The Common heat treatments for steel wires are -

1. ANNEALING

2. HARDENING

3. TEMPERING

4. NORMALISING

5. PATENTING

[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

deformed ?

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 workhardening ?

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 treatmentsHeat to Soaking cooling Quench

ANNEALING Above R C Temp. Yes Slow ×

1

NORMALISING Above R C Temp. Yes Normal ×

HARDENING Above R C Temp. Yes Very fast √

TEMPERINGBelow R.C. Temp.

Yes Normal ×

400-500 ˙C

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).

TERMINOLOGY: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 drawing

(B) Additionally, the pearlitic structure enhances the tensile properties. This is imparted by the directional alignment of the plates .

Parameters of pearlite transformation –1. Temperature

2. Time

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 “Patenting”

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 PATENTINGTypical difficulties faced are –

(1) Oxidation in austenitizing furnace. It can be controlled and prevented by the adjustment of air and fuel ratio.

(2) Lead bath length is smaller. It can be addressed either by changing to a longer lead tank or reducing the line Speed.

(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 drawing

(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 of quenching.