1. Heat Treatment

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

6. SUMMARY: PROCESSING OPTIONS Adapted from Fig. 10.27, Callister 6e. 19

7. Spheroidite AS: Alloy Steel PCS: Plain-carbon Steel Austenite Rapid Quench Martensite Slow Cooling Re-heat Moderate cooling (AS) Isothermal treatment (PCS) Tempered Martensite Re-heat coarse fine Pearlite Bainite

8. Time in region indicates amount of microconstituent! Slow Cooling

9. Cooling Rate, R, is Change in Temp / Time °C/s Medium Cooling

10. Fast Cooling This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

11. Classification of heat-treatment processes 1. annealing 2. Normalizing 3. Hardening 4. Tempering

16. Hardenability 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. Hardenability may also be remembered as the ability of a steel to become unifoirmily hard or to harden in depth direction. 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

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.

18. Factors affecting hardenability Composition & menthod of manufacture Quenching media and method of quenching The mean composition of the austentite before quenching, including the nature and amount of alloying elements. All alloying elements, except Co, tend to increase the hardenability. 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

19. The size of austentite grains before quenching. The larger the grains prior, greater is the degree of hardenability. The homogeneity of the austentite before quenching. Method to determine the Hardenability

20. Jominy/ End Quench Test 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 Softest Hardest

21. Case Hardening & Surface Treatment 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. No carbon steel can posses both these requirements at the same time. Low carbon steel 0.1% C will be tough and High carbon steel 0.9%C will possess adequate hardness when suitably heat treated

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. Processes used to create hardened cases are 1. Carburizing 2. Nitriding Cyaniding Carbonitriding Flame hardening 6. Induction hardening

23. Surface treatments involve applying coatings to the surface of metals/alloys..

24. Assignment (Submission date- 9/4/2010) Write about (a) Surface Hardening (b) Case Hardening (c) Age Hardening (d) Precipitate Hardening 2. Elaborate Jominy Hardening Test.