The document discusses the process of thermomechanical treatment (TMT) of steel. It defines TMT as a surface quenching process used to produce steel bars with high strength. The key aspects of TMT are surface quenching to form martensite, self-tempering to refine the microstructure, and final cooling. The mechanical properties of TMT bars depend on factors like the martensite volume fraction, cooling rate, and microstructure of the core.

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

3. Higher yield strength Improved toughness Improved weldability Higher resistance to brittle cleavage Higher resistance to low-energy ductile fractures Good cold forming, particularly by bending Lower costs which are possible by using hot-rolled rather than heat treated sections. Achieving desired properties with fewer amounts of alloying elements thereby reducing the production cost. Presented by Ansar Hussain Rizvi

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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 (  ) iron, delta (  ) iron, and gamma (  ) iron. The a-iron form exists below 1625 o F (885 o C) while  -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. Presented by Ansar Hussain Rizvi

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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: Pure metals Intermetallic compound Solid solution Presented 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 3 C or Cementite. Presented 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. Presented 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. Presented 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:- Presented by Ansar Hussain Rizvi Constituents Temperature range Pearlite 705 o C to 535 o C Bainite 535 o C to 230 o C Martensite Below 230 o C

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. Presented 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. Presented by Ansar Hussain Rizvi

15. The hardness increases as the transformation temperature decreases. This is due to a finer distribution of carbide in Bainite formed at lower temperature. Presented by Ansar Hussain Rizvi

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. Presented 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% Presented 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. Presented 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 Presented by Ansar Hussain Rizvi

20. The Process Presented by Ansar Hussain Rizvi

21. It is a surface quenching process Makes use of the rolling heat Involves three stages:- Surface quenching stage Self tempering stage Final cooling stage Controlling parameters:- Finishing rolling temperature Quenching time Water flow rate Microstructure of TMT bars consists of:- Surface layer  tempered martensite Core  ferrite, Pearlite and/or Bainite Presented by Ansar Hussain Rizvi

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24. Technological characteristics depend upon:- Volume fraction of the martensite Tensile properties of martensite 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) Presented 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. Presented by Ansar Hussain Rizvi

26. The core of the bar The mechanical properties depend on two groups of parameters: The chemical composition of the steel giving specific TTT diagrams-microstructure-mechanical properties relationship The process conditions along the three stages of the process Presented 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:- The pressure The flow rate The filing factor Presented by Ansar Hussain Rizvi

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 Presented 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. Presented by Ansar Hussain Rizvi

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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 Presented by Ansar Hussain Rizvi