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Heat treatment

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analysis of annealing hardenig and tempering

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Heat treatment

  1. 1. Heat Treatment Analysing of Annealing Hardening Tempering By M.Sabari Venkat Narayanan
  2. 2. Definition • Annealing is a heat process whereby a metal is heated to a specific temperature /colour and then allowed to cool slowly. This softens the metal which means it can be cut and shaped more easily. Mild steel, is heated to a red heat and allowed to cool slowly.
  3. 3. Annealing  Makes a metal as soft as possible  Hypoeutectoid steels (less than 0.83% carbon) are heated above upper critical temp., soaked and cooled slowly.  Hypereutecoid (above 0.83%) are heated above lower critical temp., soaked and allowed to cool slowly
  4. 4. Process of Annealing • Process Annealing. Low carbon steels may harden through cold working. They can be heated to around 100 degrees below lower critical temp., soaked and allowed to cool in air. • Spheroidising. High carbon steels may be annealed just below the lower critical temp. to improve machinability
  5. 5. Full Annealing • A full anneal typically results in the second most ductile state a metal can assume for metal alloy. It creates a new uniform microstructure with good dynamic properties. To perform a full anneal on steel for example, steel is heated to 50°C above the austenic temperature and held for sufficient time to allow the material to fully form austenite or austenite-cementite grain structure. The material is then allowed to cool slowly so that the equilibrium microstructure is obtained. In some cases this means the material is allowed to air cool. In other cases the material is allowed to furnace cool. The details of the process depend on the type of metal and the precise alloy involved. In any case the result is a more ductile material but a lower yield strengthand a lower tensile strength.
  6. 6. Normalizing • Normalizing is a technique used to provide uniformity in grain size and composition throughout an alloy. The term is often used for ferrous alloys that have been austenitized and then cooled in open air.[18] Normalizing not only produces pearlite, but also sometimes martensite, which gives harder and stronger steel, but with less ductility for the same composition than full annealing.
  7. 7. Stress Relief • Stress relieving is a technique to remove or reduce the internal stresses created in a metal. These stresses may be caused in a number of ways, ranging from cold working to non-uniform cooling. Stress relieving is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly
  8. 8. Hardening  Medium and High carbon steels (0.4 – 1.2%) can be heated until red hot and then quenched in water producing a very hard and brittle metal. At 723 degrees, the BCC ferrite changes into Austenite with a FCC structure.
  9. 9. Hardening 0.6% carbon steel  The metal is heated to over 780 degrees, which allows the carbon to dissolve into the FCC Austenite.  Quenching the metal quickly in water prevents the structure from changing back into BCC.  A different structure, Body Centre Tectragonal (BCT) is formed. It is called Martensite and is extremely hard and brittle with a needle-like microstructure.
  10. 10. Quenching • Quenching is a process of cooling a metal at a rapid rate. This is most often done to produce a martensite transformation. In ferrous alloys, this will often produce a harder metal, while non-ferrous alloys will usually become softer than normal.
  11. 11. Tempering  To remove some of the brittleness from hardened steels, tempering is used. The metal is heated to the range of 220-300 degrees and cooled.  Tempering colours are an indicator of temperature on polished metals. Colours range from yellow to brown to violet and blue.
  12. 12. Heat Treatments  A – Normalising  B – Annealing or Hardening  C – Spheroidising or Process Annealing  D - Tempering

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