grain

boundaries} Change in mechanical properties such as

ductility maximized in expense ofstrength(UTS)


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3-grain growth} New crystals are grow in expense of old

ones} As new crystals growth increased as metal

ductility increases


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Types of Annealing1. Stress-Relief Annealing ( o r Stress-relieving)

2. Normalizing3. Homogenization annealing4. Spheroidizing Annealing ( o r Spheroidizing


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1. S tre ss-R elief Annealing} It is an annealing process

below the transformationtemperature A c1 , withsubsequent slow cooling,the aim of which is toreduce the internal re s iduals tre ss e s in a workpiece

without intentionallychanging its structure andmechanical properties


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Causes of Residual Stresses1. Thermal factors (e.g., thermal stressescaused by temperature gradients within theworkpiece during heating or cooling)2. Mechanical factors (e.g., cold-working)3. Metallurgical factors (e.g., transformationof the microstructure)


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How to Remove Residual Stresses?} R.S. can be reduced only by a plastic

deformation in the microstructure .} This requires that the y ield strength of the material

be lowered below the value of the residualstresses .

} The more the yield strength is lowered, the greater the plastic deformation and correspondingly thegreater the possibility or reducing the residual

stresses} The yield strength and the ultimate tensile

strength of the steel both decrease withincreasing temperature


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Stress-Relief Annealing

Process} For plain carbon and low-alloy steels the

temperature to which the specimen is heated

is usually between450

and650C

, whereas for hot-working tool steels and high-speed steels itis between 600 and 750C

} This treatment will not cause any phasechanges, but re c ry s tallization may take place.

} Machining allowance sufficient tocompensate for any warping resulting fromstress relieving should be provided


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Stress-Relief Annealing R.S.} In the heat treatment of metals, quenching or

rapid cooling is the cause of the greatest residualstresses

} To activate plastic deformations, the localresidual stresses must be above the yield strengthof the material.

} Because of this fact, steels that have a high yieldstrength at elevated temperatures can withstand

higher levels of residual stress than those thathave a low yield strength at elevatedtemperatures

} Soaking time also has an influence on the effectof stress-relief annealing


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Relation between heatingtemperature and Reductionin Residual Stresses

} Higher temperatures andlonger times of annealingmay reduce residualstresses to lower levels


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Stress Relief Annealing -

C ooling} The residual stress level after stress-relief annealing will

be maintained only if the cool down from theannealing temperature is controlled and slow enough

that no new internal stresses arise.} New stresses that may be induced during cooling

depend on the (1) c ooling rate , (2) on the c ro ss-s e c tional s ize of the workpiece, and (3)on thec ompo s ition of the s teel


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2. N ormalizing} A heat treatment process consisting of

austenitizing at temperatures of 3 080Cabove the A C 3 transformationtemperature followed by slow cooling(usually in air)

} The aim of which is to obtain a fine-grained, uniformly distributed, ferrite pearlite structure

} Normalizing is applied mainly tounalloyed and low-alloy hypoeutectoid

steels} For hypereutectoid steels theaustenitizing temperature is 3 080C above the A C 1 or A C m transformationtemperature


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Normalizing Heating andCooling


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Normalizing AustenitizingTemperature Range


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Effect of Normalizing on Grain Size} Normalizing refines the grain of a steel that has

become coarse-grained as a result of heatingto a high temperature, e.g., for forging or welding

Carbon steel of 0.5% C. (a) As-rolled or forged;(b) normalized. Magnification 500


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Need for Normalizing} Grain refinement or homogenization of the

structure by normalizing is usually performedeither to improve the mechanical propertiesof the workpiece or (previous to hardening)to obtain better and more uniform resultsafter hardening

} Normalizing is also applied for better machinability of low-carbon steels


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Normalizing after Rolling} After hot rolling, the

structure of steel isusually oriented in therolling direction

} To remove the orientedstructure and obtain thesame mechanicalproperties in alldirections, a normalizingannealing has to beperformed


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Normalizing after Forging

} After forging at high temperatures ,especially with workpieces that varywidely in crosssectional size, becauseof the different rates of cooling fromthe forging temperature, aheterogeneous structure is obtainedthat can be made uniform by

normalizing


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Normalizing - Cooling} Care should be taken to ensure that the cooling

rate within the workpiece is in a rangecorresponding to the transformation behavior ofthe steel-in-question that results in a pure ferrite pearlite structure

} If, for round bars of different diameters cooled inair, the cooling curves in the core have beenexperimentally measured and recorded, then by

using the appropriate CCT diagram for the steelgrade in question, it is possible to predict thes tru c ture and hardne ss after normalizing


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3. Homogenization Annealing} Cooling of catings produces dendritic

structure which have a segregation

} This segregation can be eliminated byannealing by which diffusion and atomtransfer from area to another untilconcentration be equal


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4.S

pheroidizing Annealing} It is also called as Soft

Annealing} Any process of heating and

cooling steel that producesa rounded or globular formof carbide

} It is an annealing process at

temperatures close below or close above the A C 1temperature, withsubsequent slow cooling


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Spheroidizing - Purpose} The aim is to produce a soft structure by changing allhard constituents like pearlite, bainite, and

martensite into a structure of spheroidized carbidesin a ferritic matrix

(a) a medium-carbon low-alloy steel after soft annealing at 720C;(b) a high-speed steel annealed at 820C.


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Spheroidizing - Uses

} Such a soft structure is required for goodmachinability of steels having more than0.6%C and for all cold-working processesthat include plastic deformation.

} Spheroidite steel is the softest and mostductile form of steel


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Spheroidizing - Process} Prolonged heating at a temperature just bel

ow the lower critical temperature , usually followed by relatively slow cooling

} In the case of small objects of high C steels,the spheroidizing result is achieved more ra

pidly by prolonged heating to temperaturesalternately within and slightly below the criticaltemperature range

} Tool steel is generally spheroidized by heatingto a temperature of 749 - 804C and higher for many alloy tool steels , holding at heat from 1 to4 hours , and cooling slowly in the furnace


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CASE HARDENING} C a s e hardening or s urfa c e hardening is the

process of hardening the surface of ametal, often a low carbon steel, by infusingelements into the material's surface,forming a thin layer of a harder alloy.

} C a s e hardening is usually done after the

part in question has been formed into itsfinal shape


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Case-Hardening - Processes} Flame/Indu c tion Hardening} C

arburizing} N itriding} C yaniding


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F lame and indu c tion hardening

} Flame or induction hardening are processes inwhich the surface of the steel is heated to hightemperatures ( by direct applicati o n of a f lame,

o r by inducti o n heating ) then cooled rapidly,generally using water } This creates a case of marten s ite on the

surface.} A carbon content of 0 .40 .6 wt% C is needed

for this type of hardening} Application E xamples -> Lo ck s ha ck le and

Gear s


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Carburizing

} Carburizing is a process used to case hardensteel with a carbon content between 0 .1 and0 .3 wt% C .

} Steel is introduced to a carbon richenvironment and elevated temperatures for acertain amount of time, and then quenchedso that the carbon is locked in the structure

} E xample -> Heat a part with an acetylenetorch set with a fuel-rich flame and quench itin a carbon-rich fluid such as oil


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C arburizing} Carburization is a diffusion-controlled

process, so the longer the steel is held in

the carbon-rich environment the greater the c arbon penetration will be and thehigher the c arbon c ontent .

} The carburized section will have a carboncontent high enough that it can behardened again through flame orindu c tion hardening


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C arburizing} The carbon can come from a solid , liquid or gaseous source} Solid source -> pa ck c arburizing . Packing low

carbon steel parts with a carbonaceous materialand heating for some time diffuses carbon intothe outer layers.

} A heating period of a few hours might form ahigh-carbon layer about one millimeter thick

} Liquid Source -> involves placing parts in a bathof a molten c arbon - c ontaining material, often ametal cyanide

} Gaseous Source -> involves placing the parts in afurnace maintained with a methane - ric h interior


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N itriding} Nitriding heats the steel part to 48262 1 C in an

atmosphere of N H3 gas and broken N H3 .} The time the part spends in this environment

dictates the depth of the case.} The hardness is achieved by the formation ofnitrides.

} Nitride forming elements must be present in theworkpiece for this method to work.

} Advantage -> it causes little distortion , so the partcan be case hardened after being quenched,tempered and machined


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C yaniding} Cyaniding is mainly used on low carbon steels.} The part is heated to 870 - 950C in a bath of

sodium cyanide (N a C N )and then is quenchedand rinsed, in water or oil , to remove anyresidual cyanide.

} The process produces a thin, hard shell ( 0 .5 -0 .75 mm ) that is harder than the oneproduced by carburizing , and can becompleted in 20 to 30 minutes compared toseveral hours.

} It is typically used on small parts.} The major drawback of cyaniding is that

cyanide salts are poisonous


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hardening} Metals or alloys can be hardened by:} 1-heat treatment(quenching and

tempering)} 2-cold working hardening


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Quenching} Cooling speeds, from fastest to slowest , go

from polymer, brine, fresh water, oil, andforced air

} However, quenching a certain steel too fastcan result in cracking , which is why high-tensilesteels such as AIS I 4 1 40 should be quenched inoil, tool steels such as H1 3 should be quenchedin forced air, and low alloy such as AIS I 1 040

should be quenched in brine} Metals such as au s teniti c s tainle ss s teel (304,

316), and c opper , produce an opposite effectwhen these are quenched: they anneal


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Tempering} Untempered martensite , while very hard, is

too brittle to be useful for most applications.} In tempering, it is required that quenched

parts be tempered (heat treated at a lowtemperature, often 150C) to impart sometoughness.

} Higher tempering temperatures (may be upto 700C, depending on alloy and

application) are sometimes used to impartfurther ductility, although some yield strengthis lost


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Tempering} Tempering is done to toughen the metal by

transforming brittle martensite or bainite into acombination of ferrite and cementite or

sometimes Tempered martensite} Tempered martensite is much finer-grained

than just-quenched martensite} The brittle martensite becomes tough and

ductile after it is tempered.} Carbon atoms were trapped in the austenite

when it was rapidly cooled, typically by oil or water quenching, forming the martensite


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