Thermo Mechanical Treatment

Purpose of TMTDefinition of steelTerminology of Physical MetallurgyPhase diagram

1Presented by Ansar Hussain Rizvi

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).


1. Higher yield strength2. Improved toughness3. Improved weldability4. Higher resistance to brittle cleavage5. Higher resistance to low-energy ductile

fractures6. Good cold forming, particularly by bending7. 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.




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 () iron, delta () iron, and gamma () iron. The a-iron form exists below 1625oF (885oC) while -iron is stable above 2540oF ( 1395oC). 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.



A phase is a distinct and physically, chemically or crystographically homogeneous portion of an alloy. There are three types of phases:

1. Pure metals2. Intermetallic compound3. Solid solution


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 Fe3C or Cementite.


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.


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.


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:-ConstituenConstituen

tstsTemperature rangeTemperature range

PearlitePearlite 705705oo C to 535 C to 535oo C C

BainiteBainite 535535oo C to 230 C to 230oo C C

MartensiteMartensite Below 230Below 230oo C C 12Presented by Ansar Hussain Rizvi

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.


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.


The hardness increases as the transformation temperature decreases. This is due to a finer distribution of carbide in Bainite formed at lower temperature.


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.


http://en.wikipedia.org/wiki/Phase_%28matter%29

Interstitial compound of Iron and Carbon Formula Fe3C A very hard compound Tensile strength = 5000 psi approx Elongation in 2 inches = 0%


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.


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


The Process


It is a surface quenching process Makes use of the rolling heat Involves three stages:-

1. Surface quenching stage2. Self tempering stage3. Final cooling stage

Controlling parameters:-1. Finishing rolling temperature2. Quenching time3. Water flow rate

Microstructure of TMT bars consists of:-1. Surface layer tempered martensite2. Core ferrite, Pearlite and/or Bainite




Technological characteristics depend upon:-

1. Volume fraction of the martensite2. Tensile properties of martensite3. 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 oC = 512-456*C-16.9*Ni+15*Cr-9.5*Mo+217*C2-71.5*(C*Mn)-67.6(C*Cr)


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.


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


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 pressure2. The flow rate3. The filing factor


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


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.



Water temperature No significant effect The temperature of the cooling water after

cooling should be less than 60o C. Impurities

Water impurities have no influence on the formation of martensite


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Thermo Mechanical Treatment