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MAIN HEAT TREATMENTS

Annealing

The annealing of a metal alloy is a heat treatment
consisting in heating to a temperature usually
lower than the melting point, followed by an
appropriate length permanence and by a slow
cooling, usually in furnace.[1] It must achieve
one of the following objectives:

– chemical equilibrium: minor segregation
reduction;
– structural equilibrium: metastable phases
transformation;
– mechanical equilibrium: internal residual
stress reduction, including work hardening.

It is mainly used on steel and copper to prepare
them for the later processing stages, making
the material softer and smoother. In the case of
steel, it is heated to a temperature slightly higher
than the austenitizing one and it is kept at that
temperature for a time sufficient to completely
transform it into austenite; this is followed by a
slow cooling in the furnace.
Through the annealing process the microstructure
of the material is altered, causing changes in its
properties such as flexibility and hardness. The
typical result is the removal of crystalline structure
defects. The annealing can also have the purpose
of standardizing the steel chemical composition,
in this case the heating process is performed at a
higher temperature and for longer times.
The processes that lead to changes in the crystal
lattice during annealing have inspired, in
computing, the simulated annealing technique,
which is a methaeuristic similar to genetic
algorithms.

Normalization

In metallurgy normalization is a heat treatment
consisting in heating the material to a temperature
slightly higher than the austenizing (Ac3 + 50-70
°C), followed by a 15-minute permanence at this
temperature so as to achieve the microstructural
balance and finally by a cooling process in still
air.
This process is similar to annealing, but in this
case the cooling is faster.
The main goal is to refine the crystalline grain of
the steel, to uniform the microstructure and to
mitigate the extension of layered bands of different
phases (structure that is likely to emphasize the
behavior of the weakest phase).
Usually the achieved structures are similar to those
of a material that has undergone an annealing
process: the perlite obtained with normalization,
however, is made up of tinier crystals (due to
faster cooling).
Consequently, resistance is improved and the
ductile-fragile transition is displaced.
Usually this process is performed as the last
operation; it may be the remedy to an overheating
of the grain.
It should always be carried out on jets of carbon
steel and low-alloy and on those already subjected
to homogenizing annealing, to refine the coarse
structure.
It is useful to cancel any heat or mechanical
treatment (eg, quenching and work hardening).
The grain refining that follows is a useful
preparation for subsequent quenching and
carburizing.
For steels to be used at low temperatures, double
normalization is appropriate: a first one at a
higher temperature and a second one at a lower
temperature to refine the grain.
It is performed preferably on hypoeutectoid steels.

Tempering

It is a heat treatment consisting in heating followed
by cooling at a controlled rate, to which may
be subjected steels and light alloys to reduce the
brittleness induced by hardening, though to the
detriment of hardness. The tempering is divided
into different stages depending on the temperature
at which the steel is brought: the higher is this
temperature the better are the mechanical properties
of the structure obtained (apart from the aforementioned
reduction in hardness). Usually the
tempering treatment is carried out immediately
after the quenching.
The quenching treatment followed by 4º stage
tempering is called “quenching and tempering”
(“bonifica” in Italian). It applies to martensitic
materials and leads to the formation of bainite.
The change is from a bct structure to a cubic ferrite
and cementite.

Stress relieving

It is a heating treatment to 150-180 °C, which
causes a slight reduction of the internal stress, without
decreasing too much the hardness. Practically,
it is a particular type of tempering.

Quenching and Tempering (Bonifica)

The expression “quenching and tempering” means
a set of heat treatments carried out on particular
types of steels, consisting of a quenching
followed by a tempering.[1]
During the quenching of steels, martensite is
obtained, a structure with high hardness and
considerable ultimate strength, bit with a rather
low resilience which may give rise to breakage
due to impact. Given the dangerousness of these
events, which result in a practically instantaneous
collapse of the structure, the steel is subjected to
a tempering heat treatment, to transform part of
the martensite into tempered martensite. In fact,
the martensite is a metastable phase, i.e. it comes
out only because the carbon atoms are not able
to escape from the lattice due to the high cooling
rate that prevents diffusive motions.
Since the martensite, in the absence of alloying
elements, is formed only with a carbon percentage
higher than 0.2%, below this limit steels cannot
practically be hardened, therefore a “quenching
and tempering” treatment is not worth to
be carried out, since it is as much useful as the
higher is the carbon percentage. Typically, the
steels used in this treatment have a carbon percentage
of 0.4 – 0.6% and are called “quenching
and tempering steels”.
This sequence of treatments is defined “quenching
and tempering” only in the case the tempering
takes place at a temperature higher than 550
°C (except in the case of steels for springs, which
are tempered at around 450 °C). When brought to
this temperaure, the martensite tempers, turning
into sorbite, a structure which combines a good
tensile strength, even if lower than that of martensite,
with a greater toughness.

Quenching

The quenching[1] treatment in general consists
in the suddeng cooling of a material after it is
brought to an austenization temperature.
The high cooling speed inhibits the diffusive action
apt to restore the equilibrium and the number
of vacancies (therefore of clusters, i.e. groups of
point defects) that competes to the quenches temperature
is kept at room temperature. More generally
it can be said that the quenching, inhibiting
the diffusive processes necessary to thermodinamic
stabilization, transfers to room temperature
a state thermodinamically competent at a higher
temperature.
A so treated single crystal has higher mechanical
strength than a single crystal cooled slowly.
Due to the quenching, for example. the pearlitic
structure of the steel is transformed into martensitic:
you bring the alloy to be quenched at a temperature
of about 50 °C above the austenization
one and you cool it very quickly until room temperature
(it is not necessarily reached). Not having
sufficient time to diffuse, the carbon remains
trapped into gamma cell, which is transformed
into alfa cell at room temperature. This leads to a
tetrahedral structure, which is precisely the martensite.
In ancient times the quenching was performed,
besides in water or in oil, in various urines, whoch
were able to provide a certain amount of nitrates
and nitrites to have also a nitrogen atoms diffusion
(partial nitriding).

Notes on alloy steels

The steel with a carbon concentration greater than
0.3-0.5% shows a high risk of cracking. The presence
of alloying elements slows down the dissolution
of carbides during the austenizing process.
The use of alloy steels in bodies subjected to fatigue
or bending stress can be dangerous because
of the cracking risk, and it is therefore nod recommended
if not essential.

Solution or solubilization quenching

Consider an alloy formed by the solute B dissolved
in the matrix A. If you heat it until complete
dissolution of B and then you cool it suddenly,
usually in water or oil, up to room temperature,
the atoms of B paralyze in metastable conditions,
resulting in a softer and more plastic alloy.
It is applied to austenitic stainless steels (e.g.: :
AISI 304 o X5CrNi1810, AISI 316 o X5CrNi-
Mo1712) to improve the corrosion resistance. A
slow cooling, in fact, would cause the separation
of chromium carbides at the grain boundaries, resulting
in depletion under the 12% (the limit for
passivation) and interganular corrosion.
In the austenitic steel, manganese improves the
toughness. The solution quenching is also carried
out on heat treatment aluminum alloys, before
starting the aging process. Carrying out an aging
process, whether natural or artificial, on an aluminum
alloy considerably improves the mechanical
properties, because it will form very faint precipitates
which block the movement of dislocations.
The aluminum alloys subjected to aging are recognized
by the initial T6 (artificial aging) or T4
(natural aging). For example, AA-2xxx-T6 (aluminum
alloy, 2000 series, heat treatment aluminumcopper,
artificially aged).

Hardness quenching

It is a heat treatment that suppresses the eutectoid
transformation and leads to the formation of
martensite due to continuous cooling. Considering
the graph of the CCT curves, the cooling rate
curve must not cross the CCT curves at each point
of the piece, so that you only get to the formation
of martensite.
It must therefore be kept in mind that the cooling
curve depends on

– quenching bath
– thermal properties of steel
– geometric properties of the treated piece
while the CCT curves depend on
– steel composition (for example, carbon moves
them to the right
– grain size
– non metallic inclusions, carbides, azides or
segregations
The “quenching depth” is detectable through two
methods, based on the principle that the hardness
depends solely on the amount of martensite and
the carbon content.
– “Ideal diameter”. The critic diameter (diameter
of the bar that, after quenching, is 50% martensite
in its core), deducing it from the ideal diameter of
a bar quenched in an ideal quenching bath, with
an infinite severity index H, thanks to the diagram
proposed by Grossmann.
– “Jominy Curve”. A cylindrical sample is quenched
and cooled according to a standard method,
then the Rockwell C hardness is measured along
its axis and a hardness-distance from the extreme
graphic is created. The latter allows to evaluate
and compare the quenchability of different steels
(e.g. 40CrMo4 is more quenchable than C40).
The quenching penetration is obtainable when
you know the hardness corresponding to 50%
martensite.
It is possible to deduce the results of the first method
from those of the second, thanks to standard
correlations encoded in ISO standards.

Heating environment

Oxidation and decarburization of the quenched
piece must be prevented. It can be protected
with: balsamic oil
– solids (chips of grey cast iron, coal) suitable
for electric furnaces, for carbon steels, low-allow
steels up to 0.6% of C, at high chromium (e.g.
X210Cr13) and quenching temperature lower
than 1050°C;
– liquids (molten salt) for valuable pieces, such
as cutting tools or machine parts, which require
uniformity and accuracy of heating;
– gaseous substances (CO, CO2, H2, N2, inert
gases) for economical treatments on a large scale;
a particular case is the vacuum.

Heating rate

Graduality is needed to avoid cracking and thermal
stresses.

Quenching temperature

Attention must be paid to excessively raising the
temperature (to increase the austenitizing speed),
as you may risk overheating of the crystal grain,
burning of grains edges due to oxygen infiltration,
oxidation, decarburization, obtainable excessive
fragility of martensite, residual austenite. That
6 7 being stated, the temperature is 30 °C, 50 °C, 70 °C higher than Ac3 depending on whether the cooling medium is water, oil, air or salt baths.

Permanence in temperature

The permanence time depends on the desired degree
of carbides dissolution:
– carbon and low-alloyed structural steels: a
few minutes;
– medium-alloyed structural steels: at least 15
minutes
– carbon and low-alloyed tool steels: 0.5
minutes per mm of thickness, for one hour
max
– chromium high-alloyed steels: 0.8 minutes
per mm, for one hour max
– steels for hot-workings: half an hour at most,
given the low volume of carbides
– high-speed steels: they are heated at the
highest temperature, so the permanence
must be limited to a minimum (time depends
on thickness)

Quenching media
The optimal fluid should ensure:
– high cooling rate in the A1 – Ms range to
avoid the formation of pearlite or bainite;
– low speed in the Ms – Mf range (but not too
low so as not to create excessive residual
austenite); this feature is proportional to the
difference between the fluid temperature and
its boiling point;
– the fluid must not decompose on contact with
the hot metal.
The most common media are water, oil, molten
salts and air and they are classified according to
the “drastic H index”.
There are 3 cooling stages for liquids subjected
to boiling:
1 – at the first contact between the medium and
the piece a film of steam will form
(calefaction), with relatively slow cooling;
2 – when the fim of steam breaks down, new li
quid touches the piece, absorbs the latent
evaporation heat and the maximum energy
removal is reached;
3 – coming below the boiling temperature, there
will be a drop in the heat removal.

Water is the most widespread extinguishing medium,
especially for carbon steels and some lowalloyed
steels, but is not the ideal fluid. Its action
can be improved with the addition of substances
that would raise the boiling point, for example
NaCl o NaOH.
Mineral oil is suitable for low and medium alloyed
steels, likely to form stable austenite then
convertible with low quenching critical speed.
Mineral oil is closer to the ideal fluid, reducing
internal stresses and quenching defects.
Air is recommended for high-alloyed steels and
for the low and medium-alloyed ones in complex
parts.
Molten salts, suitable for pieces not too big and
made of well-quenchable steel, excel especially
in isothermal treatments that replace quenching.

Induction quenching

A body that is a good conductor of electricity, placed
within an alternating magnetic field, is heated
by the Joule effect due to induced currents:
this event allows to bring a steel object to high
temperature, thus to austenitize it.
Due to the skin effect of alternating current, the
thickness of the heated layer varies with the frequency
of the current (but it also depends of the
material conductivity); in manufacturing are generally
used low frequency generators (below 5
kHz), medium frequency (5 to 30 kHz) and high
frequency (200 kHz). The material layer affected
by heating is inversely proportional to the generated
frequency (low frequency corresponds to
deeper layers).
Then we have the cooling phase, which can be
done by immersion or spray; examples of “localized
quenching” are: scissors or mowers blades,
pliers cutting parts, plowshares, big gears teeth
and mainly pieces that can rotate during heating.
“Progressive quenching”, instead, involves the
sliding of the piece with respect to the coil and
immediate cooling of the outgoing surface. The
method is used for pallets guides, hacksaw blades,
big gears teeth, propeller shafts…
The last step in the process is the induction tempering,
at 160-200 °C.
To avoid cracking, the steels that can be subjected
to this treatment are carbon steels or low-alloyed
steels (39NiCrMo3) with C = 0,30-0,50% (exception:
if the quenching must reach the core of the
piece, 100Cr6 and 100CrMn4 can be used, for
example in rolling bearings). The “quenching and
tempering (bonifica) is used to obtain a starting
structure with faint carbides, which would dissolve
soon in austenite during the fast heating, and
a firm core. For opposite reasons, annealed steels
are excluded (coarse carbides and poorly firm
core).

Austempering

short, the standing in the thermal bath, at a temperature
slightly above Ms (martensite formation
starting temperature). leads to the complete transformation
of austenite in lower bainite, resulting
in a more tenacious material, less tensioned and
without the need for tempering. It can be said that
the lower bainite which is obtained from this process
is the structure with the best mechanical features
compared to all the other structures obtained
from different heat treatments. Of course, this
is also a relative concept, in facts it depends on
what the project requires. However, if we could
be able and wanted to classify the mechanical
properties obtained by the different structures,
lower bainite would be on the top step of the podium.
However, the thermal process to obtain it must
be isothermal and, given the complexity, it becomes
expensive and therefore not widely used.
It is often chosen the alternative closer to lower
bainite, that is sorbitol, which is obtained by tempering
(typically at T≈550 °C – 600 °C) preceded,
of course, by a hardness quenching.

Patenting

It is a variant of austempering, consisting in passing
an armonic steel wire, with a continuous
movement, inside a thermal bath of molten lead
at 500 °C. You get faint pearlite, suitable for wire
drawing.

Martempering

The temepering at a low temperature does not
always adequately eliminate cracks and distorsions.
It is then useful to use martempering, that
is the isothermal break at a temperature slightly
above Ms, in a bath of salts, for the time strictly
necessary to make the piece temperature uniform
but not enough for the formation of bainite. Then
the air cooling and tempering follow.

Advantages: simultaneous formation of martensite,
no oxidation nor decarburization if the final
cooling takes place in a protective atmosphere,
higher toughness at the expense of a bit of
hardness.

Disadvantages: higher installation costs, more residual
austenite.

It is to remember that the martempering is often
used in cases where it is necessary to quench
large pieces (for example gears of marine engines,
molds for plastics) that come to occupy volumes
in the vicinity of cubic meters. Given the
size of the piece, it is immediate to imagine how
the temperature difference between surface and
core of the piece can acquire very high values if
a traditional quenching treatment is carried out
(without the isothermal break of martempering).
The stress states, then, would take on very high
values and would therefore be dangerous for the
integrity of the piece.

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