Thermo Mechanical Treatment

This presentation is an introduction to the Thermo-mechanical treatment of steel bars.

1. Purpose of TMT
Definition of steel
Terminology of Physical Metallurgy
Phase diagram
1Presented by Ansar Hussain Rizvi

2. There are 14 different types of crystal unit cell
structures or lattices are found in nature.
However most metals and many other solids
have unit cell structures described as body center
cubic (bcc), face centered cubic (fcc) or Hexagonal
Close Packed (hcp).
2Presented by Ansar Hussain Rizvi

3. 1.Higher yield strength
2.Improved toughness
3.Improved weldability
4.Higher resistance to brittle cleavage
5.Higher resistance to low-energy ductile fractures
6.Good cold forming, particularly by bending
7.Lower costs which are possible by using hot-
rolled rather than heat treated sections.
8.Achieving desired properties with fewer amounts
of alloying elements thereby reducing the
production cost.
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6. When a metal undergoes a
transformation from one
crystalline pattern to
another, it is known as an
allotropic change. Iron exists
in three crystal (atomic)
allotropes, namely: alpha (a)
iron, delta (d) iron, and
gamma (g) iron. The a-iron
form exists below 1625
o
F
(885
o
C) while d-iron is stable
above 2540
o
F ( 1395
o
C).  
Gamma iron exists at the
temperatures between these
two ranges.   It is the
allotropy of iron that allows
for these crystal structures to
change with temperature.
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8. A phase is a distinct and physically, chemically
or crystographically homogeneous portion of an
alloy. There are three types of phases:
2.Pure metals
3.Intermetallic compound
4.Solid solution
8Presented by Ansar Hussain Rizvi

9. Chemical compounds between metals and
metalloids are known as Intermetallic
compounds. A large portion of the known
Intermetallic compounds contain one of the
following metalloids: carbon, phosphorus,
silicon, sulfur, arsenic or the metal aluminum.
The important compound present in alloys of
iron and carbon is the carbide Fe
3C or Cementite.
9Presented by Ansar Hussain Rizvi

10. A complete merging in the solid state of the two
phases, pure metals and Intermetallic
compounds, are known as the solid solutions.
There can be solid solution of two metals, of a
metal and an Intermetallic compound, or of two
compounds.
10Presented by Ansar Hussain Rizvi

11. In Iron-Carbon alloys austenite is the solid
solution formed when carbon dissolves in face-
centered cubic (gamma) iron in amounts up to
2%. Its microstructure is usually large grained.
11Presented by Ansar Hussain Rizvi

12. Austenite transforms to Pearlite when it is cooled
slowly below the Ar critical temperature. When
more rapidly cooled, however, this
transformation is retarded. The faster the cooling
rate, the lower the temperature at which the
transformation occurs resulting in a formation of
the micro-constituents given hereunder:-
ConstituentsConstituents Temperature rangeTemperature range
PearlitePearlite 705705
oo
C to 535 C to 535
oo
C C
BainiteBainite 535535
oo
C to 230 C to 230
oo
C C
MartensiteMartensite Below 230Below 230
oo
C C
12Presented by Ansar Hussain Rizvi

13. Pearlite is a lamellar aggregate of ferrite and
cementite. It is a result of the eutectoid reaction
which takes place when a plain carbon steel of
approximately 0.8% carbon is cooled slowly from
the temperature range at which austenite is
stable.
13Presented by Ansar Hussain Rizvi

14. Bainite is a decomposition of austenite consisting
of an aggregate of ferrite and carbide. Its
appearance is featherlike if formed in the upper
part of the temperature range and acicular if
formed in the lower part.
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15. The hardness increases as the transformation
temperature decreases. This is due to a finer
distribution of carbide in Bainite formed at lower
temperature.
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16. It is formed by the rapid cooling of Austenite
Has a body centered tetragonal cubic structure
Martensite is not shown in the equilibrium phase
diagram of the iron-carbon system because it is a
metastable phase, the kinetic product of rapid cooling of
steel containing sufficient carbon.
16Presented by Ansar Hussain Rizvi

17. Interstitial compound of Iron and Carbon
Formula Fe
3
C
A very hard compound
Tensile strength = 5000 psi approx
Elongation in 2 inches = 0%
17Presented by Ansar Hussain Rizvi

18. Carbon
Gives steel the properties that make it valuable
Makes steel responsive to heat treatment
Restricts the motion of dislocations thereby
increasing the resistance t deformation.
Reduces the ductility and toughness
Increases the hardness, tensile strength and yield
point
Reduces the percentage of elongation, reduction in
area and impact strength.
18Presented by Ansar Hussain Rizvi

19. Manganese
A deoxidizer
Its content ranges from 0.5 to 0.8% in plain carbon steels
Raises the strength of steel without practically reducing
its ductility.
Reduces the RED SHORTNESS, i.e. brittleness at high
temperature due to the effect sulphur.
Silicon
A common deoxidizer
In plain carbon steels – up to 1%
Increases tensile strength and yield point
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20. The Process
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21. It is a surface quenching process
Makes use of the rolling heat
Involves three stages:-
1.Surface quenching stage
2.Self tempering stage
3.Final cooling stage
Controlling parameters:-
1.Finishing rolling temperature
2.Quenching time
3.Water flow rate
Microstructure of TMT bars consists of:-
 Surface layer  tempered martensite
 Core ferrite, Pearlite and/or Bainite
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24. Technological characteristics depend upon:-
1.Volume fraction of the martensite
2.Tensile properties of martensite
3.Tensile properties of the ferritc-pearlitic (Bainite)
structure of the core
Volume fraction of the martensite
Depends upon the temperature at which the
martensitic transformation starts (Ms)
Ms is a function of the chemical composition and of
the temperature field in the cross-section of the bar at
the exit of the water box.
Ms
o
C = 512-456*C-16.9*Ni+15*Cr-9.5*Mo+217*C
2
-71.5*(C*Mn)-67.6(C*Cr)
24Presented by Ansar Hussain Rizvi

25. Tensile properties of martensite
The yield strength level of the martensitic layer
depends on the chemical composition and on the
tempering temperature.
Lower tempering temperature  higher yield
strength, lower ductility
Tempering temperature is the maximum surface
temperature achieved at the end of the second stage
and it is directly depending on the water quenching
procedure adopted in the first stage.
25Presented by Ansar Hussain Rizvi

26. The core of the bar
The mechanical properties depend on two groups
of parameters:
1.The chemical composition of the steel giving specific
TTT diagrams-microstructure-mechanical properties
relationship
2.The process conditions along the three stages of the
process
26Presented by Ansar Hussain Rizvi

27. This is one of the most important factors of the
TMT process
The heat removal from the surface of the bar
in the water box is characterized as the Heat
Exchange Factor
Parameters to reach a high heat exchange
factor:-
1.The pressure
2.The flow rate
3.The filing factor
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28. Pressure
Pressure is most important
Less pressure gives lower heat exchange and thus
less cooling efficiency
A certain pressure range is necessary to obtain given
mechanical properties
Flow rate
Flow rate is function of pressure and gap setting
between the nozzles of TMT
The relative velocity between the water and the bar
contributes significantly the heat exchange
28Presented by Ansar Hussain Rizvi

29. It is the relative water flow on the surface of the
roll stock
It is defined as the square of the ratio between
bar diameter and pipe diameter
Low filling factor results in a thick steam jacket
around the bar thus limiting the heat exchange
Lower D/d ratio (the ratio between the bar
diameter and the pipe diameter) results in thin
water film causing the water to boil. This
causes vibration in the bar consequently there
might be a mis-roll.
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31. Water temperature
No significant effect
The temperature of the cooling water after cooling
should be less than 60
o
C.
Impurities
Water impurities have no influence on the formation
of martensite
31Presented by Ansar Hussain Rizvi