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UNIT- IV --HEAT TREATMENT.pptx
- 2. UNIT IV HEAT TREATMENT Definition – Full annealing, glass annealing, stress relief, recrystallization and spheroidizing – normalising, quenching, hardening and tempering of steel. Isothermal transformation diagrams – cooling curves superimposed on Time Temperature Transformation (TTT) diagram, Critical Cooling Rate (CCR) – Hardenability, Jominy end quench test – austempering, martempering – case hardening, carburizing, nitriding, cyaniding, carbonitriding – flame and induction hardening. SYLLABUS
- 3. HEAT TREATMENT PROCESS Its a operation or combination of operations involving heating and cooling of metal / alloy in solid steels to obtain desired conditions ( Relieve stress ) and properties (like Machinability, ductility etc. ).
- 4. Heat Treatment Processes Softening Processes - Annealing Normalizing Hardening Processes - Hardening Tempering Surface Hardening Process -
- 5. HEAT TREATMENT OF STEEL The object of heat treatment is to make the steel suitable for some specific application. The shape of the grains can be altered by heating the steel to a temperature above that of recrystallization and the duration of heating and the rate of cooling. Heat treatment process classified into following: 1. Treatment that produces equilibrium condition 2. Treatment that produces non equilibrium condition
- 6. Treatments that Produce Equilibrium Condition • The mechanical properties of steel depend upon the carbon content, the heat treatment temperature, the holding time and the cooling rate. • Steel is heated to a temperature up to 500°C, small residual stresses will be relieved to a small extent to cause only a slight reduction in hardness and strength.
- 7. ANNEALING
- 8. TYPES OF ANNEALING PROCESS • Full Annealing • Glass Annealing • Stress Relief Annealing • Process Annealing • Spherodising Annealing • Isothermal Annealing
- 9. FULL ANNEALING • Heating the steel to a temperature at or near the critical point, holding there for a time period and then allowing it to cool slowly in the furnace itself.
- 10. FULL ANNEALING ,
- 11. Glass Annealing Here the external surface of the glass is rapidly cooled. Because the inner material cools at a slower rate compared to the outer material, the thermal differential creates compressive stress on the glass surface and tensile stress at the inner layer of the glass.
- 12. Glass Annealing
- 13. Stress Relief Annealing • This process involves heating the casting or structure to about 650°C. The temperature is maintained constantly for a few hours and allowed to cool down slowly.
- 14. Process Annealing (for removing cold working effects) • Process of annealing consists of heating steel uniformly to a temperature of 650°C–723°C and holding at that temperature for sufficient time, followed by slow cooling.
- 15. Spheroidal Annealing • Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels. To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied.
- 16. Normalizing (Cooling in still Air) • To achieve high hardness and strength. • Metal is heated above Upper critical temperature. • At that temperature, the structure is converted into Austenite and removed from the furnace then it is cooled under natural convection at controlled room temperature. • This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite.
- 17. Hardening of Steels • Its a process of heating the steel above or below the critical temperature for a particular period and then allow to cool by oil or water rapidly
- 18. TEMPERING • For minimizing the harness and removing the internal stresses we heat the metal near to upper critical temp once again and let it for some time then cool by air
- 19. WHY – TTT & CCT DIAGRAMS ? • The phases martensite and bainite are non-equilibrium phase that do not appear in fe-fe3 c (iron-iron carbon) phase diagram • also strengthening treatment like hardening and tempering are non-equilibrium process. • in order to show the influence of varying cooling rates, that is time, on the transformation of austenite other types of diagrams are necessary. • The time temperature transformation or TTT diagram and the continuous cooling transformation or CCT diagram are used to explain the things in the cooling operation
- 20. . • Non-equilibrium cooling will result in different microstructures hence altered properties. • TTT diagrams are the tools that we can use to take into account the kinetics of the transformation. • They show the relationship between time, temperature and (percent) transformation. • There are two types of TTT diagrams: isothermal transformation (IT) TTT diagrams continuous cooling transformation (CCT) TTT diagrams
- 21. TIME TEMPERATURE PATH ON ISOTHERMAL TRANSFORMATION DIAGRAM
- 22. EFFECT OF COOLING RATE ON TTT DIAGRAM Austenite transforming into pearlite, bainite and martensite with different cooling rates is an example of this.