Heat treatment

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Heat treatment

  1. 1. Heat Treatment<br />
  2. 2. Heat treatment may be defined as an operation or combination of operations involving heating and cooling of a metal/alloy to obtain desirable conditions, e.g., that of relieved stresses, properties, e.g., better machinability, improved ductility, homogeneous structures, etc<br />
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  6. 6. SUMMARY: PROCESSING OPTIONS<br />Adapted from Fig. 10.27, Callister 6e.<br />19<br />
  7. 7. Spheroidite<br />AS: Alloy Steel<br />PCS: Plain-carbon Steel<br />Austenite<br />Rapid<br />Quench<br />Martensite<br />Slow <br />Cooling<br />Re-heat<br />Moderate cooling (AS)<br />Isothermal treatment (PCS)<br />Tempered <br />Martensite<br />Re-heat<br />coarse<br />fine<br />Pearlite<br />Bainite<br />
  8. 8. Time in region indicates amount of microconstituent!<br />Slow Cooling<br />
  9. 9. Cooling Rate, R, is Change in Temp / Time °C/s<br />Medium Cooling<br />
  10. 10. Fast Cooling<br />This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!<br />
  11. 11. Classification of heat-treatment processes <br />1. annealing<br />2. Normalizing<br />3. Hardening<br />4. Tempering<br />
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  16. 16. Hardenability<br />Whereas hardness is a measure of resistance to plastic deformation (by indentation), hardenability is the ease with which hardness may be attained in the depth direction of an object.<br />Hardenability may also be remembered as the ability of a steel to become unifoirmily hard or to harden in depth direction.<br />It should be remembered that hardenability is not an indication of the hardeness of a steel, rather hardenability is an index of the depth to which martensite can be formed in a given steel aas the result of quenching<br />
  17. 17. Hardenability value for a given steel is the diameter in inches of a cylindrical bar that will form 50% martensite t the centre during an ideal quench.<br />
  18. 18. Factors affecting hardenability<br />Composition & menthod of manufacture<br />Quenching media and method of quenching<br />The mean composition of the austentite before quenching, including the nature and amount of alloying elements.<br /> All alloying elements, except Co, tend to increase the hardenability. <br /> As compared to carbon steels, alloy steels harden to a considerably larger depth due to the high stability of the supercooledaustentite and the corresponding lower critical cooling rate<br />
  19. 19. The size of austentite grains before quenching. The larger the grains prior, greater is the degree of hardenability.<br />The homogeneity of the austentite before quenching.<br /> Method to determine the Hardenability<br />
  20. 20. Jominy/ End Quench Test<br />A 25-mm dia by 100-mm long bar is properly austenitized and quenched on the end in a standardized way as shown in figure<br />Softest<br />Hardest<br />
  21. 21. Case Hardening & Surface Treatment<br />Many industrial application such as cams, gears, etc., require a hard wear resistant surface called the case and a relatively soft, tough and shock resistant inside, called the core.<br /> No carbon steel can posses both these requirements at the same time.<br />Low carbon steel 0.1% C will be tough and High carbon steel 0.9%C will possess adequate hardness when suitably heat treated<br />
  22. 22. However, both these requirements may be met by employing a low carbon steel with suitable core properties and then adding (or penetrating) Carbon, N or both to the surface of the steel part in order to provide a hardened case (or layer) of definite depth. These treatments are known as Case hardening.<br />Processes used to create hardened cases are<br />1. Carburizing<br />2. Nitriding<br />Cyaniding<br />Carbonitriding<br />Flame hardening<br />6. Induction hardening<br />
  23. 23. Surface treatments involve applying coatings to the surface of metals/alloys..<br />
  24. 24. Assignment (Submission date- 9/4/2010)<br />Write about <br /> (a) Surface Hardening<br /> (b) Case Hardening<br /> (c) Age Hardening<br /> (d) Precipitate Hardening<br />2. Elaborate Jominy Hardening Test.<br />

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