HEAT TREATMENT

Unit -2
Syllabi

• UNIT II HEAT TREATMENT 9

Definition – Full annealing, stress relief, recrystallisation and spheroidising –normalizing, hardening
and tempering of steel. Isothermal transformation diagrams – cooling curves superimposed on I.T.
diagram– continuous cooling Transformation (CCT) diagram – Austempering, Martempering –
Hardenability, Jominy end quench test -case hardening, carburizing, Nitriding, cyaniding,
carbonitriding – Flame and Induction hardening – Vacuum and Plasma hardening – Thermo-
mechanical treatments- elementary ideas on sintering.

•
Heat Treatment

• A combination of heating & cooling operation timed & applied to

a metal or alloy in the solid state in a way that will produce
desired properties
• Often associated with increasing the strength of material
• •Can also be used to obtain certain manufacturing objectives like
• To improve machining & formability,
• To restore ductility
• To recover grain size etc.
–Known as Process Heat Treatment
Heat Treatment

• Heat treatment done for one of the following objective: –

• Hardening
• Softening.
• Property modification.
• Hardening heat treatments particularly suitable for Steels
• Many phase transformation involved even in plain carbon steel and
low-alloy steel.
• Other type of heat treatments equally applicable to ferrous &
non-ferrous
Hardening Heat Treatment

• Hardening of steels is done to increase the strength and wear

properties.
• Hardening (Quenching followed by Tempering) is intended for
improving the mechanical properties of steel.
• Generally increases hardness at the cost of toughness
• Pre-requisites for hardening is sufficient
• carbon and/or alloy content.
• Sufficient Carbon - Direct hardening/Case hardening.
• Otherwise- Case hardening
Hardening Heat Treatment

• Common Hardening Heat Treatments:

• Direct Hardening –Heating,Quenching,Tempering
• Austempering
• Martempering
• Case Hardening –Case carburizing,Case Nitriding,Case Carbo-nitriding or
Cyaniding,Flame hardening,Induction hardening etc
• Precipitation Hardening
Heat Treatment
Full Annealing

• Annealing is a process involving heating and cooling, usually

applied to produce softening.
• The term also refers to treatments intended to alter mechanical or
physical properties, produce a definite microstructure, or remove
gases.
• The temperature of the operation and the rate of cooling depend
upon the material being annealed and the purpose of the treatment.
Full Annealing

• Full annealing is a softening process in which a steel is heated to

a temperature above the transformation range (Acs) and, after being
held for a sufficient time at this temperature, is cooled slowly to
a temperature below the transformation range (Ari).
• The steel is ordinarily allowed to cool slowly in the furnace,
although it may be removed and cooled in some medium such as mica,
lime, or ashes, that insures a slow rate of cooling.
• Since the transformation temperatures are affected by the carbon
content, it is apparent that the higher carbon steels can be fully
annealed at lower temperatures than the lower carbon steels.
• The temperature range normally used for full annealing is 25 to 50
deg F above the austenizing temperature
Full Annealing

• Video
Stress Relief

• The heat treatment that relieves residual stresses is commonly

called "aging," "normalizing," or "mild annealing;" the term
"stress relieving" is more accurately descriptive.
• This treatment can be accomplished by heating the cast iron to
between
• 800° and 1,100° F, holding at temperature from 30 min. to 5 hr, the
time depending on the size and temperature, and cooling slowly in
the furnace.
• Such treatment will cause only a slight decrease in hardness, very
little decomposition of cementite, and only a slight change in the
strength of the metal.
Recrystalisation – Process Annealing

• Temperature raised near the lower critical temperature line A1 i. e. 650ºC to

700ºC
• Holding for sufficient time, followed by still air cooling
• Initially, the strained lattices reorient to reduce internal stresses
• (recovery)
• When held long enough, new crystals grow (recrystallisation)
• Material stays in the same phase through out the process
Only change in size, shape and distribution of the grain structure
• This process is cheaper than either full annealing or normalizing
• As material is not heated to a very high temperature or cooled in a furnace.
Spheroidization

• Spheroidizing is a process of heating and cooling steel that

produces a rounded or globular form of carbide m a matrix of
ferrite.
• It is usually accomplished by prolonged heating at temperatures
• just below the Aci but may be facilitated by alternately heating to
temperatures just above the Aci and cooling to just below the Ari.
• The final step, however, should consist of holding at a temperature
just below the critical (Ari).
• The rate of cooling is immaterial after slowly cooling
• to about 1,000° F.
Spheroidization

• The rate of spheroicolization is affected by the

• initial structure.
• The finer the pearlite, the more readily
spheroidization is accomplished.
• A martensitic structure is very amenable to
spheroidization.
• This treatment is usually applied to the high carbon
steels (0.60% of carbon and higher).
• The purpose of the treatment is to improve
machinability and it is also used to condition high-
carbon steel for cold-drawing into wire.
Normalising

• Normalizing heat treatment helps to remove impurities and improve

ductility and toughness.
• During the normalizing process, material is heated to between 750-980 °C
(1320-1796 °F).
• The exact heat applied for treatment will vary and is determined based on
the amount of carbon content in the metal.
Normalising

• After heating, material is cooled to room temperature.

• The rate of cooling significantly influences both the amount of pearlite
and the size and spacing of the pearlite lamellae.
• At higher cooling rates, more pearlite forms, and the lamellae are finer
and more closely spaced.
• Both the increased amount of pearlite and the greater fineness of the
pearlite result in higher strength and higher hardness.
• Conversely, lower cooling rates result in softer parts within the metal
normalizing process.
•
Normalising

• Desired outcomes will typically produce a uniform pearlitic structure in

combination with either ferrite grains or grain-boundary carbides present
depending on the base material’s carbon content.
• Improved machinability, grain-structure refinement, homogenization, and
reduction of residual stresses are the primary reasons that normalizing is
performed.
Normalising vs Annealing vs Stress Relief

• The shorter cooling time produces metal that is less ductile and has a
higher hardness value than the annealing process.
• The stress relief process uses heat treatment to reduce, as the name
suggests, stresses caused by rolling or cutting, but is not heated enough to
produce any significant changes to the material properties as with the
normalizing and annealing processes.
• The process requires more intense heating than both annealing and stress
relief, but the cooling process is significantly faster
Materials Suitable for Normalization

• Aluminum
• Brass
• Copper
• Iron alloys
• Nickel alloys
• Steel
Hardening

• Also named martensitic or quench hardening, neutral hardening is a heat

treatment used to achieve high hardness/strength on steel.
• It consists of austenitising, quenching and tempering, in order to retain a
tempered martensite or bainite structure.
• The hardening processes described here are typically neutral, which
means that the chemical composition of the steel surface of the parts is
not intended to be changed during the process.
Hardening

• Direct quench hardening is the most common practice for hardening of steel.
• The first step is to heat up in stages to the hardening temperature which is, depending on
steel type, between 800 and 1220°C.
• At a temperature between 730 and 900°C (depending on steel type) a transformation of
the microstructure into austenite takes place.
• The second step is to hold at this hardening, austenitising temperature to simultaneously
fully equalise the temperature of the parts, and transform the microstructure into
austenite
• The third step is quenching the part direct from the austenitising temperature in a cold
medium. This kind of quench medium is usually water, liquid salt, oil or high pressure
nitrogen, depending on the steel type and part dimensions.
• The quenching speed must be high enough to prevent the material from transforming
back into the original soft structure.
Tempering

• Tempering is a low temperature (below A1) heat treatment process

normally performed after neutral hardening, double hardening,
atmospheric carburising, carbonitriding or induction hardening in order to
reach a desired hardness/toughness ratio.
• The tempering temperature may vary, depending on the requirements
and the steel grade, from 160°C to 500°C or higher.
• Tempering is normally performed in furnaces which can be equipped with
a protective gas option.
• Protective gas will prevent the surface from oxidation during the process
and is mainly used for higher temperatures.
Tempering

• For some types of steels the holding time at the tempering temperature is
of great importance; an extended holding time will correspond to a higher
temperature.
• Depending on the steel grade a phenomenon known as temper brittleness
can occur in certain temperature intervals.
• Tempering inside this temperature interval should normally be avoided.
• These areas are shown in the steel suppliers steel catalogues, as well as
the most suitable temperature depending on hardness requirements.
Case Hardening

• Case hardening is a material processing method that is used to

increase the hardness of the outer surface of a metal.
• Case hardening results in a very thin layer of metal that is notably
harder than the larger volume of metal underneath of the hardened
layer.
• Case hardening almost always requires elevated temperatures to
perform.
• Through heating, the hardening can be caused by altering the
crystal structure of a metal or adding new elements to the
composition of the exterior surface of a metal.
Case Hardening

• Since hardening processes reduce formability and machinability,

case hardening is typically done once most other fabrication
processes have been completed.
• One of the most popular methods for higher carbon steels or other
heat-treatable metals is through heating and quenching.
• Heating and quenching involves using some sort of heat source,
such as induction coils or an oxyfuel flame to get the outer surface
of a steel up past the temperature where its microstructure begins
to change, known as its critical temperature (generally somewhere
around 700 degrees Celsius).
Case Hardening

• Once this has been accomplished, the steel surface needs to be

rapidly cooled by being placed into contact with a quenching
medium.
• This can be brine, water, oil, or air.
• Different media will be used for different applications depending on
the cooling rate required.
• This rapid cooling causes the steel to form martensite, which is a
very hard, abrasion-resistant microstructure.
Nitriding

• Another case hardening method is nitriding.

• Nitriding gets its name from the formation of nitrides that the process forms
on the surface of a metal.
• To perform the nitriding process, metals are heated to an elevated
temperature and exposed to ammonia or other nitrogen carrying substances.
• The elevated temperature and exposure to nitrogen promote the formation of
nitrides, which by nature are very hard and resistant to abrasion.
• This process only works when there are elements on the metal being
hardened that can form nitrides such as chromium and molybdenum.
• Nitriding generally requires lower temperatures than heating and quenching
and also does not require a quench process, resulting in less distortion.
Carburizing

• Carburizing is another form of case hardening that is widely used to

improve the mechanical properties of a steel substrate.
• During carburizing, a steel alloy is heated to an elevated temperature
and then is exposed to high amounts of carbon on its surface.
• The external carbon source can be a gas, liquid, or solid depending on
the application requirements.
• The high amounts of external carbon will then form carbides with other
elements on the surface of the steel.
• These carbides provide increased hardness and wear resistance.
• Similar to nitriding, the heating requirements are generally less,
potentially resulting in less distortion.
Cyaniding

• Cyaniding is a case hardening process in which both C and N2 in form of

cyaniding salt are added to surface of low and medium carbon steel.
• Sodium cyanide or potassium cyanide may be used as the hardening
medium.
• It is a process of superficial case hardening which combines the absorption
of carbon and nitrogen to obtain surface hardness.
• The components to be case hardened are immersed in a bath having fused
sodium cyanide salts kept at 800-850°C.
Cyaniding

• The component is then quenched in bath or water.

• This method is very much effective for increasing the fatigue limit of
medium and small sized parts such as gears, spindle, shaft etc.
Carbo Nitriding

• carbonitriding is a thermochemical treatment involving the incorporation

of both carbon and nitrogen into the surface of the component, usually
simultaneously.
• The process is carried out at lower temperatures, and generally for shorter
times than carburising, and therefore components are less prone to
distortion.
• The diffused nitrogen has a stabilising effect on austenite and lowers the
critical quenching speed and, as a consequence, the hardenability of the
steel.
Carbo Nitriding

• Less severe quenching media like oil, instead of water quenching needed
for mild steel, can be applied for reducing distortion.
• Carbonitriding is usually carried out in a temperature range of 820-900°C
in a gaseous atmosphere adding between 0.5 to 0.8% carbon and 0.2-0.4%
(< 5%) nitrogen to the surface of plain carbon steel or low alloy steel.
• After diffusion time the components are directly quenched in oil.
• The heat treatment is completed by low temperature tempering between
150-200°C for the higher case depth range reducing brittleness and
depending on tribological circumstances.
Types of Metals Can Be Case Hardened

• Low carbon steel

• High carbon steel
• Cast iron
• High strength low alloy steel
• Tool steel
• Stainless steels
Applications

Some common components that are case hardened include:

• Gears
• Fasteners
• Camshafts
• Rods
• Pins
Induction Hardening

• Induction hardening is a process used for the surface hardening of

steel and other alloy components.
• The parts to be heat treated are placed inside a water cooled
copper coil and then heated above their transformation temperature
by applying an alternating current to the coil.
• The alternating current in the coil induces an alternating magnetic
field within the work piece, which is made from steel, caused the
outer surface of the part to heat to a temperature above the
transformation range.
Induction Hardening

• Parts are held at that temperature until the appropriate depth of

hardening has been achieved, and then quenched in oil, or another
media, depending upon the steel type and hardness desired.
• The core of the component remains unaffected by the treatment and
its physical properties are those of the bar from which it was
machined or preheat treated.
Flame Hardening

• Flame hardening is similar to induction hardening, in that it is a surface

hardening process.
• Heat is applied to the part being hardened, using an oxy- acetylene (or
similar gas) flame on the surface of the steel being hardened and heating the
surface above the upper critical temperature before quenching the steel in a
spray of water.
• The result is a hard surface layer ranging from 0.050" to 0.250" deep.
• The composition of the steel is not changed; therefore core mechanical
properties are unaffected.
• Flame hardening produces results similar to conventional hardening
processes but with less hardness penetration.
Vacuum Hardening

• Vacuum hardening is a heat treatment process of metal components carried out

under controlled partial pressure, during which temperatures of up to 1,300 °C
can be reached.
• The quenching methods will differ with regards to the material treated but gas
quenching using nitrogen is most common.
• The aim of this process variant is the creation of bright metallic workpiece
surfaces which render further mechanical processing unnecessary.
• In most cases hardening takes place in conjunction with subsequent reheating,
the tempering.
• Depending on the material, hardening improves the hardness and wear
resistance or regulates the ratio of toughness to hardness.
Vacuum Hardening

• It minimizes workpiece distortion since there is no direct contact cooling.

• The process control is precise and reproducible, leading to consistent results.
• The vacuum prevents oxidation or discoloration on the surface of the workpiece.
• The hardening depth in vacuum hardening refers to the depth to which the
workpiece is hardened.
• It measures how deeply hardening processes, such as martensite formation,
penetrate into the material.
• The hardening depth depends on various factors, such as material composition
and cooling rate.
Vacuum Hardening

• There are some limitations to vacuum hardening.

• The process is not suitable for all materials, especially mild and low-alloy
steels. In addition, vacuum hardening can be more expensive than other
hardening processes due to the equipment required and the longer
process cycle.
Plasma Hardening

• Plasma carburising and plasma nitriding are thermochemical

processes for increasing the surface hardness of low carbon
steels.
• Both processes produce little distortion of components and
hence minimise the need for post-treatment machining or
grinding.
• Due to its lower process temperatures, plasma nitriding gives
the least amount of distortion and is particularly suitable for
surface hardening of tools, dies and precision parts.
• Any steel which can be gas carburised can also be surface
hardened using plasma carburisation.
• The principle of the process is the same and it aims to increase
the carbon content at the surface of a component to allow
formation of a deep and hard layer on quenching.
JOMINY END QUENCH TEST

• The Jominy end quench test is used to measure the hardenability of a

steel, which is a measure of the capacity of the steel to harden in depth
under a given set of conditions.
• The test sample is a cylinder with a length of 102 mm (4 inches) and a
diameter of 25.4 mm (1 inch).
• The steel sample is normalised to eliminate differences in microstructure
due to previous forging, and then austenitised.
• This is usually at a temperature of 800 to 900°C.
JOMINY END QUENCH TEST
JOMINY END QUENCH TEST

• The test sample is quickly transferred to the test machine, where it is held
vertically and sprayed with a controlled flow of water onto one end of the
sample.
• This cools the specimen from one end, simulating the effect of quenching
a larger steel component in water.
• The cooling rate varies along the length of the sample from very rapid at
the quenched end, to rates equivalent to air cooling at the other end.
JOMINY END QUENCH TEST
JOMINY END QUENCH TEST
JOMINY END QUENCH TEST

• The round specimen is then ground flat along its length to a depth of
0.38 mm (15 thousandths of an inch) to remove decarburised material.
• The hardness is measured at intervals from the quenched end.
• The interval is typically 1.5 mm for alloy steels and 0.75 mm for carbon
steels.
• High hardness occurs where high volume fractions of martensite develop.
• Lower hardness indicates transformation to bainite or ferrite/pearlite
microstructures.
Isothermal Transformation Diagram
Isothermal Transformation Diagram
Isothermal Transformation Diagram
Isothermal Transformation Diagram
Isothermal Transformation Diagram