05. industrial materials

Jamal UmerAssistant Professor

M.Sc. Mechanical Engineering (2010-12) KU Leuven Belgium, Uminho Portugal & ULj Slovenia

B.Sc. Mechanical Engineering (2005-09)UET Lahore

Heat treatment is an operation or combination ofoperations involving heating at a specific rate,soaking at a temperature for a period of time andcooling at some specified rate. The aim is to obtain adesired microstructure to achieve certainpredetermined properties (physical, mechanical,magnetic or electrical).

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JAMAL UMERAssistant Professor

Mech. Engg. Deptt. UET Lahore 2

Various heating and cooling processes performedto effect structural changes in a material, which inturn affect its mechanical properties

Most common applications are on Metals

Similar treatments are performed on Glass-ceramics

Tempered glass

Powder metals and ceramics

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Heat treatment operations are performed on metalwork-parts at various times during theirmanufacturing sequence

To soften a metal for forming prior to shaping

To relieve strain hardening that occurs duringforming

To strengthen and harden the metal near the endof the manufacturing sequence

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Annealing

Martensite formation in steel

Tempering of martensite

Precipitation hardening

Surface hardening

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Heating and soaking metal at suitable temperaturefor a certain time, and slowly cooling

Reasons for annealing: Reduce hardness and brittleness Alter microstructure to obtain desirable

mechanical properties Soften metals to improve machinability or

formability Recrystallize cold worked metals Relieve residual stresses induced by shaping

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Full annealing - heating and soaking the alloy inthe austenite region, followed by slow cooling toproduce coarse pearlite

Usually associated with low and medium carbonsteels

Normalizing - similar heating and soaking cycle asin full annealing, but faster cooling rates,

Results in fine pearlite, higher strength andhardness, but lower ductility

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Cold worked parts are often annealed to reducestrain hardening and increase ductility by allowingstrain-hardened metal to recrystallize partially orcompletely

When annealing is performed to allow for furthercold working of the part, it is called a processanneal

When no subsequent deformation will beaccomplished, it is simply called an anneal

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Annealing operations are sometimes performed solelyto relieve residual stresses caused by prior shapeprocessing or fusion welding

Called stress relief annealing

They help to reduce distortion and dimensionalvariations that might otherwise result in the stressedparts

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The iron-carbon phase diagram shows the phases ofiron and iron carbide under equilibrium conditions

Assumes cooling from high temperature is slowenough to permit austenite to transform into ferriteand cementite (Fe3C) mixture

However, under rapid cooling, so that equilibrium isprevented, austenite transforms into anonequilibrium phase called martensite, which ishard and brittle

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Figure: The TTT curve, showing transformation ofaustenite into other phases as function of time andtemperature for a composition of about 0.80% C steel.Cooling trajectory shown yields martensite.

Time-Temperature-Transformation Curve

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P= Pearlite

B= Bainite (Alternative mixture

of peralite)

M= Martensite

A = Austenite

s = start

f = finish

A unique phase consisting of an iron-carbonsolution whose composition is the same as theaustenite from which it was derived

Face-centered cubic (FCC) structure

of austenite is transformed into

body-centered tetragonal (BCT)

structure of martensite

The extreme hardness of martensite results fromthe lattice strain created by carbon atoms trapped inthe BCT structure, thus providing a barrier to slip

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Figure: Hardness of plain carbon steel as afunction of carbon content in martensiteand pearlite (annealed).

Hardness of Plain Carbon Steel

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Martensite has significant effect

on hardness of plain carbon steel

Consists of two steps:

1. Austenitizing - heating the steel to a sufficientlyhigh temperature for a long enough time toconvert it entirely or partially to austenite

2. Quenching - cooling the austenite rapidly enoughto avoid passing through the nose of the TTTcurve

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Various quenching media are used to effect cooling rate

Brine -salt water, usually agitated (fastest cooling rate)

Still fresh water

Still oil

Air (slowest cooling rate)

The faster the cooling, the more likely are internal stresses, distortion, and cracks in the product

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A heat treatment applied to martensite to reducebrittleness, increase toughness, and relieve stresses

Treatment involves heating and soaking at atemperature below the eutectoid for about one hour,followed by slow cooling

Results in precipitation of very fine carbide particlesfrom the martensite iron-carbon solution, graduallytransforming the crystal structure from BCT to BCC

New structure is called tempered martensite

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The relative capacity of a steel to be hardened bytransformation to martensite

It determines the depth below the quenched surfaceto which the steel is hardened

Steels with good hardenability can be hardenedmore deeply below the surface and do not requirehigh cooling rates

Hardenability does not refer to the maximumhardness that can be attained

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Hardenability of steel is increased through alloying

Alloying elements having the greatest effect arechromium, manganese, molybdenum

The mechanism by which these alloying elementswork is to extend the time before the start of theaustenite-to-pearlite transformation

In effect, the TTT curve is moved to the right, thuspermitting slower quenching rates

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Figure: Jominy end-quench test: (a) setup, showing end quench of the testspecimen; and (b) typical pattern of hardness readings as a function ofdistance from quenched end.

Jominy End-Quench Test for Hardenability

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Heat treatment that precipitates fine particles thatblock the movement of dislocations and thusstrengthen and harden the metal

Principal heat treatment for strengthening alloys ofaluminum, copper, magnesium, nickel, and othernonferrous metals

Also utilized to strengthen a number of steel alloysthat cannot form martensite by the usual heattreatment

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The necessary condition for whether an alloysystem can be strengthened by precipitationhardening is the presence of sloping solvus line inthe phase diagram

A composition in this system that can beprecipitation hardened is one that contains twoequilibrium phases at room temperature, but whichcan be heated to a temperature that dissolves thesecond phase

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Figure: Precipitation hardening: (a) phase diagram of an alloy system consisting of metals A and B that can be precipitation hardened; and (b) heat treatment: (1) solution treatment, (2) quenching, and (3)

precipitation treatment.

Precipitation Hardening

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1. Solution treatment - alloy is heated to atemperature Ts above the solvus line into the alphaphase region and held for a period sufficient todissolve the beta phase

2. Quenching - to room temperature to create asupersaturated solid solution

3. Precipitation treatment - alloy is heated to atemperature Tp, below Ts, to cause precipitation offine particles of the beta phase

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Thermochemical treatments applied to steels in whichthe composition of the part surface is altered byadding various elements

Often called case hardening

Most common treatments are carburizing, nitriding,and carbonitriding

Commonly applied to low carbon steel parts toachieve a hard, wear-resistant outer shell whileretaining a tough inner core

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Heating a part of low carbon steel in a carbon-richenvironment so that C is diffused into surface

In effect the surface is converted to a high carbonsteel, capable of higher hardness than the low-Ccore

Carburizing followed by quenching produces acase hardness of around HRC = 60

Internal regions are low-C steel, with lowhardenability, so it is unaffected by quench andremains relatively tough and ductile

Most common surface hardening treatment

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Treatment in which nitrogen is diffused into surfaceof special alloy steels to produce a thin hard casingwithout quenching

Carried out at around 500C (950F)

To be most effective, steel must have alloyingingredients such as aluminum or chromium to formnitride compounds that precipitate as very fineparticles in the casing to harden the steel

Hardness up to HRC 70

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Requires higher temperatures and longer treatmenttimes than the preceding hardening treatments

Usually applied to low carbon steels

Casing is not only hard and wear resistant; it is alsoheat and corrosion resistant

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Fuel-fired furnaces

Normally direct fired - the work is exposeddirectly to combustion products

Fuels: natural gas or propane and fuel oils thatcan be atomized

Electric furnaces

Electric resistance for heating

Cleaner, quieter, and more uniform heating

More expensive to purchase and operate

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Batch furnaces

Heating system in an insulated chamber, with a door for loading and unloading

Production in batches

Continuous furnaces

Generally for higher production rates

Mechanisms for transporting work through furnace include rotating hearths and straight-throughconveyors

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Atmospheric control furnaces

Desirable in conventional heat treatment to avoidexcessive oxidation or decarburization

Include C and/or N rich environments fordiffusion into work surface

Vacuum furnaces

Radiant energy is used to heat the parts

Disadvantage: time needed each cycle to drawvacuum

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These methods heat only the work surface, or localareas of the work surface

They differ from surface hardening methods in thatno chemical changes occur

Methods include:

Flame hardening

Induction hardening

High-frequency resistance heating

Electron beam heating

Laser beam heating

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Heating of work surface by one or more torchesfollowed by rapid quenching

Applied to carbon and alloy steels, tool steels, andcast irons

Fuels include acetylene (C2H2), propane (C3H8), andother gases

Lends itself to high production as well as bigcomponents such as large gears that exceed thesize capacity of furnaces

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Application of electromagnetically induced energysupplied by an induction coil to an electricallyconductive workpart

Widely used for brazing, soldering, adhesivecuring, and various heat treatments

When used for steel hardening treatments,quenching follows heating

Cycle times are short, so process lends itself tohigh production

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Figure: Typical induction heating setup. High frequency alternating current in acoil induces current in the workpart to effect heating.

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Used to harden specific areas of steel work surfaces by application of localized resistance heating at high frequency (400 kHz typical)

Contacts are attached to workpart at outer edges of the area

When HF current is applied, region under conductor is heated quickly to high temperature - heating to austenite range typically takes less than a second

When power is turned off, area is quenched by heat transfer to the surrounding metal

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Figure: Typical setup for high-frequency resistance heating.

High-frequency Resistance Heating

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Electron beam focused onto small area, resulting inrapid heat buildup

Involves localized surface hardening of steel - highenergy densities in a small region of part so thataustenitizing temperatures can be achieved oftenin less than a second

When beam is removed, heated area is immediatelyquenched and hardened by heat transfer tosurrounding metal

Disadvantage: best results are achieved whenperformed in a vacuum

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High-density beam of coherent light focused on asmall area - the beam is usually moved along adefined path on the work surface

Laser - acronym for light amplification bystimulated emission of radiation

When beam is moved, area is immediatelyquenched by heat conduction to surrounding metal

Advantage of LB over EB heating is that laser beamsdo not require a vacuum

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Mech. Engg. Deptt. UET Lahore

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05. industrial materials

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