Different Types of Heat Treatment Processes

Dr. Riya Mondal

MME Dept.
NIAMT, Ranchi
Classification:
 Annealing:

 Annealing is a heat treatment in which the metal is heated to a temperature above its
recrystallization temperature, kept at that temperature for some time for homogenization of
temperature followed by very slow cooling to develop equilibrium structure in the metal or
alloy.
 The steel is heated 30 to 50°C above Ac3 temperature in case of hypo-eutectoid steels and 30 to
50°C above A1 temperature in case of hyper-eutectoid temperature
 The cooling is done in the furnace itself.
Microstructural Changes during Annealing:
Types of Annealing:
 Full annealing
 Process annealing
 Stress relief annealing
 Recrystallization annealing
 Spheroidise annealing
 Diffusion Annealing
 Bright Annealing and Black Annealing
Aims of Annealing:
 Improve or restore ductility and toughness
 Reduce hardness
 Improving formability
 Recrystallize cold worked (strain hardened) metals
 Remove internal stresses
 Decrease brittleness
 Increase machinability
 Decrease electrical resistance
 Improve magnetic properties
 Reduces segregation
 Reduce the gaseous contents in steel
 Enhance Machinability,
 Eliminate chemical non-uniformity,
  In full annealing, hypoeutectoid steels (less than 0.8%
Full Annealing: C) are heated to 30 to 60°C above the upper critical
point i.e., A3 line (i.e., between 723°C and 910°C), as
shown in Fig. This is to convert the structure to
homogeneous single-phase austenite of uniform
composition and temperature, held at this temperature
for a period of time, and then slowly cooled to room
temperature.

 Cooling is usually done in the furnace itself by

decreasing the temperature 10 to 30°C per hour to at
least 30°C below the A line. Then, the alloy is
removed from the furnace and air cooled to room
temperature.

 Now the resulting structure is coarse pearlite with

excess ferrite. In this condition, the steel is quite soft
and more ductile.
 Full Annealing:
 The procedure for hypereutectoid steels (i.e., greater than 0.8% C) are the same, except that the hypereutectoid
steels are heated to 30 to 60°C above the A1 line (i.e., between 723°C and 1138°C), as shown in Fig.

 In this case, the resulting structure of a hypereutectoid steel will be coarse pearlite plus excess cementite in
dispersed spheroidal form.

 This structure imparts much improvement in mechanical properties, high ductility and high toughness.

The main objects of full annealing are:

 To soften the metal,

 To refine its crystalline structure, and

 To relieve the stresses.

 Material: The full annealing is specially adopted for steel castings and steel ingots
 Process Annealing (or Subcritical Annealing):
 Purpose: Process annealing is a heat treatment that is often used to soften and increase the ductility of a previously
strain- hardened metal.

 Material: Process annealing is extensively employed for steel wires and sheet products (especially low carbon
steels).
 Operation: In process annealing, the low carbon steels (< 0.25% C) are
heated to a temperature slightly below the A1 line (i.e., between 550°C
and 650°C), as shown in Fig.2.3. Then they are held for period of time to
achieve softening, and then cooled at any desired rate. This process
induces single phase morphology.

 Because the material is not heated to as high temperature as in full

annealing, the process annealing is comparatively cheaper, more rapid,
and tends to produce less scaling.

 Application: This process has wide application in preparing steel sheets

and wires for drawing.
 Parts which are fabricated by cold forming such as stamping, extrusion,
upsetting and drawing are frequently given this treatment as an
intermediate step.
 Stress Relief Annealing (or Commercial Annealing):
Purpose: The stress relief annealing is a heat treatment proces that is employed to eliminate internal residual
stresses induced by casting, quenching, machining, cold working, welding, etc.

✓ Causes of internal residual stresses: Internal residual stresses may develop in metals/alloys due to:

1. Plastic deformation processes such as machining and grinding;

2. Non-uniform cooling of a metal that was processed or fabricated at an elevated temperature, such as a weld or a
casting; and

3. A phase transformation that is induced upon cooling wherein parent and product phases have different densities.

✓ Effects of internal residual stresses: If the internal residual stresses are not removed, then distortion↑ or
warpage↑↑ of the -material may result.

✓ Operation: In stress relief annealing, the steel parts are heated in the range of 550°C to 650°C, held for a period of
time, and then cooled slowly. For plain carbon and low-alloy steels the temperature to which the specimen is heated
is usually between 450 and 650˚C, whereas for hot-working tool steels and high-speed steels it is between 600 and
750˚C. This treatment will not cause any phase changes
 ✓ Recrystallisation temperature: The temperature at which
Recrystallization Annealing: crystallisation takes place i.e., new grains are formed is called
recrystallisation temperature. This temperature is a function
of the particular metal, its purity, amount of prior
deformation, and annealing time.

✓ Operation: In this heat treatment process, cold worked

steel is heated to a temperature above recrystallisation
temperature, held at this temperature for some time, and
then cooled.

It may be noted that the recrystallisation process does not

produce new structures. But it produces strain-free new
grains. This results in increase in ductility as well as
decrease in the hardness and strength.
Application:
This process is employed as an intermediate
operation or as final treatment in industries manufacturing
steel wires, sheets or strips.
 Spheroidizing Annealing:
Purpose:
 The purpose of this process is to improve the machinability and ductility of medium and high carbon
steels (>0.60 wt% C) and air hardening alloy steels.
 To soften steels
 To reduce hardness, strength and wear resistance.
Process / Method:
Various methods are available to produce spheroidised structure
 The first method consists of heating steel to a temperature just below the lower critical temperature,
holding at that temperature for a prolonged period and followed by slow cooling.
 The second method involves heating and cooling the steel alternatively just above and below the lower
critical temperature.
 The third method (In the case of tool or high alloy steels) consists of heating the steel to a temperature
above the lower critical temperature (750°C to 800°C or higher) followed by slow cooling to a
temperature below the lower critical temperature and holding at that temperature for a long period.
 Formation of small globular cementite (spheroids) dispersed throughout the ferrite matrix.
 Diffusion Annealing:
 This process also known as homogenizing annealing, is employed to remove any structural non-uniformity.
 Dendrites, columnar grains and chemical in-homogeneities are generally observed in the case of ingots, heavy
plain carbon steel casting, and high alloy steel castings. These defects promote brittleness and reduce ductility and
toughness of steel.
 In diffusion annealing treatment, steel is heated sufficiently above the upper critical temperature (say, 1000-
1200°C), and is held at this temperature for prolonged periods, usually 10-20 hours, followed by slow cooling.
 Segregated zones are eliminated and a chemically homogeneous coarse grain steel is obtained by this treatment as
a result of diffusion.
 The coarse grained structure can be refined either by plastic working for ingots or by employing a second heat
treatment for castings.
 Hypoeutectoid and eutectoid steel castings are given full annealing treatment, whereas hypereutectoid steel
castings are either normalized or partially annealed for this purpose.
 Partial Annealing:
 Partial annealing is also referred to as intercritical annealing or incomplete annealing. In this
process, steel is heated between the A1 and the A3 or Acm. It is followed by slow cooling.

 Generally, hypereutectoid steels are subjected to this treatment. Resultant microstructure

consists of fine pearlite and cementite. The reason for this is that grain refinement takes place
at a temperature of about 10 to 30°C above Ac1 for hypereutectoid steels.
 As low temperature are involved in this process, so it is cost effective than full annealing

Bright Annealing and Black Annealing:

 Annealing process carried out in protective atmosphere prevents dis-colouration of the steel.
This process is called as Bright Annealing.
 When components to be annealed are surrounded by reducing agents like charcoal and
annealed in a box then oxidation of these components is reduced. This process is Box or Black
annealing.
Normalizing:
 The process consists of heating the steel to above A3 temperature for hypoeutectoid steels and above Acm (to
break the network of pro eutectoid cementite) for hypereutectoid steels by 30-50oC, holding at this temperature
for a definite period and slow cooling to below A1 or room temperature usually in air.
 Due to air cooling which is slightly fast as compared to furnace cooling employed in annealing, normalised
components show slightly different structure and properties than annealed components.
 Results a grain structure of fine Pearlite with pro-eutectoid Ferrite or Cementite
 When steel is heated to a high temperature, the carbon can readily diffuse throughout, and the result is a
reasonably uniform composition from one area to the next.
 Steel is then more homogeneous and will respond to the heat treatment in a more uniform way.
 Normalizing treatment is more frequently applied to ingots prior to working, and to steel castings and
forgings prior to hardening for homogenizing or grain-refining treatment.
Purpose:
 To refine the grain structure.
 To increase the strength of the steel.
 To provide a more uniform structure in castings
and forgings.
 To relieve internal residual stresses due to cold
working.
 To achieve certain mechanical and electrical
properties.
Structural Change:

 In this process, the homogeneity of austenite increases since the temperature involved is more that
that for annealing.

 It results in better dispersion of ferrite and cementite in the final structure.

 The grain size is finer in normalized structure than the annealed one.

 This results in a slightly higher strength and hardness but lower ductility than full annealing.

Application:

Rolled and forged steels possessing coarse grains are subjected to normalising treatment for grain
refinement.
 NORMALISING ANNEALING / FULL ANNEALING
Normalising is more economical than full annealing since no Full annealing is costly
furnace is required to control the cooling rate.
Normalising is less time consuming Full annealing is more time consuming

Normalising temperature is higher than full annealing Annealing temperature is lower than normalising

It provides fine grain structure It provides coarse grain structure

In normalising, the cooling is established in still air. So the In full annealing, the furnace cooling ensures identical cooling
cooling will be different at different locations. Thus properties conditions at all locations within the metal, which produces
will vary between surface and interior identical properties

Comparatively higher strength and hardness Comparatively lower strength and hardness

Normalising improves the machinability of low carbon steel Annealing improves the machinability of medium carbon steel

In this metal is heated 30 to 50°C above higher critical In this hypo-eutectoid steel is heated to a temperature
temperature. approximately 20 to 30°C above temperature the higher
critical temperature and for hypereutectoid steel is heated
20 to 30°C above the lower critical temperature.
It induces gives higher ultimate strength, yield point and It gives high ductility.
impact strength in ferrous material.
 Hardening and Hardness are two very different things. One is a process of
Hardening: heat treatment and other is a extrinsic property of a material.
Steel is hardened by heating 20-30°C above the upper critical point for hypo eutectoid steel and 20-30°C above the
lower critical point for hyper eutectoid steel and held at this temperature for some time and then quenched in water
or oil or molten salt bath. It is some time said as rapid quenching also.
QUENCHING:
 The Process of fast or instant Cooling is Knows as Quenching.
 The cooling can be accomplished by contact with a quenching medium which may be a gas, liquid or solid.
 Most of the times, liquid quenching media is widely used to achieve rapid cooling.
Types of Quenching medium:
5 – 10% Caustic soda
5 – 20% brine (NaCl)
Cold water
Warm water
Mineral oil – obtained during the refining of crude petroleum
Animal oil
Vegetable oil
Air
Thus water produces the most severe quench followed by oil, which is more effective than air.
 The high hardness developed by this process is due to the phase transformation accompanying rapid cooling.
Rapid cooling results in the transformation of austenite at considerably low temperature into nonequilibrium
products.
 Hardening is applied to cutting tools and machine parts where high hardness and wear resistance are important.

The Process Variables:

 Hardening Temperature: The steel should be heat treated to optimum austenitising temperature. A lower
temperature results lower hardness due to incomplete transformation t austenite. If this temperature is too high
will also results lower hardness due to a coarse grained structure.
 Soaking Time: Soaking time at hardening temperature should be long enough to transform homogenous austenite
structure. Soaking time increases with increase in section thickness and the amount of alloying element.
 Delay in quenching: After soaking, the steel is immediately quenched. Delay in quenching may reduce hardness
due to partial transformation of austenite.
 Type of quenching medium also has a profound effect, which will be discussed briefly
Quenching:
 Quenching is a process of rapid cooling of materials from high temperature to room temperature or even lower.
In steels quenching results in transformation of austenite to martensite (a non-equilibrium constituent).
 During cooling, heat must be extracted at a very fast rate from the steel piece. This is possible only when a steel
piece is allowed to come in contact with some medium which can absorb heat from the steel piece with in a short
period.
 Under ideal conditions, all the heat absorbed by the medium should be rejected to the surroundings immediately.
 The removal of heat during quenching is complex in the sense that heat is removed in three stages.
1) Vapor Blanket,
2) Nucleate Boiling,
3) Convection.
Vapor Blanket (stage 1)
 As soon as the work-piece comes into contact with a liquid coolant (quenchant), the surrounding quenchant layer
is instantaneously heated up to the boiling point of the quenchant and gets vaporized due to the high temperature
of the work- piece.
 This acts as an insulator, preventing the quenching oil from contacting the metal surface. As a consequence, the
rate of cooling during this stage is slow.
 At this stage the work piece is cooled only by conduction and radiation through the vapor film. Only the surface
is cooled considerably prior to the formation of vapor envelop.
Nucleate Boiling (stage 2)
 This second stage is also called as transport cooling stage or liquid boiling stage. The
temperature of the work-piece comes down, through very slowly and the vapor blanket is no
longer stable and collapses.
 Metal surface comes into contact with the liquid/ quenchant. Violent boiling quickly removes
heat from the quenched component while forming bubbles and being pushed away, resulting
in the cooler fluid coming into contact with the work piece.
 This happens till the temperature of the work piece comes down to the boiling point of the
liquid.
 Maximum cooling rate is achieved during this stage.

Convection (stage 3)
 The third stage is called as the liquid cooling stage or the convection stage.
 It starts when the temperature of the surface becomes equal to the boiling point of the
quenchant.
 Cooling at this stage takes place via conduction and convection processes.
 The rate of cooling is the slowest at this stage.
APPLICATIONS OF QUENCHING MEDIUM
Quenching medium Applications Selection of Quenching
Mineral oils Used in hardening alloy steels
medium:
Based on the following factors,
Water or aqueous solution of Used for quenching carbon and  Desired rate of heat removal
NaOH or NaCl low alloy steels
 Required temperature
Water and Air Used for rails, pipes and heavy interval
forgings
 Boiling point
 Viscosity
 Flash point
 Stability under repeated use
 Possible reactions with the
material being quenched
 Cost
Effect of Quenching Medium

 Quenching medium has the profound effect on the final phase of the material. Quenching
medium is directly related to the rate of the cooling of the material.
 Some of the widely employed quenching media are water, aqueous solutions, oils (mineral,
vegetable and even animal oils), molten salts and air.

Quenching Medium (Water):

 Water has maximum cooling rate amongst all common quenchants except few aqueous solutions.
 It is very cheap and easily disposed off compared to other quenchants.
 Hence water is used for carbon steels, alloy steels and non-ferrous alloys.
 The layer if scale formed on the surface during heating is also broken by water quenching, thus
eliminating an additional process of surface cooling.
Quenching Medium (Oil):
 Most of the Oils used as quenchants are mineral oils. These are in general paraffin based and do not
possess any fatty oils.
 Quenching in oil provides slower cooling rates as compared to those achieved by water quenching.
 The slower cooling rate reduces the possibility of hardening defects.
 The temperature difference between core and the case of work piece is less for oil quenching than for
water quenching.

Effect of Quenching Medium :

 Just the drastic water quench generates a fully martensite structure.
 Although quenched in oil the austensite converts into suitably fine pearlite.
 Accurate pearlite also results if the austenised eutectoid steel is air-cooled.

 Though, if allowed to cool in furnace coarse pearlite is appearance.

Tempering:

 Hardened steels are so brittle that even a small impact will cause fracture. Toughness of such a steel
can be improved by tempering. However there is small reduction in strength and hardness.

 The purpose of tempering is to relieve the residual stresses and improve ductility and toughness of the
hardened steel.

 The gain in ductility and toughness is usually attained at the loss of hardness and wear resistance.

 Tempering is a sub-critical heat treatment process used to improve the toughness of hardened steel.

 Tempering consists of reheating of hardened steel to a temperature below Lower critical temperature
and is held for a period of time, and then slowly cooled in air to room temperature.

 At tempering temperature, carbon atoms diffuses out and form fine cementite and softer ferrite
structure left behind. Thus the structure of tempered steel consists of ferrite and fine cementite.

 Thus tempering allows to precipitate carbon as very fine carbide and allow the microstructure to
return to BCC
 Three stages of tempering are distinguished:
Depending on temperatures, tempering processes can be classified as:
1) Low- temperature tempering (150 - 250°C),
2) Medium – temperature tempering (350 - 450°C),
3) High – temperature tempering (500 - 650°C).
First stage of Tempering/Low temperature tempering (1-2 Hours at a Temperature up to 250°C)
 The tempering reactions in steels, containing carbon less than 0.2%, if carbon atoms have not yet segregated
(during quenching) to dislocations, these diffuse and segregate around the dislocations and lath boundaries in the
first stage of tempering. No ε-carbide forms as all the carbon gets locked up to the dislocations (defects).
 Martensite in steels with more than 0.2% carbon is highly unstable because of super saturation, and interstitial
diffusion of carbon in BCT martensite can occur. Thus in the first stage of tempering, the decomposition of
martensite into low-tetragonality martensite (containing ~0.2%C, c/a ~ 1.014) and ε-carbide, Fe2.4C occurs.
 The structure at this stage referred to as tempered martensite, which is double phase mixture of low tetragonal
martensite and ε-carbide. This structure is fine and etches are dark. This is known as black martensite. In this stage
volume ↓ because specific volume of martensite ↓ due to rejecting of C atoms.
Second stage of Tempering / Medium temperature tempering:(350 C to 500°C):
 In the second stage of tempering retained austenite transforms to lower bainite (the carbide in bainite is ε-carbide).
This structure is called troostite. The matrix in lower bainite is cubic ferrite (c/a = 1), where as in tempered
martensite, the low tetragonal martensite has c/a ~ 1.014
 When retained austenite changes to lower bainite, their takes place increase in volume.
 These changes in microstructure result in increase of ductility and toughness with a corresponding decrease in
hardness and strength.
Third stage of Tempering / High temperature tempering(500-650°C):
 In this stage of tempering, ε-carbide dissolves in matrix, and low tetragonal martensite losses its completely its
carbon and thus, the tetragonality to become ferrite .
 the cementite particles coalesce and grow. Cementite forms as rods at interfaces of ε-carbide and matrix, twin
boundaries, interlath boundaries, or original austenite grain boundaries. The resultant structure is known as
sorbite.
 This treated steel has better tensile, yield and impact strength and is free from internal stresses.
 During this stage, volume decreases just as in stage one, due to complete loss of tetragonality. In a 1% carbon
steel, the total decrease in length in the first and third stages in around 0.25%
Applications:
 Low temperature tempering is useful for high carbon and low alloy steels in manufacturing cutting
and measuring tools.

 Medium temperature tempering is best suited for coil and laminated springs.

 High temperature tempering is suited for medium carbon steel and medium carbon low alloy steels.
Connecting rods, shafts and gears are frequently subjected to this treatment.
Tempering of alloy steels : Secondary Hardening:
 In alloy steels, having larger amounts of strong carbide forming elements like Mo, Ti, V, Nb, W, Cr etc., and
carbon , a peculiar phenomena occurs, the hardness of the as-quenched martensite (called primary hardness) on
tempering, decreases initially, as the tempering temperatures is raised, but starts increasing again to often become
higher than the as quenched hardness, with much improved toughness, when tempered in the range of 500 to
600°C. This increase in hardness is called secondary hardness (also called red hardness).
 This is great importance in high speed steels, as these are able to continue machining, at high speeds (as these are
able to resist fall in hardness and thus, the cutting property ) even when they become red hot.
 Secondary hardening is a process, similar to age hardening, in which coarse cementite particles are replace by a
new and much finer alloy carbide dispersion of V4C3, Mo2C, W2C (which normally form on dislocations). As in
aging a critical dispersion causes a peak in the hardness and strength of the alloy, and as over aging takes place,
i.e., carbide dispersion slowly coarsens, the hardness decreases.
 Secondary hardening is best shown in steels containing Mo, V, W, Ti and also in Cr steels at high chromium
concentrations.
 The amount of secondary hardening in an alloy steel is directly proportional to the volume fraction of the alloy
carbides, and thus is directly proportional to the concentration of strong carbide forming elements present in
steels. The alloy carbides must precipitate as fine dispersion in ferrite matrix rather than massive carbide particles.
Temperature and colours for Heating and Tempering of Steel: