Heat Treatment Techniques-Annealing of Cold Worked Steel

Well, by now you have got a good understanding of the Annealing process. Now we will learn about
Annealing of the cold-rolled steel.
Annealing of the cold-rolled steel normally is designed to produce a recrystallized ferrite microstructure
from the highly stressed grains resulting from cold work.
Cold Working: anne During cold-working, strain-hardening takes place because it is done below
recrystallization temperature which results in the increase of internal stress and decrease in electrical
conductivity.Annealing cycle comprises of Heating, Soaking and Cooling cycles. The annealing process
comprises of three stages, namely:1. Recovery2. Recrystallization and3. Grain Growth4. Recovery
5. During heating of the steel, and in the first segment of the holding portion of the cycle, the first
metallurgical process to occur is recovery. During this process, internal strains are relieved, ductility is
moderately increased, and strength is slightly decreased
6. Recrystallization
7.As annealing continues, the process nne of recrystallization occurs, and new, more equiaxed ferrite
grains are formed from the elongated grains. During recrystallization, strength decreases rapidly, with a
corresponding increase in ductility.

8. Grain Growth

9. Further at times the temperature causes some of the newly formed grains to grow at the expense of
other grains; this is termed grain growth and results in modest decreases in strength and small but often
significant increases in ductility. Most plain carbon steels are given an annealing treatment that
promotes full recrystallization, but care must be taken to avoid excessive grain growth, which can lead to
surface defects such as orange peel in formed parts.
Heat Treatment Techniques-Annealing Cycle-Continuous Annealing
In the previous screen we have learnt about batch annealing cycle. Now we are going to learn about
continuous annealing cycle.
During continuous annealing, uncoiled steel sheet is passed through a two-stage furnace for a few
minutes duration. In the first stage, steel is heated to accomplish its recrystallization, while in the second
stages it is heated at a lower temperature to overage the steel and remove carbon from the solution
effectively. This carbon would eventually cause strain or quench aging and reduces sheet formability.
Continuously annealed sheet steels are very susceptible to aging effects, because the thin sheet cools
rapidly from the annealing temperature in contrast to batch annealing. As a result, various types of
overaging treatments are applied to continuously annealed steels, as shown in the Figure here. These
treatments are designed to remove carbon and nitrogen from solid solution by the precipitation of
relatively coarse carbide and nitride particles.

Heat Treatment Techniques Normalizing

In the previous screens we have learnt about Annealing process and its various types. Now we will learn
about the second important heat treatment technique: Normalizing.
Normalizing is used to refine grains to produce a uniformly distributed fine grain pearlite steel, which are
tougher than coarse grained pearlite steel as produced in annealing process.
Normalizing is the process of raising the temperature to over 60 degree Celsius or 108 degree
Fahrenheit, above the A3 or Acm line fully into the Austenite range.
It is held at this temperature to fully convert the structure into Austenite, and then it is removed from
the furnace and cooled at room temperature under natural convection.
This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite.
This process is considerably cheaper than full annealing since there is no added cost of controlled
furnace cooling. A Partial iron-iron carbide phase diagram showing typical normalizing range for plain
carbon steels is displayed on the screen.
As you can see Normalizing is accomplished by heating the steel at least 60 degree Celsius above the
upper critical temperature A3 for compositions less than the eutectoid and above Acm temperature for
compositions greater than the eutectoid as represented.
Heat Treatment Techniques Normalizing

Having understood what hardening means let us take a look at tempering and understand why
hardening has to be followed tempering.Tempering is a process done subsequent to quench
hardening.Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of
Martensite. This brittleness is removed by tempering.When the steel cools to about 40 °C after
quenching, the part o be tempered is reheated to a temperature of 150 to 400 °C. In this region a softer
and tougher structure Troostite is formed. Tempering results in a desired combination of hardness,
ductility, toughness, strength, and structural stability.Note: Alternatively, the steel can be heated to a
temperature of 400 to 700 °C (752 to 1292 °F) that results in a softer structure known as Sorbite. This has
less strength than Troostite but more ductility and toughness.Quench Hardening As you can see in this
diagram, in steel quenched to a microstructure consisting essentially of martensite, the iron lattice is
strained by the carbon atoms, producing the high hardness of quenched steels.

Heating

Upon heating, the carbon atoms diffuse and react in a series of distinct steps that eventually form Fe3C
or an alloy carbide in a ferrite matrix of gradually decreasing stress level.

Soaking While soaking, the properties of the tempered steel are primarily determined by the size, shape,
composition, and distribution of the carbides that form, with a relatively minor contribution from solid-
solution hardening of the ferrite. These changes in microstructure usually decrease hardness, tensile
strength, and yield strength but increase ductility and toughness.