Annealing heat treatment and Normalizing heat treatment compared

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.

Prepared by: Mohammad Minhajul Anwar email: [email protected]


Figure: Annealing in a typical furnace

2. Annealing


Figure: Annealing at a glance

Annealing is a heat treatment process that alters the physical and sometimes chemical properties of
a material to increase its ductility and reduce its hardness, making it more workable. It involves
heating a material above its re-crystallization temperature, maintaining a suitable temperature for
an appropriate amount of time and then cooling.
In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading
to a change in ductility and hardness. As the material cools it re-crystallizes. For many alloys,
including carbon steel, the crystal grain size and phase composition, which ultimately determine the
material properties, are dependent on the heating rate and cooling rate. Hot working or cold
working after the annealing process alters the metal structure, so further heat treatments may be
used to achieve the properties required. With knowledge of the composition and phase diagram,
heat treatment can be used to adjust from harder and more brittle to softer and more ductile.
In the case of ferrous metals, such as steel, annealing is performed by heating the material (generally
until glowing) for a while and then slowly letting it cool to room temperature in still air. Copper,
silver and brass can be either cooled slowly in air, or quickly by quenching in water.[1] In this
fashion, the metal is softened and prepared for further work such as shaping, stamping, or forming.
Alloys are annealed at temperatures of between 300-410°C, depending on the alloy, with heating
times ranging from 0.5 to 3 hours, depending on the size of the work piece and the type of alloy.
Alloys need to be cooled at a maximum rate of 20°C per hour until the temperature is reduced to
290°C, after which the cooling rate is not important.
The annealing process is not same for all the metals; the difference lies in the cooling mechanism.

3. For example, steel is heated to red hot 1000 ᵒC (1900 ᵒF) and cooled slowly for achieving desired
properties of the metal. The combination of heating and cooling is used to obtain desired
mechanical properties of metal.
Annealing works in three stages – the recovery stage, re-crystallization stage and the grain growth
stage. These work as follows:
1. Recovery Stage
This stage is where the furnace or other heating device is used to raise the temperature of the
material to such a point that the internal stresses are relieved.

Figure: Stored energy of cold work and fraction of the total work of 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-crystallization Stage
Heating the material above its re-crystallization temperature but below its melting point causes new
grains to form without any residual 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. Grain Growth Stage
Cooling the material at a specific rate causes new grains to develop. After which the material will be
more workable. Subsequent operations to alter mechanical properties can be carried out following
annealing.

Figure: Effect of temperature on re-crystallized grain size

Types of annealing:
Full Annealing In full annealing the carbon steel is slowly heated to a temperature of 50 C (122 F)
above the austenitic temperature (Lies between 750-900 °C / 1320-1652 °F) also
known as “holding temperature,” and then cooled down slowly to the room
temperature. The cooling rate recommended is 20 °C (68 °F) per hour.
The long annealing time produces ideal softening. Full annealing is done inside the
furnace. After reaching the holding temperature the furnace is turned off, and

6. metal anneals inside the furnace.
Once the metal is reached at 50 °C (122 °F) it can further be cooled to room
temperature using air with natural draft. The basic heat treatment processes for
carbon steel involve the decomposition or conversion of austenite.

The appearance of these conversion products determines the mechanical and
physical properties of any metal.

Soft Annealing

The soft annealing heat treatment can be performed on steel and alloys of copper
and brass. Steel with high carbon content is typically treated with soft annealing
process which gives it softer and easier to work structure.

The process takes about 12 to 48 hours and can be performed in continuous or
batch-wise in the oven. The load is heated to the temperature of 800 °C (1472 °F).
The metal is held at this temperature for 2-4 hours so that the structure fully
converts into austenite.

The temperature of the metal is quickly brought down to 790 C (1454 °F). Further
cooling of this steel is performed at a controlled and steady rate of 10 °C (50 °F) per
hour until the temperature reaches 690 °C (1274 °F). The metal is then cooled to
ambient temperature. The structural changes in the steel make it soft.

The cooling condition defines the degree of softness attained. The advantage of
this process is that a soft and ductile carbon steel is obtained that has a good ability
to form.
Stress Relief
Annealing
The worked pieces of metals tend to have stresses due to work hardening or
thermal cycling. The large castings such as cold formed, welded parts, etc. are
heated up to the temperature of 600 to 650 C (1112 to 1202 F) and kept in this
condition for hour or more.

The metal is allowed to cool in the furnace till the temperature drops to 426 °C
(800 °F) then cooled to ambient temperature slowly in the still air.

Process
Annealing

Process annealing is similar to stress relief annealing. The process is used in wire
and sheet industries to soften the steel by re-crystallization for further working
without fracture. It is also used in treating the hardened parts of low carbon steel.

The process involves the heating of steel to the temperature of 700 °C (1292 °F).
The time is given for re-crystallization and re-structuring of the ferrite phase. The
steel is then cooled slowly.


Effects and advantages of annealing:
Annealing is used to reverse the effects of work hardening, which can occur during processes such as
bending, cold forming or drawing. If the material becomes too hard it can make working impossible
or result in cracking.

7. By heating the material above the re-crystallization temperature, it is made more ductile and
therefore ready to be worked once more. Annealing also removes stresses that can occur when
welds solidify. Hot rolled steel is also shaped and formed by heating it above the re-crystallization
temperature. While steel and alloy steel annealing is common, other metals can also benefit from
the process, such as aluminum, brass, and copper.
Metal fabricators use annealing to help create complex parts, keeping the material workable by
returning them close to their pre-worked state. The process is important in maintaining ductility and
reducing hardness after cold working. In addition, some metals are annealed to increase their
electrical conductivity.
The main advantages of annealing are in how the process improves the workability of a material,
increasing toughness, reducing hardness and increasing the ductility and machineability of a metal.
The heating and cooling process also reduces the brittleness of metals while enhancing their
magnetic properties and electrical 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) After cooling 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

Normalizing of steels is carried out by heating approximately 100 degree above the upper critical
temperature A3 or ACM line followed by cooling in still air to room temperature.

10. Figure: Annealing and Normalizing range for plain carbon steels
The purpose of normalizing is to produce a harder and stronger steel than full annealing so that for
some applications normalizing may be a final heat treatment. Therefore for hyper-eutectoid steels it
is necessary to heat above the ACM line in order to dissolve the cementite network.
Normalizing may also be used to improve machineability, modify and refine cast dendritic structure
and refine the grain and homogenize the microstructure in order to improve the response in
hardening operations. The increase in cooling rate due to air cooling as compared with furnace
cooling affects the transformation of austenite and the resultant microstructure in several ways.
Since we are no longer cooling under equilibrium conditions the iron-iron carbide diagram cannot be
used to predict the properties of pro-eutectoid ferrite and pearlite or pro-eutectoid cementite and
pearlite that will exist at room temperature.
There is less time for the formation of the pro-eutectoid constituent consequently there will be less
pro-eutectoid ferrite in normalized hypo-eutectoid steels and less pro-eutectoid cementite in
hypereutectoid steels as compared with annealed ones.

11. Figure: Normalized .5 percent Carbon steel heated to 1800 degree F and air cooled, 100X,
Pro-eutectoid ferrite surrounding pearlite areas
Figure above shows the microstructure of normalized 0.50 percent carbon steel. In the annealed
condition this steel would have approximately 62 percent pearlite and 38 percent pro-eutectoid
ferrite. Due to air cooling, this sample has only about 10 percent pro-eutectoid ferrite, which is the
white network surrounding the dark pearlite areas.

For hypereutectoid steels, normalizing will reduce the continuity of the pro-eutectoid cementite
network, and in some cases it may be suppressed entirely. Since it was the presence of the
cementite network which reduced the strength of annealed hypereutectoid steels, normalized steels
should show an increase in strength. This is illustrated by the strength values given in Table below

12. Figure: mechanical properties of annealed vs normalized steel

Aside from influencing the amount of pro-eutectoid constituent that will form, the faster cooling
rate in normalizing will also affect the temperature of austenite transformation. and the fineness of
the pearlite. In general, the faster the cooling rate, the lower the temperature of austenite trans-
formation and the finer the pearlite. The difference in spacing of the cementite plates in the pearlite
between annealing and normalizing is shown schematically in Figure below:

Figure: Schematic picture of the difference in pearlitic structure due to annealing and normalizing

13. Ferrite is very soft, while cementite is very hard. With the cementite plates closer together in the
case of normalized medium pearlite; they tend to stiffen the ferrite so it will not yield as easily, thus
increasing hardness. If the annealed coarse pearlite has a hardness of about Rockwell C10, then the
normalized medium pearlite will be about Rockwell C20. Non-equilibrium cooling also shifts the
eutectoid point toward lower carbon content in hypo-eutectoid steels and toward higher carbon
content in hypereutectoid steels. The net effect is that normalizing produces a finer and more
abundant pearlite structure than is obtained by annealing, which results in a harder and stronger
steel. While annealing, spheroidizing, and normalizing may be employed to improve machineability,
the process to be used will depend upon carbon content. Based on many studies, the optimum
microstructures for machining steels of different carbon contents are usually as follows:

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
Normalizing differs from annealing in that the metal is heated to a higher temperature and then
removed from the furnace for air cooling rather than furnace cooling. For many manufacturing
engineers there is often a great deal of confusion as to when to specify normalizing and when to call
out annealing. There is a logical reason for this because, in many instances, the procedure for
normalizing and that of annealing are one and the same. For example, very-low-carbon steel can be
almost fully annealed by heating above the transformation range and cooling in air.
Normalizing is a process that improves part quality and plays an important role in controlling
dimensional variation in hardening and case hardening. It should be done whenever dimensional
stability is important or when manufacturing operations are expected to impart significant amounts
of stress into the material. It helps avoid many heat-treating problems.















Thank You

20. Source:
[1] Book: Introduction to Physical Metallurgy by Sidney H Avner
[2] Wikipedia Article: 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/