Annealing is a heat treatment where the material is taken to a high temperature, kept
there for some time and then cooled. High temperatures allow diffusion processes to
occur fast. The time at the high temperature (soaking time) is long enough to allow
the desired transformation to occur. Cooling is done slowly to avoid the distortion
(warping) of the metal piece, or even cracking, caused by stresses induced by differential
contraction due to thermal inhomogeneities. Benefits of annealing are:
- relieve stresses
- increase softness, ductility and toughness
- produce a specific microstructure
11.2 Process Annealing
Deforming a piece that has been strengthened by cold working requires a lot of
energy. Reverting the effect of cold work by process annealing eases further
deformation. Heating allows recovery and recrystallization but is usually limited to
avoid excessive grain growth and oxidation.
11.3 Stress Relief
Stresses resulting from machining operations of non-uniform cooling can be eliminated
by stress relief annealing at moderately low temperatures, such that the effect of cold
working and other heat treatments is maintained.
11.4 Annealing of Ferrous Alloys
Normalizing (or austenitizing) consists in taking the Fe-C alloy to the austenitic
phase which makes the grain size more uniform, followed by cooling in air.
Full anneal involves taking hypoeutectoid alloys to the austenite phase and
hypereutectoid alloys over the eutectoid temperature (Fig. 11.1) to soften pieces which
have been hardened by plastic deformation, and which need to be machined.
Spheroidizing consists in prolongued heating just below the eutectoid
temperature, which results in the soft spheroidite structure discussed in Sect. 10.5. This
achieves maximum softness that minimizes the energy needed in subsequent forming
Heat Treatment of Steels
To achieve a full conversion of austenite into hard martensite, cooling needs to be
fast enough to avoid partial conversion into perlite or bainite. If the piece is
thick, the interior may cool too slowly so that full martensitic conversion is not
achieved. Thus, the martensitic content, and the hardness, will drop from a high
value at the surface to a lower value in the interior of the piece. Hardenability is
the ability of the material to be hardened by forming martensite.
Hardenability is measured by the Jominy end-quench test (Fig. 11.2).
Hardenability is then given as the dependence of hardness on distance from the quenched
end. High hardenability means that the hardness curve is relatively flat.
11.6 Influence of Quenching Medium, Specimen Size, and Geometry
The cooling rate depends on the cooling medium. Cooling is fastest using water,
then oil, and then air. Fast cooling brings the danger of warping and formation of
cracks, since it is usually accompanied by large thermal gradients.
The shape and size of the piece, together with the heat capacity and heat
conductivity are important in determining the cooling rate for different parts of the
metal piece. Heat capacity is the energy content of a heated mass, which needs to be
removed for cooling. Heat conductivity measures how fast this energy is transported
to the colder regions of the piece.
Hardening can be enhanced by extremely small precipitates that hinder dislocation
motion. The precipitates form when the solubility limit is exceeded.
Precipitation hardening is also called age hardening because it involves the hardening of
the material over a prolonged time.
11.7 Heat Treatments
Precipitation hardening is achieved by:
a) solution heat treatment where all the solute atoms are dissolved to form a
b) rapid cooling across the solvus line to exceed the solubility limit. This leads to a
supersaturated solid solution that remains stable (metastable) due to the low
temperatures, which prevent diffusion.
c) precipitation heat treatment where the supersaturated solution is heated to an
intermediate temperature to induce precipitation and kept there for some time (aging).
If the process is continued for a very long time, eventually the hardness decreases.
This is called overaging.
The requirements for precipitation hardening are:
- appreciable maximum solubility
- solubility curve that falls fast with temperature
- composition of the alloy that is less than the maximum solubility
11.8 Mechanism of Hardening
Strengthening involves the formation of a large number of microscopic nuclei, called
zones. It is accelerated at high temperatures. Hardening occurs because the
deformation of the lattice around the precipitates hinder slip. Aging that occurs at
room temperature is called natural aging, to distinguish from the artificial aging caused
by premeditated heating.
11.9 Miscellaneous Considerations
Since forming, machining, etc. uses more energy when the material is hard, the steps in
the processing of alloys are usually:
- solution heat treat and quench
- do needed cold working before hardening
- do precipitation hardening
Exposure of precipitation-hardened alloys to high temperatures may lead to loss of
strength by overaging.
Jominy end-quench test
Precipitation heat treatment
Solution heat treatment