Heat treatment is the controlled heating and cooling of metals for the purpose of altering their properties.
REVIEW AND SUMMARY
✓ Heat treatment is the controlled heating and cooling of metals for the purpose of altering their properties. In this unit, the theory and the various processes of heat treatment have been presented.
✓ The theory of heat treatment is based on the fact that a change takes place in the internal structure of metal by heating and cooling which induces desired properties in it.
✓ The important heat treatment processes are as follows:
1. Annealing
(a) Full annealing,
(b) Process annealing,
(c) Stress relief annealing,
(d) Recrystallisation annealing, and
(e) Spheroidise annealing.
2. Normalising
3. Hardening
4. Tempering
5. Austempering
6. Martempering
7. Case hardening
(a) Carburising,
(b) Nitriding,
(c) Cyaniding,
(d) Carbonitriding,
(e) Flame hardening, and
(f) Induction hardening.
✓ Annealing is a heat treatment for the general purpose of softening or stress-relieving a material.
✓ Normaling is a heat treatment obtained by austenitising and air cooling to produce a fine pearlite structure. The objective of this heat treatment is to enhance toughness by refining the grain size.
✓ Quenching refers rapid cooling. Some of the quenching medium that are widely used are caustic soda, brine solution, water, oil, and air.
✓ Hardening refers to the heat treatment which increases the 2l hardness by quenching.
✓ Tempering is a heat treatment in which martensite is reheated. nesIn this, the ductility and toughness of martensite can be enhanced by reducing the hardness of martensite.
✓ The time-temperature transformation (TTT) diagram, also bris known as isothermal transformation (IT) diagram or C-curve, is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformation begins and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.
✓ The continuous cooling transformation (CCT) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to indicate when transformations occur as the initially austenitised material is continuously cooled at a specified rate. In addition, using the CCT diagram the final microstructure and mechanical grisqus characteristics may be predicted.
✓ Hardenability refers to the ease with which hardness may be attained. That is it is a measure of ease of forming martensite. The Jominy end-quench test method is widely used to determine the hardenability.
✓ Martempering is a heat treatment of a steel involving a slow to cool through the martensitic transformation range to reduce stresses associated with that crystallographic change.
✓ Austempering is a heat treatment of a steel involving holding to just above the martensitic transformation range large enough to completely form bainite.
✓ The surface heat treatment is used to achieve (i) the hard outer surface (or case), and (ii) the soft inside (or core), of the components.
No comments:
Post a Comment