The Common heat treatments for steel wires are -
[ISO thermal phase transformation]
Q. How are metals different from other materials ?
A. Briefly, metals are crystalline & therefore can be repeatedly
deformed & restored. Metals can be easily deformed
permanently (Plastic deformation)
Q. What happens when metals are plastically
A. When deformed, plastically, metals take new shapes and
new dimensions, but additionally they get hardened -
known as ‘work hardening .‘ Which means the UTS and
Hardness go up and the ductility goes down.
Q. What is done to address the issue of work
A. This is where the “Heat treatment” comes in to picture.
The crystalline structure is strained due to the
deformation and the same is restored by heating the
deformed metals to a solid solution phase and then
recrystallizing by VARIOUS steps of cooling. The re-
crystallized structure is same as that before the crystals
were strained in the deformation process. The re-
crystallisation process restores the mechanical and micro
structural properties to the same as it started before the
deformation. Thus after the heat –treatment the metal is
ready for further deformation.
Q. What are basic differences in the different heat treatments
Heat to Soaking cooling Quench
ANNEALING Above R C Temp. Yes Slow ×
NORMALISING Above R C Temp. Yes Normal ×
HARDENING Above R C Temp. Yes Very fast √
Below R.C. Temp.
Yes Normal ×
PATENTING Above R C Temp.
Yes Very fast
530 - 600 ˙C
STEP-1 900 - 1000 ˙C
STEP-2 Hold @ 530 - 600 ˙C Yes Normal ×
Q. What are the objectives of a heat - treatment ?
A. Primary objectives of any heat - treatment are -
TREATMENT OBJECTIVE APPLICATIONS
ANNEALING INCREASE DUCTILITY-REWORRING FORGING,FLATTENING,RIVETTES,REDRAWING
NORMALISING RESTORE STRUCTURE SEAMLESS PIPES
HARDENING INCREASE STRENGTH,WEAR-RESISTANCE TOOLS, SPRINGS.
TEMPERING RELIEVE EXT.STRESSES ARISING OUT OF ALL HARDENED STEEL, COLD DRAWN WIRES
COLD WORKING OR HARDENING TREATMENT
CASE-HARDENING INCREASE WEAR-RESISTANCE SELECTIVELY ON RAZOR BLADE, TOOLS, WIRE DRG.DRUMs
SURFACE OR EDGE
PATENTING INCREASE UTS AND DRAWABILITY HIGH CARBON STEEL WIRES
Q.What is steel (carbon steel)?
A. Plain carbon steel is a binary system of Iron and carbon .
However, carbon in steel never exists as free carbon and it
always exists as Iron carbide (Fe3C). So, essentially, plain
carbon steel is an alloy of Iron and Iron-carbide. The 2 phases
that can exist in carbon steel are ferrite (pure Iron) and
cementite( Iron carbide).
FERRITE Crystals of α- iron (pure iron)
CEMENTITE Crystals of iron carbide (Fe3C)
AUSTENITE Solid solution of carbon in iron > 723˙c.
PEARLITE Lamellar or plate-like colony of ferrite and
cementite alternatively placed.
How Pearlite is formed –
When we heat the steel to above the re- crystallization
temperature (say around 900 – 1000 ˙C) austenite
forms. As we know that in austenite the two
components of the system are (iron & carbon) are in
single phase i.e. Carbon is completely dissolved is
Iron. Now, if we cool the steel to room temp. in a
normal way the carbon will precipitate from the
solution as cementite(fe3c) and the remaining will be
the pure iron (ferrite) – so we will have cementite
crystals distributed in the ferrite Matrix
Now on the other hand if we modify this cooling
of austenite in 2 steps i.e.-
First rapidly cool (quench) the steel to a temperature
below the re-crystallization temp. (say 530 – 600˙c). By
doing this rapidly be do not allow austenite to transform
in to ferrite and cementite and we actually end up with
“retained austenite” which is unstable at this temperature
( 530-600˙c). This retained austenite due to being
unstable transform in to ferrite and cementite. But due to
un-natural circumstances the ferrite and cementite
precipitate as alternative plates. These alternative plates of
ferrite & cementite are called pearlite. Now depending
upon the plate thickness of the pearlite we can classify the
same as fine pearlite or coarse pearlite.
Why Pearlite Structure is important –
(A) Most significant aspect of the pearlitic structure is
that it facilitates wire – drawing to very small
diameters. As we draw to thinner and thinner dias.
The pearlitic plates keep collapsing in the drawing
direction and hence offer minimum resistance to
(B) Additionally, the pearlitic structure enhances the
tensile properties. This is imparted by the
directional alignment of the plates .
Parameters of pearlite transformation –
Accordingly, T-T-T-(Time – Temp.- Transformation )
curves are availalele to arrive at right temp.(Lead bath)
and time (head bath length-vs-Line speed)
Q. What exactly is Patenting ?
A. It is actually an “ISO thermal Phase Transformation”
process. This is a unique process and depends upon Time &
temperature. So for the Pearlitic transformation time is very
important and sufficient time has to be given at a constant
temp for a good pearlitic structure. Since the phase
transformation takes place at a constant temp. it is known as
ISO thermal phase transformation. It so happened that this
heat treatment process was patented by someone in 19th
century in the U. S. and hence it came to be called as
As we have already understood the patenting gives lamellas
microstructure which is conducive to easy –drawing and also
to improved tensility typically, the UTS values improve as
given below :
C % 3.00MM As Unpatented Kg/MM2 As Patented Kg/MM2
0.5 80 95
0.6 90 105
0.7 100 115
0.8 110 125
Note :- As unpatented is not as drawn but as Normalised.
DIFFICULTIES IN PATENTING
Typical difficulties faced are –
(1) Oxidation in austenitizing furnace. It can be
controlled and prevented by the adjustment of air and
(2) Lead bath length is smaller. It can be addressed either
by changing to a longer lead tank or reducing the line
(3) Variation of Lead bath temp.-thus making it
undesirably a “Non Iso thermal system”. This has to be
controlled by circulating the molten Lead within the
bath and installing cooling nest within the bath.
(4) Low UTS and high RA% in the patented wire. This
happens when the line speed is substantially higher
than that is permitted by the Furnace DXV. The higher
line speed makes the wire temp. quite low when it is
entering the lead bath making the quenching rate very
low. This very low quenching rate lead to a situation
which is between patenting and normalizing. This kind
of wire begins to break after 75%-80% reduction in wire
(5) Patented wire becomes very brittle with high UTS and
low R.A %. This happens when the line speed is very low
or lead temp is low-say 520∙c or 510. This is a case of very
high quenching rate. Here a very fine pearlite is farmed
which is brittle. This very fine pearlite is called : Bainite”
OTHER HEAT – TRATMENTS
(1) Annealing- There can be many kinds of annealing e.g. full
annealing , process-annealing, spherodized annealing . Annealing is
done to make the steel softer and more ductile. Unlike patenting it
does not increase the tensile values. The full annealing imparts the
best ductility and is best done in a batch process. Above 3hrs. Of
soaking above the RC temp and cooling within the furnace are
necessary parts of “Full annealing:. In process- annealing the soaking
time & cooling are not very stringent. Process annealing is usually
done in-line . This in-line/ strand annealing does not have any
possibility of long soaking hours (such as 3 hours) hence process-
annealing is suitable here.
There is one more commonly used method of annealing. This is”
spherodized or Globular Annealing". As the Name suggest this kind
of annealing imparts a spherical (Globular) structure. The Fe3C is
present in the Ferrite matrix as spheres. The method to achieve this is
that after soaking the steel is cooled in 2 or 3 steps. This kind of
annealing is very important in the steels which are subsequently cold
forged or cold - flattened
(2)Stress – Relieving – Also known as “Tampering” this treatment
dose not make any changes in the microstructure but only relieves
the stresses developed on the body of steel due to cold –working.
The degree of stresses present in the body of steel before and after
the “stress relieving” operations is not measureable. Hence,
success of the stress relieving operation is measured by the
improvement in % Elongation and the % yield Point.
(3) Hardening – this is commonly employed in tool steels and is
always followed by a stress – relieving or Tempering operation. In
this treatment steel is heated above the re – crystallization temp.
and is directly quenched to room temp.. This gives rise to
precipitation of martensite. For Hardening treatment steel of high
carbon and /or high chromium are suitable. Martensite is a
structure where carbon atoms get entrapped within the Ferrite
crystals as they do not get not time to escape out due to very rate