Heat Treatment involves various heating and cooling procedures performed to effect structural changes in a material, which turn affect its mechanical properties

2.  Heat Treatment involves various heating and
cooling procedures performed to effect
structural changes in a material, which turn
affect its mechanical properties.
Heat treatment is an important operation in the
manufacturing process of many machine
parts and tools. Only by it we can impart high
mechanical properties to the various metal.

3.  Improve hardness, strength, ductility,
toughness etc.
 Relieve internal stress.
 Eliminate structural inhomogenity.
 Improve machinability.
 Refine grain size and structure.
 Improve wear resistance.

5.  Overheating- Heating for long periods at
temperatures considerably exceeding the
normal values.
Remedies:-
1. For slight overheating-apply normal
annealing and normalising.
2. For more apply double annealing and
normalising.

6.  Burning- Heating for a long period at a high
temperature in an oxidation atmosphere or at
temperatures near the melting point.
Remedies:-
1. 1st stage- use homogenising followed by
double annealing.
2. 2nd stage- use forging followed by
annealing.
3. 3rd stage- defect can not remedied.

7.  Oxidation- Thick layer of scale on the surface
of the steel article. Oxidation atmosphere in
the heating furnace is the caused.
Remedies:-
1. Heat in furnaces with reducing , neutral or
protective atmospheres.
2. Heat in boxes with used carburising agent
or cast iron chips.
3. Heat in molten salt bathes.

8.  Decarburisation- loss of carbon in the suface
layers of the articles. This is due to the
oxidising atmosphere in the heating furnace.
Remedies:-
1. Heat in furnaces with reducing, neutral or
protective atmosphere.
2. Machine off the decarburised layer if the
machining allowances is sufficiently large.
3. Heat in molten salt bath.
4. Heat in boxes with used carburising agent.

9.  Quenching cracks- this is due to the internal
stresses due to volume changes at
temperature, caused by martensitic transfo
rmation.
Remedies:-
1. Avoiding sharp projections ,sharp corners
,and sudden transitions from thick to thin
sections .
2. Apply austempering.
3. Temper immidiately after quenching.
4. Heat to the minimum suitable temperature
for hardening.

10.  Warping- asymmetrical deformation of the
articles in quenching. This is due to volume
changes in cooling or heating, non-uniform
heating, internal stresses etc.
Remedies:-
1. Cool slowly in the martensitic range.
2. Apply surface hardening.
3. Heat uniformly for hardening.
4. Quench as uniformly as possible in
hardening.

11.  Soft spots- zones on the surface of hardened
articles with lower hardness due to
decarburizations and inhomogeneity of
structure.
Remedies:-
1. Using more effective quenching medium.
2. Obtaining a more homogenous structure ,
employing annealing or normalising before
hardening.
3. Protecting against decarburisation in
heating.

12.  Erosion –reduction in size of the articles or
loss of metals from its surface due to
chemical reactios of chlorous salts and
oxidation of metals.
Remedies-
1. Deoxidised salt bath with borax or
ferrosilicon should be used.
2. Properly position the article in reference to
the electrodes of electrical salt bath.

13.  Corrosion- pitting or groove or nonuniform
scale formation type destruction of the
surface of an article due to heating in flame
furnace , salt bath composition and
oxidation.
Remedies –
1. Carefully control salt composition.
2. Deoxidise the bath.
3. Eliminate the oxidising temperature
atmosphere when heating in flame
hardening.

14. The main types of rejects in annealing and
normalising are due to faulty regulation of
heating temerature, including overheating,
burning, underheating, etc….
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