Process annealing is performed to improve the cold-working properties of low-carbon steels (up to 0.25% carbon) or to soften high-carbon and alloy steels to facilitate shearing, turning or straightening processes. Process annealing involves heating the steel to a temperature below (typically 10–20°C below) the lower critical temperature (Ac1) and is often known as ‘subcritical’ annealing. After heating, the steel is cooled to room temperature in still air. The process annealing temperatures for plain carbon and low alloy steels is typically limited to about 700°C to prevent partial reaustenitisation. In some cases this is limited to about 680°C for steel compositions, such as high-nickel containing steels, where the nickel further reduces the Ac1 temperature[Ref. .31]. This process can be used to temper martensitic and bainitic microstructures to produce a softened microstructure containing spheroidal carbides in ferrite[Ref. 31]. Fine pearlite is also relatively easily softened by process annealing, while coarse pearlite is too stable to be softened by this process. Annealing of Steels When a metal is cold worked (deformed at room temperature), the microstructure becomes severely distorted because of an increased dislocation density resulting from the deformation. Cold working is also referred to as work hardening or strain hardening. As a metal is cold worked, the strength and hardness increase while ductility decreases. Grain growth It is the growth of some recrystallized grains, and it can only happen at the expense of other recrystallized grains. Because fine grain size leads to the best combination of strength and ductility, in almost all cases, grain growth is an undesirable process. Although excessive grain growth can occur by holding the material for too long at the annealing temperature, it is normally a result of heating at too high a temperature.

1. Microstructure and Process Annealing of Steels
Dr. R.Narayanasamy, B.E., M.Tech., M.Engg., Ph.D., (D.Sc.)
Retired Professor (HAG),
Department of Production Engineering,
National Institute of Technology,
Tiruchirappalli-620015, Tamil Nadu, India.
Email: narayan19355@gmail.com
Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.)
Assistant Professor,
Department of Mechanical Engineering,
Mookambigai College of Engineering,
Pudukkottai-622502, Tamil Nadu, India.
Email:mv8128351@gmail.com

2. Iron-carbon binary phase diagram

3. Annealing Temperature Ranges

4. PROCESS (SUBCRITICAL) ANNEALING
 Process annealing is performed to improve the cold-working properties of low-carbon steels
(up to 0.25% carbon) or to soften high-carbon and alloy steels to facilitate shearing, turning
or straightening processes [Ref. 30,31].
 Process annealing involves heating the steel to a temperature below (typically 10–20°C
below) the lower critical temperature (Ac1) and is often known as ‘subcritical’ annealing.
 After heating, the steel is cooled to room temperature in still air.
 The process annealing temperatures for plain carbon and low alloy steels is typically limited
to about 700°C to prevent partial reaustenitisation.
 In some cases this is limited to about 680°C for steel compositions, such as high-nickel
containing steels, where the nickel further reduces the Ac1 temperature[Ref. .31].
 This process can be used to temper martensitic and bainitic microstructures to produce a
softened microstructure containing spheroidal carbides in ferrite[Ref. 31].
 Fine pearlite is also relatively easily softened by process annealing, while coarse pearlite is
too stable to be softened by this process

5. Deep Drawable Steels
Importance of Chemical Composition
 The short annealing time during Close Annealed (CA) process imposes close control
over alloying elements.
 Stricter composition control in CA-AK (Aluminium Killed) steels helps steel
manufacturers to obtain the right combination of grain size, texture, and
microstructure for deep drawing property.
 Amongst alloying elements, Carbon is very crucial.
 Free carbon in solution directly affects {111} texture formation adversely.
 It has been observed that in spite of the over-aging treatment, some amount of
carbon can still remain in solid solution.

6.  Ono et al. [16] have shown that n-value progressively increases with reduction of
carbon in solution but rm value initially increases and reaches a plateau at C ~0.02
wt.% and below.
 Moreover, reduction in carbon below this level causes an increase in the aging
index, and yield and tensile strength and a reduction in the elongation value.
 These are attributed to the low driving force of carbide precipitation during over-
aging process of strips containing a very low solute carbon.
Contd.

7. Contd.
 Taking the above facts into consideration, the carbon content in CA steels is normally kept to
a level of around 0.02–0.03 wt.%, which is 0.01–0.02 wt.% less than that of Batch
Annealed(BA) AK steels.
 The effects of carbon content on the mechanical properties of CA-AK steels are illustrated in
Fig. 5.8.

8. Contd.
Figure 5.8. Variation of the mechanical properties of aluminum killed steel with carbon content,
continuously annealed at 700°C and 850°C. S. Ono, O. Nozoe, T. Shimomura, K. Matsudo, B.L.
Bramfitt, P.L. Manganon (Eds.), Metallurgy of Continuously Annealed Sheet Steel, AIME, Dallas,
1982, p. 99.
Source: Reprinted with permission of The Minerals, Metals & Materials Society.

9. In low carbon steel, smaller grain size is found to be detrimental
to rm value.
Presence of excess solutes makes grain boundary movement difficult
by solute drag effect.
In CA, due to the very short annealing time, this problem is more and
thus CA steels generally yield smaller grain sizes.
Apart from increasing the annealing temperature, the other effective
way to compensate for this effect is the removal of solid solution
elements from the matrix.
Both of these practices promote grain growth.

10. In this context, it is beneficial to keep manganese content to a level
just adequate to fix the amount of sulfur which is present as impurity
elements in steels.
Toda et al. [17] defined a parameter, called K, which is related to Mn
and S (both in wt.%) content as follows:
K =Mn−55/32S–55/16O ………Equ (1)
Contd.

11.  So, when K=0, manganese is at stoichiometric level to fix all the sulfur
and oxygen present in solution.
 When K is negative, some sulfur and oxygen will still be there in the
matrix, whereas when K is positive, excess manganese will be in solid
solution.
 Although the above equation was developed for rimming steels, it
was found to be equally applicable for aluminum killed steels as well,
provided the oxygen term is set at 0.
Contd.

12. Process-annealed microstructure – low-carbon steel[8] (Photomicrograph courtesy of
Aston Metallurgical Services Co., Inc.)
Ferrite
Pearlite

13. Microstructures of steel showing effect of annealing

14. Microstructures of steel showing effect of annealing

15. Microstructures of steel showing effect of annealing

17.  Generally, in plain carbon (C) steels, annealing produces a ferritic-
pearlitic microstructure (Fig. 1).
 Steels can be annealed to facilitate cold working or machining, to
improve mechanical or electrical properties, or to promote
dimensional stability.
 The choice of an annealing treatment which provides an adequate
combination of such properties at minimum expense often involves a
compromise.
Contd.

18.  Terms used to denote specific types of annealing applied to steels are
descriptive of the method used, the equipment used, or the
condition of the material after treatment.
 Fig 1 shows microstructures of steel showing the effect of annealing.
Contd.

19. Annealing of Steels
 When a metal is cold worked (deformed at room temperature), the
microstructure becomes severely distorted because of an increased
dislocation density resulting from the deformation.
 Cold working is also referred to as work hardening or strain
hardening.
 As a metal is cold worked, the strength and hardness increase while
ductility decreases.

20.  Eventually, it is necessary to anneal the piece to allow further forming
operations without the risk of breaking it.
 In addition, some steels are strengthened primarily by cold working.
 In this case, it is important that the steel not soften appreciably when
placed in service.
Contd.

21.  Cold-worked steels with highly distorted microstructures are in a
high-energy state and are thermodynamically unstable.
 Annealing is the heat treatment process which softens a metal that
has been hardened by cold working.
 Annealing consists of three distinct process stages namely (i)
recovery, (ii) recrystallization, and (iii) grain growth.
 Although a reduction in stored energy provides the driving force,
annealing normally does not spontaneously occur at room
temperature.

22.  This is because the reduction in stored energy occurs by diffusion and
the activation energy needed to start the diffusion process is
normally insufficient at room temperature.
 Hence, heating is necessary to provide the thermal activation energy
needed to transform the material to a lower-energy state.
 As the internal lattice strains are relieved during annealing, the
strength decreases while the ductility increases.
Contd.

23. Recovery
 During recovery, there is a rearrangement of internal defects,
known as dislocations, into lower-energy configurations.
 However, the grain shape and orientation remain the same.
 There is also a significant reduction in residual stresses, but the
strength and ductility are largely unaffected.

24.  Because there is a large decrease in residual stress during recovery,
recovery-type processes are normally conducted to reduce residual
stresses, often to prevent stress-corrosion cracking or minimize
distortion.
 During stress-relief operations, the temperature and time are
controlled so there is not a major reduction in strength or hardness.
Contd.

25. Recrystallization
 It is characterized by the nucleation and growth of strain-free grains
out of the matrix of the cold-worked metal.
 During recrystallization, the badly deformed cold-worked grains are
replaced by new, strain-free grains.
 New orientations, new grain sizes, and new grain morphologies are
formed during recrystallization.
 The driving force for recrystallization is the remaining stored energy
which is not expended during recovery.
 The strength reduces and the ductility increases to levels similar to
those of the metal before cold working.

26.  Recrystallization is considered complete when the mechanical
properties of the recrystallized metal approach those of the metal
before it was cold worked.
 Recrystallization and the resulting mechanical softening completely
cancel the effects of cold working on the mechanical properties of
the work piece.
 An annealing curve for an alloy, such as a typical steel, show minimal
changes in mechanical properties during recovery and large changes
in properties which occur during recrystallization.
Contd.

27.  Mechanical properties, such as hardness, yield strength, tensile
strength, percent elongation, and reduction in area, change
drastically over a very small temperature range.
 Although physical properties, such as electrical conductivity, undergo
large increases during recovery, they also continue to increase during
recrystallization.
Contd.

28. Grain growth
 It is the growth of some recrystallized grains, and it can only happen
at the expense of other recrystallized grains.
 Because fine grain size leads to the best combination of strength and
ductility, in almost all cases, grain growth is an undesirable process.
 Although excessive grain growth can occur by holding the material for
too long at the annealing temperature, it is normally a result of
heating at too high a temperature.

29.  Annealing is a generic term denoting a treatment which consists of
heating to and holding at a suitable temperature followed by cooling
at an appropriate rate, primarily for the softening of metallic
materials.
 It is a process involving heating and cooling, normally 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.

30. Microstructure of the annealed Ti-IF-steel before processing.

31. Figure 6. Schematic illustration showing microstructural changes before
and after cold rolling and annealing (Ex: Austenite Stainless Steel)

32. Figure 1. Schematic illustration of annealing process (Deep Drawing Dual-Phase Steel).

33. Thank You
References:
Authors of Technical articles and Scopus Journals are
Acknowledged.